1. INTRODUCTION
Page 1
© 1998-2007 DH Instruments, a Fluke Company
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1.1
PRODUCT OVERVIEW
molbox RFM is a support unit for making low mass flow measurements using molbloc mass
flow elements. molbox RFM reads calibration data off the molbloc EEPROM and measures molbloc
upstream and downstream pressure using built-in high accuracy Reference Pressure
Transducers (RPTs). An ohmic measurement system reads the resistance of the molbloc platinum
resistance thermometers from which molbloc temperature is calculated. Using the molbloc calibration
data, pressures, temperature and gas properties stored in memory, the flow rate of the gas flowing
through the molbloc is calculated. A microrange option is available to increase resolution and accuracy
below 10 % FS of the molbloc-L flow range.
Internal molbox RFM valving supports on-board PRESSURE TRANSDUCER TARING, LEAK TESTING
and SELF PROTECTION functions as well as a gas purge routine.
molbox RFM provides a local user interface via a front panel key pad and display and includes advanced
on-board functions. Remote communication capability is supported with RS232 and IEEE-488 interfaces.
molbox RFM is intended for applications in which a highly compact presentation, high range ability and
lower cost are the primary considerations. A second model, molbox1, is available for applications where
higher accuracy is the most important requirement.
1.1.1
MOLBLOC FLOW ELEMENTS
Two different types of molblocs may be used with molbox RFM; molbloc-L (laminar) and
molbloc-S (sonic).
1.1.1.1
MOLBLOC-L FLOW ELEMENT
molbloc-L is the original molbloc laminar flow element. molbloc-L covers the
lower portion of the molbloc/molbox system flow range. The key molbloc-L
measurement is the differential pressure across the element, which is roughly
proportional to the mass flow rate through it. molbloc-L elements are calibrated
to be used at an absolute pressure that remains nearly constant, while the
differential pressure varies with flow rate. Different operating pressure options
and their effect on molbloc flow range are described in Section 1.2.4.1.2.
1.1.1.2
MOLBLOC-S FLOW ELEMENT
molbloc-S elements use critical (sonic) flow venturi nozzle technology to
measure flows, which overlap with the higher molbloc-L ranges and extend the
high end of the molbloc/molbox RFM system flow range. The mass flow rate
through a molbloc-S element is roughly proportional to the upstream absolute
pressure when the flow is “choked”, so the molbloc-S operating pressure can
vary widely as the mass flow rate is changed throughout the flow range. The
limits of molbloc-S operating pressure and flow ranges are defined by the
molbloc-S calibration type, described in Section 1.2.4.2.2.
