Instructions
H82.0.03.6C-08 Operating Manual GMH 5155 page 13 of 22
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7.7 CALC: Pitot measuring of Velocity and Flow (only for sensor 1)
7.7.1 Velocity measurement in air: “CALC SPEd”
The air velocity is calculated by means of the pressure difference at a Prandtl pitot tube or comparable
measuring setup.
with
v = velocity (m/s)
p = dynamic pressure (Pa) => p(A)-p(B) = measured pressure difference
s = pitot tube factor (= 1.0 for Prandtl pitot tube) value input at “COEF” in the CALC menue
= air density (kg/m³)
p
a
= air pressure (Pa) value input at “P.abs” in the CALC menue
R
i
=
T = absolute temperature (K) value input in °C at “t” in the CALC menue
(0 °C = 231.15K)
Generally the pressure differences evaluated are very small, therefore it is recommended to use pressure
sensors with high resolution.
Recommended sensors for use with a Prandtl pitot tube:
Sensor
Meas. range
Pmax
Vmin
Vmax
CALC-Menu
CALC-Menu
ConF-Menu
V
GMSD 2,5 MR - K51
2.500 mbar
1 m/s
4 km/h
20 m/s
72 km/h
000.0
1.0
2
GMSD 25 MR - K51
25.00 mbar
5 m/s
18 km/h
64 m/s
230 km/h
0000
5
2
It is not recommended to measure velocities below Vmin, because therefore the resolution of the sensors is
too low.
-:
If -1 digit is displayed at velocity of flow measurement the connected sensor gives a negative
pressure value -> check offset!
Velocity or flow calculation cannot be done with negative values.
7.7.2 Volumetric flow measurement in air: “CALC FLo”
The flow measurement is based on the velocity measurement (see above). Velocity multiplied with the cross-
section area (input in CALC menu by