Mechanical Gauges

Mechanical gauges are those pressure measuring devices, which embody an elastic element, which deflects under the action of the applied pressure and this movement mechanically magnified, operates a pointer moving against a graduated circumferential scale. Generally these gauges are used for measuring high pressures and where high precision is not required. Some of the mechanical pressure gauges which are commonly used are as noted below.

i) Bourdon Tube Pressure Gauge

It is the most common type of pressure gage which was invented by E. Bourdon (1808–84). The pressure responsive element in this gauge is a tube of steel or bronze which is of elliptic cross-section and is curved into a circular arc. The tube is closed at its outer end and this end of the tube is free to move. The other end of the tube, through which the fluid enters, is rigidly fixed to the frame as shown in Fig. 1. When the gauge is connected to the gauge point, under pressure enters the tube. Due to increase in internal pressure, the elliptical cross-section of the tube tends to become circular, thus causing the tube to straighten out slightly. The small outward movement of the free end of the tube is transmitted, through a link, quadrant and pinion, to a pointer which by moving clockwise on the graduated circular dial indicates the pressure intensity of the fluid.

The dial of the gage is so calibrated that it reads zero when the pressure inside the tube equals the local atmospheric pressure and the elastic deformation of the tube causes the pointer to be displaced on the dial in proportion to the pressure intensity of the fluid. By using tubes of appropriate stiffness, gauges for wide range of pressures may be made. Further by suitably modifying the graduations of the dial and adjusting the pointer Bourdon tube vacuum gauges can also be made.

When a vacuum gauge is connected to a partial vacuum, the tube tends to close, thereby moving the pointer in anti-clockwise direction, indicating the negative or vacuum pressure. The gauge dials are usually calibrated to read Newton per square metre (N/m²),or pascal (Pa), or kilogram (f) per square centimetre [kg(f)/cm²]. However other units of pressure, such as metres of water or centimetres of mercury, are also frequently used.

Fig. 1 Bourdon Tube Pressure Gauge

ii) Diaphragm Pressure Gauge

The pressure responsive element in this gage is an elastic steel corrugated diaphragm. The elastic deformation of the diaphragm under pressure is transmitted to a pointer by a similar arrangement as in the case of Bourdon tube pressure gauge. However, this gauge is used to measure relatively low pressure intensities. The Aneroid barometer operates on a similar principle.

Fig. 2 Diaphragm Pressure Gauge

iii) Bellows Pressure Gauge

In this gauge, the pressure responsive element is made up of a thin metallic tube having deep circumferential corrugations. In response to the pressure changes this elastic element expands or contracts, thereby moving the pointer on a graduated circular dial.

Fig.3 Bellows Pressure Gauge

iv) Dead -Weight Pressure Gauge

A simple form of a dead-weight pressure gauge consists of a plunger of diameter d, which can slide within a vertical cylinder, as shown in Fig. 4. The fluid under pressure, entering the cylinder, exerts a force on the plunger, which is balanced by the weights loaded on the top of the plunger. If the weight required to balance the fluid under pressure is W, then the pressure intensity ‘p’ of the fluid may be determined as, 

The only error that may be involved is due to frictional resistance offered to motion of the plunger in the cylinder. But this error can be avoided if the plunger is carefully ground, so as to fit with the least permissible clearance in the cylinder. Moreover, the whole mass can be rotated by hand before final readings are taken.

Fig. 4 Dead-Weight Pressure Gauge

Dead-weight gauges are generally not used so much to measure the pressure intensity at a particular point as to serve as standards of comparison. Hence as shown in Fig. 4, a pressure gauge which is to be checked or calibrated is set in parallel with the dead-weight gauge. Oil under pressure is pumped into the gauges, thereby lifting the plunger and balancing it against the oil pressure by loading it with known weights. The pressure intensity of the oil being thus known, the attached pressure gauge can either be tested for its accuracy or it can be calibrated.

A dead-weight gauge which can be used for measuring pressure at a point with more convenience. In this gauge a lever, same as in some of the weighing machines, is provided to magnify the pull of the weights. The load required to balance the force due to fluid pressure is first roughly adjusted by hanging weights from the end of the main beam. Then a smaller jockey weight is slide along to give precise balance. In more precise type of gauge the sliding motion may be contrived automatically by an electric motor.

The following points should be kept in view while making connections for the various pressure measuring devices.

• At the gauge point the hole should be drilled normal to the surface and it should flush with the inner surface.
• The diameters of the holes at the gauge points should be about 3 to 6 mm.
• The holes should not disturb the internal surface and no burrs or irregularities must be left.
• There should be no air pockets left over in the connecting tubes, which should be completely filled with the liquid. The presence of air bubbles can easily be detected if the connecting tubes are made of polythene or similar transparent material.