Heat mat or heated propagator

[mrbadexample]

I've stripped out the old lining which was brittle and nasty, so I know I need to re-line it.

I've got plenty of sand - what depth does it need please?

How deep should the cable be buried? Should it be on the bottom of the box?

Please assist, if you know what you're doing.

[quanglewangle]

[ATTACH=CONFIG]89814[/ATTACH]

I've stripped out the old lining which was brittle and nasty, so I know I need to re-line it.

I've got plenty of sand - what depth does it need please?

How deep should the cable be buried? Should it be on the bottom of the box?

Please assist, if you know what you're doing.

"If you know what you are doing" That would be a first for me, but here is advice anyway.

It's worth standing the propagator on some insulation board. I know theory says not much heat should go downwards, but I found it made a difference. Off-cuts can be scrounged from building sites.

Depth of sand: remember purpose of sand is to even out heat distribution; to provide some "thermal ballast" so temperature doesn't vary to much; and support the pots. You don't need any more than will achieve that. Mine is about 40mm deep.

How deep should cable be? Mine is about 2/3 the depth of the sand. I would be worried about putting it on the very bottom, since you want the heat to go up.

Remember to keep it very moist. It's the water that conducts the heat not the sand.

Don't over think it. It will be fine.

[mrbadexample]

Thanks, I've got some thick polystyrene in the way in the garage so that takes care of that.

I'll get it going tomorrow and see how we get on.

[broadway]

I have not had an electric propagator for years but with the new allotment decided to treat myself to the one below.

I also have the unheated version so will be able to utilise the additional trays

[rary]

If I was to make a hot bed again it would be 2in cover of sand for the base then place heating cable, cover with an other 2ins.of sand and if using a rod thermostat cover with a further 2ins. if you are using one of those newer pin? probes make your second layer 3ins and as quangle has said keep it moist, and always use sand several years ago peat was recommended for these beds but if it dries out it will ignite, so no combustible material what so ever

[quanglewangle]

I put the thermostat into the one of pots rather than into the sand. After all it's the compost in the pots you want at the right temperature.

My measurements show that the pots run a couple of degrees cooler than the sand.

[rary]

Looking at your photo (which I should have done at the start) I see its a rod thermostat you use, personally I would place it higher in the box and if you always keep the medium moist the plastic should last for years, I found that using vizqueen? was the best material to use for lining

[mrbadexample]

Thank you. I've re-lined it (as is, thermostat in the same place) and filled it with the (sieved) sand it came with. It's about 4" deep, and the cable is around 1.5" from the bottom. I've plugged in, and the light came on which is encouraging.

Will I be able to tell (by touch) that the sand is warm once it's got up to temperature? I've set it to 35 at the moment. I've sown a dozen tomato seeds in a module and sat it on top.

[quanglewangle]

Will I be able to tell (by touch) that the sand is warm once it's got up to temperature? I've set it to 35 at the moment. I've sown a dozen tomato seeds in a module and sat it on top.

Sounds good.

You may be able to tell by touch, but don't bank on it. The thermostat light should go off periodically. A thermometer would be best.

[edit] Give it a day whatever

Covering the wbole thing with fleece/polythene/bubble wrap will make a big difference.

[quanglewangle]

Thinking more about it - depending on the ambient temperature the thermostat light may never go off if it is set to 35°C. The heating cable may not be powerful enough to get the sand to that temperature in cold surroundings. Not a problem in itself, just the lifgt won't go off.

[mrbadexample]

Hmmm...this doesn't seem to be working, unfortunately.

The sand at the top is cold to the touch, and when I dig down to the cable, it's cold there too.

There doesn't seem to be much to go wrong with this - is there a way of testing the cable? I was given this last year and have no idea as to its history.

[quanglewangle]

Hmmm...this doesn't seem to be working, unfortunately.

The sand at the top is cold to the touch, and when I dig down to the cable, it's cold there too.

There doesn't seem to be much to go wrong with this - is there a way of testing the cable? I was given this last year and have no idea as to its history.

The cable should be warm. You need some sort of continuity tester or muilt-meter (about £10 from eg Amazon or screwfix)

I use one of these to check temperature (then sneak it back in to the kitchen)
Meat Thermometer, Habor 192 Ultra-Fast Read Digital Food Cooking Thermometer with Backlight LCD, IP67 Waterproof
Learn more: https://www.amazon.co.uk/dp/B07RLXK7..._wESkEbVZPMBGT

[mrbadexample]

You need some sort of continuity tester or muilt-meter

I've got one. Somewhere.

[quanglewangle]

I've got one. Somewhere.

For 25W cable expect a resistance of about 2k Ohms or
for 50W about 1k Ohms

Phew - brain hot now.

[mrbadexample]

For 25W cable expect a resistance of about 2k Ohms or
for 50W about 1k Ohms

Phew - brain hot now.

I didn't say I know how to use it. I was going to come back for instructions once I'd found it.