Manometer well type

Several other variations of the fluid inanomerer provide a higher sensitivity, The inclined well-type manometer (Fig. 12.17) has a large-cross-section container for the manometer fluid connected ro an inclined rube with a scale. The pressure difference becomes... 

Figure 6.2. (a) The simple manometer (b) The well-type manometer... 

The question is often asked. How often should calibration be carried out Is it sufficient to do it once, or should it be repeated The answer to this question depends on the instrument type. A very simple instrument that is robust and stable may require calibrating only once during its lifetime. Some fundamental meters do not need calibration at all. A Pitot-static tube or a liquid U-tube manometer are examples of such simple instruments. On the other hand, complicated instruments with many components or sensitive components may need calibration at short intervals. Also fouling and wearing are reasons not only for maintenance but also calibration. Thus the proper calibration interval depends on the instrument itself and its use. The manufacturers recommendations as well as past experience are often the only guidelines. 

The sensitivity of the well manometer can be adjusted by changing the angle or and can be some 30 times that of the U-tube. This type is also often called a micromanometer due to its ability to measure very small pressure differences. Several other types of micromanometers and fluid manometers are also available. 

The most frequently applied mechanical manometers in ventilation applications are fluid manometers, bur the following types are also used. The Bourdon tube is a small-voiume tube with an elliptic cross-section bent to the shape of a circular arc, the C-type. One end is open to the applied pressure while the other end is closed. The pressure inside the tube causes an elastic defonnation ot the tube and displaces the closed end, which is then converted, by means of a linkage mechanism, into the movement of a pointer. The Bourdon tube may be of a spiral or helical design as well. 

Electrical manometers have developed during the last 30 years. Modern electrical manometers are well suited for ventilation applications, both in the laboratory and in the field. The advantage of this type of instrument is fhat they are sensitive enough to measure small pressure differences with electrical output, enabling monitoring. A convenient feature, especially in the field is that the instrument is hand-held and there is no need for leveling on a bench, as for fluid manometers. The conversion of the pressure difference into an electrical signal can be based on several different phenomena. 

Equation (51) can equally well be applied to a U-tube type viscometer, in which a capillary is connected to essentially a manometer (see, for example, Maron and Belner, 1955). The only modification is that the pressure head, Ah, is calculated from the difference in the decreasing meniscu of the capillary arm and that arising in the manometer arm. 

Manometers of this type may ordinarily be read to 0.25 mm if a well-designed scale is used and are satisfactory for pressures greater than 10 mm. For lower pressures or higher accuracy, the sensitivity may be increased by one of two methods. First, the manometer may be tilted so that the mercury will rise a considerable distance along the length of the tube per unit of vertical rise. Second, the manometer ... 

The front panel of the casing of the instrument contains the adjustment and control units (control switches for the UV lamps (254 nm and 366 nm), timing unit, and temperature), with a keyboard of the sensor type, as well as the power switch for the instrument, and the switch for the nitrogen overpressure with a manometer (0-2.5 bar). The rotation speed may be varied between 80 and 2000 rpm in steps of 10 and 100 rpm. 

In the well- or cistern-type manometer one leg has a cross section much larger than that of the other leg. The zero adjustment in the cistern is usually made manually with an adjusting screw. Then the pressure is found by the following formula... 

Special types of manometers sometimes used for more accimate measurement include the inclined manometer, the micromanometer, and U tubes installed with hook gauges, as well as many special types of manometers for vacuum measurement, which will be mentioned later. 

In many cases a Fortin-type barometer (Fig. M-13) is suitable. In this case the merexuy in the well is exposed to the atmosphere with the other end evacuated and sealed. All barometer readings shotdd be corrected for temperature, local gravity, and capiUary effect. Atmospheric pressure can also be measured by an aneroid barometer, which is a special type of elastic gauge. It is sometimes used in place of a manometer-type barometer because of the ease of transportation. 

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