RPM4™ OPERATION AND MAINTENANCE MANUAL
3.5.1 <1AU T O Z>
PURPOSE
To offset the RPM4 Q-RPT(s) readings relative to a reference value in order to compensate for possible changes in Q-RPT zero between full recalibrations. In differential measurement mode, to compensate for the difference between the reading of the Hi and Lo Q-RPTs at the “line pressure” at which differential measurements are made.
To assure operation within measurement uncertainty specifications (see Section 1.2.2), it is
recommended that AutoZ be run (the value of Poffset updated) whenever vented in gauge mode, whenever the line pressure is changed in differential mode and at least every 30 days or when
RPM4 has been exposed to temperature changes exceeding 15 ºC (36 ºF) in absolute mode.
PRINCIPLE
AutoZ Purpose and Principle
The main component of the change over time of the RPM4 Q-RPTs is change in zero or offset, independent of span. Offsetting or “rezeroing” RPM4 Q-RPTs relative to a reference between recalibrations allows measurement uncertainty specifications to be maintained with less frequent full calibrations. AutoZ is also used to compensate for the difference between the Hi and Lo Q-RPT at the “line pressure” at which differential measurements are made. The RPM4 AutoZero function (AutoZ) provides full on-board support for the rezeroing process to simplify its application by the user.
The AutoZero function uses three values:
1.Pstd,0: The pressure value indicated by the AutoZ reference, the device that is acting as the reference relative to which to offset the RPM4 Q-RPT.
For Axxx (absolute) Q-RPTs in absolute measurement mode, the pressure at which
AutoZ is performed is normally atmospheric pressure and the Pstd,0 value can be supplied
a)by manual entry, b) automatically from a DHI RPMx connected to RPM4 COM2 communications port, or c) automatically from an Axxx Lo Q-RPT if one is present in the RPM4.
For Gxxx or BGxxx (gauge) Q-RPTs or Axxx (absolute) Q-RPTs in gauge measurement mode, Pstd,0 is always zero (atmospheric pressure) which is supplied by definition when the Q-RPT is vented to atmosphere.
For RPM4s with two Q-RPTs of the same type (Axxx, Gxxx, BGxxx) with the Hi Q-RPT in differential measurement mode, Pstd,0 is the pressure indicated by the Lo Q-RPT when the Hi and Lo Q-RPTs are connected to a common pressure.
2.Pu,0: The pressure reading of the Q-RPT, with no AutoZ offset, at the time AutoZ is performed.
3.Poffset,G , Poffset,A and Poffset,D: The difference between the reading of the Q-RPT with no AutoZ offset (Pu,0) and the indication of the AutoZ reference (Pstd,0) for gauge (G), absolute(A) or differential (D) measurement mode:
Poffset = Pu,0 - Pstd,0
Poffset represents the difference between the Q-RPT reading (Pu,0) and the AutoZ standard (Pstd,0). The AutoZ function manages the determination, storage and application of Poffset to correct the zero offset in absolute, gauge and differential measurement modes.
When the Q-RPT is calibrated, Poffset is set to zero. Poffset is then redetermined at regular intervals using the AutoZ function. The most recent value of Poffset is applied to the Q-RPT reading to correct for change in zero.
