The purpose of the measurement system is to present the observer with a numerical price value that corresponds to the variable being measured. The measurement system consists of several elements or blocks. It is possible to recognize 4 types of elements, even if a system that is given one type of element may be lost or can occur more than once.
Sensing element
This is in contact with the process and gives one output that depends on several ways the variable is measured. If there is more than one feeling element cascade, the elements that come into contact with the process are called primary sensing elements, others are called secondary sensing elements. The output from a sensor can be a change in resistance, change in voltage, change in current, frequency etc.
Signal conditioning element
This is what makes the output of the sensing element and converts it to a form that is more suitable for further processing, usually a DC voltage, DC current or frequency signal. An example is a deflection bridge that changes an impedance change to a voltage change; an amplifier which amplifies milli-volt voltage to volts; the oscillator that converts the impedance change into a variable frequency voltage. In most cases, the output signal conditioning element output follows the standard signal level, which is 0-10 Volts or 0-5 Volts. If the signal will be sent via way to the Control Room, the output of the signal conditioning element is 4-20 mA. In this case, a combination of sensors and signal conditioning elements is called a Transmitter. For temperature transmitters that measure temperatures between 0-120 OC, output 4mA corresponds to 0 OC, and output 20 mA corresponds to 120 OC.
Signal processing element:
This is what makes the output from the conditioning element and changes it and turns it into a form that is more suitable for further presentation. For example an analogue to digital ADC converter that converts the voltage into a digital form as input to the computer; a microcomputer that calculates the value of a measured variable from the incoming digital data. Typical calculations are the calculation of total mass flow from volume flow rates and density data; harmonic component analysis of vibration measurement, and correction of sensing element nonlinearity.
Data presentation element:
Presentation data elements present measured values in a form that can be easily understood by observers. For example elements such as indicator, pointer scale indicator; chart recorders; alphanumeric displays; and computer monitors.
