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Temperature measurement reliability challenges: Part 1

Hydrocarbon Engineering,

According to a recent report by Honeywell, temperature is one of the most common yet critical measurements in any process industrial plant. Challenges that impact measurement reliability include:

  • Measurement variation due to drift in the device.
  • Noise interference.
  • Degradation of sensor performance.
  • Ambient temperature variation.

Measurement variation

Honeywell explains that drift of zero or span in temperature transmitters is a major source of measurement variation. It is impossible to trust the reliability of a device that is not stable.

Calibration is a proven solution to eliminate drift, this cannot be done frequently enough considering there can be thousands of installed devices that are often located in remote and difficult to reach areas.

This becomes even more critical when temperature measurements are used for fiscal metering in gas pipelines. If there is a variation of 0.1 °C in a temperature measurement, this can create a variation of 0.02% in flow measurement resulting in the loss of several thousands of dollars.

Noise interference

Process and power plant environments are inherently ‘noisy’. Since temperature transmitters employ low voltage sensors like thermocouples and are often connected by compensating cables, the entire installation from sensing to the actual temperature device is susceptible to interference from electrical surges.

This problem is compounded when grounded sensors are used. Grounded sensors create ground loops when subjected to electrical transients of lightning strikes. These transients can cause a difference in ground voltages that range from a few hundred volts up to a few thousand volts.

Galvanic isolation in the temperature transmitter is a proven solution to eliminate noise and ground loops. Galvanic isolations separate signal currents from noise currents and will eliminate interference.

Another source of noise is caused by thermocouples when they are attached to a conductive material such as steel or are submerged in conductive liquids such as water. When they come into contact with any conductive material, thermocouples are susceptible to common-mode noise.

Isolation can dramatically improve the rejection of common mode noise. With conductive materials that have a large common mode voltage, isolation is required. In the absence of isolation, amplifiers cannot measure signals with large common mode voltages.

In addition, Honeywell highlights that all plants have electromagnetic interference (EMI) and radio frequency interference (RFI). They interfere with the low level signals generated by temperature sensors and carried through extension wires. It is very important that the temperature sensors are immune to these interferences. EMC compliance to IEC61326 is mandatory for use in noisy environments.

Adapted from a report by Emma McAleavey.

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