pH meter - How it works

 Glass electrode

From John Palmer 

If the mash pH at room temp is 5.2 to 5.6

At mash temp (~ @67 °c) is lower,  4.9 to 5.3

So measure at room temp, gold standard.  YouTube 

Must read: AN-1852 designing with pH electrodes.

pH electrode transfer function. input: electrode voltages, temperature. output: pH values 

PH electrode sensitivity, which is dependent on temperature.

LMP7721 is recommended op amp. TI forum

Horiba FAQ. This is aim of buffer calibration

So ATC Calibration is for pH electrode, using Nernst equation  to adjust for respective electrode output voltage at that temperature. Of course, ATC cannot compensate for characteristics of various samples eg mash  ,  at different temperatures.

Nerst equation.  input: pH and temperature, output: electrode voltages

John Palmer's water chemistry book:

slope should fall between 95 and 103%, and the pH(0 mV input) should lie between pH 6.8 and 7.2 (Uoffset within ± 15 mV).

CA3140 is another op amp widely used  in 80s, which has input bias current of 10 pA

Sensorex

acid error pH<2

care and maintenance of pH electrodes. 

Patent