connected to an operating unit housing circuitry, meter, batteries, and so forth, and are
about the size of a cigar box. They are quite portable and commercially available.
Problems with these instruments are related primarily to maintaining the integrity of the
probe. Heavy dust loadings or corrosive materials, as well as mechanical shock, can
damage the delicate wires in the probe. These instruments also require periodic
(4) Heated wire anemometer. Heated wire anemometers depend upon the
change in resistance of a wire with a change in temperature. The degree of
temperature change is proportional to the velocity of air passing the wire. Velocity is
read directly on a meter that is actuated by a change in voltage, resulting from the
temperature change. Generally, the advantages and limitations of these instruments
are the same as those described for heated thermocouple anemometers.
All too often the need for calibration is not applied to devices for measuring
airflow and velocity, yet, as a group (with the exception of the Pitot tube), such devices
require periodic calibration. Calibration needs to be executed before measurements are
performed. Generally, airflow-measuring instruments are based on electrical or
mechanical systems, which are sensitive to shock. In addition, use of these instruments
in corrosive or dusty atmospheres affects their reliability.
4-10. MEASURING AIR FLOW WITHIN THE SYSTEM
a. It is frequently necessary to determine precise measurements that
characterize the performance of an air flow system within the system. Instead of
measuring the velocity of air going into exhaust hoods or coming from air outlets,
measurements are made inside the ductwork leading to the point of entry of discharge.
Such measurements are usually made to determine static pressure drops associated
with hood entries, ducts, and across air cleaners, as well as velocity pressures.
b. The Pitot tube is the standard instrument for measuring the velocity of air in
ducts. The Pitot tube consists of two concentric tubes. The opening of the inner tube is
axial to the flow of air and measures total pressure, while the large tube with
circumferential openings measures static pressure. The difference between the total
pressure and the static pressure is the velocity pressure. Figure 4-6 shows the
relationships between the various pressures in an exhausting system, while Figure 4-7
illustrates the construction of a standard Pitot tube. The major disadvantage of the Pitot
tube is that it is not direct reading. The Pitot tube measures velocity pressure in inches
NOTE: An inch of water is a unit of pressure equal to the pressure exerted by a column
of liquid one inch high at a standard temperature.