Gauge, pressure, manometers differential

Pressure Manometer Differential pressure cell (mechanical or semi-conductor) Bourdon gauge... 

Gas pressure is measured using gauges and pressure transducers. Differential pressures are measured using calibrated manometers in conjunction with differential pressure transducers. 

Under most conditions when measuring the pressure drop of a gas, its density may be ignored, as pm > /on- When the second leg is open to atmosphere, the manometer reading represents the gauge pressure—the absolute pressure will equal the pressure differential plus the barometric pressure. 

Vacuum gauge, capacitance manometer gauge A vacuum gauge that uses the deflection of a diaphragm, as measured by the changing capacitance (distance) between surfaces, as an indicator of the pressure differential across the diaphragm, the pressure on one side being a known value. 

Pressure reducing valves should be of steel constmction, designed for minimum and maximum operation conditions. Pressure gauges should be of ak-kon constmction. Pressure rehef valves should be of the spring-loaded type. Rupture disks may be used only as auxkiary equipment. Differential pressure measurements using mercury manometers should be avoided in ammonia service. 

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. 

Chattock gauge. A differential manometer that uses the pressure difference of two columns of liquid of nearly equal density. 

In order to define the gas volumes more precisely, and overcome the problems associated with the mercury in contact with the gas and the limited temperatme range because gas and mercury were at the same temperatme, expansion methods were later devised where the gas was allowed to expand from one vessel into another, previously evacuated, vessel (and in some cases into a series of other vessels). The volumes had previously been accurately determined by weighing with water or mercury. The gas was separated from the manometer, which can be at room temperature, by a differential pressure gauge. 

Pressure can be measured by means of manometers which show the pressure in terms of the different levels of a liquid in a U-tube, by mechanical pressure gauges which record the differential effect of pressure forces on the inside and outside surfaces of a coiled tube, and electronic devices which measure the change of electrical characteristic of an element with pressure. 

Corrective Actions If the manometers or pressure gauges demonstrate a reduction in pressure differential below the required level, work should cease and the reason for the change investigated and appropriate changes made. The air flow patterns should be retested before work begins again. 

Schafer and co-workers have described a variant of the gas-thermo-meter principle, in which one glass bulb A (Figure 2) is filled with a known amount of nitrogen, the other B with a known amount of the gas under investigation. A differential pressure gauge C measures the difference in pressure between the two bulbs. The volumes of bulbs A and B, which are almost equal, are accurately measured. If the pressures in the two bulbs are equal at temperature Ti, then at temperature T, the pressures will, in general, differ. The pressures can be equalized by alteration of the mercury level in the calibrated tube D of the manometer, and from this change of volume can be found a value for the second virial coefficient of the gas at temperature Tg. 

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