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xanderphillips
01-28-2014, 11:29 PM
I've just been working on re-calibrating my pH meter and re-checking my must to see if I screwed up and dropped the pH too much,

The temperature of the calibration solution and the temperature of the thing being measured (in this case the must) will impact the reading you get from your pH measurements a fair amount.

For example the pH calibration reference solution for my meter indicates that a calibration solution temperature ranging from 10c-60c (50f-140f) will vary the meter's output from 7.07-6.96 using the same 7.0 test solution.

The warmer the temperature the lower the pH reading.

This temperature / pH variation also occurs in our brews.

This is not an error in measurement, the pH will ACTUALLY change depending on the temperature of the must. Apparently many pH meters are calibrated to 25c (77f) so unless my must is ALSO at 77f my measurement will not be 100% accurate. I'm assuming there is a temperature adjustment you can do for if my meter is calibrated at X, then my reading at Y will be off by Z amount, but I have not found one yet. I know that having pH 'in the ballpark' of where you want is probably a +/- horseshoes and hand-grenades kind of thing, but being a pretty scientifically minded person I tend to like to over analyze.

I found this webpage that describes the situation much better than I have:

http://www.all-about-ph.com/ph-measurement.html

fatbloke
01-29-2014, 02:55 AM
I'm guessing that you're reading too much into this.

Your post indicates +/- 0.1 pH........ So fine there's a small discrepancy depending on temp but you are suggesting quite a wide temp differential, so while you could be OCD accurate in your measurements, generally it's fine to be about a tenth out.

Maybe just calibrate the pH meter at the optimum temp, then depending on the scenario you're testing for e.g. a slow ferment, still reasonable amount of sugars to ferment but a result that suggests the pH is close to 3.0, then logic would dictate that it'd be best to add a little Potassium Carbonate to raise it a few tenths to let the yeast be likely a little less strssed.

I've got to get some more calibration solution, but as generally I'm only checking wine range pH, I'll likely find some of the 4.01 pH liquid and set to that......

GntlKnigt1
01-29-2014, 05:21 AM
I completely agree with Fatbloke. As a meadmaker that (perhaps) pays more attention to pH than most, the one-tenth reading is close enough, as measured by the el cheapo pH meter like this one...
http://www.amazon.com/BestDealUSA-PH-009-Meter-Digital-Tester/dp/B0087LOWIA/ref=sr_1_4?s=industrial&ie=UTF8&qid=1390986942&sr=1-4&keywords=ph+meter

Yeast don't seem to be sensitive to variations of less than .1, and most people would be hard pressed to taste that small a difference. While it might not be precise, such measurements are close enough for MY meadmaking anyway. If you have the tools/expertise/equipment/finances to be more exact, then meadmaking could provide a great environment to exercise it.

Bob1016
01-29-2014, 08:50 AM
I have the mw102 pH meter (0.01 resolution, +/- 0.02 accuracy), and that thing is accurate! I just got it and have been playing around with it, and while temp plays a big role, as long as all temps are close it is good enough. 25C is the standard so I get most of my sample to 25 +/- 1C, and it works great. Don't worry about yeast until the environment is <3.2pH (in the case of a +/-0.1 accuracy I'd say 3.3), and even then, many wines are fermented lower (santorini is a prime example at 2.8-3.1 typically).
As far as a correction for temp, it is not a truly linear process. Temp will change the volume of the liquid (thus changing H concentration which is what pH measures), and it will change the reaction equilibriums causing a non linear shift. There are estimates and they're close, but not exact.
Just pull a sample, put in erlenmyer, swirl to degas, chill for hydrometer reading, then put in water bath at 30C, wait, make sure sample temp is close to 25C, use calibrated pH meter to test (allow time for accurate reading), the add any nutrients that are required based on recipe and SG, pour sample back into fermenter. (I usually discard the pH sample as I don't sanitize the probe, but use the gravity sample to dissolve the nutrients) (I guess that's not very fast, but it's great for data collection! ;))