Posted: 03/02/2014 at 19:00
Hi all, the temperature results are in. I left it a couple of days to make sure things had settled down.
With the probe placed in the sand and showing 25C, the temperature on the surface of a tray of compost placed on top of the sand and covered with a clear lid varied over a night and a day from between 15.5C to 19.5C (measured using a mix-min thermometer placed on top of the compost.) Clearly, the air temperature in the shed has a fair effect on the compost temperature even though the sand is absolutely constant at 25C, so tonight I'm raising the sand temperature to 30C and will report back tomorrow.
Because I have a moveable thermostat probe, in order to keep an exact temperature in the seed trays themselves, I do have the option of putting the probe in the actual seed tray rather than in the sand below and will try that tomorrow night.