Manometer common fluids

Common fluids used in the manometer are mercury, water, and oil. Maximum pressure differentials achievable are higher with mercury but the precision is much less than one-tenth compared to water. Certain types of oils have a density of around 800 kg m and thus the measured A P has a 25% higher sensitivity versus water. In order to increase the sensitivity, the second leg may be inclined, as shown in Figure 4.10. 

The most common manometer fluids arc water, alcohol, and mercury. The density of water and alcohol arc quite close to each other, whereas the density of mercury is much higher. Many factors have to be considered when selecting a fluid for a manometer, including... 

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

Manometers consisting of liquid columns of, commonly, mercury or a fluid such as silicone oil, have been used extensively in the past to measure gas mixtures in, for example, experimental, static investigations of the overall kinetics of gas-phase reactions. They continue to be used in many applications, including the establishment of primary pressure standards in several countries. 

Diaphragm pressure transmitters are most commonly used when an electrical signal is desired for data acquisition. These devices are also durable and reliable but require additional calibration and maintenance compared to a fluid manometer or spring gauge. 

For low pressure differences, the inclined manometer shown in Fig. 2.36 is often used (this device is used so often to measure the draft of a furnace that its common name is a draft tube). If the scale is set to read zero length at P = P and the manometer fluid is colored water, what will the reading be at - Pg =0.1 Ibf/in What would be the reading of an ordinary manometer with vertical legs for this pressure difference ... 

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