f. There are fewer limitations for Pitot tubes than for other air velocity measuring
devices. Although they can be used in corrosive or variable temperature conditions, the
impact and static openings can become clogged with particulate matter. Also, as with
other instruments, corrections will have to be made if the temperature is plus or minus
30F from standard; if the altitude is greater than 10,000 feet, or if the moisture content
is 0.02 pound per pound or higher. They cannot be used to measure low velocities
(less than 600-fpm) and require an inclined manometer that must be level and free of
vibration. They are not applicable for use in small diameter ducts (less than 3 inches) or
in orifice type openings.
g. In addition to inclined manometer, there are other instruments available for
pressure measurements within an exhaust system.
(1) Aneroid gauges. The most common and best known of the aneroid
gauges is the Magnehelic gauge. Aneroid gauges can be used for total, static, and, in
conjunction with a Pitot tube, velocity pressure measurements. They are small,
extremely portable, and not as sensitive to vibration and leveling as liquid filled
manometers. Since the inches of water pressure is a function of the location of an
indicating needle on a dial, they are extremely easy to read. The principal limitations
are accuracy and calibration. Accuracy is usually below plus or minus two is a need to
calibrate these devices periodically.
(2) Manometers. Manometers range from the simple U-tube to the inclined
manometers mentioned earlier. A range of sizes and varieties of U-tube manometers is
available and they may be filled with a variety of media ranging from alcohol to mercury.
Readings can be converted to inches of water simply by correcting for differences in
density (that is, 1 inch of mercury is equal to 13.61 inches of water). When extreme
accuracy is not essential or in high-pressure systems, U-tube manometers will suffice.
However, for greater accuracy and in low-pressure systems, inclined manometers are
required. Figure 4-8 illustrates an inclined manometer used with a Pitot tube.
h. It is frequently necessary to make static pressure measurements within a
ventilation system. Instruments used in making static pressure measurements include
the static leg of the Pitot tube as well as any pressure-measuring device connected to a
hole in the side of a duct. U-tube manometers and Magnehelic gauges are quite
acceptable. Whereas the exact location of the hole is not extremely critical, the type of
hole is. Generally, the holes should not be located in points where there is some basis
for turbulency or nonlinear flow such as the heel of an elbow. Holes should be flush
with the inside of the duct, with no projections or burrs. Thus, holes should be drilled
and not punched. The location of holes 90 degrees apart will allow for the averaging of
multiple readings to provide an improved estimate of static pressure. Taps can vary in
complexity from a simple soft rubber hose held tightly against a 1/16th-inch hole, to
soldered petcocks for use in high-pressure systems. Figure 4-9 illustrates the traverse
of a round and a rectangular duct.