3. OPERATION

In differential measurement mode (see Section 3.3.3.1), the LED Ready/Not Ready indication applies to the differential pressures.

In parallel measurement mode (see Section 3.2.4), the LED Ready/Not Ready indication applies to the average pressure read by the <HL> Q-RPT.

When RPM4 is being used as an external measurement reference of a PPC3 pressure controller, the Ready/Not Ready indication is still based on the RPM4 stability and stability limit only. The RPM4’s Ready/Not Ready LED does not indicate whether the PPC3 pressure control is Ready.

3.2.2 GAUGE AND NEGATIVE GAUGE MODES WITH AN AXXX (ABSOLUTE) Q-RPT, DYNAMIC COMPENSATION FOR ATMOSPHERIC PRESSURE

Q-RPTs with the designation Axxx are intrinsically absolute but they are also used for gauge and negative gauge measurement modes (see Section 3.3.3, PRINCIPLE). Gauge measurement modes are achieved by subtracting the value of atmospheric pressure, Poffset,G, from the Q-RPT’s absolute reading using AutoZ (see Section 3.2.2). The AutoZ routine that measures Poffset,G, is run by pressing [AutoZ] whenever RPM4 is in the vented condition. This assures the continuous updating of the Poffset,G value corresponding to atmospheric pressure. Gauge pressure is the measured absolute pressure, Pu, minus the atmospheric offset.

Pgauge = Pu - Poffset,G

However, atmospheric pressure may change between opportunities to run AutoZ and update the value of Poffset,G, for example when running an extended test without venting. RPM4 uses dynamic compensation for atmospheric pressure to correct for changes in atmospheric pressure between opportunities to run AutoZ and update Poffset,G. When AutoZ runs, and Poffset,G is determined, the reading of RPM4’s on board barometer, Patm,0, is also recorded. Later, when no longer vented, the change in atmospheric pressure, ΔPatm, since Poffset,G was updated, is the difference between the current barometer reading, Patm, and the barometer reading at the time of AutoZ execution, Patm,0:

ΔPatm = Patm - Patm,0

Dynamic compensation for atmospheric pressure uses ΔPatm to correct the value of Poffset,G, thus always compensating real time for changes in atmospheric pressure:

Pgauge = Pu - Poffset,G - ΔPatm

Gauge pressure measurement on an Axxx (absolute) Q-RPT allows instantaneous switching between gauge and absolute measurements modes. The additional uncertainty in gauge pressure mode due to the dynamic compensation for atmospheric pressure technique is a function of the resolution and short term stability of the on-board barometer, not its absolute measurement uncertainty. This additional uncertainty is ± 1 Pa (0.00015 psi).

3.2.3 MULTIPE RANGES (Q-RPTS, AUTORANGE AND INFINITE RANGING)

An RPM4 may have one or two Q-RPTs. Position indication of the currently active Q-RPT is continuously displayed in the upper right hand corner of the MAIN RUN screen and most other screens. See Table 3 for position designation protocol for the Q-RPTs in an RPM4.

See Table 1 for a complete listing of RPM4/RPM4 Q-RPT choices and their default ranges.

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© 2004-2007 DH Instruments, a Fluke Company

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Fluke RPM4 manual Multipe Ranges Q-RPTS, Autorange and Infinite Ranging