By defination a pressure or vacuum gauge usually consists of a closed coiled tube (called a Bourdon tube) connected to the chamber or pipe in which pressure is to be sensed. As the pressure increases the tube will tend to uncoil, while a reduced pressure will cause the tube to coil more tightly. This motion is transferred through a link to a gear train connected to an indicating needle. The needle is presented in front of a card face inscribed with the pressure indications associated with particular needle de.

Elastic Pressure Transducers

This type of pressure transducers use elastic primary sensing elements such as the Bourdon tube, bellows and diaphragm.

the C-type Bourdon Tube Pressure Gauge

The Bourdon tube is the most frequently used pressure gauge because of its simplicity and rugged construction. It covers ranges from 0-15 psig to 0-100,000 psig, as well as vacua from 0 to 30 inches of mercury.

Adjustments Basically there are two types of adjustments of the Bourdon tube:

(a) Multiplication Adjustment Because of compund stresses developed in the Bourdon tube, actual travel is non-linear in nature. However, for a small travel of the tip this can be considered to be linear and parallel to the axis of the link. The small linear tip movement is matched with a rotational pointer movement. This is known as 'multiplication' and can be adjusted by adjusting the length of the lever. A shorter lever gives larger rotation for the same amount of tip travel.

(b) Angularity When the approximately linear motion of the tip is converted to a circular motion with the link lever and pinion attachment, a one to one correspondence between them may not occur and a distortion results. This is known as "angularity". Angularity can be minimized by adjusting the length of the link.

Advantages Following are the advantages of Bourdon tubes:

( i ) Their cost is low.

( ii ) They have simple construction.

( iii ) They have been time-tested in applications.

( iv ) These tubes are available in a wide variety of ranges, including vey high ranges.

( v ) They are adaptable to transducer designs for electronic instruments.

( vi ) They allow high accuracy, especially in relation to cost.

Disadvantages Following are the disadvantages of Bourdon tubes:

( i ) They have low spring gradient (i.e. below 50 psig).

( ii ) They are susceptible to shock and vibration.

( iii ) they are susceptible to hysteresis.