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Forum Index : Other Stuff : Heating with Wire/ Cable.
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| Davo99 Guru  Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
Short story long and I'm not sure about what I'm ultimately asking.... Thought I'd get a head start on my summer crops to have them ready for Christmas. Planted about 8 Different types of Tomatoes, Corn, Melons, sunflowers and others. Most were slow to germinate, a lot didn't.... Till I Figured it was the soil temps holding them back and got the bright Idea of getting an unused pet heating pad and putting the starter Punnets on that in a plastic tub and bringing it inside to get the soil temp up. Worked very well although I still have lost the best part of a month I wanted to be ahead on but anyway. Soon as they germinated I put an LED Downlight in there which gave a bit more heat as well. Suffice to say that worked a charm and things that hadn't germinated have done well and some that did gone nuts. I bought an extra heating pad for another tub and more powerful than the original But I was thinking, Could I not just use a wire with a PMW controller to regulate the amps and so the temps to do the same job? I bought some 2.5MM silicone wire recently which has a rating of 200o. 20-25 would be plenty for what I want so I don't see the need for the silicone stuff as even the core temp shouldn't get that high. This also comes in smaller gauges and I also saw some Carbon Fibre heating wire. This stuff is noted as being 33 ohm/ meter. I have tried to look up how to calculate the heat value of that per certain length at different voltages but I must be putting the terms wrong cause I'm not getting anything helpful. Can anyone steer me to the right place for that answer please? Also, for the heating value of a length of wire, if I get a PWM and a Power meter and feed in say 3A@ 20V, am I going to get 60W over the length of that wire being dissipated? Seems straightforward but wondering if there are any other considerations here at play I'm missing? If that's right, seems I can use any old bit of wire including unshielded tie wire which is Cheap. Could I also not just use a bit of sheet metal with connections on Opposite sides? If it's just the watts being fed into it that are going to create the heat, seems anything conductive is fine? I'm thinking of this for sinking in trays I plant seeds in to warm them directly and was also thinking of Laying a loop of wire in the garden to keep the soil warm when I plant out so the seedlings will Continue to grow as the Night and soil temps are still below Optimum for what I want to grow. Also, provided the wire is capable of carrying a given load, would it make any difference what the gauge of said wire is? Would I be better with a lighter or smaller gauge or is the watts going to be the watts regardless of wire gauge up to it's capacity? Does anyone know why they have the carbon fibre wire for heating and specify it's resistance? I am guessing this is to spread the heat out over more cable/area and reduce the load on controllers? If the watts pumped in over a certain length are the same, does it matter if it's " Normal" Wire or the ohm rated stuff? |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I am not really into growing stuff myself, not worth the trouble here, but... A greenhouse of some kind is the traditional way to grow out of season crops. Direct sunlight is about 800 watts per square metre, and its free. Electrically heating the soil should be possible, but be very careful to avoid passing any current through the soil itself. As with electroplating, the current flows one way and ions go the other way, and that can be particularly devastating to the root system of any tree or plant over time. I think you would definitely want insulated wire and keep the voltage as low as possible. Safest way might be to bury some plastic pipe and circulate water that is kept only a few degrees above ambient. Heat the water electrically in maybe a 44 gallon drum and fit a circulating pump. It may be possible to get enough heat most of the time during the day from just a coil of black plastic pipe in the sun. With a few temperature controls one or more pumps and a part time electric heating element, should be possible to build something pretty interesting and unique. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
I had the little greenhouse before but it takes a lot to keep it heated being just plastic and outside in the cold. Nowhere to put it inside like the Tubs which obviously take up much less space. The tubs are using about 20W on the heating mats and the LED's are 13W and working well. Less than 100W in total, 2.4Kwh a day call it where I was pumping 12Kwh from memory with the GH. I had a 100L drum with a water heater element in it on a thermostat going through a PWM to limit the current so I could run it from a regular outlet. Worked Great but the power is excessive for winter when I need it most. In summer I wouldn't care but then of course don't need it. Apart from the temps being up, the aim is to have the plants well established early in the season so I can have them ready to harvest at Christmas. I like to have home grown produce for Christmas Dinner and it's been a real hit with the family when I can do it. I was thinking that bare wire/ Metal may cause problems but as you say, easy to get around using the insulated wire which the silicone may be best for or have it like I do now under the tubs with a sheet of styro underneath so the heat does not go to the floor or whatever I'm sitting the tubs on. Using wire, is the heat directly related to the amps pumped in regardless of wire gauge? Would there be any benefit to using thicker or narrower gauge ( other than cost?) or again is it irrelevant and watts are watts? If I used sheet metal, would that work or would it heat the cables going to it instead of the plate the cables being the smallest conductor? I can see how the heated water would be best but that is not a practical option for the moment. It is something to keep in mind for the home heating system though. Have the return water go though the pipes in the garden which would mean the burner could pretty much run all the time and use the garden as a Dump load and I could just control the fan for the indoor temp. That would work quite well on several fronts. I was thinking of a radiator on the return if the fan for the house was off to stop boiling but this would be far better and allow me to have out of season vegetables growing. Ideally the soil needs to be in the 20-25C range. Anything under 12 is useless as most plants become dormant at that point. I also like to keep plants alive instead of them having die off like normal. I had a couple of Tomatoes before I came here that were 3 years old I was very proud of. Unfortunately the frost here was too much for them. If I could put some heating in the container to keep them going through winter that would be great. I did manage to find some things on using wire as heating elements but they are all referring to high temps and large outputs rather than the low power I am needing. I would like to DIY this if I can do it simple and cheap ( and possibly Direct solar with the PWM) rather than just using the bought heat mats. |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 378 |
Hi Davo, cable used for underfloor heating in houses is made of resistance wire, probably nichrome. I am not up on PWM modulation but am wondering how you plan to keep the current limited. The heat dissipated in cable is a function of current and resistance. That is why cable losses are called I squared R losses. Sounds like an interesting project, I don't know if underfloor heating cable would work in your situation as it is usually designed for 240 volts and the resistance is varied to suit the room size. Not sure if it is waterproof either. You could try just putting something Black like weedmat on the soil so that the sun heats the soil directly. Using normal copper cable, well I am not sure about how you control the current. I would think the resistance is too low to give much heating, unless you pass a ridiculously high current through it. Pete |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Problem with a wire will be the high temperature gradient. Bloody hot 1mm from the wire, and cold 100mm from the wire. Really needs something much larger that is only just slightly warm to spread the heat more evenly through the soil. First thing you need to do is work out how much power is required in watts. Then the total length of your wire. If you can work those out, then its possible to choose an appropriate wire gauge and voltage. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
For the main thing I want, that will Be OK. I have the heating mats now under the trays with the soil in them that Might not even be quite 100MM Deep and the heat seems to spread quite well because the soil is kept very moist. Also being in the tubs tends to help with the over all gradient as I keep the lids on which ups the Humidity, keeps more warmth in and the soil does not dry out. I found with the greenhouse I had last year I could have it 50o in there and as long as the condensation was running down the walls, All very good. When it got warmer and I made the mistake of opening the thing up and the Humidity dropped, I burnt the seedlings at 30o..... which was a real frustration. I'm sure that against what the purists would say with the temps but I have read nothing on this and done it all hands on and by observation and that was what I saw. Tomatoes, Capsicums, corn and other things I grew didn't care about 50o temp as long as the HUMIDITY was there. I didn't mean to go to 50 but though being slack that's what it hit which amazed me anything survived but not only did they survive, they seems completely unphased. I backed it down to 30 and saw no difference Till I let the Humidity out THEN i had problems. I can well see what you mean though in a garden with heat transfer. I did a furnace a while back out of nothing but dirt and the inside was glowing red and the outside was a plastic drum and was fine till the dirt blew away from the burner and eroded though. I don't think most of that was even 4"! I was pretty surprised at how good an Insulator the dirt was. The key will obviously moisture in the soil to transfer the heat. Dry dirt insulated well but the moisture is what allows the thermal bleed. Yes, I was thinking about what you said with water, the Ideal HE would be a radiator. Good for the seedling trays, just sit them on top and I'd have a broad area. Could bury them in the garden too but have to put them in plastic Bags to stop the roots getting in and clogging or damaging them causing leaks. Spose some hose would be OK too either side of a row. Thing is I aminly need to get the seedlings up and going so when they are ready to plant out, the soil is getting warm enough. It's the early start I'm most concerned about so I can have them established a month or 6 weeks then plant them out rather than wait for the soil to be warm enough and start them from seed and be that month/. 6 weeks behind. It makes a difference for where I am between having the produce on the Table at Christmas and not yet ready. Yes I saw a question about this online and they went into great detail about thermal losses and the weight of the soil, ambient temp and moisture content and the science of It all. That's for smart people like you that have the knowledge to work it out and know the answer before they start. The pelicans like me say well I have a 20W heating mat now and that's OK so if I have a PWM and can adjust the input, then I can pick a number, take the temp and make adjustments to get to where I want without ever having to work any of that out Or even knowing what the power input is! I would certainly take reading but that would be so I could replicate the solution more easily but by the same token, If I used everything the same far as power supply, PWM and wire, then I'd be probably close enough setting the Dial on the PWM to the same place.  I don't need precision, close enough is plenty near enough. I have to work within my abilities which are low but I still think I can Kludge my way to the end goal..... With a bit of help on the basics. :0) That's where I am lacking info. Most things, all really, I can find are working on the direct input method and not having a Controller. I understand the concept of working out the resistance in the first place, the numbers are another thing which is why I'm looking to bypass doing it the proper way with the science and bluff my way thought it by cheating with a PWM where I can adjust the input as I need instead. If I have a 1mm 1M long cable and say a 1.5MM cable same length and I pass the same amount of wattage, say 50w which either would handle with ease, does it make any difference what the cable gauge is as long as it's under the cable's rating which it would be for any normal size cable? |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
If you put 50 watts into a wire only one metre long, it will need to get smoking hot to dissipate 50 watts. A 1mm (square mm) copper cable 1 metre long has a resistance of 0.172 ohms. Current requires will be the square root of power over resistance. That is 50 watts divided by .0172 ohms = 2907 Square root of that = 53.9 amps. Its going to get very hot indeed, I don't know what kind of insulation would withstand that, but its what is needed to dissipate 50 watts per metre through a 1mm squared copper wire. Voltage would be .0172 ohms multiplied by 53.9 amps = 0.93v A practical way to try this would be to find a 50 watt or larger transformer, wind a one or maybe two turn 50 amp secondary to produce 0.9 volts. Don't be too surprised when your 1mm copper wire smokes, sags, and turns black. If you use something like nichrome resistance wire stripped from an old bar radiator element, the resistance will be a lot higher and it will work with a higher voltage and less current. But the wire will still need to reach a similar temperature to dissipate 50 watts per metre length. Cheers, Tony. |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1122 |
We looked at using resistance cable in the plant propagation area as a heater to warm the soil; but decided against it. The wire temperature must be kept only a few degrees above the wanted final soil temperature in order to prevent burning the fine roots. Means we would have had to use hundreds of meters of wire to get enough surface area for effective heat transfer into the soil; All overly too complex. Intension now is to use warmed water running in rubber irrigation tubing throughout the soil bed, that sits upon a sheet of polystyrene for insulation, the water heated by solar hot water panels. Cheers Mike |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
My thoughts exactly Mike. One Idea I have been pondering would be to run a resistance wire through a length of flooded plastic pipe. No pump required, the heating would be even along the whole length, and the plastic pipe would have enough surface area to transfer heat with a very low temperature gradient. Plastic pipe would also electrically insulate the soil from the heating element. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
Thanks for that Gents. I now understand why they have the Carbon fibre wire that is 33 OHm. Can someone tell me what sort of power that would dissipate over a meter @ 12V? Does any one know of an online calculator to work this out? IF I'm following the formula Correct, ( which I think I'm not) The 33 ohm wire would be 1.23 Amps. Voltage would be 40.59V Can I not turn the amps up and the voltage down so I can use a 12V supply? Say around 4A? The silicone wire is 200 oC rated so would it be likley to handle the resulting temp? Is there any way to work that out? Probably not going to need 50W but 30w or a bit above would probably be OK. This is the wire I'm looking at There is also a 60 Ohm version. I believe it's rated at 25W/ M but that's at 240V. Confused now if 12/14V will make any difference except to the amps. If it comes in around 4 as I calculated, that would be great. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
If I am reading that right its 33 ohms per linear metre. If you put 12v across that you get 12/33 = 0.364 amps Power will be 0.364 x 12v = 4.36 watts If you put 24v across it 24/33 = 0.727 amps Power will be 0.727 x 24v = 17.45 watts Might be preferable to run it at 12v but run multiple wires in parallel. That would spread the heat better and put less heat stress on the wire. Cheers, Tony. |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 378 |
I heard about people using Reptile Tank heating cable for raising seedlings. Have a look online, it seems that it is waterproof and comes in different lengths and wattages. I don't know what voltage it operates on but suspect it would have a power supply to lower the voltage from mains Pete |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Some of that heating cable works in a very different way. Its actually a cable pair, and you connect 120/240v directly across the pair. Heat is produced from a controlled even leakage current all along the cable. The advantage is, you can cut the cable and connect together any lengths needed. The longer it is, the greater the current and power distributed evenly along the length. Its used mainly to wind around water pipes to prevent freezing. I have never seen it in Australia, but I believe its common in America and Europe. Cheers, Tony. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Here you go: https://www.ebay.com.au/itm/284354889390?hash=item4234df46ae:g:VQAAAOSwED1g3bjS Sixty watts per metre at 240v. An ordinary standard light dimmer should work fine with a resistive load. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
Thanks. I'll look into that some more to see if there is a more economical alternative as that particular one seems a bit overkill for what I need. Had another couple of ideas today that I think will suit my needs and working methods... Have plenty of water heating elements up the shed and can now even find them! 3600W is 17 Ohms. @ 24V, that's 33w. Pretty much on my Guesstimate. First I thought I can Bury the element in the dirt and seal the connected ends with silicone o stop electrical problems. Next thought was just bury the element in a tray of sand and then sit the tray with the seeds and soil on top. No electrical issues then. I pulled out an element and connected it to a shattered solar panel I mounted on the wall of the shed to drive some more fans. The thing heated up pretty well and even though it was cloudy and late in the afternoon. I also found some laptop power supplies which are 19.5V @3.3A which would give about 22 W so may be ok. also have the 4800W/ 12 Ohm elements which would give 31w on the same power supply. May not need it though.... I modified my setup today which I think will allow less power for the desired result. I was putting the plastic tubs with the lid on top of the pet heating mats which were sitting on a sheet of styrofoam. The tubs have ridges on the bottom for strength I take it which I never noticed so don't properly contact the mat flat and directly as I assumed. I Put the mat in a garbage bag to try and keep some of the condensation out as the Tubs with the lids on are Like terrariums where the water re condenses, and put them IN the tub and with the tray direct on top of that. I put in some Cucumbers and Zucchini, beetroot and Rhubarb seeds today and put them in the tub with this new setup. Came back after just an hour or so and the soil was at 23-25 depending on where I measured. Might have gone up now it's been about 6 hours but that's OK anyhow and the heat mats are still the same 20W. Might just have to put the sand tray with the element in it in a garbage bag as well to keep the moisture out. Easy enough to do. I was thinking I could do a similar setup for my little Dinosaurs. Tray of sand with an element in it and a bit of thin ply or plastic over the top and their bark chips and straw. Would be nice and gently warm for them and different to having the light on all the time. Was also thinking some tail light globes sunk in the sand might be OK as well. Use the sand to spread the heat a bit but basically use it like a Target top stove, hot in the middle and cooler further out. if I put the globes about 2/3rd down one end of the tray, they could find their own Comfort level of the warmth they wanted depending where they sat. The Tired snails I give them move faster than they do when they are cold but they are pretty lively when they are warm. They sure demolished some big ripe strawberry's and banana I gave them today! Thanks for all the feedback and Input, especial Tony for your help as usual. I was pretty confident I could use any old ordinary wire with the PWM but I can see how very far off the mark I was with that now and have a much better understanding of what is involved. I have learned a lot from this simple but not as easy as I presumed exercise and am sure it's something I'll be able to refer back to with other things. Yes, I could use any old wire but the precision of control and the output I'd get without frying it would not be practical. I will seal up some elements and sink them in bigger foam Containers that I will Put some tomatoes in. When Winter comes, I can just put them under cover and plug them in and should have no trouble keeping them going!  |
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Technophiliac Regular Member  Joined: 18/12/2020 Location: New ZealandPosts: 89 |
I guess you are aware low voltage hot water elements are available including in 12V, 24V and 48V? Elements are available single, dual and 3 phase for power switching options as well. From memory they range from 100W to 1500W and can be customised to order, and earthed. With the element positioned very low you may well get a passive Thermo-siphon working. The repitle pads seem to work well with controlled temperature (for obvious reasons!). Another option may be bathroom mirror heaters. Davo, Wellington. You can have it perfect, on time, and at the best price. Choose any two. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
Yes, I have looked at the low Volt elements for use with solar. I might get one to try them out in a water heater but still not sure of their effectiveness on panels when varying light levels comes into play and the ohm correlation changes. For this they would be OK in a water Circulation system but I can get something around a KW Just by Ohm matching the 240V elements. Might order a 36V element just to learn with. The pads I am using are for Pets. Had them a few years now for the cats during winter. Just put one in a cardboard Box and put a towel over the top and make a little tent out of it. They love it and prefer to be outside in that than in the house. Having the heat mats inside the Plastic tubs seems to make a big difference understandably. I'm getting the temps I want so all Good. I'll have to prepare my garden beds this week as the early seedlings are getting big. Having some conduit and circulating water though it in the garden could really make a difference to getting early or all year round Crops. |
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