Hi Bruce, You would have to be careful with your constantin wire as there is a thermocouple junction of 40 microvolts/K at each copper/constantin connection. If the pairs of junctions are kept together thermally all cancels out. Stainless steel is also very non-conductive of heat, but would have similar thermocouple problems. cheers, Neville Michie
On 28/06/2008, at 12:26 PM, Bruce Griffiths wrote: > wje wrote: >> In this case, the temp thermistor bridge is outside the oven >> cavity >> itself. The cable only passes power and the already-processed >> bridge >> delta to the heater power amp. So, there's no particular >> benefit from >> having the cable stuck to the heater wrap. (at least, I think >> so; my >> basic failure was because the cable fried and shorted power to >> ground) >> Bill Ezell >> ---------- >> > Bill > > If the temperature bridge is outside the oven cavity then its critical > that the temperature sensor leads are thermally shunted to the oven. > Substituting constantan wire for copper wire also helps as the thermal > conductivity of constantant is significantly lower than that of > copper. > > For example if the temperature sensor has a thermal resistance of 1K/W > to the oven and the leads have a thermal resistance of 100K/W to > ambient > then ambient temperature fluctuations of 10 K will induce temperature > sensor temperature variations of 0.1K which the oven controller will > correct by varying the oven temperature by 0.1K. > The lead themal resistance would have to be > 1E4K/W to maintain oven > temperature fluctuations below 1mK when the ambient temperature varies > by 10K. > Such a high thermal resistance is difficult to achieve. > > The thermal resistance of a length of wire can be estimated by > measuring > its electrical resistance and dividing it by the product of the > thermal > conductivity and electrical resistivity. > > For copper wire thermal resistance ~ 1.6E5 x Electrical resistance > > For 10 cm of 20swg Cu wire the electrical resistance (at 20C) is about > 4.4 milliohms and the corresponding thermal resistance is about 700 > W/K. > > Bruce > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ > time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
