3. OPERATION
Page 35
© 1998-2007 DH Instruments, a Fluke Company
as well as units of volume flow (i.e., ccm). See Table 11 for a complete listing of the unit
conversions available. molbox RFM can also display the measured flow in two different units
of measure simultaneously (see Section 3.4.6.5).
OPERATION
The UNIT function is used to set the unit of measure of the molbox RFM flow display. To
change the flow unit of measure press
[UNIT]
.
The display is:
1sccm 2slm 3uccm
4pccm 5mg/s 6vlm
Select the desired unit. For all units except
<uxxx>
or
<vlm>
, operation then returns to the
run screen with the flow unit of measure changed to the selected unit. When user units (i.e.,
uxxx) are selected, the reference temperature must be specified before the unit is activated
(see Section 3.4.3.3). When “vlm” (
volume
) units are selected, a menu of
volume
units is
accessed. The desired volume unit must be selected and then the temperature and pressure
of the flowing gas must be specified (see Section 3.4.3.4).
See Section 7.1.3 for specific molbox RFM flow unit conversion calculations.
molbox RFM supports many more flow units of measure than the six default units of the
UNIT function. The six units available under the UNIT function can be customized to
include any molbox RFM supported units in any order (see Section 3.4.3.5).
Many different types of flow units are commonly used including a wide variety of mass
flow units as well as volume flow units. Please read Sections 3.4.3.1 through 3.4.3.5 for
additional information on the various unit definitions and how they are handled by molbox
RFM before making unit of measure selections.
3.4.3.1
MASS FLOW VS. VOLUME FLOW
COMPASS for molbox software users: conversions to volume (sometimes
called actual) flow units are handled in COMPASS. When using COMPASS,
the molbox always operates in mass flow units.
molbox RFM measures
mass flow
(quantity of material per quantity of time).
molbox RFM always calculates flow in terms of kg/second [kg/s]. It also supports
conversions of kg/second to a variety of other flow units. These include other
mass flow units such as g/s and mole/s as well as volumetrically based mass
flow units (i.e., sccm and slm) (see Section 3.4.3).
In steady state flow, mass
flow is the same at different points in the flow system independent of gas
pressure and temperature
. Therefore, the measurement of mass flow made by
the molbloc/molbox represents the mass flow at the same time at other points in
a steady state flow system.
molbox RFM can also make conversions to
volume flow
under specific pressure
and temperature conditions by dividing the mass flow by the density of the gas
under the specific pressure and temperature conditions (see Section 3.4.3.4).
Volume flow is sometimes referred to as
actual
flow.
