Digital Zeus™ HVAC Tool & Instruments Journal

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A manometer is read by adding the difference between both sides of the water column. In other words, if the watercolumn moves down 1” on the pressure side and up 1” on the other side, that would equal 2” of water. The reading of a manometer that equals 2” of water column is .07226 PSI.
One inch of water column equals .03613 pounds per square inch (PSI). Twelve times .03613 PSI equals .43356 PSI, so one foot of water equals .43356 PSI or almost a half of a pound.
On the manometer that would read as 6” on one side and 6” on the other.
The manometer should be mounted in a vertical position and tied off or otherwise fastened to prevent false readings andthe possibility of spilling the water solution. The water solution can be colored with dye or a small amount of antifreezeto make it easier to see. Only use enough to tint the water as antifreeze has a different specific gravity and could affect the readings by as much as 12%.

Air Velocity Measurement

Sensing Static Pressure

In air conditioning, heating and ventilating work, it is helpful to understand the techniques used to determine air velocity. In this field, air velocity (distance traveled per unit of time) is usually expressed in feet per minute (FPM). By multiplying air velocity by the cross section area of a duct, you can determine the air volume flowing past a point in the duct per unit of time. Volume flow is usually measured in cubic feet per minute (CFM).
Velocity or volume measurements can often be used with engineering handbook or design information to reveal proper or improper performance of an airflow system. The same principles used to determine velocity are also valuable in working with pneumatic conveying, flue gas low and process gas systems. However, in these fields the common units of velocity and volume are sometimes different from those used in air conditioning work.

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Measurement of Pressure with the Manometer

Pressure is defined as a force per unit area - and the most accurate 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 so 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 about 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 one leg, the liquid is forced down in that leg and up in the other. The difference in height, “h,” which is the sum of the readings above and below zero, indicates the pressure.

Fig. 2-3. When a vacuum is applied 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 above and below zero, indicates the amount of vacuum.

Instruments employing this principle are called manometers. The simplest form is the basic and well-known U-tube manometer. (Fig. 2-1). This device indicates the difference between two pressures (differential pressure), or between a single pressure and atmosphere (gage pressure), when one side is open to atmosphere. If a U-tube is filled to the half way point with water and air pressure is exerted on one of the columns, the fluid will be displaced. Thus one leg of water column will rise and the other falls. The difference in height “h” which is the sum of the readings above and below the half way point, indicates the pressure 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 pressure has been imposed on the liquid in the well causing the level to go down very slightly. Liquid level in indicating tube has risen substantially. Reading is taken directly from scale at liquid level in indicating tube. The scale has been compensated for the drop in level in the well.

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