Large number and variety of variables are involved in measurement in practice. These variables may be constant with time or time varying. Measurement of the weight of an object is the example of time constant measurement. The measurement of pressure inside an I.C. engine is an example of time varying measurement. Output of a time varying variable cannot be read on a scale and a pointer or on a digital display device. Therefore, the types of measuring instruments that are available are quite large and diverse. Also, they work in many different principles making a comprehensive classification of instrument rather difficult.
A broad classification of these instruments based on their application, mode of operation, manner of energy conversion and the nature of output signal is given below:
- Deflection and Null type instruments
- Analog and Digital instruments
- Active and passive type instruments
- Automatic and manually operated instruments
- Absolute and secondary instruments
- Contacting and non-contacting instruments
- Intelligent instruments
Deflection and Null Type Instruments
In a deflection type instrument, the measured quantity generates some effect which can be ultimately related by the deflection of a pointer or displayed as a number, to its magnitude. Consider a simple example of measuring the weight of an object by a simple spring balance as shown in fig.
The weight of the object is indicated by the deflection or movement of a pointer on a graduated scale. Therefore, it is called deflection type instrument. In this case, the weight of the object generates the effect of elongation of the spring which is indicated by a pointer a on a scale.
In null type instruments, the effect caused by the quantity to be measured is nullified. The required nullifying effect provides a measure of the magnitude quantity being measured. For example, consider the measurement of weight by an ordinary beam balance as shown in fig. The unknown weight placed in one-side causes the beam and the pointer to deflect. Weights of known value are placed on the other side till a balanced or null condition is indicated by the pointer.
The null type instrument has more accuracy than deflection type. But from the user point of view, deflection type instruments are more convenient to use. The sensitivity of null type instrument is high as the null pointer has to cover a small range around the null point. Dynamic response of the null type instrument is poor as some adjustment to achieve
the null balance is required.
Analog and Digital Instruments
The analog instrument gives the output which varies in a continuous manner as the quantity being measured changes, and can take infinite values in a given range. An ordinary D’Arsonval type voltmeter and a pressure gauge indicate the voltage and pressure respectively in analog form.
The digital instrument gives the output which varies in discrete steps and can take only finite number of values in a given range. The output of digital instrument is generally displayed numerically as digits.
The advantages of using digital instruments are:
- It gives direct and precise readings.
- Digital signals are noise resistance during transmission.
- Digital circuits operate on relatively low voltage and they are particularly suitable for digital computer processing.
Active and Passive Type Instruments
In active instruments, the quantity being measured just activates the magnitude of some external power input source which in turn produces the measurement. In this type of instruments, another external energy input source is present apart from the quantity to be measured. The liquid level indicator is the best suitable example of active instrument which is shown in fig.
In passive type instruments, output is produced entirely by the quantity being measured. The example of such a system is shown in fig. The resolution of the passive instrument is less and cannot be increased very easily.
But in case of active instrument, the control over resolution can be obtained by adjusting the magnitude of the external energy input. The design of passive instrument is easy and hence it is cheaper where as the design of active instrument is complicated and hence costly.
Automatic and Manually Operated Instruments
Manually operated instruments require the services of a human operator. If some auxiliary devices are incorporated in the instrument to dispense with the human operator it is termed as automatic measuring instruments. For example, a ‘null balance’ type instrument in which null balance is achieved manually termed as manually operated instruments. When the process of null balance is automated, it is known termed as automatic instruments. The electronic weighting scale is the best example for automatic type null balance.
Absolute and Secondary Instruments
Absolute instruments are those which give the value of the electrical quantity to be measured in terms of the constant of the instruments and their deflection only. Example: tangent galvanometer, Rayleigh current balance etc. These instruments are rarely used except in standard laboratories.
Secondary instruments are those which have been precalibrated by comparison with an absolute instrument. The value of the electrical quantity to be measured in these instruments can be determined from the deflection of the instruments. Without calibration of such an instrument, the deflection is meaningless. These type of instruments are widely used.
Contacting and Non-Contacting Instruments
The instrument in which some parts of the instrument are in contact with the measuring medium is called as contacting type instrument. Few example of contacting type instruments are thermometer(mercury-in-glass type), Bourdon type pressure gauge and voltmeter. These instruments are placed in an electrical circuit.
The instrument in which no part of the instrument is in contact with the measuring medium is called non-contacting type instrument. In this instrument, the measurement can be considered to be a measurement of distance. For example, an optical pyrometer which can be used to determine the temperature of a hot body from a distance without being in physical contact with the body can be termed as non-contact type.
In conventional instrument, the basic functional elements are present whereas the intelligent instrument incorporates a microprocessor. This microprocessor based measuring instruments facilitates the programmed signal processing and the application of data manipulation algorithms to the measured variables. Some of the facilities provided by the intelligent instruments are as follows:
- Time constants may be selected.
- Non-linear output can be linearised.
- Suitable for various ranges of input signal strengths.
- Fault detection can be diagnostic.
- Remote can be used to operate the instrument.
- Programming is possible to correct the output of transducers due to change in environmental conditions and signal loading.
- Final output can be provided in a desired form and units.