pH Meter Accuracy - Page 2 - Home Brew Forums

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10-24-2011, 10:15 PM   #11
ajdelange
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Aug 2010
McLean/Ogden, Virginia/Quebec
Posts: 9,409
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I only mention this because as you took the trouble to monitor pH readings over time it is clear that you have an intrinsic curiosity about how these things work but you can do your own calibration and ATC if you can put the meter in millivolt mode. You can do this with may meters but I am not sure you can do it with the cheapies. The general concept is that you measure mV in 7 buffer, mV in 4 buffer, compute the offset and slope and then use those to convert the mV reading of the sample to a pH value. As you are doing the slope and offset calculations you can decide when to accept the mV readings for the calibrations. If you are interested in this I can post the formulas (or at least I think I can - I'm out of the country at the moment.)

10-24-2011, 11:53 PM   #12
Student7
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Sep 2011
Knoxville, TN
Posts: 22

The manual doesn't mention millivolt mode and there's no obvious way to switch to it: mode button only controls Cº/Fº and buffers used. No other switches in battery compartment or anywhere on the thing.

I've sent a message to tech support asking if it can be done with this meter and will post back once I hear. I'm definitely interested in doing this if it's possible.

10-25-2011, 05:15 AM   #13
ajdelange
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Aug 2010
McLean/Ogden, Virginia/Quebec
Posts: 9,409
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You don't really need mV mode though it makes understanding of what is going on easier. The electrode produces a voltage

E = E0 + s*T(7-pH)

where E0 is the offset, s is the slope and T is the temperature (in Kelvins - add 273.15 to centigrade). When you use a pair of buffers of known pH you get two voltage observations and can solve 2 equations for E0 and s and then in response to sample voltage Es calculate the pH responsible as pH = (E0 - E)/(s*T) + 7. If you turn ATC off the meter will use 293.15 (20 °C) as the temperature. What is important is that the estimates of E0 and s are fixed if the meter is not recalibrated and for our purposes it does not matter that the values are incorrect. You will calibrate the calibration.

1. Turn ATC off
2. Measure the first buffer's temperature and pH. Convert temperature to Kelvins. Call the measurements T1_ and pH2_.
3. Do the same for the second buffer. Call the results T2_ and pH2_.
4. Determine the actual pH's of the buffers at T1 and T2. There is usually a table on the packaging or the values can be found on the net. Look for NIST technical buffer data. I have some simple formulas but not sure I can dig them out (I'm not at home).Call these pH1 and pH2.
5. Compute beta = (T1_*pH1_ - T2_*pH2_)/(T1_*pH1 - T2_*pH2). If the meter were calibrated perfectly pH1_ would equal pH1 and pH2_ would equal pH2 so beta would be 1.000
6. Compute alpha = T1_*(pH1_ - beta*pH1). If the meter were calibrated perfectly beta would be equal 1 and alpha would be 0.
7. Measure a sample. Call the pH measurement pHs_ and the Kelvin temperature Ts_
8. Compute (pHs_/beta) - (alpha/beta)(1/Ts_). This is the temperature corrected calibrated pH estimate. When alpha = 0 and beta = 1 (meter perfectly calibrated) the estimate is identical to the reading.

I promise to check this tomorrow for algebra errors and edit as necessary.

10-25-2011, 04:10 PM   #14
Student7
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Sep 2011
Knoxville, TN
Posts: 22

This meter will not display raw voltage measurements from the probe per mfgr tech support.

Quote:
 Originally Posted by ajdelange 1. Turn ATC off
Unfortunately it is also not possible to turn off the ATC. I did some research and learned a little about the Nernst equation and how ATC is calculated hoping I could factor it in to your equations given that the meter readings will always show up as temp. adjusted, but TBH doing so is beyond me at this point. Seems to me I would need to know the meter's calculated calibration slope and offset numbers to be able to adjust for the ATC reading, but I'm probably missing something here, my math is not great...

I guess I could use the method and formulas you provided and do all my readings at 25C. That would work to get more familiar with the readings, but I'm not sure how practical it would be to try and hold all my samples at 25C on brewday.

10-27-2011, 10:33 PM   #15
Student7
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Sep 2011
Knoxville, TN
Posts: 22

Brewed a batch today and this meter was working much better. After calibrating it, measured both the buffer solutions and within 30-45 seconds the meter was within .02 of the correct reading. Leaving it for a couple minutes it moved exact or within .01 and stayed there.

Not sure why it's working better, all I can think of is that the electrode benefitted from soaking in the storage solution for some more time. Before my initial post, it had already been in storage solution for over a day, and calibrated multiple times, but always showed the inaccurate, slow pattern. Seems fine now and I hope it stays this way!

10-27-2011, 11:13 PM   #16
ajdelange
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McLean/Ogden, Virginia/Quebec
Posts: 9,409
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I'm not sure how new your meter was when you began using it but the "work" is done in a "hydrated layer" in the glass membrane. This needs to be formed for the electrode to work stably and I suppose it is possible that in your case it was not fully formed for those first couple of brews. Extended soaking in the storage solution is definitely a good way to form that layer and be sure the appropriate ions are held within it. pH meters are definitely subject to vagaries and it just takes some experience to be able to tell when things are kosher and when they are not.

Even though it now seems less important in light of better behavior it appears that it is possible to "calibrate the calibration" in a meter that does not display mV and in which you cannot disable ATC. It's a bit more complicated than what I posted earlier but not that much. I'm writing it up and think it would be better to transmit a .pdf than to try to post it here so if you are interested pm me an e-mail address. Don't know when I'll finish the analysis.