Download CE 150 LAB - CSU, Chico

CE 150 LABORATORY
Fall 2003
Fluid Mechanics Lab #3
Survey of Flow Measurement Devices
Fluid flow measurement is necessary in a wide variety of applications such as arterial blood
flow, liquid oxygen flow in rockets, aerodynamic drag/lift, wind loading on buildings, soil
erosion, dispersion of pollutants, and food/beverage processing. The selection of the proper
instrument depends upon many variables that include the nature of the flow, required accuracy,
and cost. In all cases, measurement uncertainty will be present and an estimate of its value should
be known or computed. Furthermore, a reliable estimate of the result (e.g., velocity, volume flow
rate, etc.) should be made before the measurement is made. This will enable the user to evaluate
the measured results for gross errors.
The purpose of this laboratory is to present a variety of flow measurement devices that are
currently used in the field of fluid mechanics. The operation of the devices will be either
described or demonstrated; three demonstrations will require data taking and analysis. The
following devices in the Civil and Mechanical Engineering laboratories at CSU, Chico will be
covered:
Velocity Measurement Devices
1. Pitot-Static Tube – a tube inserted into a flow stream that allows measurement of the static
and stagnation pressures with a manometer or transducer. Velocity is determined by
application of the Bernoulli equation. This device is often used as a standard by which other
velocity measurement devices are calibrated. Accuracy depends upon the type of manometer
used, but is typically less than 1%.
2. Rotating Cup Meters – utilizes the drag force on an array of cups attached to a common
rotating shaft adjacent to a 4-pole magnet or Hall effect sensor that produces a sine wave or
square wave electric signal. It is often used for wind speed and water current measurements.
Requires calibration. Accuracy of calibrated meters is typically 1 to 2%.
3. Thermal Air Velocity Probe – consists of two sensors inside a slotted metallic sheath. One
sensor is an electric resistance heater that is maintained at a constant temperature by a
Wheatstone bridge circuit. The velocity (cooling effect) of the air stream is directly related to
the electric power necessary to maintain the sensor temperature. The other sensor measures
temperature, which compensates the circuit over a wide range of gas temperature. The circuit
outputs a dc voltage that is linearly related to velocity under standard conditions (1 atm,
25C). This particular model is designed for use in high temperature, corrosive, particulateladen flows, such as combustion gases. Requires calibration. Accuracy is typically 2 to 3%.
4. Magnetic Water Current Meter – measures the velocity of conducting fluids (e.g., water)
based upon Faraday’s law, i.e., a conductor moving through a static magnetic field will
induce a voltage that is directly proportional to the velocity of the conductor. A sensing
probe contains an electromagnetic coil that produces the magnetic field and a pair of
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electrodes that are perpendicular to the magnetic field vector measure the induced voltage. It
has replaced the rotating cup water meter as the standard measurement device for open
channel water flows. Requires calibration. Accuracy is typically 2%.
5. Swing Vane Anemometer – small, lightweight, low-priced instrument based upon the drag
force on a pivoting plastic vane with calibrated scale. It is specifically designed for low air
velocities (25-400 ft/min or 0.1-2 m/s) and is often used to check safety ventilation
requirements for spray booths and laboratory fume exhaust hoods. Accuracy is 5 to 10%.
Volume Flow Rate Measurement Devices
1. Weigh-Tank w/Stopwatch – provides a time-average volume or mass flow rate by
measuring the time necessary to collect a quantity of liquid in a tank. The weight of liquid is
measured by an accurate scale that is integrated with the tank. This device is often used as a
standard by which other volume flow measurement devices are calibrated. Accuracy depends
upon length of time and quantity of liquid, but is typically less than 0.5%.
2. Orifice meter – an obstruction-type device placed in a liquid or gas pipe flow. The
obstruction is a stainless-steel plate with a circular hole with a precisely-measured loss
coefficient. The pressure drop across the orifice plate is accurately measured with a
manometer, dial gage, or transducer. Volume flow rate is determined by application of the
Bernoulli equation. Accuracy depends upon pressure measurement and knowledge of orifice
loss coefficient; can be less than 1%.
3. Venturi meter – an obstruction-type device placed in a liquid or gas pipe flow. The
obstruction is a converging-diverging pipe section with a precisely-measured loss
coefficient. The pressure drop across the converging section is accurately measured with a
manometer, dial gage, or transducer. Volume flow rate is determined by application of the
Bernoulli equation. Accuracy depends upon pressure measurement and knowledge of orifice
loss coefficient, but is typically less than 1%.
4. Rotameter – a drag-effect device consisting of a tapered vertical tube that contains a float.
As fluid (gas or liquid) enters the bottom of the tube, the float rises until the drag is balanced
by the weight and buoyancy forces. The position of the float is measured with a calibrated
scale that indicates volume flow rate under standard conditions (1 atm, 25C). Accuracy is
typically 2 to 5% of full scale.
5. Turbine Flowmeter – a device placed in a pipeline that contains a small turbine wheel that
rotates as gas or liquid moves through the meter. A permanent magnet is built into the
turbine-wheel body; a reluctance pickup attached to the stationary housing detects a pulse for
each rotation of the wheel. The pulse rate is proportional to the volume flow rate and the
proportionality constant (flow coefficient) is found by calibration. Accuracy of 0.5% is
achievable.
6. Sharp-Crested Weir – an obstruction-type device for measuring open channel water flow
rate. The weir may be a rectangular, triangular, or trapezoidal plate, over which the water
flows. The height of the water above the weir is measured, from which the volume flow rate
can be calculated using the Bernoulli equation and the flow coefficient for the particular
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weir. Accuracy depends upon the type of weir and height measurement; typically from 2 to
5%.
A. Orifice Meter Demonstration
The instructor will set the gate valve at three predetermined positions and students will read
the pressure drop across the orifice plate for each setting with a differential mercury-water
manometer. The orifice meter equation with loss coefficient is
2( p1  p 2 )
 w [1  (d 2 / d 1 ) 4 ]
Q  Co A2
Refer to Figure 3.18 and equation (3.20) in text. This equation will be discussed during lab.
Assignment: Determine the volume flow rate [ft3/s] through the pipe for each valve setting.
Values for the loss coefficient, pipe diameter, and orifice diameter will be given during the lab.
B. Pitot-Static Tube Demonstration
The instructor will set the speed control on the closed-loop wind tunnel at three
predetermined positions and students will measure the differential pressure from the Pitot-static
tube with the inclined manometer for each setting. An air temperature and barometer reading will
also be made. The Pitot-static tube equation is
V
2( p 2  p1 )
a

2  w gh
a
Refer to Figure 3.6 and equation 3.16 in text. This equation will be discussed during the lab.
Assignment: Determine the velocity of the air flow [m/s] at each setting from the Pitot-static
tube measurements.
C. Pitot-Static Tube and Thermal Air Velocity Probe Demonstration
The instructor will set the guillotine valve on the low-velocity wind tunnel to a
predetermined position and students will measure the differential pressure from the Pitot-static
tube with the micromanometer. Note: The liquid level deflection from the micromanometer must
be multiplied by two to get the total differential pressure. The voltage output of the thermal air
velocity probe will then be recorded. An air temperature and barometer reading will also be
made. The velocity-voltage relationship for the thermal air velocity probe is
V  1.2  volts
[standard m/s]
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Assignment: Determine the velocity of the air flow [m/s] from the pitot-static tube measurement
and the air velocity probe. Calculate the percent difference based upon the pitot-static tube
velocity. Which measurement do you think is most accurate?
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