July 1950 ET-287
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine
CONSTRUCTION OF A SIMPLE AIR FLOWMETER
By R. A. Fulton and Frank S. Phinney,
Division of Insecticide Investigations
In laboratory or field investigations there is often need for a flow-
meter with a known delivery rate. A series of durable meters has been
designed and constructed by the use of simplified flow equations that
govern gases or liquids flowing in a straight line and passing through
any constriction in the line of flow. The pressure drop across the con-
striction is dependent on the rate of flow. The flowmeter described
herein maybe constructed of parts available from any hardware or
The following equation was found suitable in measuring flow of air
with orifice meters:
Q = 611 D2 V/R-
where Q = air flow rate in cubic centimeters per second
Do = orifice diameter in centimeters
R = manometer reading in centimeters of water
The constant 611 is a simplified value for meters of the type des-
cribed, derived from more complex, precise equations by the use of
certain simplifying assumptions.
With this equation orifice meters may be constructed with a wide
range of flow rates.
For purposes of illustration, suppose a flowmeter is to be constructed
that will deliver 10 liters of air per minute with a water-manometer
differential reading of 10 centimeters.
Q = 10 liters per minute, or 166. 7 centimeters per second
R = 10 cm. of water
Substituting in this equation,
166.7 = 611 D2 viO0, D2 = 166. = 0 0863
0 0 611 in.
D = \4.60863 = 0.294 cm., or 0.116 in.
Any unknown conditions may be calculated from the following equation
if the flow rate corresponding to any given reading is known:
Q l 1
Q2 ^ Ki
The calculated and calibrated values for four flowmeters of this
type are shown below:
Do, inch Q, liters Q, liters
DQper lminuter R, centimetersper minute R, centimeters
______per m inute ______per m inute _____
0.313 74 -23.3 10-1 79 -24.1 10-1
.209 33.5-10.6 10-1 39 -12.3 10-1
. 164 20 -10 10-2.5 21 -10.6 10-2.5
.116 10 3.16 10-1 8.3- 3.5 10-1
The type of meter shown in figure 1 is designed to measure the flow
of air at 765 mm. of mercury and 75 F. (23.9 C.). It is constructed
from two 1-inch IPT pipe flanges (A and A'), two 1-inch standard pipe
nipples 6 inches in length (D and D'), and a 3 1/2-inch brass disk (I) made
from 1/32-inch brass sheet. The pipe flanges are attached to the nipples,
and to insure a tight unit the threads are sealed with pipe-sealing com-
pound or solder. A hole of the size calculated for the orifice is drilled
in the center of the disk I. The drilling may be done on an ordinary drill
press with new bits. Four additional holes are then drilled near the
circumference of the disk to match the holes of the two pipe flanges.
Two 21/64-inch holes (E and E') are drilled in the 6-inch pipe nipples
15/16 inch from the edge of the flange nearest to the disk and tapped with
1/8-inch IPT. The refrigeration-flared unions (F and F') 1/8-inch
IPT x 1/4-inch SAE are used as outlets to the manometer T1 and P2).
The threads are made airtight with sealing compound. One-fourth-inch
OD-copper tubing (H, H') is connected to the 1/4-inch SAE flared nuts, (G and G').
This copper tubing is connected to the manometer by means of rubber tubing.
Two gaskets (C and C') are cut from standard fiber gasket stock,
1/32 to 1/16 inch, with holes to match the inside of the pipe and the holes
in the pipe flange. The unit is assembled with 1-inch flat-head stove
The orifice meter can be calibrated with a standardized wet-test
meter or a precalibrated gas meter. The precalibrated gas meter may
be obtained from your local gas company. Both types of meters are
satisfactory, but for air flows above 10 liters per minute the gas meter
has less air resistance. Water displacement, dilution methods, or
,ilh,.r accepted types of calibration also may be used.
Figure 1. --Construction of a simple air flowmeter.
See text for explanation of letters.
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