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pH Meter Calibration

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ajdelange

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I frequently get asked about how to use and calibrate a pH meter. Let's start with use and then move on to calibration.

Measurement of mash pH is the use to which meters are most often put by brewers. Assuming the meter is calibrated (see below) here is how that is done.

1. Stir the mash thoroughly. This is especially important if the measurement is to check on the effects of an acid or alkali addition. Withdraw a small sample of the liquid. It doesn't matter if some grain is included.
2. Cool the sample to room temperature, ideally the same temperature as the buffers you used for calibration. This prolongs electrode life and reduces the burden on ATC (more on ATC below). If you use a small metal saucepan you can achieve the cooling quickly by immersing it in cool/cold water.
3. Rinse the electrode with DI water, shake off and blot (see calibration below) and insert the electrode into the sample. Move the electrode around for a few seconds (sample rinses any water off bulb and junction) then stop and wait until the reading is stable. Some meters will decide when the reading is stable for you and beep to signal this. Record both the reading and the temperature.
4. Repeat the process every 5 minutes or so until the readings stop changing. This usually 15 - 30 minutes after strike.
5. Rinse the electrode with DI water and return to storage solution or just tap water for short term (i.e. between readings) storage.

For calibration the overall instructions are simple: follow the manufacturer's instructions. For those who don't have a meter in hand and want to have an idea as to what is involved or for whom the supplied instructions are less than adequate the following is offered. Buffers and samples should be at room temperature.

1. Store the electrode in a storage solution recommended by the manufacturer. This will often be a saturated or nearly saturated solution of potassium chloride (see below for more on storage solution).
2. Prepare fresh 4 and 7 buffer solutions using deionized water. Several manufacturers sell capsules of powder which contain the buffers’ chemical components. These are simply added to a specified amount of DI water (50 or 100 mL) just before use. Premixed buffers are also sold in sealed packages (similar to the ketchup packages from fast food restaurants). These work as well as the buffers one mixes on the spot and are obviously more convenient but tend to be, because of the packaging, more expensive. Premixed buffers are also sold in bulk i.e. 1 L bottles or 4 L jugs or cubitainers. If buffers in this form are being used check that they are not beyond their expiration dates and pour small amounts of each into a clean beaker or preferably, sealed container, at the beginning of each brew day. Do not return used buffer to the bulk storage.
3. Remove the storage cap from the electrode. If the electrode is the refillable type, insure that it contains adequate fill solution, top up if neecessary and, whether you top up or not, open the fill hole so air can enter the electrode body allowing fill solution to freely flow out through the reference junction.
4. Rinse the electrode with a stream of DI water from a wash bottle. Blot dry with clean tissue or paper towel. Don’t touch the actual electrode bulb when you do this. You don’t need to get all the adhering water, just the bulk of it. Wicking of water into the paper is adequate.
5. Turn the meter on, allow it to stabilize for a few minutes, and then lower the electrode into the first buffer solution. With most modern meters it does not matter which one you go into first as these meters have automatic buffer recognition. Following the manufacturer’s instructions put the meter in calibration mode and initiate calibration if necessary (e.g. press the ‘read’ or ‘Cal’ button).
6. Move the electrode around in the buffer a little to rinse any adhering DI water off the bulb and away from the reference junction.
7. Wait until the reading stabilizes. Modern instruments tend to have stability indicators which beep or otherwise alert the operator when the reading is stable (hasn’t changed by more than a threshold amount in a given period of time). These often also instruct the operator to move on to the next buffer when stability is detected. In others you may have to determine when the reading is stable yourself and indicate this to the meter by pressing a button. Follow the manufacturers instructions and/or prompts on the meter’s display.
8. When instructed to move to the second buffer, remove the electrode from the first buffer, shake adhering buffer off and rinse with a stream of DI water. Blot away as above and insert the electrode in the second buffer. Move electrode around in second buffer.
9. When the second reading is stable, take whatever action is necessary to complete the measurement as above. In some meters there will be an option for a third buffer. In those meters you will have to do something (e.g. press an ‘exit’ button) to indicate to the meter that calibration is complete if you are doing a 2 buffer calibration.
10. The instrument will now calculate the calibration parameters (slope and offset) and, in some cases, display these to you in the case of slope either as a percentage (should be near 100) or a number like 57.3 which is the number of millivolts change per unit change in pH at some reference temperature. The offset will be a millivolt number which should be small i.e. a few millivolts (it can be negative). If the meter presents those numbers, write them in your log book. They represent a record of the rate at which your electrode is aging. Fancy meters will automatically store the calibration data, tagged with time and date, in the meter’s memory.
11. Take whatever action is necessary to indicate that the calibration is to be accepted (e.g. press a ‘store’ or ‘exit’ or other button as directed by the manual).
12. Remove the electrode from the second buffer. Shake, rinse and blot as before. Place in sample.
13. Press ‘read’ button if necessary. Otherwise monitor display. Move electrode around in sample.
14. When reading is stable (as determined by you or meter electronics) record pH and temperature. Fancy meters will automatically store these in memory and some will even transmit them to an external computer.
15. Remove from sample, rinse and blot dry as before. Move to next sample. If finished, rinse extra thoroughly. After shaking and blotting dry insure that cap contains sufficient storage solution to cover bulb and replace cap. Turn meter off if finished for the day. If not finished for the day the probe can be left in the last sample.


11b. As a check on the calibration you can measure the 4 and 7 buffers again at room temperature. You may wish to do this after some time has passed or even after you have finished measuring samples for the day. When specifically checking a meter for stability as in the case of a new meter or one that is suspected of degraded performance you should run a fresh calibration and then remeasure the pH 4 buffer over time every couple of minutes up to about 20 minutes and then every 10 minutes or so for couple of hours or more. Record the values and if possible plot them against time. Readings all over the graph (e.g. 4.00, 4.05, 4.07, 3.98, 3.89, 4.10, 3.90...) are indicative of general instability. Readings that cluster around a straight line with a slope are indicative of drift (e.g. 4.00, 4.04, 4.06, 4.08, 4.11, 4.15...).

pH values are often printed on the buffer package. Sometimes they are not. If not and assuming you are using NIST traceable pH 4 and 7 technical buffers the pH values of the buffers are:

pH 7: 1911.4/K -5.5538 + 0.022635*K - 6.8146E-6*K*K

pH 4: 1617.3/K -9.2852 + 0.033311*K - 2.3211E-5*K*K

where K = °C + 273.15 (i.e. K is the temperature in Kelvins).
The values you read should be close to those given by the formulas or on the buffer package. If they are not then your meter is drifting.

Cool the 4 buffer to about 40 °F and measure its pH. Do this right after completing calibration. If your meter reads off by more than a few hundredths then its isoelectric point is not equal to 7 and you must be careful to measure buffers and samples at close to the same temperature (ATC won't work well).

ATC - What Is It?

This is a subject that has confused many a brewer encountering pH meters for the first time. A pH meter has an electrode which produces a voltage that depends on the pH and the temperature of the sample being measured. The voltage is

V = Vo - s*(R/F)*K*(pH - pHi)

Vo is a small 'offset' voltage. It is determined during calibration. s is the 'slope', close to 1, also determined during calibration. R and F are physical constants. K is the temperature in Kelvins (K = °C + 273.15). pHi is the 'isoelectric pH', i.e. the pH at which the electrode response in insensitive to temperature. It is 7, or very close to it in modern meters and is always assumed to be 7 by such instruments. The pH meter calculates and dispalys

pH = pHi - (V-Vo)/(s*R*K/F)

Thus, as the temperature increases, the magnitude of the electrode voltage increases but the reported pH does not because the meter divides by the temperature in calculating its interpretation of the electrode voltage. This is ATC - Automatic Temperature Compensation. Since almost all meters are now digital which implies a calculation like that of the last formula being done in a microprocessor nearly all modern meters are ATC capable. ATC only compensates for the electrode response to temperature. It does not respond to changes in the pH of a sample as temperature changes. As noted in 11b above, even buffers designed for meter calibration have pH values which change with temperature as the formulas given there indicate. To repeat: ATC does not compensate for those changes. The actual pH of mash, for example, decreases by about 0.0055 pH/°C. Even if your meter is equipped with properly functioning ATC you need to allow for actual mash temperature if it is more than a few degrees above room temperature. All pH measurements in brewing are referred to room temperature. If measured at other than room temperature they should be corrected by 0.0055 pH per centigrade degree of difference.

Measure at room temperature. You won't have to do the corrections that way, room temperature avoids thermal stresses on your electrode and it will last longer and, as your buffers are at room temperature, ATC will work well even if your meter's pHi is not exactly 7.


Storage Solution


It is important that the electrode be stored in a manner consistent with the manufacturers instructions. These can vary somewhat but the most usual recommendation is that one buy a storage solution made by the manufacturer for his electrode. These solutions are, as noted above, usually saturated or near saturated solutions of potassium chloride which may include mold inhibitors and a buffer. The reason most electrodes use saturated KCl is because the electrode's reference half cell contains saturated KCl. The interior of the reference half cell communicates with the sample through a 'junction' which is often a small piece of fritted glass though on the electrode I am looking at as I write this it is more like a wick that passes between the body of the electrode and the face through which the temperature sensor and pH bulb protrudes.

When the electrode is placed in sample the concentration of potassium and chloride ions is lower in the sample than in the interior of the reference half cell and ions flow to equalize those concentrations thus conducting the electric current necessary to complete the circuit: Interior of pH bulb glass -> bulb electrolyte (inside bulb) -> pH electrode (platinum wire inside bulb) -> pH meter -> reference cell electrode (platinum wire inside reference half cell) -> reference cell electrolyte (KCl), through the reference junction into the sample -> outside of pH bulb, through bulb glass to bulb interior. When the electrode is placed in storage solution the concentrations of K+, Cl- and water are the same at the outside of the reference junction as at the interior of the reference half cell and, as no concentration gradient exists, no ions or water flow. The electrolyte within the reference cell is thus preserved.

Buffering the pH to the proper value maintains the proper ion distribution within the active layer of the outer surface of the pH glass (bulb) membrane and, of course, the water keeps this layer hydrated.

Meter Recommendation

I am always being asked for recommendations for pH meters and this has, in the past, been difficult because good meters have been priced out of the reach of most home brewers. The inexpensive ones seem to suffer primarily from instability and this is why we gave the stability check instructions earlier. A hint that the meter may not be stable is an indication that the precision is 0.01 but the accuracy is appreciably worse that the precision. A meter is accurate to its precision at calibration. If the calibration does not drift then it will stay accurate. In the fairly recent past tremendous improvements have been made in electrode technology which is as much art as it is science. I believe most of this to be in the area of junctions but I think getting the impedance of the sense glass down may be part of it too. This leads to greater electrode stability. These are what I think the requirements for a good meter are:

1) Stability.
2) 2 Buffer calibration
3) Electrode longevity
4) Junction resistant to fouling by sugars, proteins
5) Lets the user decide when to accept a buffer calibration reading
6) ATC
7) Automatic buffer recognition

1) is the most important. 2) is also very important as one cannot get good accuracy half way between standard buffer values (i.e. mash pH) with single buffer calibration. Conversely with 2 buffer calibration one can get slightly better accuracy that the accuracy of the buffers themselves. 3)Electrodes tend to be the most expensive component in an inexpensive meter. You want an electrode that lasts 2 or more years. 4)This used to mean a double junction electrode but I think some of the new single junction designs are as good, in this regard, as double junction ones. 5)The Hanna pHep meter is quite stable but decides when to accept the calibration reading on its own. It grabs it too early. This throws the calibration off. The operator should make the decision as to when to accept the buffer reading and should only do so when he is positive it is stable. 6)ATC is important more because it signals that the processing is digital than because of a real need for temperature compensation. ATC isn't perfect at doing what it is intended to do unless the electrode is perfect but it is fairly easy to determine the relevant perfection and correct the ATC where very fine work (correction to 0.01 - 0.02 pH) is desired. 7)This just makes calibration a bit easier.

The only inexpensive meter I have met so far that meets these requirements is the new Hach PocketPro+. As it is new I can't of course, comment on the longevity of the electrode nor whether it will plug up with protein. Watch this space.

I hope that this unit will prove over time to be the answer so many have been looking for. Beyond that I hope it represents an advancement in the art that will result in many stable meters in this price range.

The only other inexpensive meter I have any experience with the the Hanna pHEp. It appears to meet all the requirements above except 5 and I'm not sure about 4. This is really too bad because it seems to be quite stable. One can easily overcome this problem by calibrating, then doing the cal check and finding where the meter stabilzes during the check. If, for example, the buffer pH is 4 and the meter stabilizes at 3.98 then just add 0.02 to each meter reading.

Also keep firmly in mind that I have checked only one meter of each type.

The Milwaukee MW102 is getting good grades here on HBT.

PocketPro.jpg
 

Biobrewer

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I work at Thermo Fisher Scientific in Fremont, CA as a particle scientist, and we have to calibrate our pH meters every time we use them. The unfortunate thing is that every meter is different (buttons, setup, platform), and that makes it a pain sometimes. The fortunate thing is that they should all come with extremely detailed instructions, usually pasted to the instrument as to make sure you do not lose them.

No two pH meters will have the same protocol for calibration, so just make sure you**FOLLOW THE INSTRUCTIONS**!! If you do not do this correctly, your calibration curve will be wrong, and then you can end up with some seriously incorrect sample measurements. If you get an incorrect reading, you will be making additions under false pretenses, which may or may not adversely affect your beer.

Just follow the calibration procedure and always use fresh calibration buffer (bacterial contamination in the buffers will quickly change the pH), and you will be fine.
 
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ajdelange

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...we have to calibrate our pH meters every time we use them.
That really shouldn't be necessary (unless, of course, your boss says it is). Modern electrodes are incredibly stable. I just went and stuck one in 4 and 7 buffers and read pH values using the cal in the computer which is 2-1/2 days old and the cal in the meter which is a couple of weeks old. For 4 buffer at 21.5 °C the pH is 4.0036. The 2 day old cal gave me 4.008. The weeks old cal, 3.96. For 7 buffer the pH at 21.5 °C is 7.0110. The 2 day old cal gave me 7.015 and the 2 week old cal 7.02. This is an old electrode so maybe it's aging has stabilized but I am just amazed at how stable these things seem to be.

Now I'm not advocating skipping cals but certainly once a day with a check half way through ought to be good enough for most applications. If you are using ±0.02 buffers the amount of drift I'm observing is less than the uncertainty in the buffer.

I think where people get in trouble WRT calibration is not waiting long enough i.e. the meter has it's own algorithm for determining when to measure the buffer voltage. For a good cal you really need to wait 5 - 10 minutes - especially with an older electrode.

I think the other place people get in trouble is relying on ATC. You have no way of knowing what the isoelectric pH of your meter is (well you do but it takes an extensive set of measurements and some hairy math to get a good estimate) and even if you do you can't punch that into any meter I've seen (that's why I do cal and measurement in the computer and, of course, for the recorded data). So it is imperative that sample and buffers be at the same temperature. I'll note that the electrode I spoke of earlier has an isoelectric pH of 8.6 - way out of spec.
 

Biobrewer

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That really shouldn't be necessary (unless, of course, your boss says it is). Modern electrodes are incredibly stable. I just went and stuck one in 4 and 7 buffers and read pH values using the cal in the computer which is 2-1/2 days old and the cal in the meter which is a couple of weeks old. For 4 buffer at 21.5 °C the pH is 4.0036. The 2 day old cal gave me 4.008. The weeks old cal, 3.96. For 7 buffer the pH at 21.5 °C is 7.0110. The 2 day old cal gave me 7.015 and the 2 week old cal 7.02. This is an old electrode so maybe it's aging has stabilized but I am just amazed at how stable these things seem to be.

Now I'm not advocating skipping cals but certainly once a day with a check half way through ought to be good enough for most applications. If you are using ±0.02 buffers the amount of drift I'm observing is less than the uncertainty in the buffer.

I think where people get in trouble WRT calibration is not waiting long enough i.e. the meter has it's own algorithm for determining when to measure the buffer voltage. For a good cal you really need to wait 5 - 10 minutes - especially with an older electrode.

I think the other place people get in trouble is relying on ATC. You have no way of knowing what the isoelectric pH of your meter is (well you do but it takes an extensive set of measurements and some hairy math to get a good estimate) and even if you do you can't punch that into any meter I've seen (that's why I do cal and measurement in the computer and, of course, for the recorded data). So it is imperative that sample and buffers be at the same temperature. I'll note that the electrode I spoke of earlier has an isoelectric pH of 8.6 - way out of spec.
In fact it is very necessary. Because we are ISO 13485 certified, we have very strict standard operating procedures (SOPs) we have to follow. The branch of Thermo Fisher Scientific I work for makes clinical diagnostics, and if we don't follow the SOP and the FDA or some OEM customer does an audit, we are in serious trouble. We're talkin' corrective actions, fines, and potentially losing your job for negligence. In a highly regulated facility, the thought is: how do you know that someone else didn't mess with it? How do you know the calibration is still good? In a good manufacturing procedure (GMP) lab, the only way to know is to calibrate before every use.

I'm sure fly-by-night labs or academic labs can do what ever they want. In fact, I know they can because I just completed my graduate studies a year ago, and we only calibrated when it gave an electrode error. However, being an ISO 13485 facility, there are rules, and if they are not followed, there is hell to pay.
 

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I have a question about step 5.

5. "Turn the meter on, allow it to stabilize for a few minutes, and then lower the electrode into the first buffer solution."
This implies it is OK to have power applied to the electrode without the electrode being submerged.

There was an article by Chris Bible in the September, 2010 issue of BYO magazine "How a pH Meter Works", in which he said to place the electrode in the sample before turning on the meter, and stated under the subheading "Practical tips for using a pH meter":
"6. Turn on the power to the electrode. The power to the electrode should never be on unless the electrode is submerged. Applying power to an electrode when it is not submerged will decrease electrode life."

Is this really a bad thing?
 

Woodbury419

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I pre and post cal all of my equipment when i brew, from my pH meter to my thermometer. although on the pH buffer solutions I just cheat and buy the solutions instead. But yeah, I do industrial hygiene and environmental work..so all of my equipment I operate must be pre-calibrated or ops checked before use and post-cal'd afterwards. besides, there are some equipment that "drift" when in use. The Sam-935 (radiation meter) drifts every 30 minutes and has to be re-calibrated. thankfully my pH pen does conductivity, pH, temp, and turbidity!
 
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ajdelange

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In fact it is very necessary. Because we are ISO 13485 certified....
Figured it was some deal like that.

So I am prompted to ask do you really calibrate the meter i.e. measure slope, offset and isoelectric pH or just slope and offset? And if not the former are all measurements, buffer and sample, made at the same temperature?
 
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ajdelange

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There was an article by Chris Bible in the September, 2010 issue of BYO magazine "How a pH Meter Works", in which he said to place the electrode in the sample before turning on the meter, and stated under the subheading "Practical tips for using a pH meter":
"6. Turn on the power to the electrode. The power to the electrode should never be on unless the electrode is submerged. Applying power to an electrode when it is not submerged will decrease electrode life."
With all due respect to the gentleman apparently he doesn't understand "How a pH Meter Works". The electrode is a completely passive device (unless it's an ISFET type and those still aren't really competing with glass). IOW no power is applied to it. It is, in fact, a feeble battery that supplies power to the meter (though precious little).

Now modern electrodes may put some of the electronics in the electrode body. I have one which apparently contains the preamp, A/D and microprocessor in the electrode as once you calibrate it you can move it to any other meter in that family and resume using it without recalibration. The cal is stored in the electrode itself. In this case power is supplied to the head of the electrode to run all the electronics in it but no power is supplied to the actual electrode.

So not to worry about this. You can safely take the electrode in and out of solutions while the power is on or off. What you should not do, however, is allow the bulb to dry.
 
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ajdelange

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I understand that he is buying buffer solutions but he says he is buying them instead of doing something else or instead of something else and that this is a 'cheat'. My question is "Instead of what?"
 

DeafSmith

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So not to worry about this. You can safely take the electrode in and out of solutions while the power is on or off. What you should not do, however, is allow the bulb to dry.
Thanks for clearing that up. Now I can stop worrying that the two or three times I "slipped up" and turned on power without the electrode submerged might have damaged it. I wondered why the instructions that came with my meter didn't have any cautions about applying power to the electrode without having it submerged.
 

Biobrewer

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Figured it was some deal like that.

So I am prompted to ask do you really calibrate the meter i.e. measure slope, offset and isoelectric pH or just slope and offset? And if not the former are all measurements, buffer and sample, made at the same temperature?
We always calibrate and make sure our percent slope is within the acceptable range.

It is ideal to have the calibrators and the samples at the same temperature, but we have pH meters that correct for variation in temperature. Basically, it takes the temperature the calibrators we run at, and corrects accordingly based upon the temperature of the sample.
 
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ajdelange

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What I am getting at here is that there are really three calibration parameters slope, offset and isoelectric pH (pHi). Modern manufacturers assume pHi = 7.00 and modern electrode manufacturers strive to produce electrodes for which 6.5 < pHi < 7.5. I have checked the 2 electrodes I use most. One has pHi = 6.81 but for the other pHi = 8.38!.

The uncertainty in pHi causes uncertainty in any measurement made with a meter unless the buffer and sample temperatures are all the same. If the buffer temperature is Tb and the sample temperature Ts this uncertainty is sigma_pHi*abs(Tb-Ts)/Tb where sigma_phi is the uncertainty in the pHi of the electrode. This amounts to 0.034*sigma_pHi for each 10 °C in difference between buffer and sample. Assuming that pHi uncertainty is about .3 (for easy math and not terribly inconsistent with 6.5 < pHi < 7.5 or the observed pHi for my 'good' electrode that's 0.01 uncertainty for each 10 °C. If the buffers were at different temperatures when you did the cal there is additional uncertainty associated with this effect from that. If you are taking measurements 20 or 30° away from the calibrating temperature you are kidding yourself if you think frequent calibration is a benefit as the uncertainty in the readings caused by pHi uncertainty is greater than the stability related uncertainty. This is why the calibration instructions contain step 11b in which you check for pHi related ATC problems. This is also why some authors insist that that buffers and sample all be at the same temperature. If Ts = Tb the uncertainty from pHi goes away no matter how large it is.

While I am certainly not familiar with your labs procedures this strikes me as possibly another example of regulations that attempt to suppress one problem without regard to another potentially greater one. I worked in the DC area for years and I've seen plenty of this. Having vented I will say that it is my opinion that where drugs, airplanes and similar things are involved overdoing it a bit on the side of safety is justifiable.

Note that if you calibrate your meter for pHi and insert the proper pHi into your ATC algorithm you can use ATC with confidence. I use the electrode with pHi = 8.38 all the time at varying temperature but ATC is done in the computer - not the meter.
 

Biobrewer

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What I am getting at here is that there are really three calibration parameters slope, offset and isoelectric pH (pHi). Modern manufacturers assume pHi = 7.00 and modern electrode manufacturers strive to produce electrodes for which 6.5 < pHi < 7.5. I have checked the 2 electrodes I use most. One has pHi = 6.81 but for the other pHi = 8.38!.

The uncertainty in pHi causes uncertainty in any measurement made with a meter unless the buffer and sample temperatures are all the same. If the buffer temperature is Tb and the sample temperature Ts this uncertainty is sigma_pHi*abs(Tb-Ts)/Tb where sigma_phi is the uncertainty in the pHi of the electrode. This amounts to 0.034*sigma_pHi for each 10 °C in difference between buffer and sample. Assuming that pHi uncertainty is about .3 (for easy math and not terribly inconsistent with 6.5 < pHi < 7.5 or the observed pHi for my 'good' electrode that's 0.01 uncertainty for each 10 °C. If the buffers were at different temperatures when you did the cal there is additional uncertainty associated with this effect from that. If you are taking measurements 20 or 30° away from the calibrating temperature you are kidding yourself if you think frequent calibration is a benefit as the uncertainty in the readings caused by pHi uncertainty is greater than the stability related uncertainty. This is why the calibration instructions contain step 11b in which you check for pHi related ATC problems. This is also why some authors insist that that buffers and sample all be at the same temperature. If Ts = Tb the uncertainty from pHi goes away no matter how large it is.

While I am certainly not familiar with your labs procedures this strikes me as possibly another example of regulations that attempt to suppress one problem without regard to another potentially greater one. I worked in the DC area for years and I've seen plenty of this. Having vented I will say that it is my opinion that where drugs, airplanes and similar things are involved overdoing it a bit on the side of safety is justifiable.

Note that if you calibrate your meter for pHi and insert the proper pHi into your ATC algorithm you can use ATC with confidence. I use the electrode with pHi = 8.38 all the time at varying temperature but ATC is done in the computer - not the meter.

Offset error cannot be completely compensated by ATC on a meter. However, the error will be around 1/10 of a pH unit at it's worst for a good pH meter. For a home brewer, that is not a big deal. However, that is with a good pH meter. I happen to have a laboratory-grade pH meter from an old job that is highly accurate over a fairly wide temperature range, though I know that cheaper pH devices, like the ones readily available to home brewers for <$50-100, will not meet that same spec, and will likely vary more with temperature and have a larger error associated with temperature drift.

In industry (and certainly where I work), it is considered good lab practice (GLP) to read a sample within 5 C of your calibrators. In fact, it is mandatory, and any greater temperature disparity will not be accepted and will invalidate any future work completed using that material. These temperatures must be recorded and witnessed by another person, because the tests we make need to be painfully accurate, as people's lives depend on their accuracy.

However, when I am not making clinical diagnostics in a highly regulated environment and am instead brewing beer in my kitchen, I usually won't sweat over 0.1 pH units, though my sample I read is so small it is probably darn near room temp anyway by the time I get it out of the kettle and on the meter. :)

Either way, cheers! You started a good thread with some sound advice.
 
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ajdelange

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I don't think this is worth belabouring too much more but a couple of things caught my eye. They may be more a matter if terminology that anything else.

Offset error cannot be completely compensated by ATC on a meter.
A pH electrode produces, when exposed to a solution at pH, a voltage

E = Ei - 58.167*slope*((T + 273.15)/293.15)*(pH - pHi)

'slope' is a number close to 1 and Ei, technically the isolelectric voltage, is a few millivolts at most and is the voltage the meter would produce if pH - pHi. Given that the meter assumes that pHi - 7, it thinks Ei is the offset.

IOW, the meter thinks

E = Eo - 58.167*slope*((T + 273.15)/293.15)*(pH - 7)

and calculates Eo (the voltage at pH 7) and slope based on that assumptions from buffer readings. Presented a voltage E, after calibration, the meter calculates and displays

pH = pHi + (E - Eo)/( 58.167*slope*((T + 273.15)/293.15) )

If pHi is indeed 7 then Ei = Eo is not a function of temperature and ATC has no problem dealing with sample temperatures different from buffer temperatures. But if pHi != 7 then Eo is a function of temperature and the meter has no way of dealing with that because it assumes it is not a function of temperature. OTOH if you know the true value of pHi you can determine, from the calibration, what the actual value of Ei is and from that point on you are fine.

However, the error will be around 1/10 of a pH unit at it's worst for a good pH meter.
That's a lot of uncertainty. A whole pH unit of uncertainty in pHi and buffer temperatures separated from sample temperature by 32 °C would be required to induce that much pH reading uncertainty from ATC error. A more reasonable level of pHi uncertainty is probably 0.3 as I noted in a previous post (but I do own that rogue electrode) for which the ATC unceratainty is more like 0.03 with the 32 °C temperature spread. That's still enough to dominate the uncertainties associated with temperature and voltage readings and buffer pH uncertainty (± 0.02 in the technical buffers used by most home brewers).

For a home brewer, that is not a big deal.
I guess I agree for the average home brewer. But when trying to do things like figure out the titratable acidity of a malt, for example, that's much more error than I am willing to live with.

However, that is with a good pH meter. I happen to have a laboratory-grade pH meter from an old job that is highly accurate over a fairly wide temperature range,
I'm sure this one is semantics but I'll point out any way that it is not the meter that is in question here but the electrode. The meter simply dumbly plugs readings into the second equation above blissfully assuming that the offset is the isoelectric voltage and pHi = 7. It is the fact that the electrode's pHi != 7 that induces most of the error when ATC is being used. All the meter itself needs to be able to do is measure temperature with rms error less than 0.5 °C and voltage with rms error less than 0.5 mV neither of which are terribly demanding requirements. It is, of course, just good systems engineering that the meter itself produce uncertainties low enough that when rss'd with the uncertainties from buffers and electrode the latter are dominant.

though I know that cheaper pH devices, like the ones readily available to home brewers for <$50-100, will not meet that same spec, and will likely vary more with temperature and have a larger error associated with temperature drift.
I wonder about that (and I don't mean I'm challenging that statement but literally that I keep thinking about it). From what I have been able to deduce with a small sample size it is often not that cheap meters drift so much as it is that the calibration routines take readings before the results are stable enough. For a calibration I find 10 minutes is a better criterion that just waiting until the change is less than 3 mV/min or whatever the auto cal routines use. Put another way the cheap meters jump too soon and get bal cal parameter estimates so that in reading samples they display stable, but wrong, pH values. This is easily fixed. OTOH some cheap meters are just unstable. That's not fixable.

I will also assert that $ is not a defense against an unusually large pHi deviation. The electrode I have that is way off cost me over $250.

I'll close by noting again that if you know pHi you can (and I do) use ATC confidently at temperatures quite removed from the calibration buffer temperature. The problems with this are 2
1. Meter's won't let you set in a pHi value
2. Since it takes a rather large pHi error (0.3) to induce a fairly small error in pH even at a fairly high temperature excursion (0.03 pH at 32 °C differential) pHi isn't very 'observable'. It thus takes hundreds of buffer readings at various temperatures and some fairly advanced math to estimate pHi from those readings.
 

Woodbury419

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ajdelange said:
I understand that he is buying buffer solutions but he says he is buying them instead of doing something else or instead of something else and that this is a 'cheat'. My question is "Instead of what?"
Mixing my own cal juice solutions is what I meant. I cheat and purchase Hach juice instead. :)
 

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What I am getting at here is that there are really three calibration parameters slope, offset and isoelectric pH (pHi). Modern manufacturers assume pHi = 7.00 and modern electrode manufacturers strive to produce electrodes for which 6.5 < pHi < 7.5. I have checked the 2 electrodes I use most. One has pHi = 6.81 but for the other pHi = 8.38!.

..........

A pH electrode produces, when exposed to a solution at pH, a voltage

E = Ei - 58.167*slope*((T + 273.15)/293.15)*(pH - pHi)

'slope' is a number close to 1 and Ei, technically the isolelectric voltage, is a few millivolts at most and is the voltage the meter would produce if pH - pHi. Given that the meter assumes that pHi - 7, it thinks Ei is the offset.

IOW, the meter thinks

E = Eo - 58.167*slope*((T + 273.15)/293.15)*(pH - 7)

and calculates Eo (the voltage at pH 7) and slope based on that assumptions from buffer readings. Presented a voltage E, after calibration, the meter calculates and displays

pH = pHi + (E - Eo)/( 58.167*slope*((T + 273.15)/293.15) )

If pHi is indeed 7 then Ei = Eo is not a function of temperature and ATC has no problem dealing with sample temperatures different from buffer temperatures. But if pHi != 7 then Eo is a function of temperature and the meter has no way of dealing with that because it assumes it is not a function of temperature. OTOH if you know the true value of pHi you can determine, from the calibration, what the actual value of Ei is and from that point on you are fine.
I feel as though you are somehow trying to justify using a probe with a clearly out of specification pHi/offset by crunching some numbers into an equation. Yes, an offset of 1.38 will destroy your accuracy if measuring at different temperatures than you calibrate at. However, I keep thinking (literally I can't stop thinking) why in God's name anyone would use such a terrible out of spec electrode, and for that matter, why they wouldn't return it after paying $250.

The bottom line is, yes, you should read at a similar temperature to your cal to ensure the most accurate read. However, if your offset is within typical specifications of a good pH electrode and your meter has ATC, reading at a slightly higher temperature is more that accurate to hit a target mash pH within an acceptable error.

Perhaps you would want to be at the same sample/calibrator temperature when titrating acid in malt, but that sounds more like malting chemistry (lab science) and much less fun than brewing a batch of delicious beer. Perhaps it just sounds like less fun to me because my profession is being a scientist, and I spend most of my day in lab running mundane techniques like particle surface acid content titrations. If I wanted to bore myself outside of work with titrations that mean little to the bottom line of making good beer (other than the ability to tell someone I did it), I'd flashback 10 years to undergrad and retake Quantitative Analysis (I miss you, Dr. Showalter). But if titrations are your bag, that's great. Just realize that most home brewers don't need that kind of accuracy in day-to-day brewing operations. All they need is an electrode operating within the proper offset specifications of a good electrode and a correct calibration.
 
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ajdelange

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I feel as though you are somehow trying to justify using a probe with a clearly out of specification pHi/offset by crunching some numbers into an equation.
Every meter you have ever used (unless you go far enough back to have used manual meters and manual meters did the same thing but in analog circuitry rather than a microprocessor), crunches numbers through that same equation. The only difference in what I am doing and what every meter does is putting in the correct number for isoelectric pH. This allows me to use any electrode, whatever its pHi. I think that is pretty good justification. The downside is that I have to do the crunching rather than the meter. In the lab this is fine. In fact I prefer it as the computer commands readings, records and displays them as a time history and does the 'crunching'. In the brewery this would be impractical so there I use a meter with a pHi close enough to 7 that I can trust the ATC readings given that I cool all samples to room temp.


Yes, an offset of 1.38 will destroy your accuracy if measuring at different temperatures than you calibrate at.
The offset is typically around 1.38 mV and that's not a problem. It is the pHi that is the potential problem and that is not a problem if you use ATC correctly which is a simple matter of putting the correct pHi into the algorithm.
However, I keep thinking (literally I can't stop thinking) why in God's name anyone would use such a terrible out of spec electrode, and for that matter, why they wouldn't return it after paying $250.
Well $279 is $279 (replacement cost) and there is really no reason to replace this electrode as it works just fine with rock solid stability (holds cal for days), good response time, offset of less than 2 mV, slope of over 98% and a design which allows the reference junction to be renewed at the push of a button. And all this after a bit over 2.5 years in service!. That isn't bad if you recall the days when you were lucky to get a year out of an electrode. The fact that pHi is out of spec is immaterial given that I am using ATC that knows that.

Now had I known that the pHi was out of spec I doubtless would have called the manufacturer and demanded a replacement but I didn't discover that until I got interested in understanding how a pH electrode really works and ginned up an algorithm for measuring pHi (which as I noted in an earlier post isn't very observable). This (thorough understanding) is the overall major motivation. Anyway the discovery didn't come until well past the electrode's 1 year warranty's expiration. OTOH there was really no need to replace it. I just put the correct number into the ATC algorithm. An electrode with a pHi of 8.3 is not appreciably better or worse than one with pHi=7. We would actually be slightly better off if pHi = 5.4 or so i.e. within the range we as brewers are mostly interested in

Perhaps you would want to be at the same sample/calibrator temperature when titrating acid in malt, but that sounds more like malting chemistry (lab science) and much less fun than brewing a batch of delicious beer.
I think that's a matter of personal preference. There is a lot more to brewing than making good beer!

Perhaps it just sounds like less fun to me because my profession is being a scientist, and I spend most of my day in lab running mundane techniques like particle surface acid content titrations. If I wanted to bore myself outside of work with titrations that mean little to the bottom line of making good beer...
Many brewers refuse to buy a pH meter and rely on spreadsheets to predict mash pH and these are based on models of titratable acidity/alkalinity measured at one temperature with hardware store hydrochloric acid and lye from the wine supply shop. I don't personally believe it is possible to model malt titratable acidity because there are too many variables and am trying to verify that using more robust technique. Which ever way I find there is a potential to help thousands make better beer either by improving the spreadsheets or advising people as to the size of the grain of salt with which they should be taken.

As to the busman's holiday aspect: I don't think I ever took a pH reading at work but I certainly did use the estimation techniques I am using to study pH measurement quality extensively in my professional life. Any road, this bus man doesn't drive the bus any longer so this kind of exercise is great at keeping the rust out of the hinges.

Dialogues like this one are very useful to me. As a consequence of this thread I found 2 errors in a monograph on this very subject I am working on at the moment. Perhaps that will explain why I have answered several questions that no one really, perhaps, wanted answered. Helps me gain insight. So thanks![/QUOTE]
 

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ajdelange said:
Every meter you have ever used (unless you go far enough back to have used manual meters and manual meters did the same thing but in analog circuitry rather than a microprocessor), crunches numbers through that same equation. The only difference in what I am doing and what every meter does is putting in the correct number for isoelectric pH. This allows me to use any electrode, whatever its pHi. I think that is pretty good justification. The downside is that I have to do the crunching rather than the meter. In the lab this is fine. In fact I prefer it as the computer commands readings, records and displays them as a time history and does the 'crunching'. In the brewery this would be impractical so there I use a meter with a pHi close enough to 7 that I can trust the ATC readings given that I cool all samples to room temp.

The offset is typically around 1.38 mV and that's not a problem. It is the pHi that is the potential problem and that is not a problem if you use ATC correctly which is a simple matter of putting the correct pHi into the algorithm.

Well $279 is $279 (replacement cost) and there is really no reason to replace this electrode as it works just fine with rock solid stability (holds cal for days), good response time, offset of less than 2 mV, slope of over 98% and a design which allows the reference junction to be renewed at the push of a button. And all this after a bit over 2.5 years in service!. That isn't bad if you recall the days when you were lucky to get a year out of an electrode. The fact that pHi is out of spec is immaterial given that I am using ATC that knows that.

Now had I known that the pHi was out of spec I doubtless would have called the manufacturer and demanded a replacement but I didn't discover that until I got interested in understanding how a pH electrode really works and ginned up an algorithm for measuring pHi (which as I noted in an earlier post isn't very observable). This (thorough understanding) is the overall major motivation. Anyway the discovery didn't come until well past the electrode's 1 year warranty's expiration. OTOH there was really no need to replace it. I just put the correct number into the ATC algorithm. An electrode with a pHi of 8.3 is not appreciably better or worse than one with pHi=7. We would actually be slightly better off if pHi = 5.4 or so i.e. within the range we as brewers are mostly interested in

I think that's a matter of personal preference. There is a lot more to brewing than making good beer!

Many brewers refuse to buy a pH meter and rely on spreadsheets to predict mash pH and these are based on models of titratable acidity/alkalinity measured at one temperature with hardware store hydrochloric acid and lye from the wine supply shop. I don't personally believe it is possible to model malt titratable acidity because there are too many variables and am trying to verify that using more robust technique. Which ever way I find there is a potential to help thousands make better beer either by improving the spreadsheets or advising people as to the size of the grain of salt with which they should be taken.

As to the busman's holiday aspect: I don't think I ever took a pH reading at work but I certainly did use the estimation techniques I am using to study pH measurement quality extensively in my professional life. Any road, this bus man doesn't drive the bus any longer so this kind of exercise is great at keeping the rust out of the hinges.

Dialogues like this one are very useful to me. As a consequence of this thread I found 2 errors in a monograph on this very subject I am working on at the moment. Perhaps that will explain why I have answered several questions that no one really, perhaps, wanted answered. Helps me gain insight. So thanks!
[/QUOTE]

For sure! I find this stuff interesting, but I think being to worried about some of the super fine details like acid to titratability might be a little overboard. But that's just me, and clearly you enjoy it, so that's good. I suppose it's good for brewers to understand these concepts even if they do not practice them.

And by 1.38 offset, I meant an offset of pHi, which would actually be a ~ 80 mV offset, which is significant if using varying buffer and sample temps. Perhaps I am looking at this wrong, but if your pHi is 8.38 for one of your electrodes, I am confused as to how your offset would be 2 mV?

Also, what field of work are you in?

Cheers!
 
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ajdelange

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And by 1.38 offset, I meant an offset of pHi, ...
Was pretty sure you did.

... which would actually be a ~ 80 mV offset which is significant if using varying buffer and sample temps. But I guess that's why you strive to use the same temperature!
The offset (what the electrode produces at pH 7 at 20°C) is actually still quite small (a mV or 2). It is the isoelectric voltage which about -80 mV. The offset is now a function of temperture, however.

Also, do you work in a scientific field? I think it's a little rare for a standard home brewer to have this depth of knowledge on something like this. If you are self studied, that's impressive!
I was an electrical engineer - now retired. Let's not get into the are engineers scientists debate. Some certainly are not. Others lean that way. I was always accused by management of being a scientist but I got my hands dirty too.

In measuring pH you need to know slope, isoelectric voltage, and isoelectric potential which you determine by calibration. In pointing an antenna you need to know how much the pedestal is tilted to the north, how much it is tilted to the east and how much it is rotated WRT true north.You obtain these data by calibration. The math is exactly the same, easy enough to implement in any of today's computers and extremely powerful. You can solve systems of lots of non linear equations (example: finding your lat, lon, altitude and clock offset from multiple observations of GPS satellite pseudo ranges).

Thus most of what I learned about estimation theory I picked up on the job and it just maps right over into any situation where measurement and measurement quality are involved. What little chemistry I know (beyond what we got in core engineering) I've picked up outside work.

My wife does Sudoku to hold dementia at bay. I do this stuff. Whether either is effective remains to be seen.
 

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The offset (what the electrode produces at pH 7 at 20°C) is actually still quite small (a mV or 2). It is the isoelectric voltage which about -80 mV. The offset is now a function of temperture, however.
I see.

I was an electrical engineer - now retired. Let's not get into the are engineers scientists debate. Some certainly are not. Others lean that way. I was always accused by management of being a scientist but I got my hands dirty too.
HA. I did biology/chemistry in undergrad and biomedical engineering as a graduate student, so I know exactly what you mean here.

My wife does Sudoku to hold dementia at bay. I do this stuff. Whether either is effective remains to be seen.
This will definitely keep the mind sharp!
 

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Wow, I've been calibrating and using pH meters every day for 13 years and I think some of you understand how it works better than I do!!
 

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If one (not saying me, necessarily) wanted to check pH of cooled wort for under $20, would any of these options work better than pH strips (which I understand to not be particularly accurate)? The main concern is getting in "the ballpark" of correct pH (somewhere between 5.1 and 5.7), not dialing in to .01 accuracy.

1. Soil pH meter:
http://www.walmart.com/ip/Luster-Leaf-Digital-Soil-pH-Meter/19854922
Positive: durable, inexpensive, could test in less than an inch of cooled wort.
Negative: not intended for liquid, little brewer community support, likely has a lead tip (not a big issue if the brewer discards the sample after testing).

2. Yellow plastic pH tester:
http://www.amazon.com/gp/product/B00ESCZEHK/?tag=skimlinks_replacement-20
Positive: cheap (can purchase one for less than the price of a new probe for a good pH meter), plentiful, and some people seem happy with the results
Negative: flimsy, unreliable (reported), and inaccurate (reported).

Thoughts?
 
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ajdelange

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My gut feel is that you would be throwing your money away but then it isn't much money in either case. The soil testers are really just voltmeters, which a pH meter is too, with the difference being that in the soil tester the voltage is that developed across a pair of dissimilar metals stuck into the soil. If this related to actual soil pH to within 0.5 pH I'd be surprised but I've never checked one of these. In a pH meter it is the voltage across a thin glass membrane that is sensitive to hydrogen ions that is measured. For $10 I just don't see how the thing could be considered anything more than a toy. It says it is accurate to 0.1 pH and they give you some buffers to calibrate it but I'm guessing its stability is terrible. I'd be afraid that it would give you a reading so far off that you would be motivated to take corrective action when doing so would actually degrade your beer rather than improve it.
 

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Thanks for the reply. I do think the $10 yellow plastic one is too delicate and poorly made to be worth much. If it lasts three batches, the cost-per-batch is way too high to be worth it.

Still, I wonder about the soil meter. I like durable small equipment in the brewery environment, and very thin glass just begs to be broken. The dual-metal tip seems like a durable solution, provided it will work. I don't have a high quality meter, or otherwise I would test the soil meter against it.
 

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I'm shopping for a ph meter and I'm looking at the Hach PocketPro+. I noticed that it only has a one point calibration and no atc. Do the other superior features of this product make it a worthwhile buy at this price point?
 
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It is capable of 1 (don't do that) 2 or 3 (you won't need it) point calibration and it does have ATC - it's a digital meter so it automatically has it.

We collectively have data on two examples here on HBT and it seems to be stable.
 

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It is capable of 1 (don't do that) 2 or 3 (you won't need it) point calibration and it does have ATC - it's a digital meter so it automatically has it.

We collectively have data on two examples here on HBT and it seems to be stable.

3 point would be good for high pH and low pH testing correct?
( my pH in a reef tank wants 8.3-8.4 )

Where here we want around 5.X

Let me know if my assumption is correct
 

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You really just want to bracket your target. So for your fish tank, you would want to use 7 and 10 buffers, for example. Or if you did 4, 7 and 10, you'd be good for both your tank and brewing.

Using 3 buffers for calibration is better, but probably isn't necessary for what we need.
 
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ajdelange

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You really just want to bracket your target.
True.

In particular you want the buffer pH's to span the target pH and be as close to it as possible. Thus for, example, in measuring mash pH, buffers of 5 and 6 would be better (but not appreciably) than buffers at 4 and 7 but, whereas the latter are very easy to obtain, the former aren't.


Using 3 buffers for calibration is better..
Not true. A three point calibration yields two slope and 2 offset values, one set used between 4 and 7 and the other between 7 and 10. The pair used between 4 and 7 does not depend in any way on the measurement on the 10 buffer nor does the pair used between 7 and 10 depend in any way on the measurment made on pH 4 buffer. You gain nothing by making a third buffer measurement as long as your sample measurements lie between 4 and 7.

The attainable accuracy depends on the accuracy with which the buffers pH's are known, the accuracy with which they are measured and where sample pH lies relative to the buffers' pH's. Best accuracy is attained when the sample pH lies half way between the pH's of the two buffers as it does in the case of mash pH (4 + 1.5) = 5.5 = (7 - 1.5).
 

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Isn't this just a calibration curve? And isn't a 3 or 5 point calibration curve always better than single or 2 point calibration curve? Maybe pH meter calibration is different.
 
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ajdelange

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No, the assumption is that E = E0 + s*(R/F)*T(pH - pHi) i.e. that the electrode response is linear over the region spanned by the buffer pairs (in a 3 buffer measurement). Thus the calibration curve (and it is indeed a calibration curve) is considered piecewise linear. Ideally s and E0 would be the same for both segments but that is not in fact usually the case because pHi isn't exactly 7 for most electrodes.

If we were to consider a continuous calibration curve it would have to be of the form E = a1*(pH-pHi) + a2*(pH-pHi)^2 + a3*(pH - pHi)^3 and we would either have to allow (assuming that we wanted E at the buffer pH's to exhibit minumum rms error relative to buffer pH) the post calibration reading on pH 7 buffer read other than what we know its pH to be or constrain it to read the correct answer in which case the post-cal readings on 4 and 10 buffer will be off. It makes much more sense to closely surround the target pH such that the theoretical linear response is close to the truth.
 

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So what meters are recommend? (I am leaning toward the milwaukee - Model MW101)

From the discussion
Hach PocketPro+ $110 (best place to buy? Looks like direct)
Replacement pH probe is $67

Thoughts on the below
Milwaukee MW101 pH Meter w/Battery $80 (2 point) - BNC pH Probe Style
RESOLUTION 0.01 pH
ACCURACY (@25°C) ±0.02 pH
http://www.bulkreefsupply.com/catalog/product/view/id/709/

(You can get 2 Wire pH Lab Probes from HongKong for < $20 that work great 3 months and fine so far with this on my Marine Tank)
- http://www.ebay.com/itm/251124829815?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

& Here at the top 2 that get good reviews on the Marine Side

American Marine Pinpoint pH Monitor $99 (2 Point Calibration / .01 resolution / Accuracy ?)
(just needs a 4.0 cal packet added)
BNC Replaceable Probe Style (You can get Lab Probes from HongKong for $20 that work great)
http://www.marinedepot.com/American...ine_Pinpoint_Monitors-AM1111-FITEMOID-vi.html

and this one is $37.00
Hanna Instruments Checker pH Pen (2 Point Calibration / Accuracy ±0.2 pH )
http://www.marinedepot.com/Hanna_In...ums-Hanna_Instruments-HN1135-FITEMOID-vi.html
 

SnowRaven

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Northern Brewer sells these 2 ( ((EDIT ** But Shipping UPS is $25 at SCICAL EEK! - BRS and MarineDepot.com were better Prices)
Milwaukee Models pH55 and pH56


Milwaukee ph55 - $39
http://www.scical-plus.com/ph55.html?gclid=CN2O0_fegLwCFcZZ7AodyTgArQ
pH Waterproof Dual Level Tester, Unit comes with Automatic Temperature Compensation (ATC), 2 Points Automatic Calibration, +/- 0.1 pH Accuracy; Range; -2.0 TO 16.0 pH -- 7.01 & 4.01 pH Starter Calibration Solution -
- Replacement Probe Model is Mi56p $29 - http://www.amazon.com/dp/B007Z4FYLE/?tag=skimlinks_replacement-20


Milwaukee ph56 - $52
http://www.scical-plus.com/ph56.html
pH Waterproof Dual Level Tester, Unit comes with Automatic Temperature Compensation (ATC), 2 Points Automatic Calibration, +/- 0.01 pH Accuracy; Range; -2.00 TO 16.00 pH-- 7.01 & 4.01 pH Starter Calibration Solution -
- Replacement Probe Model is Mi56p $29 http://www.amazon.com/dp/B007Z4FYLE/?tag=skimlinks_replacement-20


Milwaukee Mw101 is a better price here $62.00
http://www.scical-plus.com/mw101.html
pH Meter, Range 0.00 to 14.00, With 2 Point Manual Calibration, Manual Temperature Compensation, +/-0.02 pH Accuracy, comes with a SE220 Double Junction pH probe, 9 V Battery, 20 ml pH 7.01 & 4.01 Sachet Calibration Solution & Screwdriver for Calibration
 
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