Temperature differential settings on a thermo controller

[brewbrew]

Hi,

I'm using an Inkbird 308 to control a fermentation fridge - set temp 18 degC.

The Inkbird has heat and cool differential settings (eg you can have a set value of 18.0 deg C and programme the cooling differential to 0.3 deg C - so that the cooling only cuts in when it senses a temp or 18.3 and then stops at 18.0 deg C on the sensor).

The narrowest setting is 0.3 deg C in both heat and cool directions.

I can understand that you need a band of differential - otherwise heat and cool would be running against each other all the time. However, my system seems to have slight over-run characteristics - so in the above scenario, the cooler cuts out at 18 deg C, the chamber takes time to catch up and drops to about 17.6 deg C - which if the heater is set to 0.3 deg C, comes on and that in turn over runs.

Does a narrow differential always give the best/most accurate control or is there a way of widening it with knowledge of the over run characteristics (this is something of a physics and engineering question that I can't get my head around (I got grade E at physics A level 40 years ago)).

Similarly, does a wider differential just give nothing more than better energy efficiency? In which case I'm not to worried because I'm using a domestic fridge and a 40 Watt heater.

At a practical level, I have monitored the temp of a bucket of water and it seems to make no difference as far as I can measure to its temp if I use a 0.3 or 1.0 deg C differential in both directions.

What settings do others use in this sort of set up?

Thanks

BB

[PeeBee]

Some controllers have a "dead-band" too deal with this, but not the ITC-308. I've fiddled with mine to try and reach a satisfactory compromise. 0.3C is a bit small, I've got mine set to larger differentials (but below 1.0) and not necessarily the same for heating and cooling. There's no point me dictating settings I use 'cos it will be different for each situation. You've just got to fiddle until the settings stop annoying you.

(EDIT: I'm also contemplating fitting fans to iron out the "lags". The fans coming on with the heater/cooler and with a "delay off" so they over-run a bit.).

[brewbrew]

Thanks for the advice peebee.

I have done more experimenting overnight and today with an Elitech logger sunk in a jar with a fishing weight in a 25l bucket of water - my best method of measuring what the yeast "see".

With 18 deg C as the set value, I have put the low differential at 0.8 to stop the heater coming on and causing over cycling on the fridge (unless the weather gets colder - in which case the heater kicks in at 17.2. The cooling dif is now at 0.6 and I'm getting cycle times on the fridge compressor of about 15 on then 2 hours off. I have a pair of 4 inch USB fans in there which stir the air fairly well.

I have recovered the "submarine" Elitech and it is showing that the bottom of the 25 litres is a nice steady line that runs between 17.7 and 18.1 - so I think I have found a good sweet spot.

No doubt, the winter weather (the kit is in an un heated, detached brick garage) will give a bit more swinging around as the heater kicks in - but at least the cool phases should last longer.

Now to start fermenting......