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Mastery of Measuring Devices in Process Control

Understand the importance of measuring devices in process control, learn about sensors/transducers, explore temperature measurement devices like thermocouples, thermistors, and RTDs.

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Mastery of Measuring Devices in Process Control

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  1. Module 3 Eng. Asma Abu Baker

  2. Introduction • One of the most important uses of instruments or measuring devices is in the field of process control. • If you can not measure it , you can not control it • A process is a sequence of operations that carried out to achieve a desired result.

  3. Introduction • Fixing a tire is a manual process, whereas, the process of maintaining the room temperature is an automatic process. • However, the fundamental part of any process control system is a measurement system.

  4. Elements of Measurement System Elements senses the input physical variable and converts it into a measurable (Electrical, Mechanical signal) converts the signal to a format that can be displayed or recorded by the recorder.

  5. Sensor / Transducer • A transducer is a device that converts a physical quantity into a measurable quantity, usually an electrical signal. • Examples of transducers include microphones and thermocouples. • A microphone converts sound energy into electrical energy • Thermocouple converts heat energy into electrical signal

  6. Skill 1: Sensor/Transducer Characteristics

  7. Temperature Measurement • Temperature is the degree of hotness or coldness measured on a definite scale. • The various units for measuring temperature are Celsius, Kelvin and Fahrenheit. • The SI unit of temperature measurement is the ‘Kelvin’.

  8. Temperature measuring devices • Thermometer • Thermocouple • Thermistor • RTD

  9. Thermometer • Mercury thermometer is a temperature sensor that converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. • As the temperature increases, what happens to the Mercury? • As the temperature increases, what happens to the Mercury?

  10. Thermocouple • Thermocouple is a temperature transducer / sensor • It consists of two different metals, joined together at one end. • Examples of metals used: Iron and Constantan.

  11. Thermocouple • When the metal junction is heated or cooled, the thermocouple produces a voltage proportional to the temperature. • The output voltage is very small and is in mV. • Since thermocouples are rugged, and can measure within wide temperature ranges, they are especially useful in industries.

  12. Thermocouples

  13. Choose the correct range to measure the thermocouple output ?

  14. Choose the correct DC voltage range to measure the thermocouple output ?

  15. Thermistor • Temperature sensor/transducer • The resistance of the thermistor varies with temperature. • It is a non-linear device in that the resistance of a thermistor is not proportional to the input temperature.

  16. Thermistor

  17. RTD • A Resistance Temperature Detector (RTD) • The resistance of the RTD change with temperature. • As the temperature increase, the resistance of the RTD increase • Typical elements used for RTDs include nickel (Ni) and copper (Cu), but platinum (Pt) is by far the most common because of its wide temperature range, accuracy, and stability.

  18. Skill 2: Sensor Input / Output • Identify the input physical quantity and the output

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