Explanation of Pointers and Scales in Measuring Instruments

Pointers and Scales in Measuring Instruments

Instrument scales and pointers may be considered together in two classes:

(A)    Those intended for reading quickly (in the case of switch board instruments at a distance), and

(B)     Those intended for close accurate reading for portable instruments.

Pointers: The shape and size of the pointers depends upon the type of instrument. The weight and inertia of pointer must be reduced so as to reduce the load on the bearing and to avoid necessity of damping torque. The pointer motion is usually limited by buffer stops little more than the scale. These stops are constructed as very light springs so that the pointer is not bent when it strikes them sharply on a sudden overload or reversal of the operating current.

fig 1. Pointer

The pointers for switch board and panel instruments should be so that the position of the pointer is conspicuous. The pointers are usually broad or carry broad targets at their ends. Aluminium strip or tube is used for making pointers and a type arrow head pointer construction is used to give rigidity.

Portable instruments usually have knife edge pointer which is moving over a scale marked with five lines, usually not wider than knife edge may be formed by flattering the end of a light aluminium tube or it may be a thin flat strip carried at the end of the pointer. The parallax errors van be avoided by knife edge pointers and scales mounted in the same plane as the tip of the pointer.

Scales: The scale is generally printed on enamelled surface of a metal plate or on paper or card board pasted to a metal plate.  The scale markings are legible and there is contrast with the back ground. In switch board and panel instruments, the scale marking is usually board for easy reading (i.e. marking division for a major scale is 5 to 10 parts) as shown in below fig.

fig 2. Scale

In portable instruments, division lines are laid out individually for each instrument whereas for panel instruments which are usually pre-printed. Portable instruments have been designed to give full-scale deflection through an angle of 120^o or even greater. The greater the angle of full scale deflection more clear scale of the instrument is obtained.