troubleshooting rtds and thermocouples resistance of resistor in parallel, where one resistor is the...
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© Burns Engineering Troubleshooting RTDs and Thermocouples
Troubleshooting RTDs and Thermocouples
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650-479-3208
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© Burns Engineering Troubleshooting RTDs and Thermocouples
Presenter
Bill Bergquist, Principal Applications Engineer and
RTDologistTM
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.
Your Host and Presenter
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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
What we will cover today
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© Burns Engineering Troubleshooting RTDs and Thermocouples
• Sensing element - wire wound
• resistor made from platinum wire, .0007” dia.
• 1/16” diameter x 1” long
RTD Construction
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© Burns Engineering Troubleshooting RTDs and Thermocouples
• Sensing element – thin film
• resistor made from platinum film
• .080” x 0.175” x .035” thickness
RTD Construction
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© Burns Engineering Troubleshooting RTDs and Thermocouples
• Probe sheath
• Mineral insulated cable
• Tube and wire
• Epoxy seal
• External leads
• copper with nickel or silver plating
• TFE, PVC or similar insulation
RTD Construction
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© Burns Engineering Troubleshooting RTDs and Thermocouples
RTD Troubleshooting Guide
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
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© Burns Engineering Troubleshooting RTDs and Thermocouples
RTDs colors defined by ASTM 1137 and IEC 60751
• There are also custom colors with the most prevalent being green in
place of the yellow leads in dual elements
RTD Lead Wire Colors
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© Burns Engineering Troubleshooting RTDs and Thermocouples
RTD Troubleshooting Guide
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
Step change in output after short time in service
Low insulation resistance. Moisture has entered the probe causing a low temperature indication
-Replace -Dry out and monitor IR
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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
Insulation Resistance
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© Burns Engineering Troubleshooting RTDs and Thermocouples
Test method
• Lower resistance = lower measured temperature
• Test at 50 VDC minimum
• IR should be >100 megohms at 25°C
Insulation Resistance
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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)
][
][
IRPRT
IRPRTMeasured
RR
xRRR
Where: 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 99.900 which equates to approximately -0.26ºC.
Insulation Resistance
Low insulation resistance (RIR or IR)
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© Burns Engineering Troubleshooting RTDs and Thermocouples
RTD Troubleshooting Guide
Symptom Cause Solution
Slow time response Maximum service temperature exceeded damaging potting material. May be a transmitter setting or failure.
-Replace RTD -Test transmitter for output and time delay settings
Indicated temperature increases after short time in service.
RTD drifting due to high vibration or shock.
-Test R0 to verify drift -Replace with heavy duty RTD -Relocate sensor away from vibration area -Use a thermocouple
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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
RTD Accuracy Check
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© Burns Engineering Troubleshooting RTDs and Thermocouples
Using an ice bath and ohmmeter, check resistance at 0°C
Crushed ice, purified water, and an insulated container
Ice Point Check
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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.
Ice Point Check
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© Burns Engineering Troubleshooting RTDs and Thermocouples
-4
-3
-2
-1
0
1
2
3
4
-300 -200 -100 0 100 200 300 400 500 600 700 800
Temperature (°C)
To
lera
nc
e (
±°C
)
IEC Class B
ASTM Grade B
IEC Class A
ASTM Grade A
ASTM Grade A
IEC Class A
ASTM Grade B
IEC Class B
Interchangeability
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© Burns Engineering Troubleshooting RTDs and Thermocouples
Standard Tolerance Defining Equation¹
ASTM E1137 Grade A ± [ .13 + 0.0017 | t | ]
ASTM E1137 Grade B ± [ .25 + 0.0042 | t | ]
IEC 607512 Class AA2 ± [ .1 + 0.0017 | t | ]
IEC 60751 Class A ± [ .15 + 0.002 | t | ]
IEC 60751 Class B ± [ .3 + 0.005 | t | ]
IEC 607512 Class C2 ± [ .6 + 0.01 | 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 60751 standard.
Interchangeability
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© Burns Engineering Troubleshooting RTDs and Thermocouples
RTD Troubleshooting Guide
Symptom Cause Solution
Difficulty tuning measurement loop
Probe time response too fast or too slow
-Replace RTD with a faster version -Add thermal insulating material to probe sheath -Set a time delay in transmitter software.
Probe checks out okay in calibration bath but reads high/low in service
Stem conduction -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
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© Burns Engineering Troubleshooting RTDs and Thermocouples
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6 7
Esti
mat
ed
Err
or
(°C
)
Immersion (Inches)
1/4" with Thermowell - 100°C Delta
Stem Conduction Error
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© Burns Engineering Troubleshooting RTDs and Thermocouples
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6 7
Esti
mat
ed
Err
or
(°C
)
Immersion (Inches)
1/4" Direct Immersion - 100°C Delta
Stem Conduction Error
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© Burns Engineering Troubleshooting RTDs and Thermocouples
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6 7
Esti
mat
ed
Err
or
(°C
)
Immersion (Inches)
1/8” Diameter Short Immersion Design, 100°C Delta
Stem Conduction Error
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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
Other Sources of Trouble
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© Burns Engineering Troubleshooting RTDs and Thermocouples
The good and bad
Preventive Maintenance
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© Burns Engineering Troubleshooting RTDs and Thermocouples
• 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
Thermocouple Construction
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© Burns Engineering Troubleshooting RTDs and Thermocouples
• Grounded or ungrounded junction
• Instrumentation set to accept type
Thermocouple Construction
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© Burns Engineering Troubleshooting RTDs and Thermocouples
Thermocouple Troubleshooting Guide
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
Corrosion or contamination of conductors.
Switch to different type. Change thermowell configuration.
No signal
Hot junction broken. Wires connected incorrectly.
Repair or replace sensor. Check polarity of wires.
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© Burns Engineering Troubleshooting RTDs and Thermocouples
Check polarity of the thermocouple circuit and all connection contacts.
Red lead is always negative (-).
Thermocouple Troubleshooting
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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.
Thermocouple Troubleshooting
Green Rot
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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
Thermocouple Troubleshooting
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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.
Thermocouple Troubleshooting
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© Burns Engineering Troubleshooting RTDs and Thermocouples
RFI/EMI - Thermocouples are susceptible to electrical interference
• Use shielded twisted pair cables
• Short runs
• Add a transmitter
Thermocouple Troubleshooting
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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
• 0.010” 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
Thermocouple Troubleshooting
© Burns Engineering Troubleshooting RTDs and Thermocouples
Questions?
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Contact me at 800-328-3871 ext. 6413
or visit www.burnsengineering.com
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© Burns Engineering Troubleshooting RTDs and Thermocouples
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