Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 414

15.6.3 C ommon v entilation m easurements
Duct diameters are measured to calculate duct areas. Inside duct diameter is the most important
measurement, but an outside measurement is often sufficient for a sheet metal duct. To measure the
duct, the tape should be thrown around the duct to obtain the duct circumference, and the number
should be divided by π (3.142) to obtain the diameter of the duct. Hood and duct dimensions can
be estimated from plans, drawings, and specifications. Measurements can be made with measuring
tape. If a duct is constructed of 2.5- or 4-foot sections, the sections can be counted (elbows and tees
should be included in the length). Hood-face velocities outside the hood or at the hood face can be
estimated with velometers, smoke tubes, and swinging-vane anemometers, all of which are portable
and reliable and require no batteries.

1. The minimum velocity that can be read by an anemometer is 50 feet per minute (fpm). The
meter should always be read in the upright position, and only the tubing supplied with the
equipment should be used.

2. Anemometers often cannot be used if the duct contains dust or mist because air must actu-
ally pass through the instrument for it to work. The instrument requires periodic cleaning
and calibration at least once per year. Hot-wire anemometers should not be used in air-
streams containing aerosols.

3. Hood-face velocity measurement involves the following steps:
• Mark off imaginary areas.
• Measure velocity at center of each area.
• Average all measured velocities.

4. Smoke is useful for measuring face velocity because it is visible. Nothing convinces
management and employees more quickly that the ventilation is not functioning properly
than to show smoke drifting away from the hood, escaping the hood, or traveling into the
worker's breathing zone, Smoke can be used to provide a rough estimate of face velocity.
Squeeze off a quick blast of smoke. Time the smoke plume's travel over a 2-ft distance.
Calculate the velocity in feet per minute. For example, if it takes 2 seconds for the smoke
to travel 2 feet, the velocity is 60 fpm.

Velocity = Distance/Time, or V = D / T (15.15)

Hood static pressure (SP h ) should be measured about four to six duct diameters downstream in
a straight section of the hood take-off duct. The measurement can be made with a Pitot tube or by a
static pressure tap into the duct sheet metal.

1. Pressure gauges come in a number of varieties, the simplest being the U-tube manometer.

2. Inclined manometers offer greater accuracy and greater sensitivity at low pressures than
U-tube manometers. However, manometers rarely can be used for velocities less than 800
fpm (i.e., velocity pressures less than 0.05 in. wg). Aneroid-type manometers use a cali-
brated bellows to measure pressures. They are easy to read and portable but require regular
calibration and maintenance.

Duct velocity measurements may be made directly (with velometers and anemometers) or indi-
rectly (with manometers and Pitot tubes) using duct velocity pressure.

1. Air flow in industrial ventilation ducts is almost always turbulent, with a small, nonmoving
boundary layer at the surface of the duct.

2. Because velocity varies with distance from the edge of the duct, a single measurement may
not be sufficient. However, if the measurement is taken in a straight length of round duct,