How to Calibrate an RTD or Platinum Resistance Thermometer (PRT) 2010 Fluke Corporation Presentation Title 1

Transcription

1 How to Calibrate an RTD or Platinum Resistance Thermometer (PRT) 2010 Fluke Corporation Presentation Title 1

2 Why PRTs? Thermometer types Digital Thermocouple PRT Good combination of temperature range and accuracy Thermistor Mechanical Bi-metallic Volumetric (LIG) 2010 Fluke Corporation Presentation Title 2

3 Why PRTs? Weighing the options PRT (resistive) Thermocouple (voltaic) Thermistor (resistive) Pro Best combination of temperature range and accuracy Widest temperature range; especially useful for high temperatures; least sensitive to shock and vibration Most accurate secondary temperature standard (approaches SPRT accuracy); inexpensive; rugged Con Most sensitive to shock and vibration Least accurate; may drift rapidly; base metal thermocouples not recommended for recalibration Limited temperature range 2010 Fluke Corporation Presentation Title 3

4 Two Procedures Characterization 1.Measure resistance 2.Compare to a reference 3.Make a mathematical adjustment to define the new R vs T relationship Tolerance Testing 1.Measure Temperature (resistance first) 2.Compare to a reference 3.Determine status of tolerance by comparing results to requirements 2010 Fluke Corporation Presentation Title 4

5 Characterization Procedure Probe Placement Temperature Sources Dry Block (Dry Well) Portable Fast Wide temperature range Less stabile and uniform Fluid Baths Less portable, not portable Slower to change temperatures Narrower temperature range More stable and uniform 2010 Fluke Corporation Presentation Title 5

6 Characterization Procedure Probe Placement Why is stability and uniformity important? Good measurement requires same temperature at the same time Temperature Sources Fixed-Point Inherently better uncertainty ITS-90 requirement Limited in volume Higher cost 2010 Fluke Corporation Presentation Title 6

7 Characterization Procedure Comparison C Probe F Sample K Comm Exit Menu Enter External Reference Thermometer 1502 Your Sensor and Readout Dry-Well s Thermometer/Controller 2010 Fluke Corporation Presentation Title 7

8 Characterization Procedure Connection to Readout Good contact 2-, 3-, or 4-Wire connection Proper current & voltage connections 2010 Fluke Corporation Presentation Title 8

9 Characterization Procedure Measurement of Reference Goal is to measure actual temperature Accuracy of reference probe & readout Type of Readout/Meter DMM or Thermometer Temperature conversion Efficiency, Real time Accept coefficients Appropriate source current Self heating issues Software 2010 Fluke Corporation Presentation Title 9

10 Characterization Procedure Measurement of Test Probe Similar to Reference probe measurement Goal is to measure resistance Type of Readout/Meter Accuracy required Number of test probes (scanner needed? Test probe characteristics Current required, stability of sensor, response time Software Automated measurement 2010 Fluke Corporation Presentation Title 10

11 Characterization Procedure Thermometer Curve Fitting Solving a set of simultaneous equations R vs T relationship Different equations available ITS-90 Callendar Van-Dusen Polynomial Software useful R(t)/R(0 C) Temperature ( C) Actual Pt Linear 2010 Fluke Corporation Presentation Title 11

12 Characterization Procedure What are coefficients? How are they used? 2010 Fluke Corporation Presentation Title 12

13 Tolerance Testing Procedure Low accuracy probes Low cost Electronics may not accept coefficients Replace instead of calibrate Compare to a defined values at specific temperatures. Standard conversion equations DIN/IEC 60751, ASTM 1137 Class A, B Confirm performance 2010 Fluke Corporation Presentation Title 13

14 Tolerance Testing Procedure Test Procedure similar to Calibration Compare to a reference standard Read temperature of test probe Compare to the defined values Determine if the test probe is in tolerance 2010 Fluke Corporation Presentation Title 14

15 Tolerance Testing Procedure Understanding the tolerance of the probe Determined from tables or equations Example ASTM 0.1 Class A: 1. = ( ( t)) = ( ( )) = ( ) 0.1 = Fluke Corporation Presentation Title 15

16 Tolerance Testing Procedure Understanding the tolerance of the probe 2010 Fluke Corporation Presentation Title 16

17 Tolerance Testing Procedure Determining Pass or Fail status Example: 2010 Fluke Corporation Presentation Title 17

18 Conclusion Similar procedures to measure Equipment possibly the same Different procedure to determine result No higher math required for Tolerance Test Calibration gives better measurement capability Tolerance testing is often quicker For many Industrial probes, the only choice 2010 Fluke Corporation Presentation Title 18