Instrument Classification and Characteristics

About the instrument classification and characteristics

Instruments can be subdivided into separate classes according to several criteria. These sub classifications are useful in broadly establishing several attributes of particular instruments such as accuracy, cost and general applicability to different applications.

Instruments are either active or passive according to whether the instrument output is entirely produced by the quantity being measured or whether the quantity being measured simply modulates the magnitude of some external power source. This can be illustrated by examples.

An example of a passive instrument is the pressure measuring device. The pressure of the fluid is translated into movement of a pointer against a scale. The energy expended in moving the pointer is derived entirely from the change in pressure measured: there are no other energy inputs to the system.

An example of an active instrument is a float-type petrol-tank level indicator. Here, the change in petrol level moves a potentiometer arm, and the output signal consists of a proportion of the external voltage source applied across the two ends of the potentiometer. The energy in the output signal comes from the external power source; the primary transducer float system is merely modulating the value of the voltage from this external power source.

In active instruments, the external power source is usually electrical in form, but in some cases it can be in other forms of energy such as pneumatic or hydraulic.

One very important difference between active and passive instruments is the level of measurement resolution which can be obtained. With the simple pressure gauge, the amount of movement made by the pointer for a particular pressure change is closely defined by the nature of the instrument. Whilst it is possible to increase the measurement resolution by making the pointer longer, so that the pointer tip moves through a longer arc, the scope for such improvement is clearly restricted by the practical limit of how long the pointer can conveniently be. In an active instrument, however, adjustment of the magnitude of the external energy input allows much greater control over measurement resolution. Whilst the scope for improving this resolution is much greater incidentally, it is not infinite because of the limitations placed on the magnitude of the external energy input, in considering heating effects and for safety reasons.

In terms of cost, passive instruments are normally of a simpler construction than active ones and are therefore cheaper to manufacture. The choice between active and passive instruments for a particular application therefore involves carefully balancing the measurement resolution requirements against cost.

1. Null or Deflection Type Instruments

2. Monitoring and Control Instruments

3. Analog and Digital Instruments

4. Static Characteristics of Measuring Instruments

5. Accuracy and Precision in Measuring Instruments

6. Sensitivity of Measurement to Disturbance

7. Instruments Calibration

8. Choice of Instruments

Back to Principles of Measurement