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Glossary

Absolute Pressure: The total of the indicated gage pressure plus the atmospheric pressure. Abbreviated "psia" for pounds per square inch absolute.

Atmospheric Pressure: The pressure exerted upon the earth's surface by the air because of the gravitational attraction of the earth. Standard atmosphere pressure at sea level is 14.7 pounds per square inch (psi). Measured with a barometer.

Barometer: An instrument for measuring atmospheric pressure.

Calibration: Determining or correcting the error of an existing scale.

CEFAPP: Close enough for all practical purposes.

Differential Pressure Gage: An instrument that reads the difference between two pressures directly and therefore, eliminates the need to take two separate pressures and then calculate the difference.

Electronic Instruments: Most of the mechanical analog instruments now have electronic digital counterparts. All instruments, analog or digital, should be checked against a sheltered set before each balancing project. Pressure measuring instruments should be checked against a standard liquid-filled manometer.

Gage: An instrument for measuring pressure.

Gage Pressure: The pressure that's indicated on the gage.

Harmonics: For a strobe light tachometer, harmonics are frequencies of light flashed that are a multiple or submultiple of the actual rotating speed. For example, if the light frequency is either exactly two times or exactly one-half the actual speed of the rotating equipment, the part will appear stationary but the image won't be as sharp as when the rpm is correct.

Manometer: An instrument for measuring pressures. Essentially a U-tube partly filled with a liquid, usually water, , mercury or a light oil. The pressure exerted on the liquid is indicated by the liquid displaced. A manometer can be used as a differential pressure gage.

Meniscus: The curved surface of the liquid column in a manometer. In manometers that measure air pressures, the liquid is either water or a light oil. In a manometers that measure water pressures, the liquid is mercury.

Operating Load Point: Actual system operating capacity when an instrument reading is taken.

Parallax: A false reading that happens when the eye of the reader isn't exactly perpendicular to the lines on the instrument scale.

Pitot Tube: A sensing device used to measure total pressures in a fluid stream. It was invented by a French physicist, Henri Pitot, in the 1700's.

Sheltered Set: A sheltered set of instruments is a group of instruments used only to check the calibration of field instruments.

Sensitivity: A measure of the smallest incremental change to which an instrument can respond.

Air Instruments

Air Differential Pressure Gage: Magnehelic "R" and Capsuhelic "R" are two brands of air differential gages. These gages contain no liquid, but instead work on a diaphragm and pointer system which move within a certain pressure range. Although absolutely level mounting isn't necessary, if the position of the gage is changed, resetting of the zero adjustment may be required for proper gage reading as specified by the manufacturer. These gages have two sets of tubing connector ports for different permanent mounting positions; however, only one set of ports is used for readings and the other set is capped off. The ports are stamped on the gages as "high" pressure and "l;ow" pressure and when used with a Pitot tube, static tip or other sensing device to measure total pressure, static pressure, or velocity pressure, the tubing is connected from the sensing device to the tubing connector ports in the same manner as the connection to the inclined-vertical manometer. These gages are generally used more for static pressure readings than for velocity pressures readings. They should be checked frequently against a manometer.

Anemometer: An instrument used to measure air velocities.

Capture Hood: An instrument which captures the air of a supply, return or exhaust terminal and guides it over a flow-measuring device. It measures airflow directly in cubic feet per minute. Calibration by the manufacturer should be done every 6 months, especially if the instrument isn't checked periodically against a sheltered instrument.

Compound Gages: Compound gages measure pressures both above and below atmospheric. They read in pounds per square inch above atmospheric and inches of mercury below atmospheric. Compound gages are calibrated to read zero at atmospheric pressure.

Cubic Feet Per Minute (CFM): A unit of measurement. The volume or rate of airflow.

Deflecting Vane Anemometer: The deflecting vane anemometer gives instantaneous, direct readings in feet per minute. It's used most often for determining air velocity through supply, return and exhaust air grilles, registers or diffusers. It may also have attachments for measuring low velocities in an open space or at the face of a fume hood. With other attachments, Pitot traverses and static pressures can be taken. To use this instrument refer to the manufacturer's recommendation for usage, proper attachment selection and sensor placement. A correction (Ak) factor is also needed when measuring grilles, registers or diffusers. Calibration by the manufacturer should be done every 6 months, especially if the instrument isn't checked periodically against a sheltered instrument.

Feet per Minute (FPM): A unit of measurement. The velocity of the air.

Hot Wire Anemometer: This instrument measures instantaneous air velocity in feet per minute using an electrically heated wire. As air passes over the wire, the wire's resistance is changed and this change is shown as velocity on the instrument's scale. This instrument is very position sensitive when used to measure air velocities. Therefore, it's important to ensure that the probe is held at right angles to the airflow. The hot wire anemometer is most often used to measure low velocities such as found at the face of fume hoods; however, some instruments can also measure temperatures and static pressures. Calibration by the manufacturer should be cone every 6 months, especially if the instrument isn't checked periodically against a sheltered instrument.

Inches of WAter GAge or Column (IN WG or IN WC): A unit of air pressure measurement equal to the pressure exerted by a column of water 1 inch high.

Inclined Manometer, Inclined-Vertical Manometer: The inclined manometer has an inclined scale which reads in inches of water gage in various ranges such as 0 to 0.25 in.wg, 0 to 0.50 in.wg, or 0 to 1.0 in.wg. The inclined-vertical manometer has both an inclined scale that reads 0 - 1.0 in.wg and a vertical scale for reading greater pressures such as 1.0 to 10 in.wg.

The inclined and inclined-vertical manometer have a left and right tube connection for attaching tubing from a Pitot tube or other sensing device to the manometer. The left tubing connector is called the "high" side of the manometer and the right connector is the "low" side. The manometers are filled with a colored oil which is lighter than water and, therefore, the oil will move a greater distance for a given pressure allowing more precise readings.

Magnehelice "R" Gage: An instrument for measuring air pressures and differential pressures. See air differential pressure gage.

Micromanometer: Micromanometers are instruments generally suited more to laboratory testing than to field measurements. The micromanometer is difficult to use in the filed because of the levering and mounting requirements. Micromanometers are used to measure very low pressures accurately down to plus or minus one thousandth (0.001) inch of water gage. If field measurements below 0.03 in. wg (700 fpm) are required, consider using a 0. to 0.25" inclined manometer, a micromanometer or hot wire anemometer.

Manometer: The manometer is the standard of the industry for reading air pressure. It contains no mechanical linkage and doesn't need calibration.

Other pressure measuring instruments are checked for calibration against a properly set up and accurately read manometer. Types of manometer commonly used to measure air pressures are the inclined manometer, the inclined-vertical manometer, the U-tube manometer and the micromanometer.

Pitot Tube: The standard Pitot tube has a double tube construction with a 90-degree radius bend near the tip and measures both total and static pressures. It is 5/16" in diameter and is available in various lengths from 12" to 60". The standard tube is recommended for use in duct 8" and larger diameter while a smaller (1/8" diameter) "pocket" Pitot tube is used in ducts smaller than 8". To help in taking Pitot tube traverses, the outer tube is marked with a stamped number at the even-inch points and a 1 /8" line at the odd-inch intervals. The inner tube, or impact tube, senses total pressure and runs the length of the Pitot tube to the total pressure connection at the bottom.

Static pressure is sensed by the outside tube through eight equally spaced holes around the circumference of the tube. These small openings must be kept clean and open to have accurate readings. They're located near the tip of the Pitot tube. The air space between the inner and outer tube serves to transmit the static pressure from the sensing holes to the static pressure connection at the bottom side of the Pitot tube. The static pressure connection is parallel with the tip of the 'pitot tube as an aid to aligning the tip properly.

To ensure the accurate sensing of pressures, the Pitot tube tip must be pointed so it faces directly into and parallel with the air stream. The hookups of the Pitot tube to an inclined or inclined-vertical manometer or air differential pressure gage (ADPG) are:

  1. Total Pressure (TP)
    1. Open the high and low tubing connectors on the manometer or the ADPG.
    2. Connect one piece of tubing to the total pressure connection on the Pitot tube.
    3. If the total pressure reading is on the discharge of the fan, connect the other end of the tubing to the left (high side) tubing connector on the manometer (high port of the ADPG). If the total pressure reading is on the inlet of the fan, connect the tubing to the right (low side) tubing connector (low port of the ADPG).
  2. Static Pressure (SP)
    1. Open the high and low tubing connectors on the manometers the ADPG.
    2. Connect one piece of tubing to the static pressure connection on the Pitot tube.
    3. If the static pressure reading is on the discharge side of the fan, the other end of the tubing is connected to the left (high side) tubing connector on the manometer (high port of the ADPG). If the static pressure reading is on the inlet of the fan, the tubing is connected to the right (low side) tubing connector (low port of the ADPF).

    The tubing connections to the manometer or ADPG are reversed when taking total and static pressure because the air pressures on the discharge of the fan are greater than atmospheric pressure whereas, on the inlet of the fan, the air pressures are less than atmospheric. If the tubing isn't connected to the proper side of the manometer, not only will the reading be incorrect, but also there's a good chance that oil will be blown out of the manometer.

  3. Velocity Pressure (VP)
    1. Open the high and low tubing connectors on the manometer or the ADPG.
    2. Using two pieces of tubing, connect one piece of tubing to the total pressure connection on the Pitot tube and the other piece of tubing to the static pressure connection.
    3. Connect the total pressure tubing to the left (high) side of the manometer (low port of the ADPGP).
    4. Connect the static pressure tubing to the right (low) side of the manometer (low port of the ADPG). Velocity pressure is the subtraction of static pressure from total pressure and because total pressure is always greater than or equal to static pressure, velocity pressure will always be a positive value. This means that when measuring velocity pressure using the Pitot tube,the hookup is always the same no matter if the reading is taken on the discharge of the fan or on the inlet.

ROTATING VANE ANEMOMETER (RVA): Using the rotating vane anemometer for proportional balancing is satisfactory when properly applied; however, where applicable, actual flow quantities should be verified by a 'pitot tube traverse.

The RVA is generally used for determining air velocity through supply, return and exhaust air grilles, registers or openings. It's also sometimes used to measure total airflow through coils. However, it isn't a recommended practice to measure airflow through coils or dampened registers since the air through these devices exists in thin, high-speed jets. The rotating vane anemometer is not an averaging instrument and if such measurements are attempted, it'll provide a false (high) reading.

A rotating vane anemometer measures the linear feet of the air passing through it. Because the RVA reads in feet, a stopwatch or other timing device must be used to find velocity in feed per minute. The useful velocity range of the RVA is between 200 to 2000 fpm and the accuracy of the instrument depends on the precision of use, the type of application, and its calibration. Calibration should be checked frequently against a manometer. Also, calibration by the manufacturer should be done every 6 months, especially if the instrument isn't checked periodically against a sheltered instrument. The manufacturer should return the instrument with a velocity correction chart for that instrument.

The RVA (either 3", 4" or 6" diameter [the 4"] is preferred] is placed in the air stream with the air flowing from back to front. This means that RVA is reversed for return and exhaust air readings. There's a shroud that fits on the front of the RVA which should be used when taking readings on return and exhaust air grilles to keep the dial face from being scratched. The shroud should be held tightly against the face of the grille.

All readings should be taken with the handle attachment in place to avoid interference with the airflow. All internal dampers on registers must be full open and all adjustable face bars must be at zero degrees deflection. Readings may be taken for 15, 30 or 60 seconds, depending on uniformity and velocity of flow. Follow the manufacturer's recommendations for measuring airflow.

If a manufacturer's Ak isn't available or appears erroneous, establish a new Ak by Pitot tube traverse of the duct. If this isn't practical, and a reading is needed for proportioning purposes, (1) hold the RVA 1" away from the face of a supply grille or register. This will allow the air to recover from the vena contracta effect of the air passing through the face bars. For a return grille or register, hold the RVA against the face bars; (2) use the inside dimensions of the grille (not the nominal grille size) for calculating area correction factor. Don't subtract the area of face bars from the calculated area factor. If the smallest inside dimension is less than 6", use the diameter of the rotating vane anemometer.

If the airflow is of similar quantities over the face of the opening, grille, etc., a serpentine traverse may be taken. However, if the air velocities differ greatly, the opening should be sectioned off as needed to get uniform flow. The readings are taken with the rotating vane anemometer traversing or stationary in each section. Example: A supply register is 24" x 12". There' no manufacturer's Ak. The calculated Ak is 1.9 (23.625" x 11.625" the inside dimensions, divided by 144 sq. in. per sq. ft.). The face velocities are dissimilar, 0 fpm in some areas and 500 fpm in others. This register face is divided into four 12: x 6" sections. A reading is taken in each section for 15 seconds. After each timed interval the RVA is stopped and removed from the air stream. The value on the indicating dial is read and the instrument correction factor is applied. This number is multiplied times 4. After the readings in all the sections are taken, the average velocity in feet per minute is calculated and multiplied times the Ak factor to get air volume i.e. CFM = Ak(V).

STANDARD CUBIC FEED PER MINUTE (SCFM): The volumetric rate of airflow at standard air conditions.

U-TUBE MANOMETER: a manometer with a U-shaped glass or plastic tube partly filled with tinted water, or oil. They're made in various sizes and are recommended for measuring pressures of several inches of water gage or more. They're not recommended for readings of less than 1.0 in. wg. To take a reading, open both tubing connectors. The liquid will be at the same height in each leg. A Pitot tube, impact tube or static pressure tip is connected to the manometer. When the sensing device is inserted into the duct or fan compartment the liquid is forced down in one leg and up in the other. The difference between the heights of the two legs is the pressure reading.