CALCULATING COEFFICIENTS a10 and Rtp FROM THE ITS90

[Jmpicao]

We have a 2465A primary standard from Ruska that having a temperature probe (PRT). The new calibration need to get the new coefficients a10 and Rtp through ITS90. How i can get (calculate) these data through the new calibration certificate (temperature versus resistance) issued by the entity  that calibrated the probe.

[metrologygeek]

Your Rtp (resistance at the triple point of water) is a measured value, not calculated. The ITS90 coefficients are calculated from the measured calibration points, not the temperature versus resistance table. You should be able to get these data from the calibration source, or start over and specify what you need the calibrating source to show on the report.

[Hawaii596]

I would also think in any proper calibration report, Rtpw should be listed on the report.  As for A10, I'm no ITS90 expert, but I just looked up and saw that is the 0.01 to 156.5985 Degree C range when calibrated at the Indium freeze point.  Depending on what lab calibrated it, and what freeze points were used, I am guessing they may not have calculated A10.  Depending on the range of the probe and what freeze point(s) was(were) used in the positive (Rtpw and up) range(s), it is pretty common to use A8.  But the calibration vendor should have supplied you with a positive and negative alpha numbers on the report.  A4 is common for negative temps. Don't know how much that helps.  As already mentioned, you can always ask the vendor for the alpha coefficient you need for your application.

[michthai]

jmpicao, I have a spreadsheet you might me interested in that is used by standards labs to calculate temp coefficents. Not sure you would be able to understand how to use it; but, I'm willing to send it to you if you send me your email address. Send to tolfreeroad@gmail.com.