Specifications

molbox™ RFM™ OPERATION AND MAINTENANCE MANUAL
© 1998-2007 DH Instruments, a Fluke Company Page 40
• AUTOZ function: To periodically offset the molbox RFM RPTs relative to a reference
pressure value in order to compensate for possible changes in the RPT zero between full
recalibrations (see Section 3.4.4.4).
• BPR function (present only in molbloc-S operation): To measure and display the
molbloc-S back pressure ratio (BPR) while operating in a BPR mode, which does not
continuously read and display BPR (see Section 3.4.4.5).
 OPERATION
Pressing [TARE] accesses a display with the choice
of four functions (five functions with molbloc-S).
Select the desired function. See the following for
principles and operation:
1tare 2purge
3leak check 4AutoZ ↓
5BPR
Some screens (e.g., the Tare menu) may go beyond the two lines provided by the display.
This is indicated by a flashing arrow in the second line of the display. Press the [←] and
[→] keys to move the cursor to access the lines that are NOT visible or directly enter the
number of the hidden menu choice if you know it.
3.4.4.1 <1TARE>
The purpose and operation of the tare function is different for molbloc-L and
molbloc-S operation. Tare values are generated and stored independently for
the two molbloc types. For example, a tare value generated during molbloc-L
operation is not used during molbloc-S operation, but is saved and used again
when a molbloc-L is connected to the molbox.
Tare is described separately for the two molbloc types below.
3.4.4.1.1 molbloc-L OPERATION
 PURPOSE
To zero the molbox RFM differential pressure reading at the molbloc
operating pressure. Zeros the differential between the two high pressure
absolute transducers as well as the microrange transducer if present.
 PRINCIPLE
The molbox RFM TARE function can be considered the equivalent of the
ZEROING function performed on many instruments prior to
making measurements. molbox RFM calculates the flow through the molbloc
from the differential pressure across the molbloc. The differential pressure
across the molbloc is measured by taking the difference in the absolute pressure
measured by the molbox RFM’s two internal RPTs. One RPT is connected to
the upstream molbloc pressure port and the other to the downstream molbloc
pressure port. If the molbox RFM has the microrange option (see Section 3.1.5),
when the differential pressure is under 12.5 kPa (1.8 psi), the microrange
option’s low differential RPT is used as the source of the differential pressure
measurement.
If a common pressure is applied to both absolute transducers (and both legs of
the microrange option differential RPT if present) the differential pressure
indicated should be zero. If a differential pressure is observed, the value
indicated represents an offset in the differential measurement, which will appear
as an offset or “zero error” on the flow through the molbloc calculated by the
molbox RFM. The TARE function allows the differential indication between the
two RPTs to be zeroed at the molbloc operating pressure to eliminate the zero
error in differential pressure measurement.