Temperature sensing devices:Smart Temperature Transmitters

Smart Temperature Transmitters

Standard temperature transmitters generally send a 4 to 20 milliampere signal to indicate the temperature.

They are calibrated for a specific range of temperature such as 0 to 100 degrees. Standard transmitters are designed to operate with one type of sensor such as RTD, thermocouple, and so on. Any changes to the setting require a recalibration of the unit.

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Smart transmitters contain an internal microproces- sor and can be calibrated from the control room by sending a signal to the transmitter. It is also possible to check the transmitter for problems from a remote loca- tion. A cutaway view of a smart temperature transmitter is shown in Figure 14 – 23. The transmitter illustrated in Figure 14 – 23 uses HART (Highway Addressable

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Remote Transducer) protocol. This transmitter can ac- cept RTD, differential RTD, thermocouple, ohm, and millivolt inputs. A smart temperature transmitter with meter is shown in Figure 14 – 24.

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Review Questions

1. Should a metal bar be heated or cooled to make it expand?

2. What type of metal remains in a liquid state at room temperature?

3. How is a bimetal strip made?

4. Why are bimetal strips often formed into a spiral shape?

5. Why should electrical contacts never be permitted to open or close slowly?

6. What two factors determine the amount of voltage produced by a thermocouple?

7. What is a thermopile?

8. What do the letters RTD stand for?

9. What type of wire is used to make an RTD?

10. What material is a thermistor made of?

11. Why is it difficult to measure temperature with a thermistor?

12. If the temperature of an NTC thermistor increases, will its resistance increase or decrease?

13. How can a silicon diode be made to measure temperature?

14. Assume that a silicon diode is being used as a temperature detector. If its temperature increases, will its voltage drop increase or decrease?

15. What type of chemical is used to cause a pressure change in a bellows type thermostat?

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