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I've had some problems with sensor calibration lately, and I figured I'd pass on my findings to y'all. Basically I've had some attenuation issues in one of my fermenting fridges, and problems hitting mash temps since upgrading some equipment, and I'm pretty sure it's all down to the really poor job I did of calibrating the sensors.
I have a PT100 RTD sensor for my PID controlled HLT setup, and the factory sensors that come with my STC-1000 temperature controllers for my fermentation chambers.
To properly calibrate a sensor, you're meant to put it in ice water, and it should read 0 degrees. HOWEVER, this means 70% ice, 30% water, in a thermos for at least 10 minutes - not my poky method of a few ice cubes in a teacup!
You can also do it with boiling water, but you've got a fool around with a boiling point calculator to do it accurately (take into account barometric pressure, altitiude etc).
My initial poor calibrations were out by almost 2.5 degrees - critical especially for fermentation! The "final" calibrations I did ended up being only 0.1 degrees off the factory defaults anyways, so if in doubt, just leave them alone. You're probably more likely to make things worse rather than better!
Anyways I thought it'd be worth just noting this, as I had some poor brewdays/fermentations due to this problem.
With ice in water, you just need to keep stirring it, and watch the readout. Once the readout is steady, you have achieved equilibrium between the water and the ice, hence freezing point, but you must keep stirring.
I have a digital infrared aim and shoot temp reader and find that the actual outside temp of my glass carboys is always about 1.5C lower then a STC-1000 probe Duct taped to the carboy (while around 20C), perhaps if I had insulation around it, would equalise but I just manually compensate. Its much closer when at 2C
Good tips! I should totally get one of those infrared readers.
I too am chasing my temperature probe calibration somewhat. I have a PID with a thermocouple mounted in the discharge side of my pump. I also have a cheap digital thermometer which I (thought was) calibrated in boiling/ice water. I calibrated the TC against that but now when I recirc during the last few minutes of the boil I only get 98°C at the pump! I am getting some digital probes (DS18B20) to use with an Arduino microcontroller, these are apparently calibrated at factory and are accurate to +/- 0.5°C so it will be interesting the see the difference between the TC and the DS18B20.
The other point that has been mentioned to me is if you have a probe mounted in the flow (like I do with the pump probe) is to have the flow directed along the probe not perpendicular to it. Had some good discussions on it when I was (I still am) building my setup - http://www.forum.realbeer.co.nz/profiles/blogs/new-brewery-build