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Operating Principle
All M-Series Gas Flow Meters (and MC-Series Gas Flow Controllers) 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.
Please visit the Alicat web site for a detailed explanation this principle.
alicat.com/technical-information/theory-of-operation/
STANDARD GAS DATA TABLES: Those of you who have older Alicat products
may notice small discrepancies between the gas property tables of your old and
new units. Alicat Scientific, Inc. has incorporated the latest data sets from NIST
(including their REFPROP 9 data where available) in our products’ built-in gas
property models. Be aware that the calibrators that you may be using may be
checking against older data sets such as the widely distributed Air Liquide data.
This may generate apparent calibration discrepancies of up to 0.6% of reading on
well behaved gases and as much as 3% of reading on some gases such as propane
and butane, unless the standard was directly calibrated on the gas in question.
As the older standards are phased out, this difference in readings will cease
to be a problem. If you see a difference between the Alicat meter and your in-
house standard, in addition to calling Alicat Scientific at (520) 290-6060, call the
manufacturer of your standard for clarification as to which data set they used in
their calibration. This comparison will in all likelihood resolve the problem.
Using Method 2, values between 0 and 64000 are acceptable which correspond
linearly to -100% full scale flow and 100% full scale flow, respectively. The formula
to perform linear interpolation for a bidirectional controller is as follows:
Value = (desired se full scale) X 64000 / (2 X full scale)
For example, when changing the setpoint on a 100 SLPM full scale bidirectional
controller to -35 SLPM, the following value should be entered.
20800 = (-35 SLPM + 100 SLPM) X 64000 / (2 X 100 SLPM)
Additional Serial Commands
For more advanced serial communication commands, please contact Alicat
or view the User’s Guide to Advanced Serial Programming at
alicat.com/
knowledge/documents-resources
