Construction of pressure gauges
Compressed air systems normally operate within a pressure range from 1 to 10 bar. Where indication of the static pressure is required, a Bourdon tube gauge, named after the French inventor Eug~.ne Bourdon, is the usual choice, a typical construction being shown in Figure 1. They cannot be used for measuring other than slowly changing pressures; for pulsating pressures, a pressure transducer with electronic instrumentation must be used.
The basic element is a thin walled tube, slightly flattened in section and bent into a ‘C’ shape. Under pressure the flattened section tends to become circular and the tube straightens. One end of the tube is fixed and the free end is sealed and attached to a mechanical linkage. Application of internal pressure to the tube causes it to straighten by an amount directly proportional to the internal pressure. The resulting movement of the free end is transmitted via a mechanical linkage to rotate a pointer associated with a graduated dial. This mechanical movement involves multiplication so that about 300 ° of pointer rotation is achieved over the working pressure range of the gauge.
The accuracy achieved with a Bourdon tube gauge is better than +2.5% of the full scale deflection. Greatest accuracy is achieved at the upper end of the scale. Thus selection of a gauge of this kind should be based on a pressure range where the normal reading point will be near the upper end of the scale; bear in mind that the actual pressure is likely to exceed the nominal system pressure through the operation of the compressor. The gauge should always be chosen so that its maximum pressure is in excess of any relief valve in the system. Consistency of performance is related to the fatigue characteristics of the tube material and some drift with time is inevitable. Pressure gauges need regular checking and re-calibration where necessary using a dead weight calibration method.
Bourdon tube gauges are susceptible to mechanical shock and pressure surges; they should be protected against both of these.
A simple method of protection against pressure surges is to fit a snubber or restriction
in the feed line. In severe cases, mechanical throttling or a shut off device responding directly to any sudden increase in pressure may be employed.
Another source of trouble is blocking of the tube by contaminants in the air. This can be overcome by incorporating a flexible diaphragm in the line to the gauge. The tube can then be filled with a clean fluid which is separated from the air by the diaphragm (Figure 2). The fluid used must be stable at the operating temperature. Commonly used fluids are glycerine and ethylene glycol. The principle can be extended by using the diaphragm unit as a transmitter, separated from the gauge by a length of tubing. The advantage of this is that the gauge can be some distance from the transmitter without affecting the readings.
Modifications to the simple form of Figure 1 include the use of helical tubes; some examples are shown in Figure 3.
The spiral tube is similar to the C-tube, except that its radius of curvature changes along its length, creating a greater length of tube for a given overall diameter. Because of a greater active length, the arc length of the stroke is greater for a given pressure change. A straight line pick-off from this larger angle of sweep results in non-linearity in output displacement. This may be an advantage in certain circumstances and can be compensated for in the pressure scale.
The helical tube permits a longer tube length for a given radius of curvature. The tube
may have two to six turns. The movement of the tip is greater than for the simple C-tube and is more nearly circular. The increase in length of the tube is negligible. The travel of the tip lies in a plane perpendicular to the centre line of the helix, and is proportional to the active length of the tube. The tip travel is much the same as a C-tube of the same length.
The twisted tube maintains a straight centre line along the length of the tube but is twisted about the centre line. Internal pressure causes the tube to unwind about the centre line, the angle of rotation being proportional to the applied pressure. The change in length is negligible. Different degrees of twist alter the angular rotation for any given length.
For safety pattern gauges, a solid baffle, forming part of the can structure is interposed between the element and the dial.
Refer to BS 1780 for information about the accuracy, installation, test methods and safety requirements of Bourdon tube gauges.