A Case for Periodic Calibration or Verification of RTDs

Transcription

1 A Case for Periodic Calibration or Verification of RTDs If you are experiencing audio problems please call the teleconference number below Phone Access code

2 Your Host and Presenter 2 Presenter Bill Bergquist, Principal Applications Engineer and RTDologistTM 35 years experience in temperature measurement with RTDs and thermocouples in the aerospace, industrial, and laboratory markets. Host Jeff Wigen, National Sales Manager 30 years in sales and marketing of custom designed made-to-order products for the industrial and biotech markets.

3 What We ll Cover Terminology Refresher What is Calibration? Verification? Why you should do it When you should do it How exactly do you go about doing it? 3

4 Terminology Refresher R0 = resistance at 0 C IR = insulation resistance Dewar = insulated container (Thermos bottle) SPRT = standard platinum resistance thermometer ITS-90 = International Temperature Scale of 1990 IPTS-68 = International Practical Temperature Scale 1968 TPW = triple point of water 0.01 C or Kelvin K = Kelvin temperature scale (used for ITS-90) mk or millik =.001 Kelvin NIST = National Institute of Standards and Technology NVLAP = National Voluntary Laboratory Accreditation Program A2LA = American Association for Laboratory Accreditation 4

5 A Case for Periodic Calibration/Verification No regular maintenance schedule for the measurement point Sensor began giving erratic readings Wasted energy Questionable product quality How long had this been a problem? Nobody knows for sure. 5

6 What is Calibration? Verification? 6 Calibration: Calibration is performed to verify sensor/instrument performance and characterize the R vs. T relationship Usually involves adjustment of the instrument however there typically is no adjustment to an RTD (can adjust the instrument to which it is connected) Maintain traceability of parameters with reference to national/international standards Verification: A check of the RTD to see if it meets the ASTM E1137 or IEC standards for R0, insulation resistance (RIR), and interchangeability tolerances Can be performed in the lab or field by comparison to a temperature standard, or measured with DMM

7 Why? 7 Insure accuracy of measurement Consistent product quality Meet Quality System requirements Meet 3rd party requirements Safety Efficient use of energy Poor accuracy = wasted $ Establish CONFIDENCE in the temperature measurement

8 Energy Cost and RTD Accuracy Process Fluid: Flow Rate: Control Temperature: Energy Cost: Water 25 GPM 100 F 6 / KW-hour Annual Cost of Energy Per F Error $1800 8

9 Why 9 Initial Calibration/Verification New plant or equipment commissioning Verify manufacturer data shipping and installation damage Insure accuracy of measurements Establish and record initial performance necessary to show trends Ongoing Calibration/Verification Minimize and control random and systematic errors Identify sensor and instrument errors Compare and complement the quality and reliability of measurements by comparison to standards Provide traceability to national standards, (e.g. NIST) Conformance to RTD standards ASTM 1137 and IEC Meet Regulatory Requirements (FDA, USDA, NRC)

10 When 10 Frequency of calibration Before and/or after a critical measurement Risk mitigation After an event 3rd party requirement Defined calendar schedule Manufacturer recommendation Your best guess

11 When 11 Frequency Process also dictates the calibration cycle Probe drift due to: Vibration Shock Temperature high temp = more drift Temperature cycling Product value Corrosion, erosion, product build-up

12 When 12 How time and temperature affect drift rate

13 When 13 How temperature cycling affects drift rate

14 Frequency 14 Set to meet your confidence level Guidelines Graph acceptable error vs. time std. deviations based on accumulated data Group together sensor data from similar locations Manufacturer recommendation Process characteristics Third party requirement Safety After an event Before and/or after critical measurement or batch Complete 5 cycles w/o shift then double the interval

15 How 15 Temperature scales Equations Options Methods Calibration Equipment & Software Rt= R0[1 + A * t + B * t2 + (t 100)C * t3]

16 How 16 Two types of RTD checks Verification or tolerance check Compare resistance to defined R vs. T such as IEC or ASTM Usually performed at the ice point. This may also take the form of a field check by using a standard inserted into the process or a nearby test well Characterization Calibrate at several temperatures and use equations to define R vs. T

17 How 17 Insulation resistance First and most important calibration/verification check Low IR can cause a low temperature measurement due to shunting between the sensing element wires Most IR failures are due to moisture and/or contaminants that may have entered the probe

18 Insulation Resistance Test method Lower resistance = lower measured temperature Test at 50 VDC IR should be >100 megohms at 25 C 18

19 Insulation Resistance 19 Low insulation resistance (RIR or IR) IR acts as a shunt resistor to the measurement circuit the lower the IR the higher the effect on the accuracy of the probe. The equation for calculating theoretical effect of IR on the measurement is basically the equation for calculating the overall resistance of resistor in parallel, where one resistor is the PRT (RPRT) and the other is the insulation resistance ( RIR) RMeasured Where: [ RPRT xrir ] [ RPRT RIR ] RMeasured = Resultant measured resistance RPRT = Resistance of PRT element RIR = Insulation resistance value So for example: a probe that reads 100Ω at 0ºC that then degrades to IR of 0.1 MΩ the measured resistance will be which equates to approximately -0.26ºC.

20 How 20 Ice Bath Check Resistance at 0 C is the most important and easiest to check Standard interchangeability tolerances established by either ASTM 1137, IEC 60751, or manufacturer

21 Ice Point Check 21 Using an Ice Bath and resistance readout, check resistance at 0 C Crushed ice, purified water, and an insulated container

22 Interchangeability 22 4 IEC Class B ASTM Grade B 3 2 IEC Class A ASTM Grade A Tolerance (± C) ASTM Grade A -1 IEC Class A -2 ASTM Grade B -3 IEC Class B -4 Temperature ( C) 800

23 Interchangeability 23 Standard Tolerance Defining Equation¹ ASTM E1137 ASTM E1137 IEC IEC IEC IEC Grade A Grade B Class AA Class A Class B Class C ± [ t ] ± [ t ] ± [ t ] ± [ t ] ± [ t ] ± [ t ] Note 1: t = absolute value of temperature of interest in C

24 How - Temperature Scales Evolution of standard temperature scales IPTS-27 IPTS-48 IPTS ITS-90 ITS-90 (International Temperature Scale) Released in 1990 The official international scale In better agreement with thermodynamic values than the IPTS-68 ITS-90 vs. IPTS-68 ITS-90 Uses resistance at TPW Most accurate Complex equations IPTS-68 Uses resistance at ice point Simpler equations Less accurate Callendar-Van Dusen equation

25 How - Equations 25 Callendar-Van Dusen Equation For the range between 0 C to 661 C the equation is Rt = R0(1 + At + Bt2) For the range between -200 C to 0 C the equation is Rt= R0[1 + At + Bt2 + (t 100)C * t3] t = temperature ( C) R = resistance at temperature t R0 = resistance at the ice point A, B, and C from the RTD standards: A = x 10-3 B = x 10-7 C = x 10-12

26 How - Methods 26 Methods of calibration Fixed point Comparison Laboratory Field

27 Comparison Calibration Most common method Comparison of unknown to known sensors Multiple sensors can be calibrated at the same time Equipment Meter, Standard PRT, Recorder, etc. (system) All add to uncertainty level The standard PRT should have an accuracy at least four times greater than the unit under test 27

28 Comparison Calibration 28 More practical and less expensive than fixed point temperature calibration Laboratory Typical uncertainty: C to 0.01 C Very high accuracy reference resistance bridge, standard PRT, calibration baths, etc. Uses some fixed point temperatures Field Typical uncertainty: 0.05 to 0.5 C Accurate reference meters, secondary PRTs, baths or dry-wells Instruments are field compatible

29 Typical Comparison System Setup 29

30 Equipment 30

31 Standard PRTs 31 Specifications Very fragile Used in laboratory environments Highest accuracy, high repeatability, low drift -328 to 1983 F (-200 to 1084 C), accurate to ±.0018 F (±.001 C)

32 Fluid Calibration Baths Range: -80 C to 500 C Fluids: Alcohol, Water, Silicone Oil, Salt Stability: < ±0.001 C to ±0.05 C Working depth: 12 to 18 Working diameter: 4 or Larger 32

33 Metal (hot) Block Baths Range: -30 C to 700 C Stability: ±0.02 C to ±0.05 C Working depth: 6 Portable 33

34 Thermometer Readout A readout device or data acquisition system is needed to display or record temperature data when performing a calibration 34

35 How - Calibration Options Factory Calibration Options Matched Calibration Match RTD to a transmitter Matched to a Temperature Readout Multiple Point Calibration -196, -38, 0, 100, 200, 300, 420, and 500 C 35

36 How Calibration Options Transmitters Matched to RTD Improved system accuracy 36

37 Outside Calibration Service Accredited Lab such as A2LA, NVLAP and others Defined QA program Training Documentation Complete solution RTDs Transmitters System Retention of records Scheduling Experience Support Guarantee 37

38 Summary 38 Calibrate or Verify to insure measurement confidence Verification IR and ice point check Calibration multiple points to characterize R vs. T Frequency Process dictates the calibration cycle Probe drift Vibration Shock Temperature range and cycling Product value Safety

39 Thank you for attending! Questions? Use the chat window to send us a question now Contact us later at bbergquist@burnsengineering.com or visit

40 BE educated Watch for upcoming RTDology events View previous recordings on YouTube. Search for RTDology Let us know if there is a topic you would like to see covered Find discussions from industry experts on LinkedIn: Temperature Measurement Group

41 How - Equations 41 Callendar-Van Dusen Equation Callendar-Van Dusen equations are interpolation equations which describe the temperature versus resistance relationship of industrial PRTs. Equation for the temperature range of 32 F (0 C) to 1562 F (850 C) is: For the temperature range of -392 F ( -200 C) to 32 F (0 C): Where: R(t) = the resistance of the PRT at the temperature t = temperature in deg. C. R0 = the nominal resistance of the PRT at 0 C A,B,C = calibration coefficients The nominal calibration coefficients for standard curves are: R0 A B C x x x 10-12

42 How - Equations 42 To determine the temperature from a measured resistance, a different set of equations and calibration coefficients are required. For temperatures greater than 0 C: t C = ((Rt-R0)/(Alpha*R0)) + Delta((t/100)-1)(t/100) For temperatures less than 0 C: t C = ((Rt-R0)/(Alpha*R0)) + Delta((t/100)-1)(t/100) + Beta((t/100)-1)(t/100)3 Where: t = temperature to be calculated Rt = measured resistance at unknown temperature R0 = resistance of the sensor at 0 C Alpha = , Delta = , Beta = To correctly determine the temperature from a given resistance, you must iterate the equations a minimum of five times. After each calculation, the new value of temperature (t) is plugged back into the equations. The calculated temperature value will converge on its true value. After five iterations, the calculated temperature should be within.001 C of the true value.

43 How - Equations 43 One more tidbit of information: A = Alpha x { 1+ (Delta/100)} C-1 B = -Alpha x Delta x 10-4 C-2 C = -Alpha X Beta x 10-8 C-4