Basic Definitions Related to Measuring Instruments

BASIC DEFINITIONS RELATED TO MEASURING INSTRUMENTS

1. True value

It is the actual magnitude of a signal to a measuring system which can only be approached and never be achieved.

It is defined as the average of an infinite number of measured values when the average deviation due to the various contributing factors tends to zero.

2. Accuracy

It may be defined as the closeness, with which an instrument reading approaches the true value of the quantity being measured.

3. Precision

It is the degree of reproducibility among several independent measurements of the same true valve under specified conditions. It is usually measured in terms of the deviation in measurements.

4. Error

It is defined as the difference between the measured value and the true value.

E = E_i ‒ E₀

Where,

E - Error.

E_i - Measured value of the quantity.

E₀ - True value of the quantity.

5. Sensitivity

This also means the slope of the input, output calibration curve. Sometimes it is expressed as the ratio of the magnitude of the output signal to the magnitude of the input signal.

S = magnitude of the output signal / magnitude of the input signal

6. Resolution

The smallest increment in the quantity being measured which can be detected with certainly by an instrument in its resolution or discrimination.

7. Instrument Efficiency

The efficiency of an instrument is defined as the ratio of measured quantity at full scale to the power taken by the instruments at full scale. The efficiency of an instrument should be as high as possible.

Efficiency = Measured quantity at full scale / Power taken by the instrument at full scale

The instrument which measures the current flowing in the circuit is called ammeter while the instrument which measures the voltage across any two points of a circuit is called voltmeter. The instruments which are used to measure the power are called power meters (or) wattmeters.

In this chapter, we are going to discuss the working principle, construction, advantages and disadvantages of various types of instruments used as ammeter, voltmeters, wattmeters and energy meters.

In these meters, ammeter will be connected in series with the circuit whose current is to be measured. Hence they should have a low electrical resistance so that they cause a small voltage drop across the ammeter and consequently absorb small power.

Voltmeters are connected in parallel with the circuit whose voltage is to be measured. Hence they should have a high electrical resistance so that the current drawn by them is small and consequently the power absorbed is small.