How to Read Pressure From a Manometer
Pressure is defined equally a force per unit expanse - and the most authentic way to measure low air pressure is to balance a column of liquid of known weight against it and measure the height of the liquid column then balanced. The units of measure commonly used are inches of mercury (in. Hg), using mercury as the fluid and inches of water (in. w.c.), using water or oil as the fluid.
Fig. 2-1. In its simplest form the manometer is a U-tube nearly one-half filled with liquid. With both ends of the tube open, the liquid is at the same height in each leg.
Fig. 2-2. When positive pressure is applied to i leg, the liquid is forced down in that leg and upwardly in the other. The departure in height, "h," which is the sum of the readings above and beneath nix, indicates the pressure.
Fig. 2-three. When a vacuum is practical to one leg, the liquid rises in that leg and falls in the other. The difference in height, "h," which is the sum of the readings in a higher place and below zero, indicates the amount of vacuum.
Instruments employing this principle are called manometers. The simplest grade is the basic and well-known U-tube manometer. (Fig. ii-one). This device indicates the difference between ii pressures (differential force per unit area), or between a single pressure and atmosphere (gage pressure), when one side is open to temper. If a U-tube is filled to the half style betoken with water and air pressure is exerted on ane of the columns, the fluid will exist displaced. Thus one leg of water column will rise and the other falls. The divergence in height "h" which is the sum of the readings above and below the half way point, indicates the force per unit area in inches of water column.
Fig. 2-4. At left, equal pressure is imposed on the fluid in the well and in the indicating tube. Reading is zero. At the right, a positive force per unit area has been imposed on the liquid in the well causing the level to go downwards very slightly. Liquid level in indicating tube has risen substantially. Reading is taken direct from calibration at liquid level in indicating tube. The calibration has been compensated for the driblet in level in the well.
The U-tube manometer is a primary standard because the difference in pinnacle betwixt the 2 columns is e'er a truthful indication of the pressure level regardless of variations in the internal diameter of the tubing. This principle makes even the Dwyer Slack Tube® roll-upward manometer as accurate as a laboratory musical instrument. This provides a existent convenience to the person who might otherwise take to board an plane carrying a threescore" long rigid glass U-tube manometer.
VARIATIONS IN MANOMETER Design
To overcome the U-tube requirement of readings at two different places, the well-type manometer was adult. See Fig. ii-iv. The reservoir (well) may be fabricated large plenty so that the change of level in the reservoir is negligible, or the scale may exist compensated for the change in reservoir liquid level. For purposes of a more practical instrument the Dwyer well-type manometer uses a precision bored well that requires approximately a 10% calibration correction for well drop outcome, thus avoiding an overly large and bulky reservoir.
Fig. 3-i. At left, equal pressure is imposed on the liquid in the well and the indicating tube. Reading is goose egg. At the right, a positive force per unit area has been imposed on the liquid in the indicating tube pushing it down to a point on the scale equal to the pressure. Liquid level in the well rises proportionately. Inclining the indicating tube has opened up the scale to permit more precise reading of the force per unit area.
To better and aggrandize readability, sure Dwyer U-type and well-type manometers are available with a .826 sp. gr. red oil indicating fluid, and scales compensated to read pressure direct in inches of h2o. To farther increment readability and sensitivity the well-type manometer indicating tube is inclined, as in Fig. three-one, to cause a greater linear movement along the tube for a given pressure difference. The inclined manometer is frequently chosen a Draft Gage considering information technology is widely used for determining the over-fired draft in boiler uptakes and flues.
For an inclined manometer to exist a main device, the inclined tube must be straight and uniform. Dwyer'southward precision machined solid plastic construction has been applied to a bones line of rugged manometers, inclined and inclined-vertical, which are industry accustomed equally main instruments.
Fig. iii-2. At left, with equal pressure on liquid in well and indicating tube, reading is zero. When positive pressure is imposed on liquid in indicating tube, liquid level is depressed in tube and rises slightly in well. Reading is straight since scale is compensated for change of level in well.
The combination of an inclined and a vertical manometer is very useful in air movement decision. See Fig. 3-2. For air velocity measurement, an inclined scale, mostly up to 1" w.c. is used (1" w.c. velocity pressure = 4000 fpm). In the Dwyer Durablock® inclined-vertical instrument, this scale is combined with a vertical section allowing readings of high pressures, usually one" due west.c. to 5 to ten" w.c., to exist taken. The vertical section is used primarily for determining static pressure above the range of the inclined section.
FACTORS AFFECTING MANOMETER Operation AND USAGE
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