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Agilent electronic flowmeter

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Agilent electronic flowmeter

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The Agilent Precision gas flow meter is 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 meter. Unlike other flow measuring meters, in laminar
flow meters the relationship between pressure drop and flow is
linear. The underlying principle of operation of the Agilent
Precision gas flow meter is known as the Poiseuille Equation:
Q = (P1 – P2) πr4/8ηL (Equation 1)

Where: Q = Volumetric flow rate
P1 = Static pressure at the inlet
P2 = Static pressure at the outlet
r = Radius of the restriction
η = (eta) absolute viscosity of the fluid
L = Length of the restriction

Since π, r, and L are constant, Equation 1 can be rewritten as:
Q = K (ΔP/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 (ΔP), 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 the Gas Select Mode section in
the Operating Manual for more information). This is important
because the meter 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 ratio between the two gases’ viscosities.
Gas viscosity, and thus gas composition, can be very important
to the meter’s accuracy. Anything that has an effect on the gas
viscosity (for example, water vapor, odorant additives, etc.) will
have a direct proportional effect on the meter’s 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 < .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 that of pure methane.
Absolute viscosity changes very little with pressure (within the
operating range of this meter); therefore, a true volumetric
reading does not require a correction for pressure. Changes in
gas temperature do affect viscosity. For this reason, the
AgilentPrecision gas flow meter internally compensates for this

Additional information

Weight1 kg