2010-01-17 Calibrate good times

We have a DT-470 temperature sensor in the cryostat of the project I'm currently working on. The problem is that our software is displaying the wrong readout. I'm trying to figure out how to display the correct value in Kelvin.

I've got the following to work with:

• The cryostat is filled with liquid nitrogen, which we know is around 77 K.
• The sensor was also read out when the cryostat was not cooled, so we have a combination of raw value (as read from electronics) and known temperature as well (room temperature, 20 degrees Celsius, is about 293 K).
• We've been provided with a data sheet from which a polynomial can be derived.
• The spec says we need to drive the sensor with a 10uA current, but in actuality, we drive it with a 12.368 uA current. I'm told this is not a big deal, because the difference in current is unlikely to cause a difference in warmth.
• The spec also gives us a lookup table to transform raw value to Kelvins
• Because of the way the sensor is read out, we need to apply an additional offset and factor (our electronics influence/amplify the readout).
• The sensor readout is linear for the range from 10 to 200 K, then follows a curve to 0 K.

The software has the ability to apply a polynomial to a raw value (i.e. a value that's read out from the electronics), as well as apply a user-configurable function to a value. That is usually used for convenience like electronics giving us a negative value, when we'd rather receive a positive value.

In this case, the polynomial is applied to correct the value for the way our electronics influence the raw value. Then, the user-configurable function is applied, which in this case is the polynomial that follows from the data sheet.

So the steps are:

• Remove the electronics and drive the sensor manually. Do we get the same value (raw) as we get from the electronics?
• Get the raw value, and see whether our first polynomial is OK (the first polynomial is a simple one, just an offset plus a factor). We can do this by looking up the raw value in the lookup table.
• Use that in-between value and check the lookup table to see whether the second polynomial is OK.
• The heating of the electronics board could also play a role, we need to check that as well and if so, correct the first polynomial.