Biobrewer

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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. 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.

ajdelange

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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]

Biobrewer

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[/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!

ajdelange

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Was pretty sure you did.

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 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.

Biobrewer

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I see.

HA. I did biology/chemistry in undergrad and biomedical engineering as a graduate student, so I know exactly what you mean here.

This will definitely keep the mind sharp!

Hermit

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http://www.todayifoundout.com/index.php/2010/10/alcohol-does-not-kill-brain-cells/

Mixed bag actually.

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!!