AJ, I know you recommend cooling the sample to room temp prior to measuring with pH meter, however, in Gordon Strong's new book he says "Note that mash pH is measured at mash temperatures, not cooled. If you cool the mash, the pH will read about 0.35 higher than at mash temperature." I just got a Hanna pHep 5, that has temperature compensation, yet its upper temperature limit is 140F. Can you shed some light on why he might be advocating this. How does the temperature compensation feature work?
Certainly mash pH is measured at laboratory temperature in commercial brewing but some home brewers may be measuring it at mash temperature. There are good reasons for not measuring at mash temperature such as (and probably the most important) that it is hard on the pH electrode and consequently shortens its life. Second is that mash temperature is not defined. Is it the 105 °F beta glucan rest, or 125° F protein rest or a low temp (145 °F) saccharrification rest or a higher saccharrification temperature? Conversely, lab temperature always means 20 °C or close to it.
If you read a commercial brewing textbook or any other of the published literature all pH readings you see will be for lab temp. Thus you can become confused if you try to interpret them as mash temp values.
DeClerck's book is the only one I am aware of that explicitly states that all pH values are at lab temp. You assume the other authors are doing the same thing because if they were talking other temperatures they would say what the temperatures are but you really cannot be 100% certain that this is the case. IOW everyone
should use room temp but that doesn't mean he will.
How does the temperature compensation feature work?
A pH electrode is a pair of half cells that in combination produce a voltage
V = Eo + a*(R*T/F)*(pHi -pH)
Eo is a constant (a couple of millivolts) voltage produced when pH = pHi
a is a constant near 1
R and F are physical constants
T is the absolute temperature of the solution
pHi is the pH value, close to pH 7, at which the electrode response in not dependent on temperature
pH is the pH of the sample.
a*R*T/F is about 57 mV per pH unit at 20 °C. The meters measures V and T. If immersed in 2 buffers whose pH's are known as a function of temperature, the meter can calculate a and Eo. pHi is assumed to be 7.
Thus 2 things change with temperature
1. The hydrogen ion activity ( pH = -log(activity))
2. The electrical response of the electrode to a particular pH
It is the role of ATC to compensate for the latter and only the latter.
While it is supposed to be the case that 6.5 < pHi < 7.5 this isn't always so. If it isn't, pH measurement more than a couple of degrees away from room temperature will cause ATC to introduce errors. This assumes that the buffers were at room temperature when the meter was calibrated. If calibration is done at a temperature close to that of the mash and the mash itself is measured at mash temperature then this error goes away. Much simpler to calbrate and measure at room temp (IMO).
