Nice idea. I like that. Presently the probe is sellotaped to the outside of the FV. It's not really a reflection of the liquor inside.
So; I long plastic tube. Good call.
What if the metal probe wasn't sticking out? Wouldn't the temperature be pretty accurate. That would avoid the food grade sealant issue.
Thanks again.
Yes. Steady state temperature, from a probe inside an immersed tube would be exactly the same as from an exposed probe.
But after any change in temperature, the enclosed probe reading would always lag a while longer, this would lead to increased temperature swings, around the desired temp (hysterises). So with controller hysteresis set to 0.5°C, the hysteresis, with an enclosed probes thermal lag, might be 2°C.
You could put probe down one side of a folded tube (fold at bottom), to avoid needing any sealant.
An externally mounted sensor is likely to have longer lag, than one contained in a dip tube. But both of these lags, might actually be insignificant, when compared with lag due the liquid thermal mass. Where a 24W heatpad, might take about 5 minutes to raise temp of 23L batch by 1°C.
eg. with controller hysteresis set to 0.5°C:
1) An (exposed) sensor in middle of vessel. After sensor goes 0.5°C under set temp, and heating turns on, it might take 10 minutes before heating turns off again.
By then the base temperature overshoot might easily have risen to say 10°C, ie a hot yeast/trub layer; the overshoot at sensor might end up being 3°C; and the hysterises cycle might last 20 minutes.
The average brew temperature, will stay pretty constant with variations in ambient temperature.
2) An external probe, between heatpad and base. This has the least lag. The cycle time might be 1 minute; trub layer overshoot 0.5°C; at middle middle of vessel there's virtually no temperature cycling.
Brew temperature (particularly towards top), will vary with ambient temperature.
The best would seem to be an external bottom sensor, but with the fermenter in a well insulated container, recticell box, or a defunct fridge, to keep a more consistant ambient temperature.