Types of damping force in Instruments

A Damping Torque/force is one which acts on the instrument only when it is moving and it always opposes the motion of pointer.

Damping force in any electrical instrument is necessary because it brings the pointer at rest quickly. In the absence of damping force, due to inertia of the moving system, the pointer will oscillate about its final position for some time before coming to rest in steady position. 

The degree of damping is adjusted in such a way that it makes the pointer to deflect to the final position without overshooting the final value. 

If pointer overshoots the final value, then in that case it is called dead beat.

Absence or improper damping in any instrument makes the process of measurement from the instrument slow.

It also decreases the accuracy of the instrument

METHODS OF PRODUCING DAMPING FORCE/TORQUE

Damping torque in an instrument can be produced by using

1. Air friction- It consists of a light aluminium piston attached to the moving system of instrument. This piston is placed in a chamber which is closed at one end. The chamber has a small opening. The cross section of the chamber may be rectangular or circular.

2. Fluid Friction- It is similar to the air friction . Due to greater viscosity of oil, the damping is much effective. However, oil damping is not much used because of several disadvantages such as leaking of oil, keeping instrument in vertical position etc.

3. Eddy Current- This form of damping is the most efficient of the three. It consists of thin disc of conducting but of non-magnetic material like copper or aluminium mounted on a spindle which carries the moving system and pointer of the instrument. Disc is placed between the poles of permanent magnet.