pH Meter Calibration - Page 3 - Home Brew Forums

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03-14-2012, 04:22 AM   #21
Biobrewer
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Quote:
 Originally Posted by ajdelange 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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03-14-2012, 02:39 PM   #22
ajdelange
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Quote:
 Originally Posted by Biobrewer 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.

Quote:
 Originally Posted by Biobrewer 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.
Quote:
 Originally Posted by Biobrewer 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

Quote:
 Originally Posted by Biobrewer 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!

Quote:
 Originally Posted by Biobrewer 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]

03-14-2012, 05:10 PM   #23
Biobrewer
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Quote:
[/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!

03-14-2012, 05:53 PM   #24
ajdelange
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Quote:
 Originally Posted by Biobrewer And by 1.38 offset, I meant an offset of pHi, ...
Was pretty sure you did.

Quote:
 Originally Posted by Biobrewer ... 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.

Quote:
 Originally Posted by Biobrewer 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.

03-14-2012, 07:17 PM   #25
Biobrewer
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Quote:
 Originally Posted by ajdelange 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.

Quote:
 Originally Posted by ajdelange 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.

Quote:
 Originally Posted by ajdelange 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!

03-14-2012, 07:27 PM   #26
Hermit
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Quote:
 Originally Posted by Biobrewer This will definitely keep the mind sharp!

http://www.todayifoundout.com/index....l-brain-cells/

Mixed bag actually.

12-09-2012, 02:46 PM   #27
ryanmx48
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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!!

12-01-2013, 02:04 PM   #28
Dr. Francois

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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-Lea...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/
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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12-01-2013, 02:49 PM   #29
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.

12-01-2013, 04:17 PM   #30
Dr. Francois

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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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