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3.1
GENERAL OPERATING PRINCIPLES
3.1.1
molbloc-L AND molbloc-S OPERATION
molbox RFM operates somewhat differently depending on whether a molbloc-L or molbloc-S is
connected to it. mobloc-L and molbloc-S operation use different displays and flow calculations and
some menu items are present for only one type of molbloc.
Most molbox RFM settings, such as gas, units, K factor, etc., are common to both molbloc-L and
molbloc-S operation. Changes made to these settings while operating one type of molbloc
will still be in effect when the other type of molbloc is connected. The only setting that is
used for both molbloc types but is stored independently for each type is tare. See section
3.4.4.1 for details on the tare function.
Several of the molbox RFM screen displays and functions described in this section are
different for molbloc-L and molbloc-S operation.
description of these functions is divided into two parts.
3.1.2
molbloc-S BPR LIMITS
To make flow measurements within predictable measurement uncertainty limits with a molbloc-S
flow element, critical (sonic) flow conditions must be present. Critical flow exists when the
gas velocity reaches the local speed of sound at the throat of the molbloc-S venturi nozzle.
molbox RFM uses the back pressure ratio, or BPR (the ratio of the molbloc-S downstream
absolute pressure to the upstream absolute pressure) to determine whether the flow is critical.
For venturi nozzles in general, the BPR must remain below a certain value for critical flow to
exist. Commonly accepted practice for typical venturi nozzle use suggests that this limiting
BPR value, or "choking ratio", is approximately 0.5. That is, the absolute pressure downstream
of the nozzle must be less than one half of the absolute pressure upstream of the nozzle.
Empirical study of the venturi nozzles used in molbloc-S shows that the actual choking ratio,
or maximum BPR for critical flow, varies between about 0.4 and 0.9 as a function of the
Reynolds number (Re) over which the molblocs are used.
calculates Re during flow measurement and can monitor the BPR to ensure that it does not
exceed the choking ratio at the current Re conditions. molbox RFM uses a conservative BPR
limit to indicate to the user when the BPR approaches the choking ratio, to ensure that flow
measurements are only made under "safe" critical flow conditions. molbox RFM includes
features to measure BPR, automatically alert the operator when the BPR is too high and
prevent measurements when flow is not critical (see Sections 3.1.3.2, 3.4.4.5, 3.6.9).
Maintaining a sufficiently low BPR must be considered by molbloc-S users when selecting
molbloc-S sizes and hardware setups to use for flow measurements. For example, if a
molbloc-S will be used with atmospheric pressure downstream, then the molbloc can only be
used over a range of upstream pressures starting at the maximum pressure for its calibration
type down to a minimum pressure value at which the BPR becomes equal to the BPR limit
calculated by molbox RFM.
upstream absolute pressure, the flow range for the molbloc in this application is defined by
this BPR limit. To maximize the range of a molbloc-S element, a vacuum pump can be
connected downstream to reduce the downstream pressure while flowing.
downstream pressure is kept sufficiently low, the upstream pressure, and thus the mass flow
rate, can be adjusted all the way down to the minimum value for the molbloc's pressure
dependent calibration type without being limited by the BPR value.
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Since mass flow through molbloc-S is proportional to the
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Where the differences are major, the
molbox RFM continually
© 1998-2007 DH Instruments, a Fluke Company
3. OPERATION
When the
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