Feature request: pH measurement, alarms, and logging. My equipment upgrade path is leading me to a need for a pH sensor to improve efficiency and repeatability. It occured to me how nice it would be to have that function built into this system. There are a number of probes that work with micro controllers. I imagine adding the logging function would be pretty straightforward as no output is required based on the data, though an alert of some sort would be convenient.
Excellent idea! I’ve been wanting the same. Could you link to some of the probes that you were considering?
Heres a writeup on digital pH measurement in general: https://www.sparkyswidgets.com/portfolio-item/ph-probe-interface/
Probes tend to run the gamut of pricing. Here are a couple:
http://www.atlas-scientific.com/product_pages/probes/ph_probe.html
http://www.dfrobot.com/index.php?route=product/product&product_id=1025#.VWV_Q4rXfCR
Thanks! I already have a pH probe, but not anything to connect it to the brewpi.
Looking on ebay, ready made pH boards for arduino start at $50 upwards, which imho seems pretty steep! 
@elco is much better with EE than I, perhaps he might consider building a pH board for brewpi?
EDIT; $23 module here - http://www.ebay.com/itm/Liquid-PH-Value-Detection-detect-Sensor-Module-Monitoring-Control-For-arduino-/400926133627?pt=LH_DefaultDomain_0&hash=item5d590f4d7b
The main issue with measuring pH during the brew is temperature - most pH meters are rated below 50C / 122F, which rules out mashing or boiling.
Even if you find a pH meter that tolerates the temperature, it’s advised to cool the sample to room temperature and take the reading there, as the pH value is temperature sensitive.
So, do we go to the trouble of collecting a sample, cooling it, then reading the pH?
Sadly, I am fascinated by checking pH in the brewing process! 
Currently I use some fairly expensive Hanna gear to take my measurements.
@mdma is right. Temperature plays havoc with taking readings. Even though my gear auto adjusts for temperature I always cool my samples to roughly room temperature. I find the margin for error ramps up quickly with rises in temperature.
Although I would love to see this functionality, unfortunately I think that getting BrewPi to do this could be expensive and/or tricky!
Dean.
I think, if you are using HERMS for the mash and a counter-flow chiller (plate or tube), that with a small addition in hardware you could make this work. It would require plumbing a small tube into a T fitting on the pressure side of the pump, cooling the wort in that tube, measuring the pH, then sending the wort back to the source vessel (MT or BK). The flow would be small enough that it wouldn’t effect the temperature of the source and, in the case of the MT, the wort could be re-inserted prior to the HERMS coil so there would certainly be no negative impact on the mash. The boil, however, would have to not be pH measured for the last 10 minutes or so as the system would have to be shut off so as not to introduce an infection to the wort since the wort in the pH sensing apparatus will never be hot enough to sterilize it.
There are a number of articles with correction tables. The best one I have seen is from Fisher. Goes all the way too 100 deg C. Create a correction algorithm using this table.
I would like said pH sensor to also log/measure during and after CIP (cleaning) to ensure cleaners have been rinsed adequately. In my professional work, I see how hard it is sometimes to return to a neutral pH but in home brewing I think sometimes the ‘royal we’ are lazy at rinsing sometimes. Could be easy in these complex HERMS systems which will sort of have a CIP ‘state’ anyway.
In terms of cooling during the mash for a possibly more accurate reading, I have seen very small heat exchangers used. Like a mini CCFC for example in line with your recirculation. Could be done fairly cost effectively at home brew level?
Edit: just found this: https://conical-fermenter.com/wort-thief-chiller.html
Pretty economical for Triclamp gear too.
I think making an interface board for current loop (4-20 mA) sensors might be the best way forward.
Many industrial sensors (including pressure sensors for volume measurement) have a 4-20mA output.
I am thinking microcontroller + 4 channel ADC + RS-485.
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