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molbox™ RFM™ OPERATION AND MAINTENANCE MANUAL
process gas is toxic or corrosive or simply because it is not available or convenient to use.
When the process gas cannot be used for calibration, it is common to use a different gas for
testing or calibrating (the calibration gas). In this case, a factor representing the relationship
between the calibration gas and the process gas for the test device may be applied so that
the calibration gas simulates the process gas. The calibration gas that simulates the
process gas is often called a surrogate gas for the process gas.
The relationship between a test device's process gas and calibration gas is frequently
called a K factor or gas conversion factor. The factor's value depends on specific properties
of the test device and determining the value is the responsibility of the device manufacturer.
For example, Silane (SiH4) is a frequently used gas in semiconductor processing. SiH4 is
highly toxic and requires extensive handling precautions so it is not practical for use in
calibration and testing. A major manufacturer of Mass Flow Controllers (MFCs) recommends
that MFCs that are to be used with Silane be tested with Sulfur Hexafluoride (SF6), a non-
toxic, non-flammable gas, using a conversion factor of 0.970. In other words:
The K function in molbox RFM allows a gas conversion factor to be entered by which flow
values measured by molbox RFM will be multiplied. In this example, 0.970 would be entered
as K so that the values indicated by molbox RFM when SF6 is flowing through the molbloc
simulate the flow of SiH4 for that manufacturer's MFC.
When the K function is active, molbox RFM performs all of its flow calculations normally but
multiplies the current flow value by the value of the K factor prior to displaying it.
The gas selected on molbox RFM which is displayed in the upper right corner of the molbox
RFM display should always be the gas that is actually flowing through the molbloc.
K factors or gas conversion factors are based on the properties of the device
being tested.
device's manufacturer. molbloc/molbox does not use factors or conversion coefficients
between gases. Flow is calculated from molbloc characteristics and specific gas
properties for each gas supported by molbox. The gas selected on molbox RFM
(see Section 3.4.2) which is displayed in the upper right corner of the molbox RFM display
should always be the gas that is actually flowing through the molbloc.
If the K function and the ADJ function are both active, the ADJ adder and multiplier are
first applied to the measured molbloc flow, then the result is multiplied by the K factor.
This order of operations reflects the fact that the ADJ values are intended to represent
an adjustment to be applied to the molbloc flow while measuring the calibration gas that
is flowing, and the K factor is a correction related to gas effects on the device under test
measurement (See Section 3.4.6).
OPERATION
To enable a gas conversion factor press [K] from any
run screen. The display is:
If <2off> is selected, no conversion factor will
be applied. If <1on> is selected, the next screen is:
The value of the gas conversion factor can be edited as desired. Pressing [ENTER] returns
to the MAIN run screen with the entered K factor active.
appended to the flow unit indication in the run screens when the K function is ON
(e.g., sccmK). A K factor value of 1 is handled as if the K function were OFF.
© 1998-2007 DH Instruments, a Fluke Company
SF6 flow x 0.970 = equivalent SiH4 flow for the MFC
Their
availability
and
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validity
are
the
responsibility
K Factor?
1on 2off
K Factor:
1.00000
The letter <K> is always
of
that
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