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Old 03-17-2009, 05:51 PM   #1
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Default Johnson Controls thermostat issue

I bought the Johnson Controls thermostat to maintain the temp in my keezer. I'm getting 5 degree F differences in each direction for a 10 degree total swing. I thought this was supposed to have a 3 degree differential. Is anyone one else experiencing this? Is this an issue?

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Old 03-17-2009, 08:24 PM   #2
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Where do you have your temperature sensor for your controller, and where to you have your temperature sensor for the device that is telling you that you have a 5 degree swing.

If say the controller sensor is in a glass of water (as recommended) and the temperature sensor is sensing air temp it is possible that there is a 10 degree swing in air temp, but only a 3 degree swing in water temp as it takes water a lot longer to warm or cool.

Just a thought.

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Old 03-17-2009, 08:36 PM   #3
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Sounds like you have the differential setting set at 5 degrees. I have mine set at 1. Say my set point is at 66 and I have it in cooling cut-in mode. When the temp hits 66, my fan turns on in my son of fermentation chiller which cools the wort down. When the temp hits 65 it cuts off and won't turn back on again until the temp hits 66.

Check your differential setting.

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Old 03-17-2009, 09:28 PM   #4
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Which controller are you using, analog or digital? The analog is preset at a 3 deg differential IIRC. How and with what are you measuring the temperature? I don't use the sensor in a container of water method with my analog unit. Instead, I use a wireless indoor outdoor thermometer and place the transmitter on top of the fermenter. The air temperature swings with the differential, but the fermenter stays within one degree and most of the time dead on my target temperature. Some of the swings have more to do with the refrigeration unit design than the controller. Some don't have fans inside and some do. Compressor and evaporator sizes vary considerably all making for a variety of response times and temperature swings.

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Old 03-17-2009, 09:42 PM   #5
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As others have said, analog or digital?

Analog one works fine for me, never more than 5f swings. Is your fridge empty? If you fill it up the temperature swings will be much more gradual and not as much of a problem.

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Old 03-18-2009, 02:17 PM   #6
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This is the analog and it does not have a differential setting. I have the probe in a glass of water because I saw that someone else was doing it. The thermometer is in the open air. I guess I should try taking the probe out of the water and putting a thermometer in the water to see what kind of differential I'd be getting in the water with the probe in open air.

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Old 03-18-2009, 02:41 PM   #7
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Quote:
I have the probe in a glass of water because I saw that someone else was doing it. The thermometer is in the open air.
That's why right there. Think about it: The temp rises in the fridge and your thermometer senses the air temp but the bulb is only sensing the water temp which takes time to equalize to the air temp...so your thermometer sees a big change in temp but the bulb only sees a small change. Finally when the bulb senses the high temp it kicks on the cooling. Now the fridge cools and cools and your thermometer sees a big temp drop but the bulb doesn't see that temp drop until some time has passed...meanwhile the thing just keeps cooling and your thermometer keeps dropping. Rinse and repeat.

I actually do it just the opposite of what you have...I put the thermometer probe in a jar of water and the controller bulb is exposed to the air. FWIW, I work in Chemical Lab and we keep lab standards in a tightly controlled fridge. There is a tight temp spec on these lab standards. Inside each of these fridges is a small jar of water with a calibrated thermometer in it.

EDIT: Plus by doing it the way I am...you don't see the big temp swings on your thermometer and imo you get a MUCH better idea of the temp your keg contents are at. I'll bet your kegs are not quite as cool as you think. If the air temp swings between say 40 F and 50 F...your kegs are not at 45 F...they're warmer.
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Old 03-18-2009, 03:29 PM   #8
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Quote:
Originally Posted by SpanishCastleAle View Post
That's why right there. Think about it: The temp rises in the fridge and your thermometer senses the air temp but the bulb is only sensing the water temp which takes time to equalize to the air temp...so your thermometer sees a big change in temp but the bulb only sees a small change. Finally when the bulb senses the high temp it kicks on the cooling. Now the fridge cools and cools and your thermometer sees a big temp drop but the bulb doesn't see that temp drop until some time has passed...meanwhile the thing just keeps cooling and your thermometer keeps dropping. Rinse and repeat.

I actually do it just the opposite of what you have...I put the thermometer probe in a jar of water and the controller bulb is exposed to the air. FWIW, I work in Chemical Lab and we keep lab standards in a tightly controlled fridge. There is a tight temp spec on these lab standards. Inside each of these fridges is a small jar of water with a calibrated thermometer in it.

EDIT: Plus by doing it the way I am...you don't see the big temp swings on your thermometer and imo you get a MUCH better idea of the temp your keg contents are at. I'll bet your kegs are not quite as cool as you think. If the air temp swings between say 40 F and 50 F...your kegs are not at 45 F...they're warmer.
+1 That's the way to do it. No need to dampen the response of the controller at all.
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Old 03-18-2009, 06:10 PM   #9
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Quote:
Originally Posted by SpanishCastleAle View Post
I'll bet your kegs are not quite as cool as you think. If the air temp swings between say 40 F and 50 F...your kegs are not at 45 F...they're warmer.
So why wouldn't the kegs be 45 if the swing is 40-50? Also, wouldn't the kegs be experiencing a more consistent temp due to the much higher volume relative to the glass of water. Does density play a part in this? I don't work in a chem lab and it's been a while since college.
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Old 03-18-2009, 07:10 PM   #10
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Quote:
So why wouldn't the kegs be 45 if the swing is 40-50? Also, wouldn't the kegs be experiencing a more consistent temp due to the much higher volume relative to the glass of water. Does density play a part in this? I don't work in a chem lab and it's been a while since college.
I think the main reason is because when the cooling shuts off, the fridge warms up that first 5 degrees (from 40 to 45) a lot quicker than it does the last 5 degrees (from 45 to 50). So the kegs stay above 45 longer than they stay below it. My kegerator and keezer both warm up that first degree very quickly...but when they get up to that last degree before the cooling kicks back in...it takes a long time for it to warm up that last degree. I wouldn't be surprised if it spends twice as much time above 45 as it does below it.

Yes, the kegs will be more consistent in temp than the glass of water because of their much larger thermal mass (more volume)...but they'll still be warmer than the middle of the temp swing.
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