1Tare; Figure 5. Molbox1+ Internal Pneumatic Schematic - Fluke MOLBOX1 PLUS - V 6.00 Operation And Maintenance Manual

MOLBOX1+™/MOLBOX1+S, MOLBLOC TERMINAL OPERATION AND MAINTENANCE MANUAL
3.4.4.1
The purpose and operation of the tare function is different for molbloc-L and
molbloc-S operation. It is described separately for the two modes below. The
molbox1+S does not support the ability to tare as it does not support laminar
molblocs.
3.4.4.1.1

To zero the molbox1+ at the molbloc operating pressure. Zeros the differential
reading between the two reference pressure transducers.

The molbox1+ TARE function can be considered the equivalent of the zeroing
function performed on many instruments prior to making measurements.
molbox1+ calculates the flow through a molbloc-L 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 molbox1+'s two
internal RPTs. One RPT is connected to the upstream molbloc pressure port
and the other to the downstream molbloc pressure port.
If a common pressure is applied to both RPTs the differential pressure indicated
should be zero. If a differential pressure value 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 molbloc-L calculated by the molbox1+.
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.
When the TARE function is activated, molbox1+'s internal valves operate to
pneumatically connect together the two absolute transducers at the molbloc
operating pressure (see Figure 5). The user can select whether this pressure is
the upstream or downstream molbloc pressure so that the tare can be made at
the pressure that will be held stable during operation (generally by a regulator or
because it is open to atmosphere). Activating the tare causes molbox1+ to
record the current differential as the tare value. The tare value is then used to
correct all subsequent RPT readings. The tare value is the difference between
the two transducer readings (hi - lo). After taring, the upstream RPT is corrected
by (- tare value/2) and the downstream RPT is corrected by (+ tare value/2).
© 1995 - 2010 Fluke Corporation, DH Instruments Division
<1Tare>
molbloc-L Operation
PURPOSE
PRINCIPLE
Figure 5. molbox1+ Internal Pneumatic Schematic –
TARING CHANNEL A, UPSTREAM molbloc-L OPERATION
Page 52
1. Channel A High Isolation: Open
2. Channel A Low Isolation: Closed
3. Channel B High Isolation: Closed
4. Channel B Low Isolation: Closed
5. Bypass: Open