Honeywell AUTOMATIC CONTROL Engineering Manual page 288

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
PITOT TUBE SENSORS
A pitot tube measures both total pressure and static pressure.
This device combines the total pressure sensor and the static
pressure sensor tubes into one device (Fig. 15).
DIRECTION
OF
AIRFLOW
IMPACT
OPENING
TOTAL
PRESSURE
TUBE
Fig. 15. Pitot Tube.
To obtain accurate velocity pressure readings, the pitot tube
tip must point directly into the air stream. Figure 16 shows the
error in static and total pressure readings when the pitot tube
does not point directly into the air stream. Misaligning a pitot
static pressure tube causes the static readings to first increase
and then drop off rapidly as the angle of inclination (θ) increases.
The total pressure reading drops off gradually and then more
rapidly as θ increases. (Modern Developments in Fluid
Dynamics, Volume 1, Dover Publications, Copyright 1965.)
1.2
1.1
1.0
0.9
0.8
0.7
P
θ
0.6
P o
0.5
0.4
0.3
0.2
0.1
0
0 10 20 30 40 50 60
ANGLE OF INCLINATION θ DEGREES
Fig. 16. Error Caused by Improperly Mounting
Pitot Tube in Airstream.
ENGINEERING MANUAL OF AUTOMATION CONTROL
SMALL STATIC
PRESSURE OPENINGS
STATIC
PRESSURE
TUBE
C2655
P = PRESSURE AT INCLINATION θ.
θ
P o = PRESSURE WHEN θ = 0.
STATIC TUBE
TOTAL PRESSURE
TUBE ONLY
C2656
The following are guidelines for using pitot tubes:
1. A pitot tube can be used to measure either static or total
pressure.
2. A pitot tube measures the velocity at one point. However,
several readings across the duct (a traverse) are normally
needed to obtain an accurate average velocity.
3. Accurate pressure readings cannot occur in locations with
varying swirls, eddies, or impinged air. Because of this:
— The pitot tube must be inserted into the air stream
at least 10 duct diameters downstream from
elbows, and five duct diameters upstream of bends,
elbows, or other obstructions which cause these
effects. (See DEFINITIONS for a description of
round and rectangular duct diameters.)
— Air straightening vanes must be located upstream
from a pitot tube location to ensure accurate
readings. Rotating airflow can occur in long,
straight, duct sections. If air straightening vanes
are not used, this nonparallel airflow can strike a
pitot tube at an angle and produce false total and
static pressure measurements.
4. It is impractical to use the pitot tube (or devices using its
principles) at velocities below about 700 fpm (velocity
pressure of 0.0305 in. wc).
Even with an inclined manometer, velocity pressures below
0.0305 in. wc are too small for reliable measurement outside
of a well equipped laboratory. According to the Industrial
Ventilation Manual 17th Edition, 1982, "A carefully made and
accurately leveled 10:1 inclined manometer calibrated against
a hook gauge can read to approximately ±0.005 in. wc. A
standard pitot tube with an inclined manometer can be used
with the following degree of accuracy:
Table 2. Pitot Tube Accuracy.
Velocity (fpm)
4000
3000
2000
800
600
It can be seen that the use of the pitot tube in practical
applications is limited at velocities lower than 600-800 fpm."
278
Percent Error (±)
0.25
0.3
1.0
6.3
15.0