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Module 3
Eng. Asma Abu Baker
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.
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.
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.
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
Skill 1: Sensor/Transducer Characteristics
Characteristic Definition
Linearity
Sensitivity
Accuracy
Precision
Range
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’.
Temperature measuring devices
• Thermometer• Thermocouple • Thermistor • RTD
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?
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.
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.
Thermocouples
Choose the correct range to measure the thermocouple output ?
Choose the correct DC voltage range to measure the thermocouple output ?
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.
Thermistor
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.
Skill 2: Sensor Input / Output
• Identify the input physical quantity and the outputSensor /
transducer Picture Input Physical quantity Sensor
output
Thermocouple
Thermistor
RTD