Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Optical pyrometers, was R = V/I ?



Mark said:

It's used to determine the temperature of the filament, from R = Ro(1 +
alpha*theta). For Ro we use the smallest I and V that we can measure to 2
SD, using the best meters we have. We use the tcr of tungsten as found in
the data book.

To which I said:

I will bet that this is a wildly inaccurate way to determine temperature.
Why do you expect the resistance to vary linearly with temperature over
a range of nearly a factor of ten when you already know its resistivity
varies by more than factor of ten? You need a thermometer, and the optical
pyrometer is the thermometer of choice for this purpose.

I looked up the resistivity of tungsten up to 1200 degrees C (I couldn't
find it higher). The result of using your technique for thermometry will
result in about a 200 degree error at 1200 degrees*, or about 14% when
expressed in absolute emperature units. This results in an error of
approximately 66% when T is raised to the fourth power. I have no data
for resistivity at higher temperatures, but it is likely that the error
will be even larger.

Leigh

*You will estimate 1400 degrees C when the calculated resistance is the
same as the actual resistance at 1200 degrees C.