Page 120 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 97

flowing across hot junctions of a thermopile supplied cept remains. For the gas bubble flow meters, a soapy
with constant electric power input. solution is used to fill the lower reservoir of the glass
& The measured frequency shift is related to true flow tube. The gas flow source is then connected to a
average velocity through a calibration technique. point above the bubble solution reservoir and gas
travels around to the glass flow tube. At this point, a
. What is a hot-film anemometer?
rubber bulb is either manually squeezed or a clamp is
& A hot-film anemometer consists of a platinum-sens-
used to continuously generate bubbles that travel at
ing element deposited on a glass substrate.
the same speed as the gas.
& It is less susceptible to fouling by bubbles or dirt
& When the bubble passes the lower optical sensor
when used for liquids.
within the sensor block, an internal timer is auto-
. “A thermal anemometer for gas flow measurement is
matically started, and when the bubble passes the
more universally applicable than a pitot tube for mea-
upper optical sensor, the timer is stopped. The total
suring velocity because it does not need to be adjusted
elapsed time is correlated to a gas flow rate and
for differences in the molecular weight of the gas.”
displayed on a digital readout. The small amount of
True/False?
liquid soap left over from the process collects in a
& False. The signal from a thermal anemometer varies
flow trap for disposal.
with heat loss and gases with different heat absorbing & Advantages: The major advantage of the bubble
properties (i.e., heat capacity and thermal conduc-
meter for gases is that it is not affected by the gas
tivity) will remove different amounts of heat. A pitot
composition. By contrast, most electronic meters
tube outputs a simpler property (velocity head) than a
must be calibrated for a specific gas or gas mixture.
thermal anemometer (relative heat loss).
Whether one is measuring ordinary gases such as N 2 ,
O 2 ,H 2 ,CO 2 , and Ar, or measuring a unique gas
4.1.8.4 Bubble Flow Meters mixture, one bubble meter can do it all. This versa-
tility helps to lower equipment costs and can save
. What is a bubble flow meter? What are its applications, recalibration time. However, it should be kept in
advantages, and limitations? mind that some gases may have a chemical reaction
& The bubble flow meters are of two designs, namely, with the water used to make the bubble solution and
for liquids and for gases. The design for liquids the user should be careful when specifying bubble
makes use of a timed measurement of a meniscus flow meters for such compounds.
rising between two optical sensors. & Another useful advantage of the bubble design is that
& The fluid enters the inlet and moves up inside a glass the calibration does not drift over time. The main
tube, past the sensor block and around the tube electrical parts of the system are the optical sensors
toward the outlet. As this happens, a solenoid valve for detecting the presence or absence of a bubble or
is timed to periodically open and close, thereby meniscus layer. These noncontact sensors do not
sucking a small amount of air into the tube. This wear out or experience a drift in accuracy. The glass
creates separate columns of liquid that move upward tube is fixed in diameter and will not change with
inside the tube, and toward the optical sensor block. time. In the gas chromatography market, bubble
The meniscus that is formed by these columns offluid meters can be qualified as a primary flow standard.
against the glass capillary tube walls is measured by Earlier, bubble flow meters were available only for
the optical sensors. Since the meniscus travels at the very low flow rates, but currently they are available
same rate as the column offluid, measuring the rate of also for expanded flow rate ranges. While gas flows
meniscus travel gives a direct correlation to the liquid ranging from 0.1 to 25 L/min can be accurately
flow. measured, liquid bubble meters do not have quite
& Two infrared sensors located within the sensor block the range as the gas versions and are available in sizes
time the rate of rise of the meniscus and this volume- ranging from roughly 1 –30 mL/min.
over-time measurement is then converted to a flow & Disadvantages: In order to make an in-line measure-
rate and displayed on a digital readout. As the fluid ment with a bubble flow meter, one needs to make a
moves around the top of the tube, air is vented at the break in the line where the flow reading is desired,
top while the liquid continues around and exits at the then make measurement and finally restore the line to
overflow tube. The process then repeats itself as its original condition. Bubble meters are therefore
the solenoid valve opens to create another air gap. adequate for end-of-line readings, but are not well
& By comparison, the bubble design for gas flow works suited for continuous, in-line monitoring. In some
somewhat differently although the same basic con- applications, the use of a bubble solution could be a