TEMPERATURE TRANSMITTER EXPLANATION & COMPARISON BETWEEN RTD AND THERMOCOUPLE

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TEMPERATURE TRANSMITTER EXPLANATION & COMPARISON BETWEEN RTD AND THERMOCOUPLE

RTD Advantages

Better accuracy and repeatability.
Excellent stability. Less than 0.1% in 5 years.
No reference compensation is required
Do not need special extension wires
Better linearity over temperature changes
RTD signals are less susceptible to noise-higher signal to noise ratio.

RTD Disadvantages

Less rugged when high temperature and vibration are encountered.
Limited to a temperature high limit of about 970°C ( 1800°F ).
Self heating errors can be a problem unless low current levels are used.

Thermocouple Advantages

Supports wide temperature range , -280°C to 2750°C (-440°F to 5000°F )
Faster response time than RTDs (Response time comparisons are not relevant if thermocouple installed in a thermowell )
Rugged construction resists vibration
Approx 20% less expensive than RTDs

Thermocouple Disadvantages

Electrical interference can be a concern
Reference compensation is required

TEMPERATURE TRANSMITTER EXPLANATION

To be useful for control, safety, or monitoring purposes, a temperature measurement signal must be relayed from the point of measurement to the DCS of the process. The signal is relayed back to the DCS by using transmitters..

Transmitters—The sensor is wired a short distance to the transmitter, where its signal is converted to a digital or 4–20 mA signal. The transmitter is wired back and the converted signal is relayed to the DCS.

A transmitter converts a temperature sensor’s signal from resistance or voltage into a common digital or analog 4–20 mA signal. Typically, the analog signal is linear with the process-temperature measurement. As an example, consider a transmitter with a range of 32–212 °F (0–100 °C). With this transmitter, a 4 mA output corresponds to a 32 °F (0 °C) process temperature, 12 mA corresponds to 122 °F (50 °C), and 20 mA to 212 °F (100 °C) (See Figure 29). Temperature transmitters can accept sensor inputs from a wide variety of temperature sensor types (including many types of RTDs and T/Cs).

Figure Temperature Transmitter

HART Transmitters

HART transmitters have the ability to transmit digital information simultaneously with the standard 4–20 mA analog signal. HART transmitters offer many benefits over transmitters that only communicate a 4–20 mA signal, such as reduced plant inventory, improved measurement accuracy and stability, reduced maintenance, and remote diagnostics

Please read also: CATEGORIES OF TEMPERATURE MEASUREMENT DEVICES

Please read also: THERMODYNAMIC CONSIDERATIONS & TEMPERATURE MESUREMENT SCALES

Transmitter benefits

Three benefits of using a temperature transmitter over wiring direct are:

1.A more robust signal is delivered—The 4–20 mA signal output from the transmitter is much more robust than a sensor’s signal. Noise interference has less impact on

4–20 mA signals.

2Measurement accuracy is optimal—Transmitters offer improved measurement accuracy over wiring direct. For example, sensors can be matched to transmitters (transmitter sensor matching), which improves the accuracy of the temperature measurement.

3. Time and money are saved—Transmitter installation is often less expensive than wiring a sensor direct because of savings from cabling costs and installation (sensor wire, especially T/C wire, is relatively expensive). Also, a robust signal and accurate measurements produce time

and money savings through increased functionality and diagnostic capabilities of the transmitter.