Structure of measurement systems 

The measurement system consists of several elements or blocks. It is possible to identify four types of element, although in a given system one type of element may be missing or may occur more than once. The four types are shown in Figure and can be defined as follows.

General structure of measurement system

Sensing element

This is in contact with the process and gives an output which depends in some way on the variable to be measured. Examples are:

• Thermocouple where millivolt e.m.f. depends on temperature
• Strain gauge where resistance depends on mechanical strain
• Orifice plate where pressure drop depends on flow rate.

If there is more than one sensing element in a system, the element in contact with the process is termed the primary sensing element, the others secondary sensing elements.

Signal conditioning element

This takes the output of the sensing element and converts it into a form more suitable for further processing, usually a d.c. voltage, d.c. current or frequency signal.

Examples are:

• Deflection bridge which converts an impedance change into a voltage change
• Amplifier which amplifies millivolts to volts
• Oscillator which converts an impedance change into a variable frequency voltage.

Signal processing element

This takes the output of the conditioning element and converts it into a form more

suitable for presentation. 

Examples are:

• Analogue-to-digital converter (ADC) which converts a voltage into a digital form for input to a computer
• Computer which calculates the measured value of the variable from the incoming digital data.

Typical calculations are:

• Computation of total mass of product gas from flow rate and density data
• Integration of chromatograph peaks to give the composition of a gas stream
• Correction for sensing element nonlinearity.

Data presentation element

This presents the measured value in a form which can be easily recognised by the observer. 

Examples are:

• Simple pointer–scale indicator
• Chart recorder
• Alphanumeric display
• Visual display unit (VDU).