Measurement and measurement system block diagram

Measurement is the process of comparing an unknown quantity (the measurand) with a standard of the same kind, to find its magnitude as a number and a unit. A measurement system is the set of devices that senses the measurand, converts and processes the signal, and presents the result in a form an observer or a controller can use.

Generalized measurement system

 Measurand     +-----------+   +-------------+   +------------+
 ------------->| Primary   |-->| Variable    |-->| Signal     |
 (pressure,    | sensing   |   | conversion  |   | processing |
  temp, ...)   | element   |   | element     |   | (amplify,  |
               +-----------+   +-------------+   |  filter)   |
                                                 +-----+------+
                                                       |
     +-----------+   +-----------+   +--------------+  |
     | Data      |<--| Data      |<--| Data         |<-+
     | storage/  |   | display/  |   | transmission |
     | control   |   | record    |   | (if needed)  |
     +-----------+   +-----------+   +--------------+

Functional elements

1. Primary sensing element (sensor): senses the measurand and gives a related physical response. Example: a Bourdon tube or diaphragm gives a displacement for a pressure.
2. Variable conversion element (transducer stage): converts the sensor output into a more convenient variable, usually electrical. Example: an LVDT or strain gauge converts displacement to voltage or resistance.
3. Variable manipulation / signal conditioning: amplification, filtering, modulation or demodulation, linearisation and A/D conversion, without changing the physical nature of the variable.
4. Data transmission element: carries the signal between distant stages (wires, telemetry).
5. Data presentation element: an indicator, recorder, printer or display, so the observer can read the value.
6. Data storage and control: stores the data or uses it in a feedback controller.

Simple block diagram (electrical measurement)

 Input --> [Transducer] --> [Amplifier/ --> [Indicator/
 quantity                    Conditioner]    Recorder] --> Reading

Example (temperature): a thermocouple (sensor and transducer) gives a few millivolts; an amplifier raises this; an ADC and microcontroller process it; and an LCD shows the temperature in °C.

A good measurement system has high accuracy, sensitivity and repeatability, and a low loading effect on the measured source.