Temp Controller Can't Read Air Temperature

I have 2 Ranco controllers, and I just noticed that they are way off when reading air temperature. Anyone else notice this? When I brew, I tape the controller to the side of the fermenter and cover it with insulation. It reads pretty close to what the Fermometer says. I just got a new controller for my keezer, and I just had the probe dangling about halfway down my keezer. The keezer read 44 degrees, but I measured the beer at 55 degrees. When I taped the probe to the keg and covered it, the keezer temp went up to 55. Does anyone else notice this, and if so, why is it like this? I assume that the ambient air temp in the keezer is the same as the kegs. Strange.

This is totally to be expected. The probe has a very large thermal mass compared to the air surrounding it. If it's working properly it should eventually reach equilibrium with the air. If you want a sensor that's good at sensing air temp, it needs to be really small.

Edit: What I said only makes sense if you just brought your air-immersed probe from a cooler environment (like 44F or less) into a warmer chamber.

Another thing to mention is that thermocouples can be pretty inaccurate. My temp controller originally had a J-type thermocouple that I found was off by about 5 degrees - this was relative to a well calibrated RTD. My Auber Inst controller could take an RTD, so I replaced the TC with an RTD.

It has been in the keezer for weeks. I just recently noticed that my beers were being served warm (and undercarbed because the temp was higher than what I thought). I don't get it. My plan is to make it waterproof and immerse it in a cup of water in the keezer. I am fine with doing that, but I just am wondering if someone has an explanation for why this is happening. Thanks.

As soon as I tape the thermocouple to the keg, it reads the correct temp. I think the probe is correct when touching water/beer.

bessieflames said:

It has been in the keezer for weeks. I just recently noticed that my beers were being served warm (and undercarbed because the temp was higher than what I thought). I don't get it. My plan is to make it waterproof and immerse it in a cup of water in the keezer. I am fine with doing that, but I just am wondering if someone has an explanation for why this is happening. Thanks.

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The only other thing I can think of is that when you place the probe in contact with the keg it is creating an additional electrical circuit that is altering the TC reading. TC's are based on microvolt level signals so it wouldn't take much disturbance to throw it off. Of course your observations are that the probe in-contact with the keg is giving the correct reading.

What you could do to see if it is an electrical issue is make an electrical contact similar to what happened when covered by the insulation, but don't put the insulation on. Let the bulk of the probe be in the air.

Do any of your scenarios involve both thermocouples touching the keg at the same time?

Well, I guess forget everything I said about thermocouples. It looks like the Ranco controller comes with a thermistor type sensor.

I use a type K exposed junction thermocouple with my Auber controller. Reads the same in air as in liquid, but as someone mentioned the liquid takes much longer to cool down to or warm up to the air temp.

I don't think it has anything to do with electric disturbance because it reads correctly when taped to my glass carboy. It can read correctly when touching a solid or liquid, but not a gas.

It's a long shot, but perhaps the current sent through the thermistor is too much, causing enough self-heating to change the reading in air (can't effectively dissipate the heat) but there is enough sink in the object you attach it too that the self-heating is essentially nullified, giving a more accurate reading.

Your controller is likely reading just fine. It is not surprising that it indicates an air temperature far different than the beer. The air and liquid temperatures will only read the same during two points in the cooling cycle. Once while the compressor is on and the air temperature is falling and once while the compressor is off and the air temperature is rising.

The air in the keezer has very little thermal mass, which is basically its heat retaining capacity. The kegs have a very large thermal mass. When the compressor cycles on, the air loses heat quickly and the temperature drops quickly to the setpoint. However, during this same time, the kegs lose heat very slowly and the liquid temperature only drops a small amount.

Air temperature in a keezer has widely varying temperatures during cycling as well as temperature stratification if inadequate air movement exists. IMO, if you want to control the temperature of the beer, then monitor the temperature of the beer.

Another interesting thing I found this morning: I had taped the probe to the keg last night and lowered the temperature to 40 degrees. THis morning I poured a small beer and it was 57 degrees. The second one was 40 degrees. Apparently even when the kegs are at 40 degrees, the air temp in the keezer is close to 60 and is warming the beer in me lines. Strange. I figured if the keezer was cold enough to keep my kegs at 40 that it would keep my beer lines cold too.

raouliii said:

Air temperature in a keezer has widely varying temperatures during cycling as well as temperature stratification if inadequate air movement exists. IMO, if you want to control the temperature of the beer, then monitor the temperature of the beer.

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That makes sense if the probe is reading beer temp. The air would fluctuate widely to get the beer to cool just a little bit. It doesn't explain a wide air temp range when the probe was just hanging in the keezer. If the temp range of the air fluctuated, wouldn't the probe read this? By reading the air temp with the controller, I was trying to keep the air at the constant temp. It would take longer to get the kegs cold, but it would maintain the entire system at my desired temp.

Attached is a data log I made in my fermentation chamber of the temp beer in a fermenting bucket vs. the air temp in the chamber..

The temp in the beer was an immersion probe directly into the center of the beer and the air temp was taken by an exposed junction probe. Both the immersion probe and the air probe are/were Type K thermocouples.

You can see the (approximately) sawtooth pattern of one trace which is the air temp in the chamber. The air temp rises and falls closely approximating the H and L setpoints of the controller. The controller was actually connected to a third exposed junction thermocouple co-located with the one reporting the temp to the data logger. All three thermocouples were checked against each other for consistency and equivalency of temperature readings.

The gently sloping trace is the immersion probe showing the beer temp gradually dropping into the swing range of the H and L setpoints. When this all stabilized the beer temp was essentially dead center of the swing range. Also note that as the beer temp got lower the time between cycles (the sawtooth pattern of the air temp) got longer/wider. Based on the downward slope of the sawtooths, the time the compressor ran stayed fairly consistent (slightly shorter towards end) as one might expect because the air cools fairly quickly but the time between cycles was longer due to the mass of cooling beer keeping the air cooler for a longer period of time.

What H and L setpoints are you using in the controller?

Do you have the temp control on the keezer set as cold as it will go?

raouliii said:

Air temperature in a keezer has widely varying temperatures during cycling as well as temperature stratification if inadequate air movement exists.

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I agree to some degree, but wouldn't you expect the temp (at least at the point where the sensor is) to remain essentially within the H and L setpoints of the controller? I would.

IOW, with the probe in the air and the controller set to say 45F, with +/-5F, the probe should pretty much always be returning something within the range of 40-50F. May get some swings slightly lower or higher, but I'd think not too much.

whoaru99 said:

I agree to some degree, but wouldn't you expect the temp (at least at the point where the sensor is) to remain essentially within the H and L setpoints of the controller? I would.

IOW, with the probe in the air and the controller set to say 45F, with +/-5F, the probe should pretty much always be returning something within the range of 40-50F. May get some swings slightly lower or higher, but I'd think not too much.

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That's what I would think. The kegs have been in the keezer for over 3 weeks, so they should be within the range of the H and L of my controller. Your chart is exactly what I would expect. I wouldn't expect the beer temperature line to stay above the zig-zag pattern of my controller for 3 weeks.

whoaru99 said:

I agree to some degree, but wouldn't you expect the temp (at least at the point where the sensor is) to remain essentially within the H and L setpoints of the controller? I would.

IOW, with the probe in the air and the controller set to say 45F, with +/-5F, the probe should pretty much always be returning something within the range of 40-50F. May get some swings slightly lower or higher, but I'd think not too much.

Click to expand...

I would agree as well, unless a compressor delay is allowing the temperature to rise above H. Air warms and cools very fast and H may be reached before a long compressor delay has timed out.

OP: does your controller have settings other than H and L? Like a compressor delay? Do you use a fan to stir the air up in the keezer? Is it an actual converted freezer or some other kind of unit?