DH Instruments COMPASS FOR MOLBOX, User Manual

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COMPASS

for molbox™ User’s Manual
© 1998-2003 DH Instruments, Inc .
Page 190
made and their uncertainties may be poorly quantified, testing of density sensitive devices should always
be made under conditions as close as possible to the DUT’s normal operating conditions.
Although the density correction is specified by the DUT, the actual correction is applied to the reference
flow to determine the reference flow in the DUT’s normal operating conditions, i.e., what the flow through
the DUT would be if it were operating under its normal operating conditions, not the current operating conditions.
The square root density correction multiplies the reference mass flow by the square root of the ratio of the
normal operating conditions gas density to the current measured gas density. The proportional density
correction multiplies the reference mass flow by the simple ratio of the normal operating conditions gas
density to the current measured gas density. The corrected reference mass flow is then converted to the
DUT’s flow unit of measure in the normal manner (see Section 19.2). Keep in mind that the correction
determines the flow in the normal operating conditions, so any actual flow unit conversion must assume
that these are the actual conditions.
Gas density corrections always use the reference flow converted to mg/s. The density correction ratio
is applied to this value to get the corrected DUT flow in mg/s. The flow is then converted from mg/s to
the DUT’s flow unit to define the DUT’s corrected flow under it’s normal operating conditions.
The equations below detail the two density correction types.
ρ
=
MP
ZRT
now
op
ref
Q Q
Cond ρ
ρ
× =
1
Q Q
Cond
ref
op
now
2
1
2
= ×






ρ
ρ
VARIABLE DESCRIPTION
ρ
Gas density. The subscript “op” specifies under normal operating
pressure and temperature and “now” specifies the current conditions.
M Molecular mass of the gas in g/mol.
P Absolute gas pressure in Pascal.
R Ideal gas constant
Z Gas compressibility.
T Gas temperature in Kevin.
Q(ref) Reference mass flow.
Q(cond2) Reference mass flow in the DUT’s normal operating conditions
determined using the square root density correction. This value
must be converted from the reference mass flow unit, to the DUTs
flow unit.
Q(cond1) Reference mass flow in the DUT’s normal operating conditions
determined using the proportional density correction. This value
must be converted from the reference mass flow unit, to the DUTs
flow unit