Flow Tracker Series 1000/2000
Reference Manual
9
Flow Measurement Operating Principle
The volumetric flow rate is determined by creating a pressure
drop across a unique internal restriction (known as a Laminar
Flow Element) and measuring differential pressure across it.
The restriction is designed so that the gas molecules are forced
to move in parallel paths along the entire length of the passage;
hence Laminar (streamline) flow is established for the entire
range of operation of the device.
Unlike other flow measuring devices, in laminar flowmeters the
relationship between pressure drop and flow is linear. The
underlying principle of operation of the Agilent Flow Tracker
Series flowmeters is known as the
Poiseuille Equation
:
Since
π
, r and L are constant, this equation can be rewritten as:
The second equation shows the linear relationship between
volumetric flow rate (Q), differential pressure (
∆
P), and
absolute viscosity (
η
) in a simpler form.
Q = (P
1
-P
2
)
π
r
4
/8
η
L
where:
Q = volumetric flow rate
P
1
= static pressure at the inlet
P
2
= static pressure at the outlet
r =
radius of the restriction
η
= (eta) absolute viscosity of the fluid or gas
L = length of the restriction
Q = K (
∆
P/
η
)
where:
K= a constant factor determined by the geometry of the
restriction.
