Alicat PCU Portable Calibration Unit - Manual (Page 18)

18
Operating Principle
All PCU Gas Flow Meters are based on the accurate measurement of volumetric flow. The volumetric
flow rate is determined by creating a pressure drop across a unique internal restriction, known as a
Laminar Flow Element (LFE), 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 flow meters the relationship between pressure drop and flow
is linear. The underlying principle of operation of the 16 Series flow meters is known as the Poiseuille
Equation:
Q = (P
1
- P
2
)�r 4 /8ηL (Equation 1)
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
L = Length of the restriction
Since �, r and L are constant; Equation 1 can be rewritten as:
Q = K (∆P/η) (Equation 2)
Where K is a constant factor determined by the geometry of the restriction. Equation 2 shows the linear
relationship between volumetric flow rate (Q) differential pressure (∆P) and absolute viscosity (η) in a
simpler form.
Gas Viscosity: In order to get an accurate volumetric flow rate, the gas being measured must be
selected (see Gas Select Mode, page 9). This is important because the device calculates the flow rate
based on the viscosity of the gas at the measured temperature. If the gas being measured is not what
is selected, an incorrect value for the viscosity of the gas will be used in the calculation of flow, and the
resulting output will be inaccurate in direct proportion to the difference in the two gases’ viscosities.
Gas viscosity, and thus gas composition, can be very important to the accuracy of the meter. Anything
that has an effect on the gas viscosity (e.g. water vapor, odorant additives, etc.) will have a direct
proportional effect on the accuracy. Selecting methane and measuring natural gas for instance, will
result in a fairly decent reading, but it is not highly accurate (errors are typically < 0.6%) because
natural gas contains small and varying amounts of other gases such as butane and propane that result
in a viscosity that is somewhat different than pure methane.
Absolute viscosity changes very little with pressure (within the operating ranges of these meters) therefore
a true volumetric reading does not require a correction for pressure. Changes in gas temperature do
affect viscosity. For this reason, the PCU internally compensates for this change.