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