Fluke RPM4 manual AutoZ in absolute measurement mode with an Axxx Q-RPT

3. OPERATION

The AutoZ function can be turned ON and OFF. Table 7 summarizes the effect of AutoZ ON and OFF for the three Q-RPT types in different measurement modes.

AutoZ in absolute measurement mode with an Axxx Q-RPT

Q-RPTs with the designation Axxx are intrinsically absolute. They have an evacuated and sealed reference. Axxx Q-RPTs can measure absolute pressure (difference from vacuum).

In absolute measurement mode, the source of Pstd,0 must be an absolute pressure, nominally atmospheric pressure, with uncertainty significantly better than that of the Q-RPT that is being AutoZeroed. For higher range Q-RPTs, this is easily accomplished with a variety of digital barometers or with a Lo Q-RPT in the RPM4. For lower range Q-RPTs, a piston gauge or other standard may be required to provide Pstd,0 with low enough uncertainty.

When using an Axxx (absolute) Q-RPT in absolute measurement mode, with AutoZ ON, absolute pressure is calculated as:

Pabs = Pu - Poffset,A

In absolute measurement mode, running the AutoZ function to update the AutoZ value (Poffset,A), is initiated by the operator using [AutoZ].

AutoZ in gauge and negative gauge measurement modes with an Axxx Q-RPT, dynamic compensation for atmospheric pressure

Q-RPTs with the designation Axxx are intrinsically absolute but they are also used in gauge and negative gauge measurement modes (difference from atmosphere) (see Section 3.3.3,

PRINCIPLE). Gauge measurement mode is achieved by subtracting the value of atmospheric pressure from the Q-RPT’s absolute reading using AutoZ and by dynamically compensating for changes in atmospheric pressure between opportunities for AutoZ to execute (see Section 3.2.2).

In gauge measurement modes on an Axxx Q-RPT, the value of Pstd,0 is always zero gauge pressure. Zero gauge pressure, by definition, is applied to the Q-RPT when it is vented to atmosphere. In gauge measurement modes, AutoZ is run when initiated by the operator pressing [AutoZ] when the RPM4 is vented. This assures regular updating of the Poffset,G value corresponding to atmospheric pressure. Gauge pressure is the measured absolute pressure, Pu, minus Poffset,G.

Pgauge = Pu - Poffset,G

When AutoZ is ON, dynamic compensation for atmospheric pressure is also applied to compensate for changes in atmospheric pressure as measured by a barometer (ΔPatm) between AutoZ updates (see Section 3.2.2). The measured gauge pressure is calculated using ΔPatm to correct the value of Poffset.G.

Pgauge = Pu - Poffset.G - ΔPatm

In gauge measurement mode on an Axxx RPT, running the AutoZ function to update the value of Patm,0, is initiated by the operator using [AutoZ]. Updating of ΔPatm occurs automatically and continuously.

When gauge or negative gauge mode is first activated on an Axxx Q-RPT, the value of

Poffset.G subtracted from the absolute pressure reading to achieve gauge pressure is standard atmosphere of 101.325 kPa. This can cause a large zero error when gauge mode

is activated if atmospheric pressure is significantly different from standard atmospheric pressure. When gauge mode is activated, always press [AutoZ] to run AutoZ and obtain an actual value of Poffset.G .

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