Measurement Systems

Measurement Systems

An overview of engineering measurement systems for position, speed, acceleration, vibration, strain, force, temperature, heat, pressure and flow.

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Elements of a Measurement System
Sensor: Converts a physical input quantity into an output, commonly an electrical voltage.
Signal Processor: Converts the sensor signal into a form that can be read and used.
Display or Recorder: Presents or records the signal output.
Examples of Measurement Systems
In a thermometer, the voltage produced by a thermocouple sensor is amplified, converted from analog to digital and supplied to a display.
In alarm systems, a switching element is triggered according to the value obtained from the sensor and signal processor.
Position Measurement
Potentiometer
Rheostat: a potentiometer used in high-current circuits
Proximity Sensor
Limit Switch
Digital Optical Encoder
LVDT: Linear Variable Differential Transformer
Speed Measurement
Can be derived from position measurement.
Tachometer
Speed is obtained by dividing change in position by time.
Acceleration Measurement
Accelerometer
Accelerometers may use a strain gauge.
Piezoelectric materials convert pressure into electrical voltage.
Vibration Measurement
Can be obtained from acceleration measurement.
An accelerometer is attached so that it detects vibration along the selected axis.
Vibration perpendicular to that axis should not affect the sensor.
The operating principle is based on the inertia of an additional mass attached to the vibrating object.
The added mass is connected through a spring and damper.
When the structure accelerates, relative motion occurs between the accelerometer housing and the mass.
This relative movement is detected with a displacement transducer.
Strain Measurement
Strain Gauge
Deformation can also be determined through strain measurement.
Force Measurement
Load Cell
Load cells are constructed from multiple strain gauges.
An S-type load cell is one example.
Temperature Measurement
Thermometer
Mercury thermometers operate through changes in mercury volume.
Bimetal thermometers operate through differential expansion of a metal pair.
Heat Measurement
Calorimeter
Pressure Measurement
Manometer
A U-tube manometer is used to measure fluid and gas pressure.
A Bourdon-tube manometer operates by measuring deformation in an elastic element.
Flow Measurement
Venturimeter
Pitot tubes measure fluid velocity.
Orifices, nozzles, venturimeters, weirs and narrow-section devices can be used to determine fluid quantity.
Flow rate can also be obtained from pressure and energy changes.