Troubleshooting RTDs and Thermocouples resistance of resistor in parallel, where one resistor is the PRT (R PRT) and the other is the insulation resistance ( R IR) [ ] [ ] PRT IR PRT

© Burns Engineering Troubleshooting RTDs and Thermocouples

Troubleshooting RTDs and Thermocouples

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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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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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• Sensing element - wire wound

• resistor made from platinum wire, .0007” dia.

• 1/16” diameter x 1” long

RTD Construction

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• Sensing element – thin film

• resistor made from platinum film

• .080” x 0.175” x .035” thickness

RTD Construction

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• 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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RTD Troubleshooting

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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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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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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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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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Test method

• Lower resistance = lower measured temperature

• Test at 50 VDC minimum

• IR should be >100 megohms at 25°C


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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.


Low insulation resistance (RIR or IR)

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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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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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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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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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-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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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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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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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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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


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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


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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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The good and bad

Preventive Maintenance

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• 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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• Grounded or ungrounded junction

• Instrumentation set to accept type

Thermocouple Construction

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Thermocouple Troubleshooting

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Thermocouple Troubleshooting Guide


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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Check polarity of the thermocouple circuit and all connection contacts.

Red lead is always negative (-).


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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

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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


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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.


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RFI/EMI - Thermocouples are susceptible to electrical interference

• Use shielded twisted pair cables

• Short runs

• Add a transmitter


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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



Questions?

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Contact me at 800-328-3871 ext. 6413

[email protected]

or visit www.burnsengineering.com

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Documents

Troubleshooting RTDs and Thermocouples resistance of resistor in parallel, where one resistor is the PRT (R PRT) and the other is the insulation resistance ( R IR) [ ] [ ] PRT IR PRT