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

[Mark Sylvester <msylvest@SPIN.IT>]

At 14.08 09/05/00 -0700, Leigh Palmer wrote:
> 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.

Interesting. You've sent me into the "Rubber Book" where I found a table
giving resistivity of tungsten at 100K intervals from 300K all the way to
3655K. A plot reveals an upward curve deviating from the linear
extrapolation from low temperatures in the positive direction (it's too
late for me too figure out if that's what you last sentence means). Good -
now we can normalise the resistivity-temperature curve and use this to find
the temperature from the % change in V/I with reference to room temperature.

Probably means the results will be less "good" though!


Mark



_____________________________________
Mark Sylvester
United World College of the Adriatic,
34013 Duino TS, Italy.
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