Скачать или смотреть Studio Construction - Day 18 - More Floor Insulation

Still feeling pretty bad and the weather isn't cooperating.

Rain and hail slowed proceedings, got about 2/3rds of the floor insulated now. Had to give up for the evening when the hot knife was getting so wet I was worried it would give me a strong electric shock (water had already confused its cooling fan temperature sensor, figured my cardiovascular system might be next).

The shorter element/blade in the knife does heat up quicker, but it doesn't really cut much faster, ultimately it seems to be power limited: The cutter is specified as 200 watts, the heat capacity of polystyrene is about 1 kJ/kg/K, the styrofoam sheet is about 2.5 kg for its 149 litre volume so its density is about 0.016 g/cc. The energy involved in the phase change to liquid and gas as it cuts isn't well specified, but the knife contact area is about 100 mm^2 (a bit less than 2 mm kerf, 50 mm thick sheet, angled blade, complicated transition region of melting/decomposition, but a 2.5 meter long slice or about 250 ml - a cup - of foam needs to be heated. That's only 4 grams of polystyrene or about 4 joules per kelvin and we need to heat it at least 250 kelvin probably quite a bit more, so say about 1 kJ. The cutter (if it was 100% efficient and it isn't) could deliver that in 5 seconds... Realistically it is taking about 30 seconds to make that cut. I didn't include a lot of the latent heats and the general complexity of what it actually happening at the cut point... So I think it is still fairly reasonable to assume the cut is largely thermal power limited. More power would be desirable, but 250 watts is the most powerful available at a reasonable price and I am almost finished, so no big deal.