Quantitative Air flow measurement (module 6) Flashcards
What do we measure?
- Air Pressure
- Velocity (speed + direction)
- Flow Rate
What are the 3 things that are fundamental to understanding ventilation systems?
Pressures, velocities, and flow rates:
What is pressure?
Force per unit are (PSI)
What causes air movement?
- Caused by the existence of a pressure differential
- Direction of flow is from higher pressure to lower pressure
What are different devices to measure pressure?
- Pitot Tube
- Manometer
- Digital manometer
- Magnehelic gauge
What are a pitot tube and manometer used for?
making velocity pressure measurements in a duct
What is VP?
the difference between the TP and SP measurements
What is a manometer?
- 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)
Is a manometer a primary or secondary device?
A primary device that does not need or lose calibration
What does the inclination of a manometer do?
amplifies the deflection for easier and more accurate measurement
What are the disadvantages of a inclined manometer?
- 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)
What is a digital manometer?
Uses pressure sensors and voltage meters to register changes in pressure
What is a magnehelic gage?
- 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:
What is the disadvantage of a magnehelic gage?
Not a primary standard
What are the advantages of a Magnehelic gage?
no need for leveling, easily zeroed, no fluid
How do you measure SPh?
- Measure 5-6 diameters downstream in a straight section of duct (away from turbulence)
- Use Pitot tube or manometer
is a pitot static tube a primary or secondary standard?
Primary
What does the inner tube in a pitot static tube sense?
senses Pt
What does the outer tube in a pitot static tube sense?
senses Ps
what does a pitot static tube measure?
Can be used to measure Pt, Ps and Pv from which velocity can be obtained directly from Bernouilli’s equation
What are some devices to measure velocity of air movement?
- Rotating vane anemometer
- Swinging vane anemometer
- Thermal anemometer
What is a rotating vane anemometer?
- 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
What is a swinging vane anemometer?
- 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
What is a thermal anemometer?
- 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
What are pros of thermal anemometers?
- Good sensitivity at low velocities
- Quick response time
- Probes are fairly rugged
- Easy to use
- Simultaneously measures temperature and velocity
What are cones of thermal anemometers>
- 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%)
What do you need to do before testing an LEV system with a velocimeter?
- 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)
WHat does a balometer measure?
air flow, flow rate (flowhood, capture hood)
What is a balometer?
- 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
What is a mechanical tachometer?
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
What is an electric tachometer?
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
What is stobe light?
- 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
