Gauges, pressure-measuring, types manometers

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. 

In the experiments, pressure was measured with pressure gauge of the plunger type which transmitted pressure to calibrated manometers of high precision. During the first... 

Pressure-measuring devices may be divided into two groups those which measure the pressure directly and can be calibrated without reference to another intrument and those which involve some other physical measurement which will be related to the pressure. Of the former type, the barometer, manometer, and McLeod gauge are the most common. 

Several terms for pressure measuring devices are used interchangeably including transmitters, transducers, gauges, sensors, and manometers. More precisely, a gauge is a self-contained device that converts a force from the process to a mechanical motion of needle or other type of pointer. A manometer is a term reserved for an instrument that measures the hydrostatic head of a liquid and generally operates near atmospheric pressure. A transducer or transmitter combines the sensor with a power supply and a converter— generally mechanical-to-electrical or mechanical-to-pneumatic. The sensor... 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Example 2.1. A manometer of the type shown in Fig. 2.4 is used to measure the pressure drop across an orifice (see Fig. 8.19). Liquid A is mercury (density 13,590 kg/m ) and fluid B, flowing through the orifice and filling the manometer leads, is brine (density 1260 kg/m ). When the pressures at the taps are equal, the level of the mercury in the manometer is 0.9 m below the orifice taps. Under operating conditions, the gauge pressuret at the upstream tap is 0.14 bar the pressure at the downstream tap is 250 mm Hg below atmospheric. What is the reading of the manometer in millimeters ... 

This device, unlike the manometer, requires a precise measurement of its dimensions or a calibration. Spring-type pressure gauges usually are calibrated by comparing their readings with those of manometers like the one shown in Fig. 2.16, or other equivalent devices. 

Since the cell temperature is measured with a copper-constantan thermocouple, the absolute accuracy is dependent on the accuracy of the thermocouple calibration. This calibration was done in place with reference to the triple point and boiling point of nitrogen and the triple point of argon. The resultant accuracy was estimated to be 0.1°K, while resolution of the potentiometer (L N type K-3) was 0.03 K. In regard to pressure, the accuracy was 0.025 or 0.1 psi, depending on whether a mercury manometer or Bourdon-type gauge was being used. 

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