accuracy of heated bench cables

[mark smyth]

Out of interest I brought a digital thermometer from work today to check the temperature of the heated bench in the green house. It's set to about 7C to prevent hard frost killing my Pelargoniums. What a shock when the reading peaked at 15.5C. I turned it down below 5 and the temperature now reads 10C

[Gene Mirro]

I gather you've got some kind of thermostat for sensing the temperature.  Is it sensing air or soil temp?  If it's sensing air, try putting your digital thermometer right next to it, and leave it there for 15 minutes to allow it to reach air temperature.  If it doesn't match the thermostat, then you've got an inaccurate thermostat. 

If your thermostat is mounted on a part of the greenhouse frame which is colder than the inside air, the thermostat will "see" that cold temperature and will keep the heating cable turned on for much too long.

The best setup is to have the thermostat directly sense the soil temperature.

[mark smyth]

sorry not enough info from me. My benches are filled with damp sand. The thermostat has a long probe that sits half way up the sand

[Ian Y]

Mark
the trouble with many rod thermostats is that they only have a single setting so when they trigger on the temperature has to rise quite high to turn them off again.
More accurate thermosts have two settings one is for the temperature to switch them on and the other to adjust how far the temperature must rise before they switch off again.
I have some of this type that I salvaged from an old airconditioning unit and I have the temperature set at zero and the other setting screwed back to the minimum so it goes off at +1C.

[mark smyth]

OK thanks. I'll look for a new thermostat

[Anthony Darby]

Look for the hi-tec ones used for reptile cages Mark. I don't use cables (except in snake cages) but use a 3kW fan heater set to 4^oC.