Gauges Bourdon

For measuring pressures of corrosive fluids, slurries, and similar process fluids which may foul Bourdon tubes, a cfiemical gauge, consisting of a Bourdon gauge equipped with an appropriate flexible diaphragm to seal off the process fluid, may be used. The combined volume of the tube and the connection between the diaphragm and the tube is filled with an inert liquid. These gauges are availabTe commercially. 

Bourdon gauge A pressure gauge in which the sensing element is constructed from a coiled flattened tube closed at one end. 

It may be noted that the pressure measuring devices (a) to (e) all measure a pressure difference AP(— Pj — P ). In the case of the Bourdon gauge (0, the pressure indicated is the difference between that communicated by the system to the tube and the external (ambient) pressure, and this is usually referred to as the gauge pressure. It is then necessary to add on the ambient pressure in order to obtain the (absolute) pressure. Even the mercury barometer measures, not atmospheric pressure, but the difference between atmospheric pressure and the vapour pressure of mercury which, of course, is negligible. Gauge pressures are not. however, used in the SI System of units. 

Both types of Bourdon gauge are most suitable for use with corrosive gases and both can be used most effectively as null-point instruments. Several types of mechanical gauge are available commercially which use electrical capacitance or induction to magnify the mechanical movement of a membrane. Such gauges are easily operated in a differential mode and can be used for measuring pressure differences down to ca. 10 Torr. 

All over the world instruments of the "Bourdon gauge type" are used, and many manufacturers make such devices. Some of the best instruments of the Bourdon-type are from [53] with standard ranges up to approx. 7000 bar with a tolerance of only 0.01 % of span, provided with up to 16 inch diameter dials. 

Pressure is measured extensively in the chemical processing industries and a wide variety of pressure measuring methods has been developed. Some of these have already been discussed in Volume 1, Section 6.2.2, viz. the manometer (which is an example of a gravity-balance type of meter), the Bourdon gauge (an example of an elastic transducer) and mention is made of the common first element in most pressure signal transmission systems—the differential pressure (DP) cell (Volume 1, Section 6.2.3). The latter also frequently forms part of a pneumatic transmission system and further discussion of this can be found in Section 6.3.4. 

G. Bourdon Gauges. As an alternative to mercury manometers there is a variety of gauges based on mechanical or electrical pressure transducers. This section presents a description of purely mechanical gauges which still find use in this electronic age.4 The metal Bourdon gauge (Fig. 7.5) is fashioned around a semicircular thin-walled metal tube with mechanical linkage to a pointer. Fused-quartz spiral gauges are also available. In this case, a thin spiral is sensitive to a pressure differential, and the deflection is balanced with air pressure in the surrounding envelope. The air pressure is then measured with a manometer. 

Fig. 7.5. A metal Bourdon gauge. The thin Bourdon tube is sensitive to the pressure differential between the surrounding atmosphere and the gas contained within the tube. The tube deflects an indicating needle which allows a direct pressure determination from the calibrated scale.

Bourdon gauges are used on gas cylinders and are also considered a type of aneroid gauge. These devices have a coiled tube (shown in Figure 3.5) and are used to measure the pressure difference between the pressure exerted by the gas in a cylinder and the atmospheric pressure. The coiled tube is mechanically coupled to a pointer (shown in red). As a gas at a pressure above atmospheric pressure enters the coiled tube, it causes it to slightly uncoil, kind of like those New Year s Eve paper noisemakers. This causes the pointer to move over a numerical scale, thereby indicating the gauge pressure in the tank. 

Suppose you have a cylinder of oxygen gas, and the Bourdon gauge indicates a pressure of 0 psi. If the atmospheric pressure is 14 psi, what is the pressure... 

Which of these has an operating principle that most resembles a Bourdon gauge ... 

A Bourdon gauge contains a curved metal tube connected to a pointer. Increasing pressure in the tube causes it to unwind slightly, and this causes the pointer to move around a numerical scale. 

The absolute pressure of the gas in a cylinder represents the force per unit area exerted by the gas as the gas molecules collide with the inner surface of the cylinder. Gauge pressure is registered by a Bourdon gauge on a gas cylinder, and represents the difference in pressure between the pressure exerted by the gas in the cylinder and the atmospheric pressure. 

---