Author Topic: ATC help with new meter  (Read 790 times)

Offline Andor

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ATC help with new meter
« on: February 18, 2017, 02:59:48 AM »
I got my a mw102 meter today. I calibrated it and took the ph of a few Things. Pretty nifty, seems to be working. I know ph readings should be taken at "room" temp not at mash temp. One thing I'm a little confused on is what ATC can and can't do. My calibration solutions were in the basement and were 13c/55f. Is this close enough to room temp or do samples need to be very close or exactly 25c/77f to get accurate calibration and readings. Thanks guys

Offline Stevie

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Re: ATC help with new meter
« Reply #1 on: February 18, 2017, 03:19:42 AM »
My meter disables ATC in calibration. Check your manual.

Offline tommymorris

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ATC help with new meter
« Reply #2 on: February 18, 2017, 01:24:32 PM »
The pH calibration solutions should have a table which gives the pH at different temperatures. You should use that table when calibrating.

Here is a table I put together for my calibration. Before using it you need to make sure you use the same solutions. Mine are pH 4,7, and 10 at room temp. NIST solutions are slightly different.



PS. I notice my table has an error. It should read 4.00 at 25C.
« Last Edit: February 18, 2017, 01:28:56 PM by alestateyall »

Offline Andor

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Re: ATC help with new meter
« Reply #3 on: February 18, 2017, 08:55:45 PM »
The pH calibration solutions should have a table which gives the pH at different temperatures. You should use that table when calibrating.

Here is a table I put together for my calibration. Before using it you need to make sure you use the same solutions. Mine are pH 4,7, and 10 at room temp. NIST solutions are slightly different.



PS. I notice my table has an error. It should read 4.00 at 25C.

It calibrates to 4.01 and 7.01 automatically. There's no way I see to change that.

Offline Stevie

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Re: ATC help with new meter
« Reply #4 on: February 18, 2017, 09:01:01 PM »
If that is the case, you should have your buffers as close to 25°c as possible.

Offline mabrungard

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Re: ATC help with new meter
« Reply #5 on: February 19, 2017, 04:48:53 PM »
ATC is a nice feature, but it only improves your reading accuracy by a hundredth or two in typical usage. It cannot correct the large pH shift that wort undergoes as its temperature changes. ATC only corrects for the probe's change in its sensing function. It will ALWAYS be more accurate and better for your probe to measure pH in liquids somewhere around 25C +/- 5C.

Considering that we don't really care about wort pH down to the hundredth, being a couple of hundredths off, is no big deal. Just be consistent in measuring within the preferred temp range. This is the reason why I suggest that ATC has little value in the brewery. Always have a quick-reading thermometer with you to verify your wort temp is not too hot and then insert the pH probe after that is assured.
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Offline pfabsits

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Re: ATC help with new meter
« Reply #6 on: February 20, 2017, 05:06:05 PM »
ATC corrects for the Nerstian response for a membrane potential (i.e. pH electrode). Temperature is a factor in the Nernst equation. At 25 oC a pH electrode theoretically generates 0 mV at pH 7.01 and +177.48 mV in pH 4.01. The pH 4.01 value represent 59.16 mV/pH unit and since pH 4.01 is 3 pH units away the theoretical voltage is 59.16 x 3 @ 25 oC.

pH 7 is known as the isopotential (iso = same) point in which Theoretically it should always be 0 mV. For the pH 4.01 value (or slope) the mV/pH will change with temperature. For example, 54 mV/pH unit at 0 oC and 64 mV/pH at 50 oC. Again, these values are calculated by changing the temperature value in the Nernst equation.

pH meters with ATC use the Nernst equation to correct for temperature variations as predicted by the Nernst equation.

It is critical to note that the values on the bottles/packets of pH buffer solutions are the pH of the solution at that specific temperature. It is the reason why some meters will display a different pH value after calibration. The meter is programmed to display the solution pH buffer value for for the temperature being measured. The pH at any given temperature for a NIST traceable buffer is 100% known while the pH of wort at 25 oC versus 50 oC is not known. The actual wort pH can be different at different temperatures. It cannot be predicted with 100% certainty since the composition is never 100% exactly the same.  So for a pH meter we program what we know.  This is not the same as ATC. ATC does not correct for solution pH but for the Nernstian behavior.

Temperature does impact the resistance of the glass membrane. As temperature goes up the resistance goes down. It is one of the main concerns when measuring at very high temperatures. Ohm's law stated that V = IR. (V=voltage, I = impedance, R = resistance). The voltage is predicted by Nernst and R is the resistance of the glass membrane (around 100 megaohms for general purpose glass at 25 oC). With voltage staying the same and resistance decreasing then the impedance is affected, which has to be within an ideal range for a pH meter to work correctly. It is recommended to use pH electrodes with high temperature (HT) glass for measuring above 150 oF.

Bottom line:
1) ATC will correct the slope based on the temperature of solutions used for calibration.
2) With ATC, most meters will display the solution pH (the value printed on the bottle)
3) For Manual Temp Compensation (MTC), measure the temperature of the buffer and adjust meter pH value to match the pH value for given temperature from the table printed on the bottle.

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