Measuring pressure bourdon gauge

Standard commercial iastmmentation and control devices are used ia fluorine systems. Pressure is measured usiag Bourdon-type gauges or pressure transducers. Stainless steel or Monel constmction is recommended for parts ia contact with fluoriae. Standard thermocouples are used for all fluorine temperature-measuriag equipment, such as the stainless-steel shielded type, iaserted through a threaded compression fitting welded iato the line. For high temperature service, nickel-shielded thermocouples should be used. 

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. 

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. 

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. 

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. 

The high-pressure inlet is attached to a f in. cross to provide ports for gas introduction, pressure measurement, and thermocouple placement just in front of the frit. The Bourdon gauge (0-10 bar) should be connected via a tee to a purge valve to facilitate gas changes. Before use the assembly should be tested at 10 bar for leaks. Thermal insulation such as glass wool should be wrapped around the frit assembly to keep the expansion as adiabatic as possible. 

Bourdon gauges are used to measure fluid pressures from nearly perfect vacuums to about 7000 atm. More accurate measurements of pressures below about 3 atm are provided by manometers. 

Provided that there is a change in the number of moles upon reaction and the stoichiometry of the process is known, pressure measurements may be used to determine the order of the reaction according to equation (A). Thus Letort found that the order for the decomposition of AcH was with respect to initial concentration and 2 with respect to time (see p. 2). Such direct conclusions cannot usually be drawn from pressure measurements with oxidation reactions. However, direct information may be obtained from a very neat differential system devised by Du-gleux and Frehling (Fig. 9). Vj and V2 are two RV s, of different size connected to the inside and outside of the Bourdon gauge J. Rj allows simultaneous introduction of mixtures into Vj and V2 Any fluctuation in temperature of the furnace is thus compensated for. Rapid reactions and the direct effect of promoters and inhibitors on an oxidation may be studied. This apparatus may well be useful with other systems. 

The saturated vapour pressure of a-CdSe was measured in the temperature range 975 to 1210 K. using Bourdon gauge and dew point techniques. The two methods gave identical results. The total vapour pressure was described by the expression log, (/ /bar) = 6.92 - 10020 T. No evaluations of thermodynamic properties at 298.15 K. were made. 

The total pressure was measured in systems containing a-ZnSe and l2(cr) as starting materials in the temperature range 900 to 1200 K using a Bourdon gauge. The results were analysed assuming the equilibria a-ZnSe + 12(g) Zn Cg) + /4Se2(g) and 12(g) 21(g) to be dominant. The calculation of thermodynamic quantities at 298.15 K... 

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