Ideal length of time between turning on heat and cooling down.

JohnSwift10

I don't know if this is a suitable forum for this question but here goes.

I have built a thermostat to control a heating mat in a plant propagator and can set the on and off length of times and temperature required to whatever I want.

i.e required temperature 20 deg C heating on for 10 minutes
if required temperature reached then heating off for 10 minutes
however 
if required temperature not reached heating on for 10 minutes etc. until required temperature reached then heating off for 10 minutes and if temperature above required temperature then off  for another 10 minutes.

or I could set the heating on for 5 minutes and then off for 10 minutes or then on for 15 minutes and off for 5 minutes.
or even on/off every minute.

What would be the ideal on/off times if the external temperature to the propagator was say 5 deg C?

Effician

Wouldn't you be better off with a temperature controller like the stc-1000 ( about a tenner) , I've used them for propagators , dough proofing boxes  & home brew with great results & rock steady temps. Usually set with +/- 0.5 deg c.

QrizB

JohnSwift10 said:

...
What would be the ideal on/off times if the external temperature to the propagator was say 5 deg C?

That's an unusual approach to control logic.

Fundamentally, a long interval time risks over-heating a low heat capacity propagator or over-chilling a high heat loss one.A short interval time will (or should) suit either case.

I'd be inclined to set both "heat" and "rest" periods to one minute and see how it goes.

Astria

Agreed, don't think I've ever used a thermostat which takes a reading of greater than every minute. You want to switch the heating on as soon as you see the temperature dropping and turn it off just before it's reached it's ideal temperature (to account for overshoot). The only time you wouldn't do that is if it takes forever to sense a change in temperature - eg. in a house central heating system where turning the boiler on will not make any difference to the house temperature until at least 10 minutes in.

JohnSwift10

Effician said:

Wouldn't you be better off with a temperature controller like the stc-1000 ( about a tenner) , I've used them for propagators , dough proofing boxes  & home brew with great results & rock steady temps. Usually set with +/- 0.5 deg c.

Not if you've spent ages with your grandson writing code for a raspberry pi and the temperature sensor and  relay. only cost £2.50  plus you can also add a moisture sensor and solenoid for automatic watering for another £4.