The previous cool down test was repeated over a longer time period. The tank was heated overnight with a 30 V (18 W) (as stated in previous post) and its settling temperature was 44.36±0.05 C. This gives us some idea of the expected DC heat load that the tank will require to maintain an elevated temperature of ~45 C.
The supply was turned off and a cool down curve was recorded over the next day. Plot is below. Fitted values were
a = 19.4291±0.0005 C, b=13980±730 s, c=20.75216 ± 0.00003 C
for the fitted equation  .
With this test (with a longer cool down tail), the time constant for the insulated tank to come to room temperature was 3.88 h
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The full cool down curve of the tank and a fitted curve ( )are attached below.
Comparison data of temp decay with fitted value (Values below)
Least squares fitted values are
a=23.4145±-0.0005 C, tau=7596±170 s, c=21.00947±0.00004
The variance on the time constant is very large, I think this might be due to a poor fit owing to the fact that the temperature was still rising (rather than steady) when I turned the heating off. We want the whole system to be at equalibrium at the start of the test and we also want a decent tail on the decay. I have therefore repeated the measurment this time heating overnight with 30 V (18 W) which settled on 44.36 C. I have now left the vac can to cool for the rest of the day (starting about 9.30 am).
Either way it looks like the characteitic 1/e decay time is on the order of 2.1 hours.
Full heat up and cool down, was not quite at equalibrium, but was pretty close. May or may not be an issue. The first blip is due to me changine power supplies halfway through.
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