Instrumentation Sensors and Transducers

A basic instrumentation system consists of 3 elements:

• Sensor or Input Device
• Signal Processor
• Receiver or output device

A block diagram of a basic instrumentation system is shown below:

Most modern analogue instruments work on the following standard signal ranges:

• Electric 4-20 mA 
• Pneumatic 0.2 to 1.0 bar which is equivalent to 3 -15 PSI

The old electrical equipment use 0 to 10 v. Pneumatic signals are commonly used in process industries for safety especially when there is a risk of fire or explosion.

The advantage of having a standard range or using digital signals is that all equipment may be purchased ready calibrated. For analogue systems the minimum signal (Temperature, Speed, Force, Pressure etc.) is represented by 4 mA or 0.2 bar or 3 PSI. and the maximum signal is represented by 20 mA or 1.0 bar or 15 PSI.

The physical quantities commonly measured include:

• Flow rate
• Temperature
• Pressure
• Level
• Mass or Weight
• Density
• Speed
• Strain
• Movement, Velocity and Acceleration
• Acidity/Alkalinity

Sensors may operate simple on or off switches to detect the following:

• Objects (proximity switch)
• Hot or Cold (Thermostat)
• Empty or full (level switch)
• Pressure high or low (Pressure switch)

The block diagram of a sensor as shown below:

You can also read: Introduction to Industrial Instrumentation

We have different types of sensors used in instrumentation:

• Temperature transducers e.g. Thermocouple, Resistance Temperature detectors etc. 
• Pressure transducers e.g. Bourdon Tube
• Speed transducers e.g. Tachometer, Magnetic pickup & optic types
• Flowmeters e.g. Positive Displacement meters, Differential Pressure Flowmeters, Turbine etc. 
• Force sensors: mechanical, hydraulic, electric strain gauge
• Position sensors: Resistive, Optical, Inductive
• Depth gauges: Ultrasonic, Pressure Gauge, Electronic level gauge
• Strain gauges

From the above, we learn that any instrumentation system whether basic or complex, has an input/sensor, a signal processing unit and lastly an output or receiver.