Comparison between of Thermocouple and RTD ( Advantages and Disadvantages)


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Thermocouple and RTDs are the two important temperature measuring devices used widely in industries. They are compared on their purpose.

Thermocouple:

Advantages:

Cost: Comparing to RTD, Thermocouple is lesser expensive.

Temperature Range: Extends to about 1150°C or higher. However, the thermocouple’s cryogenic range is somewhat less than that of the RTD.

Mounting cost: Lower than RTD

Ruggedness: For process environmental condition, high temperature, vibration, thermocouple are regarded highly.

Disadvantages:

Stability: Less than for the RTD. Estimated at 0.6◦C (1◦F) per year.

Calibration: Nonlinear over normal spans. Signal requires linearizing. Calibration can be changed by contamination.

Accuracy: Generally expected, after installation, ±4◦C (±7.2◦F), but there are exceptions. Thermocouple requires reference junction or special wires. The inaccuracy is due to two separate temperature measurements - the measuring junction and the cold or reference junction.

RTD:

Advantages:

Accuracy: Accepted after installation ±0.5◦C. The platinum RTD, for example, is used to define the IPTS at the oxygen point (−182.97◦C) and the antimony point (+630.74◦C).

Repeatability: Within a few hundredths of a degree; can often be achieved with platinum RTDs. Less than 0.1% drift in 5 years.

Relatively narrow spans: Some versions may have spans as narrow as 5.6◦C (10◦F).

Substantial output voltage (1 to 6 volts): This is an advantage in that the output can be controlled by adjusting the current and the bridge design in the RTD signal conversion module. Because of a higher output voltage to the RTD and controlling of temperature signals is simpler. This permits more accurate measurements without requiring complex calculations for large spans

Compensation: Not required.

Size: Generally smaller than thermocouples

Disadvantages:

Cost: Higher than the thermocouple. RTDs do not require compensation, special lead-wires, and special signal conditioning for long runs.

Less rugged: For adverse process environments, including high temperature and vibration.

Lower temperature range: Limited to about 870◦C (1600◦F).

Self-heating errors: This may be a problem unless corrected in the transistor’s electronics.