PURPOSE: Corrects for the natural zero drift of the sensor (AKA: “Q-RPT”). Without it, then the stability specification is expanded by +0.005% of Q-RPT span. (see the “Principle” sub section of sec 3.5.1 in the RPM4 Operation and Maintenance manual)
IMPLEMENTATION: AutoZero is implemented differently for absolute mode operation than it is when the RPM4 is operated in gauge mode. For both modes of operation, the AutoZero function has to first be enabled before it can be used. The top right hand corner of the RPM4 display will indicate the active Q-RPT and immediately to the left will be a lower case “z” to indicate that AutoZero is enabled. For example, “zHi” indicates the high pressure Q-RPT is active and the AutoZero function has been enabled. If the “z” is missing then the function has not yet been enabled.
ENABLING: (sec 3.5.1),(“operation” sub section)
To enable AutoZero, press the [SPECIAL] key and use the <1AutoZ> menu to enable.
NOTE: AutoZero is measurement mode dependent. Just because it’s enabled on the Hi Q-RPT in gauge mode does not mean that it will be enabled in Absolute mode.
USING AUTOZERO in GAUGE MODE: (sec 3.3.9.1)
- First, enable the function
- Press the [AUTOZ] button when the RPM4 is vented and stable. The result will be a displayed pressure of “0.000”.
USING AUTOZERO in ABSOLUTE MODE: (sec 3.3.9.2)
The AutoZero offset must be determined by comparison to an equal or higher accuracy standard. This makes the process more complex. AutoZero is determined at atmospheric pressure. Any pressure standard can be used for the comparison reference so long as it has better uncertainty at atmospheric pressure then the Q-RPT which is being zeroed.
- First, enable the function
- Press the [AUTOZ] button when the RPM4 is vented and stable.
- Choose the mode of comparison. The most common option is <1Entry>.
- If <1Entry> is selected, then type in the pressure value for atmospheric pressure as the “Pstd” value. Press [ENTER] to activate.
December 2015
M. Daniels