Temperature Control and Cycles Per Hour

[northernlad]

I put together my DIY STC-100 control box this weekend to use with a True GDM-12 merchandiser.

With the GDM turned all the way up and the STC set at 4.5C, 2 degree variance and 3 minute compressor delay the unit cycles on for 45 seconds every 6 minutes.
I looked up recommended CPH and it sounds like it shouldn't cycle more than 5 times an hour.
When I adjusted the controller to 3 degree variance it cycles on for a little over a minute every 12+ minutes.
I should mention its empty right now except for a couple thawing turkeys. I use a Speidel 30L so I suspect it will hold temp a little better with that much volume, especially since it will be at 60 instead of 40.

3 degree variance with about 5 cycles per hour should be alright, no?

 

[DurtyChemist]

I put the probe on the outside of the ferment or and cover it with a folded up paper towel that's about an inch tall and four inches wide. Put the probe on a turkey and set the temp to what the turkeys are and see how many times it cycles.

 

[northernlad]

Considered that but since the turkey was frozen the result would have been: Never.
I've seen some people who strap the probe to the side of the fermentor but I don't know if I want the tempt to fluctuate that much before engaging. Thought about strapping the probe to a pint or something smaller, though.

 

[raouliii]

I've used a GDM-10 with a built-in STC1000 style controller as my fermentation chamber for a few years now. I have found strapping the probe to the outside of the carboy under a piece of closed-cell insulation to work great. I haven't spent much effort recording its on/off time but it certainly runs for much longer than 45seconds at a time. The air in the chamber cools far too quickly to simply suspend the probe in the air. I've always operated with the position that if the beer temperature is what one wants to control, then the beer temperature must be monitored. Monitoring anything else just doesn't make sense to me. I've read threads here wherein thermowells are used to monitor the temperature of the center of the beer, but the reported cooling undershoot seemed to be an issue.

My setup allows the evaporator fan to run continuously, as the GDM is designed. This adds about 10watts of heat into the chamber, which forces it to cycle a little more often, but results in a heater not being necessary.

BTW, keep in mind that the compressor delay is just a count down timer. It delays the cooling start from the last cooling stop. It doesn't delay the cooling start from the cooling request.

 

[northernlad]

Thanks, raouliii. That makes sense.

I understand the purpose of the delay that's why I adjusted the variance to decrease the cycles per hour.
I am ok with 4-5 cycles per hour but I would like it to run longer when it is on.

\I will likely be brewing next weekend so I'll strap it up and see how it goes.

How do you have it set up to allow the fan to run?
I have the unit plugged into the controller and it cuts power to the entire unit when it reaches temp.

 

[raouliii]

I integrated the controller into the GDM using the original wiring harness. The controller does not effect operation of the evaporator fan.

Original Schematic

Modified Schematic (although the heating circuit was found to be unnecessary and is now unused)

 

[northernlad]

That looks great. I'm almost through my rocket science degree to be able to read that.