Ian's Robert's heated bed

So I'm building a custom heated bed, loosely following the instructions for Robert's heated bed. I have some questions.

I cautiously connected everything up for the first time, there were no big explosions/balls of fire. Bit smelly, I'm not entirely sure which part is causing the smell, but it wasn't burning.

I have a 3mm aluminium plate, this is painted with black stove paint. I figured it would provide a tough insulated base. The wire is haphazardly strung around the bed, there's 7 metres in total which has a resistance of 0.8ohms.

It gets warm fast, using a cheap heat measuring gun the middle part reached 160degrees. I had it on for about 3 minutes, it seemed to peak after about 30 seconds I'd guess, then just incrementally warmed.

So my multimeter gave me the 0.8ohms figure, which I think is too high. It is 7m of 24swg (.56mm) wire, which should be 0.47ohms according to this calculator. The wire is enamelled, so I burnt off the enamel AND sanded the ends before measuring. So which is more likely to be accurate?

At 0.8ohms it would draw 15 amps. 0.47 would be 307watts which would work as that power supply is 360watts.

I've figured out the electronics side of it now, I just need to built the actual bed. This one, the wire is stuck on with Kapton tape which is already peeling off, so I need to rethink that. Part of that is the way I made the loops of wire. They were springing all over the place.

How about some high temperature glue or stove cement? if that fails how about a thick layer of thermal paste and making the heater into a sandwich with a tile or glass bottom?

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I've got some on order for tomorrow, so I'll see how well that holds. I might try roughing up the painted surface too and if that doesn't work I'll try the unpainted side

Quote
UkIan
So my multimeter gave me the 0.8ohms figure, which I think is too high. It is 7m of 24swg (.56mm) wire, which should be 0.47ohms according to this calculator. The wire is enamelled, so I burnt off the enamel AND sanded the ends before measuring. So which is more likely to be accurate?

You burn off the enamel of the entire wire or just the end? Are you measuring the resistance just of the heating element wire or are you including the leads, crimps/soldered joint/whatever, etc? Are you holding the DMM leads to the wire in your hands or are you using clips of some sorts? How accurate do you think your DMM is and does it have fresh batteries? Are you POSITIVE of the wire gauge/diameter and have you actually measured it?

My guess is that it's a combination of factors but I'd probably go with what the meter read.

Burned the enamel off the ends, I want it on the main wire to help keep it insulated. One of the reasons I picked it was because one of the reviewers cooked a coil in his oven to test it at temperature
I was holding the leads in my hands rather than crimping them, and that was with the bare, sanded ends of the heating wire. I did try again once I'd attached the connecting wires which are soldered on. They're just temporary though.

I've no idea how accurate the DMM is, it's a cheapo unit and I'm guessing isn't very, but you never know. It does have a fresh battery. I haven't measured the wire diameter, that's a very good point. I will do that to be sure

I've ordered a clamp meter, so I'll have a play with that too.