Basic Terminology

Measurements: It is the process of comparison of standard quantity with the measured quantity.

Accuracy: It is defined as closeness to any value of instrument reading to true value.

Precision: It refers to the closeness of two or more measurements to each other. For example , if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise.

Error: Any measurement made with a measuring device is approximate. If you measure the same object two different times, the two measurements may not be exactly the same. The difference between two measurements is called an error. The error in measurement is a mathematical way to show the uncertainty in the measurement.

Resolution: It is defined as the smallest or least change in the input which can be detected by the measuring instrument.

Sensitivity: It is defined as the ratio of the changes in the output of an instrument to a change in the value of the quantity being measured. It denotes the smallest change in the measured variable to which the instrument responds.

Tolerance: Engineering tolerance is the permissible limit or limits of variation in a physical dimension; a measured value or physical property of a material, manufactured object, system, or service; other measured values (such as temperature, humidity, etc.).

Static Characteristics: These are used to measure the condition when it is not varying with respect to time.

Condition of Instruments:

The instruments can broadly classified into two types:

  1. Absolute Instruments

  2. Secondary Instrument

Absolute Instruments: These instruments give the magnitude of the physical constant of the instrument. These are generally not available in the market for public use and measurement is very time consuming.

Secondary Instruments: These instruments are used in general for all laboratory purposes. Some of the very widely used secondary instruments are: ammeters, voltmeter, wattmeter, energy meter (watt-hour meter), ampere-hour meters etc.

Deflection Controlling and Damping arrangements in indicating type instruments: In indicating type instruments a pointer is present which moves over the calibrated scale. In these types of instruments generally three types of torques are developed:

  1. Deflecting Torque:

  • One important requirement in indicating instruments is the arrangement for producing deflecting or operating torque (Td) when the instrument is connected in the circuit to measure the given electrical quantity. This is achieved by utilizing the various effects of electric current or voltage mentioned in the previous article.

  • The deflecting torque causes the moving system (and hence the pointer attached to it) to move from zero position to indicate on a graduated scale the value of electrical quantity being measured. The actual method of producing the deflecting torque depends upon the type of instrument and shall be discussed while dealing with particular instrument

  1. Controlling Torque:

  • If deflecting torque were acting alone, the pointer would continue to move indefinitely and would swing over to the maximum deflected position irrespective of the magnitude of current (or voltage or power) to be measured.

  • This necessitates providing some form of controlling or opposing torque (TC). This controlling torque should oppose the deflecting torque and should increase with the deflection of the moving system. The pointer will be brought to rest at a position where the two opposing torques are equal i.e. Td = TC.

  • The controlling torque performs two functions: 

  1. It increases with the deflection of the moving system so that the final position of the pointer on the scale will be according to the magnitude of current (or voltage or power) to be measured 

  2. It brings the pointer back to zero position when the deflecting torque is removed. If it were not provided, the pointer once deflected would not return to zero position on removing the deflecting torque.

  • The controlling torque in indicating instruments may be provided by one of the following two methods: 

  1. By one or more springs = spring control 

  2. By weight of moving parts =Gravity control

Spring Control:

  • This is the most common method of providing controlling torque in electrical instruments. A spiral hairspring made of some non-magnetic material like phosphor bronze is attached to the moving system of the instrument as shown in Fig. A with the deflection of the pointer, the spring is twisted in the opposite direction.

  • This twist in the spring provides the controlling torque. Since the torsion torque of a spiral spring is proportional to the angle of twist, the controlling torque is directly proportional to the deflection of the pointer i.e. TC ∝ θ

  • The pointer will come to rest at a position where controlling torque TC is equal to the deflecting torque Td i.e. Td = TC. In an instrument where the deflecting torque is uniform, spring control provides a linear or evenly-spaced scale over the whole range. For example, in a permanent-magnet moving coil instrument, the deflecting torque is directly proportional to the current flowing through the operating coil i.e. Td ∝ I

  • With spring control, TC ∝ θ

  • In the final deflected position, Td = TC

∴ θ ∝ I 

  • Since the deflection is directly proportional to I, the scale of such an instrument will be linear (uniform).

Damping Torque:

  • If the moving system is acted upon by deflecting and controlling torques alone, then the pointer, due to inertia, will oscillate about its final deflected position for quite some time before coming to rest. This is often undesirable because it makes it difficult to obtain quick and accurate readings. In order to avoid these oscillations of the pointer and to bring it quickly to its final deflected position, a damping torque is provided in the indicated instruments. This damping torque acts only when the pointer is in motion and always opposes the motion. The position of the pointer when stationary is, therefore, not affected by damping.

  • The degree of damping decides the behavior of the moving system. If the instrument is  under-damped, the pointer will oscillate about the final position for some time before coming to rest. On the other hand, if the instrument is over-damped, the pointer will become slow and lethargic. However, if the degree of damping is adjusted to such a value that the pointer comes up to the correct reading quickly without passing beyond it or oscillating about it, the instrument is said to be dead-beat or critically damped. shows graphs for under-damping, over damping and critical damping (dead-beat).

  • The damping torque in indicating instruments can be provided by 

  1. Air-friction

  2. Fluid friction

​​​​​​​Fluid friction

  1. Eddy currents

​​​​​​​Eddy currents

Calibration of Instruments:

  • It is the process of comparison of a particular instrument with known standard instruments. It is done to obtain the accuracy and errors in instruments. Static characteristics are measured by calibration method.

  • The instruments which are used for measurement must be calibrated against a sum reference instrument of higher accuracy .Calibration is the process of configuring an instrument to provide a result for a sample within an acceptable range.

  • Eliminating or minimizing factors that cause inaccurate measurements is a fundamental aspect of instrumentation design.