Meriam Laminar Flow Elements - Manual (Page 3)

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Introduction
Because of their inherently high accuracy, stable calibration, excellent response time and repeatability,
Laminar Flow Elements (LFEs) excel in critical gas and air flow measurements and are frequently utilized
in validating calibration standards. Standard models are available to measure as little as 0.2 SCCM (5.9
E-06 SCFM) to as much as 2250 SCFM at standard conditions. Custom models for up to 15,000 SCFM of
air are available. Stainless steel or aluminum materials make LFEs compatible with most gases and flow
rate of gas mixtures can also be measured when the percentages of component gases and mixture properties
are known. Higher accuracies can be achieved when used in conjunction with the Meriam MDT500
multivariable transmitter and software package. The LFE matrix is made from individual SS tubes or
windings of SS foil, these tubes are long enough, relative to their inside diameter to cause laminar flow to
occur inside each tube, the result is a near linear relationship between DP and flow rate. The DP generated
across the matrix responds very quickly to changes in flow and pressure loss to the system is reduced as
each LFE is sized to produce no greater than 8” water column at maximum flowing conditions. Individual
tube diameters are very small so flowing gases need to be clean and dry to preserve the calibration.
Filtered inlet versions of most LFE models are available to keep the matrix clean and the calibration
constant.
This manual covers models 50MK10, 50MW20, 50MR2, 50MJ10, 50MC2 and 50MY15. For descriptions,
dimensions and capacities of these elements refer to Meriam Bulletin, File No. 501:215
Special Precautions When Handling Matrix Elements
The Meriam LFE depends on the precise fabrication of a matrix metering element for its basic
accuracy. These elements are manufactured from .001 inch stainless steel stock and are carefully
fabricated. Exercise extreme care when handling the exposed element to make sure the end faces are
not gouged or damaged in any fashion. Gouges or damage to the end surfaces may produce nonlinear
resistance to flow and introduce error. If the end surface becomes damaged, the accuracy of the
element may be restored by recalibration at the factory.
Inspection
1. Make sure you have unpacked all instructions and other data that accompanied the unit.
2. Visually inspect for any signs of damage. There must be no nicks or scratches, surfaces of the
LFE should be clear.
3. Units are shipped with a cap plug in each opening which protects the ends and pressure taps.
Remove these cap plugs.
4. Visually inspect matrix surface and inside housing. No capillaries should be blocked unless
specified in special applications.
Installation
Make sure the line is free of dirt and other foreign materials. The metered gas must be clean. In-line
use of filters is recommended. Connections to the differential pressure instrument should be made
with equal lengths of 1/4" I.D hose, tubing or pipe. All instrument connections must be leak- free.
Install temperature sensor 2-diameters upstream of the element. Inlet absolute pressure instrument,
when needed, must be connected close to the LFE at the inlet pressure tap. Figure 1 shows several typical
LFE installations.
Install the LFE in the line using hose connectors, flanges, tubing or pipe, as desired. Position the LFE
in any orientation. Horizontal is the most common. Orient the high-pressure and low-pressure sensing
ports in any angular direction. Flow must be in the direction of the arrow on the LFE. Avoid
disturbances upstream of the LFE. Good measurement practices dictate an adequate straight run of the
pipe up and downstream of the element. In most installations, 10 diameters upstream and 5
diameters downstream are adequate.
Where installation makes straight pipe runs impossible, LFE's can be calibrated with piping
configurations that duplicate installation. This special calibration assures installed accuracy. In these
applications, consult Meriam regarding calibration.