Important points in RTD troubleshooting
Important points in RTD troubleshooting: Temperature sensors (RTDs) are widely used to measure temperature in various industries such as oil and gas and petrochemical, food and beverage industries and so on. In using them, we face various problems. To help resolve these issues, the following is a list of common problems with RTD applications and possible solutions or remedies. This list is by no means exhaustive (pt100 sensor recognition):
|RTD Problem||Possible Cause(s)||Remedies|
|Temperature indication generally too high||Lead resistance too high; not compensated||If possible:
(1) Install larger wire size cables.
(2) Compensate leads
(3) Use sensor head transmitters
(4) Convert to 3- or 4-wire circuits
(5) Reduce connection lead lengths
|Self heating by measuring current too high||Use a smaller measuring current(1mA current recommended)|
|Temperature indication changes with changing ambient temperature||RTD sensor in 2-wire circuit; the connection lead are subjected to a large temperature change.||(1) Convert to 3-wire circuit, which will essentially eliminate the ambient temperature effects
(2) Convert to a 4-wire circuit – in this case connection lead resistance effects completely eliminated.
|Temperature indication error increases with increasing temperature (indication too low)||Decreasing insulation resistance, acts as a shunt path for measured signal||(1) Insulation resistance of approximately 0.1MΩ in parallel with 100Ω gives an error of the same magnitude as Tolerance Class B RTD sensors.
Minimum recommendations for insulation resistance according to IEC60751 are:
(a) Insulation resistance at 20°C (68°F) must be greater or equal to 100MΩ
(b) Insulation resistance at 500°C (930°F) must be greater or equal to 2MΩ
(2) Replace defective RTD sensor
|Deviations of the temperature indication from the values in the table (parasitic and galvanic EMFs)||(1) Poor lead material, contamination, moisture,
(2) Temperature difference between the terminals of the connection leads.
(3) Corrosion at the connection terminals in the connection head.
|(1) Check installation
(2) Thermally insulate terminals (bring to same temperature)
|Indication changes over the course of time||Thermal aging (drift of the measuring resistor)||(1) Select suitable high temperature design
(2) Re-calibrate regularly
(3) Replace sensor if necessary.
RTD insulation resistance:
This is the resistance to current leakage through the insulation material of the sensor wires. High insulation resistance means the sensor is in good condition, while low insulation resistance indicates the weakness of some problems with the RTD sensor , which can lead to current leakage.
In general, changes in RTD insulation resistance over time create short parasitic circuits that operate in parallel with the actual RTD sensor resistance. The resulting shunt current causes an incorrect low measurement signal. For example, weak insulation resistance increases for higher nominal resistances of RTD sensors, for example, the effect of weak insulation resistance is felt more with Pt1000Ω than with Pt100Ω sensor.
Test an RTD sensor
Two common tests are usually performed on an RTD:
- Continuity test usually with a multimeter
- Insulation resistance test with insulation tester such as 60-100V
It is said that if the Pt100Ω RTD sensor is in good condition:
- A coupling test shows a resistance value of approximately 110Ω
- Insulation resistance test gives 100MΩ or more
- All tests are performed onthe RTD sensor at room temperature.
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