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Originally Posted by unregistered
what are your thoughts on sensor # and placement
I'm looking at 3 to 5 sensors in these calorimeters, why not 3 vs. 2 ?
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I had three on the previous fluxblock. I found that it the temperature gradient was very linear so dropped the third sensor on the current version. Still, having more would be a good idea, especially if people want to calculate heat flux, the more data points the better. The disadvantage is that if you have inline (drilled into the heatpath) the effects of the sensors
on the heatpath become more dominant. Also, a longer heatpath is then required to accomodate them, in a 10x10mm channel this would lead to very high temperatures, increasing secondary losses etc.
As far as placement relative to the interface, I am in two minds. One way is to have them quite far away, like 3mm so that the sensor effect on the surface is minimised, more length to even out the temperature at the actual surface. On the other hand going as close as possible minimises the error of any extrapolation. I think that whatever one does, assumptions need to be made and the problem modelled in order to correct for the heat shadowing. Les and I have been exploring this a bit in the Fluxdie thread.
I of the opinion that the only really safe way to position the sensor is in the centre of the heat path despite these shadowing effects. Of course, this means that the sensor needs to be as small as possible and the die CSA as large as possible to minimise errors. Since we want to test 10x10mm this limits us. Small thermistors are also not very cheap. I was intending to use Betatherm thermistors (0.457 or 1.01 mm diam.) but the cost stopped me, at ~$10 each with my tendency to destroy perfectly good sensors through mishandling this is untenable. I use Mitsubishi, RH16's which are 1.5mm in diameter and under $1.00. RTD's I am not up to speed with. Thermocouples can be any size so are probably a good solution for most people, especially since to get thermistors accurate/linear there is a convoluted calibration procedure.
Re AlSiC. I'd be very interested.