pH Meter Calibration

[Kiknjville]

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?

 

[ajdelange]

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.

 

[SnowRaven]

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

 

[mtnagel]

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.

 

[ajdelange]

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).

 

[mtnagel]

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.

 

[ajdelange]

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.

 

[mtnagel]

Duh, why didn't I think of that

I'll take your word for it

 

[SnowRaven]

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]

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

 

[ajdelange]

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

In the first post here I describe calibration and stability checks which, if passed, indicate that the meter is suitable for brewing provided, of course, that it has good reliability.

This new Hach unit and a Hanna pHEp meter are the only ones on which I have done cal and stability checks. Based on my findings on the Hach, at least two other people on here have bought them and done the stability checks. This unit looks good - so far. But we don't know how robust the thing is, how long the electrode is expected to last... I showed it to the brewer at Gordon Biersch yesterday and he went off and ordered one on the spot. He'll certainly beat it up in his normal use of it and has promised to let me know how it does for him. If he is happy with it at the end of year then my recommendation for it will go from tentative to definite.

I have also done stability checks on the Hanna pHEp. It is quite stable but you can't calibrate it because it decides for itself when to accept the calibration reading and it does so too soon. This can be worked around quite simply by subtracting the average cal check (4 and 7 pH) error to all subsequent readings but I don't recommend this meter simply because people who are using a pH meter for the first time get confused enough about having to do cals as it is.

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/

I have not done stability or calibration checks on the MW101 nor have I seen anyone else publish data. I have never had one in my hands. What is disturbing about this meter is that it gets an average review score of 2.5. Half of the reviews are 5 stars and the other half 0 stars. It seems that you either get a meter you like or a complete piece of junk with the chances being about 50/50. Another 'problem' is that these meters are analog. The advantages to digital are well known (though some stubbornly refuse to accept them) and as I have set them out so many times before, I won't do it again. I put problem in quotes because you can very well use an analog meter with manual TC (provided you are in the 50% that get a working meter). People did it for years. There is an MW102 which is the digital version. I have no stability or cal check data on it either and don't know anything about its reliability.

(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

This is just an electrode - you still need a meter (and a temperature probe unless you want to do TC manually). The electrode is, of course, where the rubber meets the road. This one doesn't have a particularly good spec.

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

I can't comment on any of these. I look for laboratory grade instruments as in mash pH measurement you really want to be as accurate as possible. When I see a laboratory grade instrument maker's label (Hach) on a $125 meter I get excited and hope wells, as they say, eternal. But I'm also a cynical SOB. If the Hach turns out to have shortcomings comparable to those I know about in other inexpensive meters I'll throw it on the heap with them.

 

[SnowRaven]

I have not done stability or calibration checks on the MW101 nor have I seen anyone else publish data. I have never had one in my hands. What is disturbing about this meter is that it gets an average review score of 2.5. Half of the reviews are 5 stars and the other half 0 stars. It seems that you either get a meter you like or a complete piece of junk with the chances being about 50/50. Another 'problem' is that these meters are analog. The advantages to digital are well known (though some stubbornly refuse to accept them) and as I have set them out so many times before, I won't do it again. I put problem in quotes because you can very well use an analog meter with manual TC (provided you are in the 50% that get a working meter). People did it for years. There is an MW102 which is the digital version. I have no stability or cal check data on it either and don't know anything about its reliability.

Where did you find reviews on the ME101 ?
or the ME102 ?

Amazon?

 

[ajdelange]

Several people have reported unfavorable comments on their experiences with the MW101 here and there are others at Amazon. OTOH some people have praised the 101 to the sky here even arguing that the fact of no ATC represents an advantage as it lets the manufacturer spend more on the 'important' parts of the meter. As I said, I have seen little on the MW102 so I don't know if they solved their QC problems in this model or not.

 

[SnowRaven]

You sold me on the Hach even though it is backordered till 2-12-14
$110+17.00 shipping from Ames Iowa -
Pocket Pro+ pH Tester with Replaceable Sensor - 9532000

Replacement $67.00 pH sensor, replacement - 9532001


Is Hach direct the only place to order from for this item?

 

[ajdelange]

No, I think they now have distributors as I have seen others selling their gear. I'd try a search on the meter and see what pops up.

 

[SnowRaven]

Google just shows Hach

 

[HopJunkieIPA]

I wondering why is ph stabilizer used?

 

[ajdelange]

I assume that by 'pH Stabilizer' you mean something like the product sold under the name '5.2'. People us it in the belief that it will do what it says it will do on the label but it won't for reasons that I have gone into so many times before here that I won't repeat it again.

If you mean buffers in the broader sense people use them to do things like calibrate pH meters, hold the pH of solutions in the region where a desired reaction proceeds most effectively etc.

 

[SnowRaven]

No, I think they now have distributors as I have seen others selling their gear. I'd try a search on the meter and see what pops up.

Not much luck besides direct, and the price is better direct so far.

 

[bleak]

You sold me on the Hach even though it is backordered till 2-12-14
$110+17.00 shipping from Ames Iowa -
Pocket Pro+ pH Tester with Replaceable Sensor - 9532000

Replacement $67.00 pH sensor, replacement - 9532001


Is Hach direct the only place to order from for this item?

$110? Wow. It didn't take long for retailers to stick it to Canadians with the very recent currency exchange rate change. For Canada the price is $123 plus $19 shipping.

 

[bleak]

I'm going to purchase a Hach Pocket Pro+, but I'm confused about the way to store the electrode. I couldn't find any storage solution on the Hach website, and so I called the company. A tech support person told me that storage solution is not required for this meter, and that the electrode only needed tap water for storage to prevent drying. The user manual also has nothing in it about storing the electrode. Does this make any sense?

 

[ajdelange]

No, not that they wouldn't put anything about it in the manual at least but I have never seen anything like this. You are not the only one puzzled. I've called them. Other people who have bought the unit have called them and the story is all pretty much the same. Store it dry or put a couple of drops of water in the cap to keep the humidity up (the cap is tightly sealed with an O - ring). So that's what I do - just put in a couple of drops of water, not enough to cover the bulb by any means, just enough to keep it humid inside the cap.

 

[ldave]

Just bought the MW102. It has ATC and is spec'd up to 70 deg C (that's not a typo). For the first time I have a pH meter in my hands that I could, in theory, measure mash pH w/out cooling samples. I know you've stated not to do this as 'it shortens the life of the probe'. And I never have with all my previous meters. But. It is very seductive to not have to cool each sample. Adding phosphoric acid in increments can be very tedious. If I didn't have to cool, I wouldn't even sample. I'd just float a small strainer in the mash and drop in the probe, removing the whole show, of course, to add acid and stir. Think of the time savings. Oooooh.

So. Based on my understanding from this thread, I should heat cal solutions up to a representative mash temp and calibrate at that temp. Assuming I do this, I don't have to offset .35, like a room temp reading. Correct? Or would it make sense to just calibrate at room temp, and observe whatever offset I get at mash temp (which would be the .35 plus whatever error is being introduced by the meter so far from it's cal point(s) temps), and just math that in every time? At this point, I have ask the deadly question: how much is the probe life shortened. These devices have come a long way since I started using them in the early 90's. Perhaps the probes have as well? After all, it's spec'd to 70 deg C. Will I be the only guinea pig here?

 

[ajdelange]

Just bought the MW102. It has ATC and is spec'd up to 70 deg C (that's not a typo). For the first time I have a pH meter in my hands that I could, in theory, measure mash pH w/out cooling samples. I know you've stated not to do this as 'it shortens the life of the probe'.

Yes, you could, and yes, it does.

First thing I hope you will do is run the stability check on your new meter. The MW101 has gotten a bad rep mostly for reliability and we hope that the newer design solves the problems.

So. Based on my understanding from this thread, I should heat cal solutions up to a representative mash temp and calibrate at that temp.

While doing the stability check you could heat the buffer and record pH error (the difference between a reported reading and the pH of the buffer - don't forget that buffer pH changes with temperature and you need the actual buffer pH to get the actual error). Now plot the error vs. temperature difference re the calibration temperature i.e. the measurement temperature minus the calibration temperature. The slope of a linear fit to the error vs temperature difference is (pHi - 7)/Tc. Multiplying the slope by Tc, the calibration temperature (in Kelvins) gives the difference between pHi and 7 and so you can estimate pHi for your meter. If it is close to 7 then you may be able to get away without calibrating at near mash temperature. If the line fitting etc. looks overwhelming just looking at the error data will give you an idea. If heating the buffer 20°C results in a pH error of, for example, 0.2 then pHi is too far from 7 and you should cal. at mash temp.

Assuming I do this, I don't have to offset .35, like a room temp reading. Correct?

No, you wouldn't unless you want to compare your pH values to what everyone else is doing. To refer your measured pH to room temperature you would multiply the difference between room temperature and the temp. at which the reading was made (°C) by 0.0055. It would actually be better if you made a series of measurements on a cooling sample in order to determine what the slope for your mashes actually is.

Or would it make sense to just calibrate at room temp, and observe whatever offset I get at mash temp (which would be the .35 plus whatever error is being introduced by the meter so far from it's cal point(s) temps), and just math that in every time?

If you cal at room temp and pHi is close to 7 then you can just take the reading as it is. If pHi not close to 7 then you can figure out what the correction is from (pHi - 7)*delta_T/T_cal (all T's in Kelvins). In other words you can correct the corrections.

At this point, I have ask the deadly question: how much is the probe life shortened. These devices have come a long way since I started using them in the early 90's. Perhaps the probes have as well? After all, it's spec'd to 70 deg C. Will I be the only guinea pig here?

Can't help you there. Is the spec for the ATC or for the electrode? If it is for the ATC I don't understand as the ATC algorithm just pushes numbers and I can't understand why 70 would be a limit. I can understand how the electrode might have a temperature limit, however.

 

[mabrungard]

I was bored on Sunday, so I performed the stability tests to see how the MW-101 performs in this respect. I guess its OK.

One added thing I note is that when in the 7.00 solution, the pH response creeps lower over time. With the 4.00 solution, the response creeps up. I'm not sure why that occurs.

 

[mtnagel]

That looks acceptable to me. Thanks for posting.

 

[mchrispen]

Martin,

What volume of solution did you use? It seems that stability is dependent on temperature - and delineated by the 3.5 hour mark. The deltas aren't horrible, but the drift from 7.01 downward seems disturbing. Did you have ATC on?

I have the MW100 and need to do a longer stability test. I think we have the same probe, I just have manual calibration and no ATC.

 

[mtnagel]

You're really "disturbed" by 3 hundredths of a pH unit? You do know we're making beer right?

 

[mabrungard]

Its the MW 101 unit, so there isnt compensation.

 

[mchrispen]

"disturbed" is strong language... but the tolerance is supposed to be within 0.01 on that meter, but no spec on over time. Mine is supposed to be within 0.02... yet has the same probe as the MW101 but no ATC function. I am wondering if the drift is mechanical (probe) or related to the algorithm.

Yeah more than good enough for brewing, better than expected for a sub $100 meter.