Temp probe position on outside of keg, has a big effect!

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chris2012
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Temp probe position on outside of keg, has a big effect!

Post by chris2012 » Mon Jun 26, 2017 2:19 pm

I had the probe positioned mid way on the keg.

The inkbird had the temperature set to 2.7C.

When I pulled a small amount of beer through a tap, it was reading 7.1C, despite the temp probe reading around 2.7C.

I've since moved the temp probe to the very bottom of the keg, and it's now reading 6.9C.

I can't believe I hadn't thought of this earlier.

I'm gonna leave the keg cooling again now, to make sure my beer becomes a lot cooler when I next take a beer temp reading.

(Note the keg isn't completely full either, but I would have still assumed the liquid in the keg was near the probe, being midway, as it's still heavyish, but I'm always gonna leave the probe on the bottom now, as that as it drains, it'll always measure some liquid's temp)

Edit: I think the reason for the temp diff. now is possibly purely due to the keg not being full, so that the temp probe might not have been next to the beer.

Because if the beer temp at the top of the keg was actually 3C, then lower down the keg it would be colder still?

Also before I was using duct tape + sponges over the probe, now I've got ratchet straps, which should be a lot tighter, and the duct tape won't fall off.

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Kev888
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Re: Temp probe position on outside of keg, has a big effect!

Post by Kev888 » Mon Jun 26, 2017 10:35 pm

Yes, it depends on how heating/cooling is applied, but usually the beer would be cooler towards the bottom.

It also depends if temperatures are changing; during periods of change (say recently put into the keggerator, or things causing the ambient temperature to fluctuate) temperatures can lag differently in different places. The keg above the beer will change at a different rate to the keg around the beer.

Though (unlike in the fermenter) finished beer doesn't appreciably self-heat so given enough time in a properly stable ambient temperature it should roughly reach equilibrium with the air temperature. In that case there shouldn't be much variation around the keg (unless it has been run off quickly releasing a lot of chilled/expanding CO2 in the headspace).
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Re: Temp probe position on outside of keg, has a big effect!

Post by chris2012 » Tue Jun 27, 2017 9:17 am

Cheers for the reply Kev.

After moving the probe to the bottom, and ratchet strapping it the oscillations seem less pronounced by my eye, as you can see
in the graph below:

Image

So I bet before, the temp probe might have been near co2 rather than liquid in the keg.

(The big spikes are when I took the keg out of the fridge to make adjustments to the probe)

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Re: Temp probe position on outside of keg, has a big effect!

Post by Kev888 » Tue Jun 27, 2017 11:43 am

Ah, yes. If the probe is being used to control the keggerator, then we are into the changing ambient temperatures scenario - though they are being changed by the inkbird.

Having the probe against the beer-filled part of the keg will give much more stable results as the thermal capacity resists fast changes. However, for the same reason the keggerator may be off and on for extended durations too; potentially stressing the fridge. It will also change as you drink the beer.

I would personally measure the air in the keggerator, and have a small (PC type) fan to ensure the air circulates. That way it will be much less dependent on beer levels and the duty cycle will be much more normal. Finished beer doesn't self heat so it will become more or less the same temperature as the air, given time. There isn't the need for super-precise accuracy (unless you are fermenting in there), so you could also set the inkbird with both a compressor delay and a wide dead-band, to ensure heating and cooling don't chase or oscillate. This will also help reduce inefficient and damaging rapid cycling of the compressor on occasions when there isn't much beer inside to offer thermal mass.
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Re: Temp probe position on outside of keg, has a big effect!

Post by chris2012 » Tue Jun 27, 2017 12:21 pm

that's interesting with respect to measuring the air temp. I guess possibly you could even measure both the beer temp and air temp and then use a dual PID algorithm, that attempts to reduce the difference between the two, which might prevent overshooting.

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Re: Temp probe position on outside of keg, has a big effect!

Post by fego » Tue Jun 27, 2017 12:34 pm

For beer that's finished fermenting, I move the probe from the side of the FV into a bottle filled with water. This seems to better represent the temperature of the beer in the Keg.
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Re: Temp probe position on outside of keg, has a big effect!

Post by Kev888 » Tue Jun 27, 2017 1:54 pm

chris2012 wrote:that's interesting with respect to measuring the air temp. I guess possibly you could even measure both the beer temp and air temp and then use a dual PID algorithm, that attempts to reduce the difference between the two, which might prevent overshooting.
People do complex things with fermenters because the fermentation causes heat (and they may also want to vary the wort temperature). Air is not a brilliant conductor, so what the fridge is doing and what the fermenting wort is doing can get away from each other.

But for kegs of finished beer there really isn't any need for great complication; they're thermally passive things. Put a keg of beer in air at your chosen temperature and give it some time, the keg will reach the same temperature. After that, the fridge is mostly just working to stop warmth from outside raising the internal air temperature. Unless you need to chill down a keg as quickly as possible, then theres no need to have the fridge set to any different temperature. All i'd suggest is the small fan (small so as not to add much heat), to make sure the air temperature is pretty even.
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Re: Temp probe position on outside of keg, has a big effect!

Post by Fil » Tue Jun 27, 2017 2:18 pm

all the top end and commercial fridges employ fans, and it improves efficiency hugely. the air inside the box is the medium of heat transfer, if its sat fairly still its not working.. move it about and it will be ;)

while you at it knock up a pc fan/phone charger arrangement for your kitchen fridge too if not already fitted.
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