Troubleshooting RTDs and Thermocouples

Transcription

1 Troubleshooting RTDs and Thermocouples If you are experiencing audio problems please call the teleconference number below Access Code:

2 Your Host and Presenter 2 Presenter Bill Bergquist, Principal Applications Engineer and RTDologist TM 34 years experience in temperature measurement with RTDs and thermocouples in the aerospace, industrial, and laboratory markets. Host Jeff Wigen, National Sales Manager 27 years in sales and marketing of custom designed made-to-order products for the industrial and biotech markets.

3 What we will cover today 3 RTDs Construction review Troubleshooting guide Ice bath resistance test Insulation resistance Error sources Stem conduction Thermocouples Construction review Troubleshooting guide Check polarity of the thermocouple Check for oxidation or corrosion Comparison test Test meter and extension wires

4 RTD Construction 4 Sensing element - wire wound resistor made from platinum wire,.0007 dia. 1/16 diameter x 1 long

5 RTD Construction 5 Sensing element thin film resistor made from platinum film.080 x x.035 thickness

6 RTD Construction 6 Probe sheath Mineral insulated cable Tube and wire Epoxy seal External leads copper with nickel or silver plating TFE, PVC or similar insulation

7 RTD Troubleshooting 7

8 RTD Troubleshooting Guide 8 Symptom Cause Solution Erratic output Sensor damage RFI or EMI -Replace -Add shielded cable, metal connection head -Route wiring away from the noise source No signal Element damage Wired incorrectly -Replace -Correct wiring

9 RTD Lead Wire Colors 9 RTDs colors defined by ASTM 1137 and IEC There are also custom colors with the most prevalent being green in place of the yellow leads in dual elements

10 RTD Troubleshooting Guide 10 Symptom Cause Solution Erratic output Sensor damage RFI or EMI -Replace -Add shielded cable, metal connection head -Route wiring away from the noise source No signal Step change in output after short time in service Element damage Wired incorrectly Low insulation resistance. Moisture has entered the probe causing a low temperature indication -Replace -Correct wiring -Replace -Dry out and monitor IR

11 Insulation Resistance 11 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

12 Insulation Resistance 12 Test method Lower resistance = lower measured temperature Test at 50 VDC minimum IR should be >100 megohms at 25 C

13 Insulation Resistance 13 Low insulation resistance (R IR 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 (R PRT ) and the other is the insulation resistance ( R IR ) R Measured [ R [ R PRT PRT xr IR R IR ] ] Where: R Measured = Resultant measured resistance R PRT = Resistance of PRT element R IR = 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.

14 RTD Troubleshooting Guide 14 Symptom Cause Solution Slow time response Maximum service temperature exceeded damaging potting material. -Replace RTD Indicated temperature increases after short time in service. May be a transmitter setting or failure. RTD drifting due to high vibration or shock. -Test transmitter for output and time delay settings -Test R 0 to verify drift -Replace with heavy duty RTD -Relocate sensor away from vibration area -Use a thermocouple

15 RTD Accuracy Check 15 Verification in an ice bath Resistance at 0 C most important and easiest to check Standard interchangeability tolerances established by either ASTM E1137, or IEC 60751

16 Ice Point Check 16 Using an ice bath and ohmmeter, check resistance at 0 C Crushed ice, purified water, and an insulated container

17 Ice Point Check 17 Ideally use an ohmmeter that has three or 4 wire input. If not available you can use a two wire meter and manually compensate for the lead resistance. 1. Measure resistance between the red leads 2. Measure resistance between white lead and one of the red leads. 3. Subtract the resistance of red leads from the white /red lead measurement and the result is the resistance of just the sensing element.

18 Tolerance (± C) Interchangeability 18 4 IEC Class B 3 ASTM Grade B 2 IEC Class A ASTM Grade A ASTM Grade A IEC Class A -2 ASTM Grade B -3 IEC Class B -4 Temperature ( C)

19 Interchangeability 19 Standard Tolerance Defining Equation¹ ASTM E1137 Grade A ± [ t ] ASTM E1137 Grade B ± [ t ] IEC Class AA2 ± [ t ] IEC Class A ± [ t ] IEC Class B ± [ t ] IEC Class C2 ± [ t ] Note 1: t = absolute value of temperature of interest in C Note 2: These tolerance classes are included in a pending change to the IEC standard.

20 RTD Troubleshooting Guide 20 Symptom Cause Solution Difficulty tuning measurement loop Probe checks out okay in calibration bath but reads high/low in service Probe time response too fast or too slow Stem conduction -Replace RTD with a faster version -Add thermal insulating material to probe sheath -Set a time delay in transmitter software. -Increase immersion length -Replace with probe designed for short immersion -Insulate exterior portions of the assembly -Check that probe contacts bottom of thermowell. -Bore diameter closely matches probe diameter

21 Estimated Error ( C) Stem Conduction Error 21 1/4" with Thermowell C Delta Immersion (Inches)

22 Estimated Error ( C) Stem Conduction Error 22 1/4" Direct Immersion C Delta Immersion (Inches)

23 Estimated Error ( C) Stem Conduction Error 23 1/8 Diameter Short Immersion Design, 100 C Delta Immersion (Inches)

24 Other Sources of Trouble 24 Corroded terminals can cause high resistance in the leads 3-wire RTD circuits are susceptible accuracy depends on each conductor having exactly the same resistance Terminals clean and tight Terminal block clean and dry, secured to head Wires are tinned, or terminated with spade lugs Connector pins connect firmly and are clean Use gold plated pins in a high quality connector 4-wire circuits also compensate for some poor maintenance Compensate fully for all lead wire resistance in the circuit

25 Preventive Maintenance 25 The good and bad

26 Thermocouple Construction 26 Mineral insulated cable Epoxy seal Lead wires are fiberglass, TFE or similar insulation Tube and wire Wire with ceramic insulators Can be used exposed or in a thermowell

27 Thermocouple Construction 27 Grounded or ungrounded junction Instrumentation set to accept type

28 Thermocouple Troubleshooting 28

29 Thermocouple Troubleshooting Guide 29 Symptom Cause Solution Erratic output or step change in accuracy RFI/EMI Use shielded cable. Add transmitter. Accuracy outside of limits Drifted out of tolerance Replace Short life No signal Corrosion or contamination of conductors. Hot junction broken. Wires connected incorrectly. Switch to different type. Change thermowell configuration. Repair or replace sensor. Check polarity of wires.

30 Thermocouple Troubleshooting 30 Check polarity of the thermocouple circuit and all connection contacts. Red lead is always negative (-).

31 Thermocouple Troubleshooting 31 Check Instrumentation Verify instrument has been set for the thermocouple type being used. Check Thermocouple Severely oxidized or corroded thermocouples should be replaced. Changes in wire composition can result from corrosion and contamination by elements such as sulfur and iron. Green rot of a type K is caused by too little oxygen at 800 C to 1,040 C. Non-magnetic chromel wire will become magnetic. Green Rot

32 Thermocouple Troubleshooting 32 Check readout/controller and extension wires Connect the extension wires to a test thermocouple of known accuracy and observe the temperature reading Use a thermocouple simulator/calibrator

33 Thermocouple Troubleshooting 33 Extension wires Check that it is the same type as the thermocouple Mismatch will cause an error Short circuit or low insulation resistance will cause an error In the case of a short circuit between conductors it acts as another measuring junction with the result being an average temperature of the two junctions.

34 Thermocouple Troubleshooting 34 RFI/EMI - Thermocouples are susceptible to electrical interference Use shielded twisted pair cables Short runs Add a transmitter

35 Thermocouple Troubleshooting 35 Slow time response, inaccurate reading Check probe and thermowell assemblies to insure that probe contacts bottom of thermowell. Applies to both RTD and TC. Bore diameter closely matches probe diameter or tighter fit 0.25 probe requires 0.26 bore Heat transfer paste can improve time response and accuracy deficiencies caused by a poor fit

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

37 BE educated Watch for upcoming RTDology events 5/9/18 Testing and Documentation for RTDs, Thermocouples and Thermowells Visit our YouTube channel, RTDology, for more full length presentations and short clips on specific topics.

38 BE educated Join our Temperature Measurement Community News: Twitter: TempTalk LinkedIn: Temperature Measurement Group