Platinum resistance thermometers converting ohms to degrees celsius hans liedberg
Download
1 / 13

Platinum resistance thermometers: converting ohms to degrees Celsius Hans LIEDBERG - PowerPoint PPT Presentation


  • 96 Views
  • Uploaded on

Platinum resistance thermometers: converting ohms to degrees Celsius Hans LIEDBERG. Overview. If converting resistance to temperature by hand, remember: PRTs are not all that linear. If using a readout that calculates temperature for you, remember: PRTs come in different sensitivities.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Platinum resistance thermometers: converting ohms to degrees Celsius Hans LIEDBERG' - aquarius


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

Overview
Overview Celsius

  • If converting resistance to temperature by hand, remember:

    • PRTs are not all that linear.

  • If using a readout that calculates temperature for you, remember:

    • PRTs come in different sensitivities.


Resistance temperature relationship of a prt
Resistance-temperature relationship of a PRT Celsius

  • PRTs are commonly characterised using two numbers,

    • the resistance at the ice point (R(0 °C) = 100 Ω for all PRTs discussed in this paper)

  • and

    • the alpha value

    • (alpha ranges from (0.00385 to 0.00393) Ω/Ω/°C for platinum of increasing purity).

  • For example, “Pt100(385)” is used to describe a 100 Ω PRT with alpha = 0.00385 Ω/Ω/°C.


Resistance temperature relationship of a prt cntd
Resistance-temperature relationship of a PRT (cntd) Celsius

  • Non-linearity:

  • PRTs decrease in sensitivity with increasing temperature, a Pt100(385) from 0.397Ω/°C at -50°C to 0.385Ω/°C at 50°C and 0.345Ω/°C at 400°C.


Resistance temperature relationship of a prt cntd1
Resistance-temperature relationship of a PRT (cntd) Celsius

Decreasing sensitivity of a PRT with increasing temperature.


Resistance temperature relationship of a prt cntd2
Resistance-temperature relationship of a PRT (cntd) Celsius

  • Different sensitivities:

  • The higher the purity of the platinum, the higher the alpha value of the PRT.


Errors arising from non linearity
Errors arising from non-linearity Celsius

Calibration data for a Pt100(385) sensor:

To calculate temperature from measured resistance, the first reaction is to interpolate linearly between these data pairs.


Errors arising from non linearity cntd
Errors arising from non-linearity (cntd) Celsius

Linear interpolation results in errors proportional to ΔT2:


Solutions to the non linearity problem
Solutions to the non-linearity problem Celsius

  • 1. Use a reference function that models the decreasing sensitivity of PRTs with increasing temperature well (e.g., ITS-90 or IEC 751).

    • The deviations of a real PRT from such a reference function should be fairly linear.

  • OR

  • 2. Fit a 2nd order polynomial to the data (e.g., using Excel’s “Add trendline” function).


Errors arising from different sensitivities
Errors arising from different sensitivities Celsius

Calibration data for a Pt100(385) sensor:

These data were measured with the readout using IEC 751 (which describes “385” PRTs) to calculate temperature.


Errors arising from different sensitivities cntd
Errors arising from different sensitivities (cntd) Celsius

If the readout is mistakenly set to “‘Pt100(3916)” or “Pt100(3923)” during use, large errors will result:


Verifying the readout sensitivity setting
Verifying the readout sensitivity setting Celsius

  • Checking the PRT + readout system at the ice point only verifies that R(0°C) or R(0.01°C) is correct.

  • To verify A, B and C coefficients, check the system at a temperature away from 0°C, using

    • a simple fixed point (e.g., boiling point of water or sublimation point of carbon dioxide)

  • or

    • a PRT + readout system for which the correct resistance-to-temperature conversion method is not in doubt.


Conclusions
Conclusions Celsius

  • PRTs capable of ±0.01°C accuracy are readily available. To achieve this, take care to

    • use an appropriate method to interpolate between resistance-temperature data pairs

  • or

    • set your readout to the same function as was used during calibration.

  • Cal lab and client should agree on the method of resistance-temperature conversion during contract review.


ad