Fluke molbox RFM Operation And Maintenance Manual Download

Page: 40 / 172

molbox™ RFM™ OPERATION AND MAINTENANCE MANUAL

Page 30

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

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:

SF6 flow x 0.970 = equivalent SiH4 flow for the MFC

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

gas conversion

factors are based on the properties of the device

being

tested. Their availability and validity are the responsibility of that

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:

K Factor?

1on 2off