Quantitative Air flow measurement (module 6)

Question 1

What do we measure?

Answer 1

• Air Pressure
• Velocity (speed + direction)
• Flow Rate

Question 2

What are the 3 things that are fundamental to understanding ventilation systems?

Answer 2

Pressures, velocities, and flow rates:

Question 3

What is pressure?

Answer 3

Force per unit are (PSI)

Question 4

What causes air movement?

Answer 4

• Caused by the existence of a pressure differential
• Direction of flow is from higher pressure to lower pressure

Question 5

What are different devices to measure pressure?

Answer 5

• Pitot Tube
• Manometer
• Digital manometer
• Magnehelic gauge

Question 6

What are a pitot tube and manometer used for?

Answer 6

making velocity pressure measurements in a duct

Question 7

What is VP?

Answer 7

the difference between the TP and SP measurements

Question 8

What is a manometer?

Answer 8

• U-tube filled with colored water
• Pressure makes water drop on one side and rise on the other
• A ‘primary’ device (does not need or lose calibration)

Question 9

Is a manometer a primary or secondary device?

Answer 9

A primary device that does not need or lose calibration

Question 10

What does the inclination of a manometer do?

Answer 10

amplifies the deflection for easier and more accurate measurement

Question 11

What are the disadvantages of a inclined manometer?

Answer 11

• Measurement between two moving points if fluctuations present
• Poor sensitivity for low airflow velocities (~0.05 in. H2O; ~900 fpm)
• Uncertainty due to meniscus
• Water evaporates relatively quickly#(but can use other fluids with known density, e.g., oil)

Question 12

What is a digital manometer?

Answer 12

Uses pressure sensors and voltage meters to register changes in pressure

Question 13

What is a magnehelic gage?

Answer 13

• A mechanic device, where a deflection of needle on a scale is produced by the motion of a diaphragm caused by a pressure difference.
• Can measure down to 0.05 ”wg.  Field practice:

Question 14

What is the disadvantage of a magnehelic gage?

Answer 14

Not a primary standard

Question 15

What are the advantages of a Magnehelic gage?

Answer 15

no need for leveling, easily zeroed, no fluid

Question 16

How do you measure SPh?

Answer 16

• Measure 5-6 diameters downstream in a straight section of duct (away from turbulence)
• Use Pitot tube or manometer

Question 17

is a pitot static tube a primary or secondary standard?

Answer 17

Primary

Question 18

What does the inner tube in a pitot static tube sense?

Answer 18

senses Pt

Question 19

What does the outer tube in a pitot static tube sense?

Answer 19

senses Ps

Question 20

what does a pitot static tube measure?

Answer 20

Can be used to measure Pt, Ps and Pv from which velocity can be obtained directly from Bernouilli’s equation

Question 21

What are some devices to measure velocity of air movement?

Answer 21

• Rotating vane anemometer
• Swinging vane anemometer
• Thermal anemometer

Question 22

What is a rotating vane anemometer?

Answer 22

• Wheel containing curved fan blades
• As air passes through wheel, fan blades rotate
• Each rotation is one unit of length (e.g. one foot, one meter)
• Each rotation is logged by the anemometer
• Time is measured simultaneously, so that average velocity =total length/total time
• Must be aligned with direction of airflow for accurate reading

Question 23

What is a swinging vane anemometer?

Answer 23

• Wheel containing curved fan blades
• As air passes through the wheel, fan blades rotate
• Each rotation is one unit of length (e.g. one foot, one meter)
• Each rotation is logged by the anemometer
• Time is measured simultaneously so that average velocity =total length/total time
• Must be aligned with the direction of airflow for accurate reading

Question 24

What is a thermal anemometer?

Answer 24

• Contains a wire or filament
• Baseline voltage applied to wire
• When air moves over wire, air movement cools the wire
• Loss in temperature produces change in voltage over the wire
• Voltage change converted to velocity

Question 25

What are pros of thermal anemometers?

Answer 25

• Good sensitivity at low velocities
• Quick response time
• Probes are fairly rugged
• Easy to use
• Simultaneously measures temperature and velocity

Question 26

What are cones of thermal anemometers>

Answer 26

• Poor sensitivity at high velocities
• Probes may be damaged by exposure to corrosive materials
• Short battery life
• Require periodic calibration
• Cannot be used at high temperature (> 200°F) or at high RH (> 80%)

Question 27

What do you need to do before testing an LEV system with a velocimeter?

Answer 27

• The instrument must b-e calibrated to be calibrated with primary standards
• Velocimeters are secondary standards
• Compare with primary standards (pressure-based) (pitot tube, calibration wind tunnel, sharp-edged orifice plate)

Question 28

WHat does a balometer measure?

Answer 28

air flow, flow rate (flowhood, capture hood)

Question 29

What is a balometer?

Answer 29

• Consists of a large hood with a rectangular opening that tapers down to a velometer
• Place balometer around air vent or hood
• Also called a capture hood because it contains all air moving into hood
• Known area of balometer at measurement opening allows for conversion of velocity to flow rate: Q = V*A

Question 30

What is a mechanical tachometer?

Answer 30

the tachometer shaft is rotated by pressing it against the center of the fan shaft so that both the tachometer shaft and fan shaft have the same speed

• measures fans and shaft RPM

Question 31

What is an electric tachometer?

Answer 31

the tachometer sends light to a shiny, rotating object, such as a silver sticker attached to a fan blade or shaft, and the reflected light is measured by the tachometer and converted to an rpm measurement

• measures fans and shaft RPM

Question 32

What is stobe light?

Answer 32

• A Strobe light produces flashes of bright light at an adjustable frequency (flashes per minute).
• As the frequency approaches the fan rpm, the blades appear to slow down, stop, and may even appear to reverse direction.
• The fan rpm is determined by adjusting the flash rate until a rotating part appears to be stopped (blade shaft or pulley)
• measures fans and shaft RP