Where to put themo probe for AC unit powered fermentation chamber

For Christmas I have built my father a fermentation chamber powered by a 5k btu air conditioner. My question is: where is the best place to put the temperature probe?

If I leave it to read the air temperature inside the chamber, the ac unit will cycle more often to get the fermenter down to the set point. But if I tape it to the side of the fermenter, won't the ac run too long, because of the massive amount of thermal energy stored in the fermenter? Would this put the unit at a greater risk for coil freeze up?

I have to decide on a permanent location for the thermo probe in the next week, as once I give it to my father, it need to 'just work'.

Any thoughts?

Rhino

You just need to be able to adjust your max on/off time. Hopefully your temp controller can let you do this. Limit the time it can run to however short you want it, and you should be able to do the same for the "off" time. With this you should have no problem submerging the probe in liquid.
For my kegerator (which is an upright freezer built into a small walk-in) I keep my probe in ambient air at the top of the cooler, with a fan inside circulating air. I use a love controller and it enables me to limit on/off times and set up a defrost cycle to prevent icing up (which seems to only happen when I let it run with the door open.)
I don't know how to set up any of the other digital controllers. I love my Love though
Cheers!

I'm using a digital thermostat controlling a SSR to power the unit on/off, so setting max times is not possible. Currently the probe is just on the end of the wires, exposed to air, so the response time is too quick, and the unit will shut of very quickly. Either the AC unit or the thermostat has a delay feature built into it, as the AC will not come back on for a couple of minutes after it has shut off, regardless of what the thermostat reads.

I'm thinking of attaching the probe to the top of the chamber, covered with a small piece of foam insulation. This should slow the response time somewhat.
But I'm open to any other suggestions.

Cheers,

Rhino

You want to control the temperature of your beer/fermentor not the air around it. I would go with taping it to the outside of the fermentor with a bit of insulation between the probe and the air. When fermentation is raging and producing extra heat the AC will run longer and cool your chamber even further to try and get the fermentor back to the proper temp.

I think you should control the air around it or maybe just a bit of water (test tube) to minimize cycling. If you controlled to a large thermal sink like 5 gallons of beer, the ambient inside the cooler would go haywire trying to bring it down to temp. The beer would also cycle by the hysteresis on the setpoint. If you control the air up and down by a couple of degrees, the beer will cycle by a very small amount. Of course, an active fermentation will cause the beer temperature to be above the mean air temp. I use a thermowell to monitor beer temp, but I control the environment. Pretty quickly you get an idea of the maximum overshoot. I tend to get about a degree over with lagers and a peak of 4 for ales in 15G stainless containers. Works pretty well.

Well, for the last 24 hours I have had the thermo probe taped to the side of the fermenter, with several layers of insulation over it. Once the liquid was brought down to 50°, the AC almost never comes on.

I think the key to this set up is to step the temperature down over a period of time. If I put 5g of 70° wort in it, and want to bring it down to 50°, I think stepping it down by 5-7° at a time is probably the best way to not overwork the ac unit.

I will see if I can fashion some sort of waterproof cover for the thermo probe, and immerse it in a couple quarts of water, and see how that goes.

Cheers,

Rhino

Tape it to the fermentor, the surrounding air temp is secondary to the wort temp.