Heat mat or heated propagator

[mrbadexample]

I went to get a photo of the inside of the control box, and noticed a couple of things. Firstly, I'd left about 4" of cable outside the box. This bit felt warm, so I'm probably losing all my heat there.



So I've pulled the cable in right up to the blue connection block. When I pulled the cable in I noticed some dry sand in patches, so I've watered the sand. I'll give it another 24 hours which might give me time to find that thermometer that's knocking abut somewhere...

Next job is to knock up a frame to support a fleece cover.

[quanglewangle]

If the cable is warm its all fine. You just haven't waited long enough. Unlikely that losses in that tail are significant.
Bodes well...

[mrbadexample]

I went out yesterday to check, and the sand was stone cold. Not a trace of heat.

Then I noticed that I had plugged it back in but not switched it on.

This did give me time to find my thermometer. Both of them actually.

I've checked today and I can immediately tell the difference, but it's not really warm. The propagator has been covered with fleece, and the thermometer tell me it's about 16�C (pushed about 1cm into the sand).

It might be a touch warmer than this, as I don't think the thermometer is particularly accurate (so could be colder, too).

So whilst it is working, I don't see that it will be hugely useful as a propagator in the greenhouse - at least, not at this time of year. I can see it being very useful to stand plants on to give them a helping hand during cold nights though.

[quanglewangle]

16�C relative to what? What is the temperature difference to the surroundings?
Do you know how many Watts the cable is rated?

[mrbadexample]

16�C relative to the ambient temperature of the greenhouse.

Oh, specifically? Greenhouse currently at about 9.5�C (max) during the day, and at time of measurement. Only dropped to 5�C last night.

It's a 27 watt cable:



I don't think it's doing a bad job, but I don't think I'll get to germinating temperatures until it has to fight less against the cold.

[quanglewangle]

16�C relative to the ambient temperature of the greenhouse.

Oh, specifically? Greenhouse currently at about 9.5�C (max) during the day, and at time of measurement. Only dropped to 5�C last night.

It's a 27 watt cable:

[ATTACH=CONFIG]89854[/ATTACH]

I don't think it's doing a bad job, but I don't think I'll get to germinating temperatures until it has to fight less against the cold.

Firstly, sorry if I come over like the secret police with tone of questions - it's easy to fall into tech-support mode.

Broadly, the performance compares with mine, which is 50W and a little over twice the area (I think)

I wouldn't be be too put off. However what you need to know is what temperature you can get in pots. Stick a pot full of damp compost on the sand, push it in a bit for good contact and look at the temperature in that. I would think the temperature at dawn would be a good stand-in for minimum night time temperature.

Remember the temperatures quoted for germination are optimums. I have good seedlings of tomatoes, aubergines, and cucumbers 20 days after sowing (cotyledons fully out and just signs of first true leaves) with an average[1] pot compost temperature of 13 �C and average[1] greenhouse air temperature of 9 �C Slow germination does leave seedlings vulnerable to damping off but good light levels, ventilation, and watering from beneath should help there.

[1] Average of temperatures every 5^m over 24^h ie not the meteorological average, which midway between daytime high and night-time low

[mrbadexample]

Firstly, sorry if I come over like the secret police with tone of questions

Fret not, I genuinely hadn't noticed.

I appreciate the help. I'll put a pot on it tomorrow and see how it gets on. I put the thermometer in the sand first just to make sure something was happening.

[rary]

Pour some hot water over the bed then check the temperature tomorrow or after a period if 4hrs has past

[mrbadexample]

Pour some hot water over the bed then check the temperature tomorrow or after a period if 4hrs has past

I've done that, although I'm not entirely sure why. Just to give it a boost?

[quanglewangle]

Top up sand in propagators with warm water

I've done that, although I'm not entirely sure why. Just to give it a boost?

I don't know about *hot* but it makes complete sense to top up the sand substrate with *warm* water.

Anyone is welcome to check the following. I did A level physics when the the world was young.

Say:

• the water from the garden tap is 10�C less than the desired sand substrate temperature. A conservative estimate .
• the heater cable is rated 25W
• you add 5l of water

It will take
P = (4.2 � 5 � 10 ) � 3600 kWh to heat the water to desired substrate temperature
or about 0.058 kWh. Not a lot, granted, but nor is 25W

so, dividing the kWh by the power:

0.056/0.025 = 2.33 or about 2^h 20^m

So two hours to recover. Worth going to the kitchen tap for.

[mrbadexample]

It was the same temperature this morning - about 16�C in the sand. I haven't tried a pot yet as I've been out all day.