Employers and employees today are concerned about the quality and safety of their work environment. In the office, the concern is for a proper supply of comfortable air, free of noxious or dangerous contaminants. In the plant the same concerns apply, plus the concern for the removal of toxic fumes. In some cases, like microprocessor fabrication, product quality itself depends on clean ambient air.

Modern hvac systems are designed to handle these demands. Older systems are being retrofitted to bring them up to current standards. Air movement in these new and retrofitted systems depends on an increased number of sensors coupled with computerized control.

All of this places more emphasis on careful system startup and, above all, a rigorous program of hvac system performance checks and preventive maintenance. Measurement of air movement in the system is critical to ascertaining proper operation.

Air Movement

The underlying measurement of air velocity and volume in an hvac system is the differential pressure output from a Pitot tube or, in some cases, the difference between the static duct pressure and atmospheric pressure. The following background information will be helpful.

Air velocity (distance traveled per unit of time) is usually expressed in feet per minute (fpm) or meters per minute. Multiplying air velocity by a cross-sectional area of duct yields the air volume flowing past a plane in the duct per unit of time. Volume flow is usually measured in cubic ft per minute (cfm) or cubic meters per minute.

Fans or blowers are used to move air in hvac systems. They impart motion and pressure to the air with either a screw propeller or paddle wheel action. When force or pressure from fan blades causes the air to move, it acquires a force or pressure component in its direction of motion due to its weight and inertia. This force is called velocity pressure and is measured in inches of water column (in. wc) or Pascal (Pa).

In operating duct systems, a second pressure is always present. This static pressure is independent of air velocity or movement. It acts equally in all directions, and in air conditioning it is also measured by in. wc or Pa.

Total pressure is the combination of static and velocity pressure, and is expressed in the same units.

A Pitot tube (Figure 1) is used to sense velocity pressure. It consists of an impact tube which receives total pressure input (Pt). Fastened concentrically outside is a second tube of slightly larger diameter that receives static pressure input (Ps) from radial sensing holes. Think of a tube within a tube; the air space between the inner and outer tubes permits the transfer of pressure from these two sensing holes to a manometer through connecting tubing. When the total pressure tube is connected to the high-pressure side of the manometer, velocity pressure is indicated directly (Figure 1).

Manometers for use with a Pitot tube are commonly available in a choice of two scale types. Some are specifically for air measurement and are calibrated directly in fpm (or meters per minute). They are correct for standard air conditions (i.e., air density of 0.075 lb/cu ft, which corresponds to dry air at 70°F and barometric pressure of 29.92 in. Hg).

To correct the velocity reading for other-than-standard air conditions requires the use of mathematical calculation, reference to charts and/or curves, or a simple slide rule.

Static Pressure Can Reveal Defects

Static pressure readings may provide the reason why air velocity is high or low. For this reason, it is an excellent diagnostic tool.

It's similar to the first diagnostic step taken at your doctor's office: the blood pressure check. When static pressure is high, air velocity usually is low. To sense static pressure, the simplest method requires a sharp, burr-free opening through the duct wall that has a tubing connection of some sort on the outside. The axis of the tap or opening must be perpendicular to the direction of the airflow. This type of tap or sensor is used where airflow is relatively slow, smooth, and without turbulence.

If turbulence exists, then impingement, aspiration, or unequal distribution of moving air at the opening can reduce the accuracy of readings significantly. In such cases, the use of a static pressure tip is recommended (Figure 2).

Pitot tubes may be permanently installed at critical duct locations, or portable tubes can be inserted through an access hole in the duct, which is plugged when it's not in use. Static pressure measurement points can be permanent or portable. The hvac system operating manual should indicate where routine measurements are to be made, whether the measurement is velocity or static pressure, and what the normal readings should be.

The basic instrument for accurate measurement of low differential pressure is the manometer. It is simply a device that allows balancing of a column of liquid of known weight against the air pressure to be measured (Figures 3, 4, and 5). While the simple U-tube manometer is, in fact, a primary standard, the small amount of column movement at the low pressure involved in measuring air makes it somewhat difficult to use in the field. With careful design, manufacture, and calibration, the inclined, well-type manometer is much more suitable; the indicating column causes a greater linear movement along the tube for a given pressure difference.

The result is an instrument that is also essentially a primary standard, but can provide the kind of resolution required for reading low differential pressure. This type of manometer is also available with a second scale calibrated directly in fpm for easy air velocity measurement.

There are several other types of manometers of interest. For instance, there is a high-range, 16-in.-wc U-tube manometer that also can be inclined and provides high-resolution readings of -0.2 to 2.5 in. wc. Another option is an inclined column and 0.10 in. wc in the vertical column. This type of manometer is often used for more precise field work, particularly in hvac system startups or for diagnosis of system problems.

There are also high-precision manometers with 0.00025-in.-wc resolution, which is necessary for applications such as the laboratory calibration of other low-pressure instruments; or, in the field, for determining pressure differences in various areas of a large room to determine overall air movement patterns. Meanwhile, the Magnehelic® gauge (offered by Dwyer Instruments, Michigan City, IN) is considered an alternative to a manometer for measuring low differential pressure. It is diaphragm-operated and, with no liquids involved, is designed to avoid the need for leveling and zeroing prior to each use.

The gauge is often specified by hvac system designers as a permanent static or velocity pressure indicator at critical points in the system. (Figure 6 illustrates the use of both a Magnehelic gauge and an inclined manometer in a permanent duct airflow measurement application.)

A more recent innovation for measuring low differential pressure, as well as positive and negative pressures, is the handheld electronic manometer with digital readout of pressure. Various makes and models offer switch selection of several ranges, different engineering units (in. wc, in. Hg, mm wc, Pa, etc.), and direct reading of air velocity as well as differential pressure.

More advanced models offer nine English/metric units, storage of 20 readings, LCD with switchable backlight, audible and visual alarms, low-battery alarms, etc. The digital manometer is a very popular field service tool despite its slightly higher price, because of its versatility and ease of use.

Air Filters

Periodic checks and troubleshooting of various other hvac system air movement controls can be accomplished with the foregoing variety of differential pressure measurement instruments.

Depending on the demands of the work environment and on the quality of return and makeup air, an hvac system may use a combination of conventional, HEPA, or electrostatic air filters.

To maintain maximum system efficiency, air filters must be replaced or cleaned when particulate accumulation causes a pressure drop across the filter in excess of the manufacturer's specifications. Thus all filters, as well as most cooling coils or duct heaters, are monitored continuously by a differential pressure switch or a transmitter or gauge. Differential pressure transmitters are also more widely used in vav systems to control fan speed.

These controls are illustrated in the applications in Figures 7, 8, and 9. They all require periodic checks and occasional troubleshooting for proper switch setpoint and transmitter full-span pressure.

A small, hand-operated calibration pump and either an appropriate range manometer (or Magnehelic gauge) makes this a simple task in the field.


Facility engineers, plant engineers, maintenance personnel, and/or those responsible for hvac system maintenance should have a selection of the above air movement instrumentation available at all times.

Technicians should be trained in the use of each instrument selected and encouraged to practice making velocity and static pressure measurements on a normally operating system until they are able to obtain consistent, proper readings in a minimum amount of time.

The benefit will be an efficiently operating hvac system and comfortable, safe working conditions. ES