Meriam Laminar Flow Elements - Manual (Page 5)

5
Calibration Curve Table
Meriam performs an air calibration of the LFE using a master flowmeter that is traceable to the National
Standards and Technology (NIST). The calibration data is standardized to an equivalent dry gas flow rate at
70ºF (21.1ºC) and 29.92 inches Hg absolute (101.3 kPa abs.). (The customer may request another standard
condition such as OºC.) It is then possible to determine the actual or standard volumetric flow rate at your
flowing conditions. Each LFE has at least one calibration curve or table. The standard curve/table is for
dry air flow rate in units of cubic feet per minute (CFM) versus the differential pressure (DP) in inches of
water referenced to 4ºC produced by the LFE. You may request a curve/table for a different gas and/or for
different flow rate units. Each curve/ table is generated using a quadratic (second order) equation.
B x DP + C x DP² = Flow
The calibration constants B and C are printed on each curve/table.
Actual Volumetric Flow Rate
The LFE determines the actual volumetric flow rate. To obtain the actual volumetric flow rate, the
differential pressure across the LFE and the inlet temperature to the LFE is measured. Using the
calibration curve/table associated with the particular LFE; a flow rate value is
obtained by either
1) reading the value from the curve/table or:
2) using the formula, B x DP + C x DP² = Flow
Each curve has unique constants B and C. Multiply this flow rate value by the ratio:
(viscosity of flowing gas at 70°F in micropoise)
(viscosity of flowing gas at flowing temperature in micropoise)
or
μstd / µf. The product is the actual volumetric flow rate. (The curve/table lists the type of gas used
to generate the curve/table.)
Actual volumetric flow rate= (B x DP +C x DP²) x (μstd μf
ൗ )
or MDT 500
Laminar Flow Systems
Figure1: Typical Installations of Laminar Flow Elements Actual, Standard, or Mass Flow