Manometers and Pitot Tubes/Static Pressure Tips

Manometers (Figures 1-37 and 1-38) are used to measure velocity and static air pressures and gas pressures. Pitot tubes and static pressure tips are probes used with manometers to take measurements inside the ductwork of an air distribution system (Figures 1-39 and 1-40).

Most manometers used for HVAC servicing are calibrated in inches of water column (in. w.c.). Manometers with pitot tubes or static pressure tip s are used to measure static pressure, velocity pressure, and total pressure in the supply, return, and branch ductwork of air distribution systems. These measurements are used to determine the air quantities supplied to each branch. The measured quantities can be compared to the design specifications to evaluate the overall system performance and to make air supply adjustments as needed. When troubleshooting systems* manometers are often used to measure the pressure drop across the individual components in the system to detect airflow restrictions.

Non-electronic manometers work on the principle that air pressure is indicated by the liquid-level difference between two columns of liquid, one on each side of the manometer. If there is a pressure difference, the column of liquid will move until the liquid level in the low-pressure side is high enough so that its weight and the low air pressure being measured will equal the higher pressure in the other tube. Manometers typically use water or oil as the measuring fluid.

Most manometers used for HVAC servicing are calibrated in inches of water column (in. w.c.) and use an oil with a specific gravity of 0.826 as the measuring fluid. The manufacturer of the gauge specifies the type of oil to be used; therefore, substitution for the specified oil is not recommended. Manometers come in many types, including U-tube, inclined, and combined U-inclined. Electronic manometers are also widely used.

Individual U-tube and inclined manometers are available in many pressure ranges. Inclined manometers are usually calibrated in the lower pressure ranges and are more sensitive than U-tube manometers. U-inclined manometers combine the sensitivity o f the inclined manometer with the high-range capability of the U-tube manometer in one instrument. Inclined-vertical manometers, often called air velocity meters, also combine an inclined section for high accuracy and a vertical manometer section for extended range. They have an additional scale that indicates air velocity in feet per minute (FPM).

Electronic manometers typically measure differential pressures of -1 to 10 in. w.c. Many can give direct air velocity readings in the range of 300 to 9,990 FPM, eliminating the need for calculations.

The standard pitot tube used for making measurements in ducts 8 inches in diameter and larger has a 5/16-inch outer tube with eight equally-spaced 0.04-inch diameter holes used to sense static pressure. For measurements in ducts smaller than eight inches, use of pocket-size pitot tubes with a 1/8-inch outer tube and four equally-spaced 0.04-inch diameter holes is recommended.

The pitot tube consists of an impact tube which receives the total pressure input. It is fastened concentrically inside a second tube of slightly larger diameter, which receives static pressure input from the radial sensing holes around the tip. The air space between the inner and outer tubes permits transfer of pressure from the sensing holes to the static pressure connection at the opposite end o f the pitot tube, and then through the connecting tubing to the low or negative pressure side of the manometer- When the total pressure tube is connected to the high side of the manometer, velocity pressure is indicated directly. To insure accurate velocity pressure readings, the pitot tube tip must be pointed directly into the duct airstream. Pitot tubes come in various lengths ranging from 6 to 60 inches with graduation marks at every inch to show the depth of insertion in the duct.

Like the pitot tube, static pressure tips are used with manometers and differential pressure gauges to measure static pressure in a duct system. They are typically L-shaped with four radially-drilled 0.04-inch sensing holes.

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