Series 160S Pitot Tubes are designed to meet the need
of the environmental testing field for an inexpensive, yet
accurate and reliable way to measure the flow of particu-
late-laden air or gas streams. These pitot tubes use large
5/16˝ diameter stainless steel tubing for both total and stat-
ic pressures to avoid plugging. Versatile 1/8˝ female NPT
connections enable use with any type of piping or tubing.
Two barbed tubing adapters are included for use with 3/16˝
I.D. rubber or vinyl tubing.
This instrument was built to allow measurement of flows by
the procedures detailed in U.S. Environmental Protection
Agency publication 40 CFR Change 1, Application A,
Method 2. For complete information, refer to that publica-
tion and the procedures contained within.
INTRODUCTION
The total pressure of a flowing air stream in a duct or pipe
is the sum of the static or bursting pressure exerted on the
sidewalls and the velocity or impact pressure of the mov-
ing air. The difference between total and static pressure is
called velocity pressure, which can be used to determine
the linear rate of air movement expressed in FPM (feet per
minute). A pitot tube has two tubes arranged to sense both
pressures simultaneously. By connecting these two tubes
differentially to a manometer, velocity pressure is indicat-
ed directly and the corresponding air velocity can be calcu-
lated after applying the appropriate correction factor.
For maximum accuracy of ±2%, as in laboratory applica-
tions, care is required and the following recommendations
should be followed.
1. Duct diameter should be 4˝ or larger.
2. Point total pressure opening upstream facing flow
and static pressure opening downstream pointing in
the direction of the flow. The faces of both openings
must be perpendicular to the airflow.
3. Make an accurate traverse per drawings; calculate the
the velocities at each point and average them.
4. Take readings in a smooth, straight duct section a
minimum of 82 duct diameters in length upstream
and 12 diameters downstream from the pitot tube.
5. Provide an egg-crate type straightener upstream from
the pitot tube.
Series 160S “S” Type Pitot Tubes
Operating Instructions
Bulletin H-12
DWYER INSTRUMENTS, INC.
Phone: 219/879-8000 www.dwyer-inst.com
P.O. BOX 373 • MICHIGAN CITY, INDIANA 46361, U.S.A. Fax: 219/872-9057 e-mail: info@dwyer-inst.com
TAKING AIR VELOCITY READINGS
To measure air velocity with a Series 160S Pitot Tube, make
a 13/16˝ (20 mm) opening in side of duct. Permanent-
mount models require a 1˝ female NPT opening. Note: per-
manent mounting is not recommended with insertion
lengths over 24˝ (61 cm) due to risk of excessive deflection.
Connect tubing from total pressure port to high pressure
side of manometer and from static pressure port to the low
pressure side. If reading is negative, reverse connections.
Make a series of readings traversing the duct in horizontal
and vertical planes. Using velocity pressures recorded at
each location, calculate velocities and average them for
final velocity value. If circumstances do not permit or
require an accurate traverse, center the pitot tube in the
duct, determine the pressure differential (velocity pressure),
calculate actual center velocity, and multiply this value by
0.9. Tests run in this manner should be accurate within
±5%.
CALCULATING VELOCITY
Air Velocity = 1096.2 (C
p
) P
v
D
where:
P
v
= Sensed pressure difference (velocity
pressure) in inches of water column
D = Air density in lbs./ft.
3
(dry air = .075)
C
p
= Pitot tube coefficient: 0.84
Air Density = 1.325 X
P
B
T
PB = Barometric pressure in inches of
mercury
T= Absolute Temperature (Indicated
Temperature in °F plus 460)
Flow in cubic feet per minute equals duct cross sectional
area in square feet x air velocity in feet per minute.
With dry air at 29.9 inches of mercury, air velocity can
be read directly from temperature correction charts on
reverse.
H-12rev 9/18/03 4:18 PM Page 1
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