DH Instruments COMPASS FOR MOLBOX, Руководство пользователя

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COMPASS
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for molbox™ User’s Manual
Page 69
© 1998-2003 DH Instruments, Inc .
8.5.3 [CORRECTION] TAB
The [Correction] DUT Definition tab is provided to accommodate DUTs that are sensitive to
gas density. COMPASS can perform a square root or proportional gas density correction
based on the characteristics of the flowing gas, the DUT’s normal operating pressure and
temperature and the current gas pressure and temperature. This feature allows a density
sensitive DUT that is intended to be operated at a given pressure and temperature to be
tested under current pressure and temperature conditions.
In general, gas density corrections do not apply to thermal mass flow controllers and meters
(MFCs and MFMs). A density correction is commonly used on rotameters, turbine meters and
other devices whose operating principle is the measurement of volume flow and a conversion
to mass flow. If the DUT outputs in mass flow units (e.g., sccm, mol/s, mg/s), density
corrections, if needed, are almost certainly performed by the DUT itself. See the molbox
Operation and Maintenance Manual, Unit Section for additional information and the Section 5
of this manual for examples.
For COMPASS to perform gas density corrections, the normal operating pressure and
temperature of the DUT along with the molecular mass and the compressibility factor of the
DUT’s process gas must be specified in the DUT Definition.
The information entered is used to determine the density of the flowing gas under the normal
operating conditions of the DUT. The density is then applied to the reference flow based on
the density correction method selected, the resultant value is the reference flow under the
DUT’s normal operating conditions. This information is displayed on the
<DUT/Ref
Comparison> window and logged in the <Data Grid> (see Sections 19.5, 18.4.1).
The purpose of the [Correction] tab fields and settings as well as instructions on how to use
them are provided in Table 17.
Selecting a <Gas Density Correction Type> other than <None> will cause the reference flow
to be corrected as a function of gas density. This correction is only valid if a) the DUT is
known to be gas density sensitive; b) the DUT’s gas density sensitivity corresponds to one
of the <Gas Density Correction Types>; c) the DUT’s normal operating pressure and
temperature can be correctly specified, d) the pressure and temperature of the gas flowing
when the DUT is tested can be determined. In any other case, specifying a gas density is
likely to cause erroneous data by causing incorrect calculation of <Ref Q DUT Conditions>.
If a density correction is selected and the <Process Gas> is also a COMPASS supported
<Calibration Gas>, all gas specific entries on the <Process Gas> panel are disabled, and
COMPASS will automatically calculate the gas density based on the normal operating
pressure and temperature data entered. Any change in the pressure and temperature
fields triggers the re-calculation of the normal operating gas density.