Fluke MOLBOX RFM Operation And Maintenance Manual page 103

For users who prefer to avoid automatic valving, there are two other BPR modes. BPR OFF
mode keeps the molbox in the BPR OFF valve state at all times. BPR ON keeps the molbox
in the BPR ON valve state at all times. BPR OFF mode should be used with caution, as it
provides no monitoring of BPR to ensure that molbloc-S measurements are valid.
When molbox RFM is used in BPR OFF mode, molbox cannot measure the molbloc-S
downstream pressure and provides no monitoring or indication of the BPR. molbox RFM
cannot warn the user when the flow through molbloc-S is not critical and flow
measurements are not valid. For example, when the molbloc is at rest in BPR OFF mode,
and atmospheric pressure is applied to both the upstream and downstream side of
molbloc-S, the molbox will indicate a false flow rate approximately equal to the molbloc-S
K value times the value of atmospheric pressure in kPa. If BPR OFF mode is used, the
F
user must make sure that a sufficiently low BPR is maintained while making molbloc-S
flow measurements (see Section 3.1.2). The [TARE], <5BPR> function can be used for
quick BPR checks during BPR OFF (see Section 3.4.4.5).
OPERATION
To access the BPR mode menu, press [SPECIAL], <9BPR>.
The display is:
Making a BPR mode selection, actives the selected BPR mode and returns to the previous
RUN screen. [ESCAPE] returns to the previous RUN screen without changing the BPR mode.
Selecting <1BPRoff> causes, the molbox RFM internal valves to immediately switch to the
BPR OFF valve state (see Figure 12), and remain there until the user changes the BPR
mode or activates a function which causes a valve change (such as the [TARE], <5BPR>
function, see Section 3.4.4.5). In BPR OFF mode, the flow is calculated from the average of
the two RPT pressure readings and the RPT pressures displayed in the P&T pressure screen
are adjusted using the dynamic tare feature, so both pressures are displayed equal to the
average value (see Section 3.1.2 and 3.4.5)
Selecting <2BPRon> causes, the molbox RFM internal valves to immediately switch to the
BPR ON valve state (see Figure 11), and remain there until the user changes the BPR mode
or activates a function which causes a valve change. In BPR ON mode, flow is calculated
from the upstream RPT measurement only. In BPR ON mode, there is no tare applied to the
RPT readings at any time.
Selecting <3Auto> does not necessarily immediately change the BPR valve state.
Depending on the current BPR mode, BPR mode changes occur as follows:
In Auto mode, in the BPR ON valve state:
molbox RFM monitors the BPR value directly. If the BPR is safely below the BPR limit
(calculated from empirical testing of molbloc-S choking ratios, see Section 3.1.2), then, once
the flow is stable within 0.5 % of reading for 3 seconds, molbox RFM switches to BPR OFF
valve state and begins live averaging of the two RPT pressure values.
In Auto mode, in the BPR OFF valve state:
molbox RFM cannot monitor BPR, but based on the proximity of the most recently measured
BPR to the calculated BPR limit, if the current flow value changes enough that it is likely that
the BPR is approaching the limit, molbox RFM switches to BPR ON valve state to check BPR
directly. It is highly unlikely that valve switching will occur during critical flow measurements
because, typically, high precision flow measurements are not made when flow is changing.
BPR mode:
1BPRoff 2BPRon 3Auto
Page 93 © 1998-2007 DH Instruments, a Fluke Company
3. OPERATION