Meriam M203 Altimeter Indicated Air Speed Tester - Manual (Page 7)

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2. Zeroing the Manometer
The M203 Precision Smart Manometer is a stable and precise
instrument. However, on occasion the M203 should have a new zero taken.
This is done to remove zero drift that can occur since the manometer was
last zeroed. The M203 can be zeroed only if the new applied zero is within ±1%
FS of the original factory calibration zero. This prevents accidental zeroing at
atmospheric pressure or other relatively high pressures. If outside this
limit a “ZERO RANGE ERROR” message appears and the manometer will not
zero.
The M203 provides three mechanisms for re-zeroing:
1. Referenced to Absolute Zero: This traditional and preferred method
takes a “snapshot” of the measured pressure when a vacuum of less than 100
microns Absolute is applied to the sensor.
2. Factory Zero: This method restores the calibration curve to the original
zero taken at the factory. Note that this feature is intended for comparison
purposes, and should not be used for real pressure measurement, as any
zero-drift since shipment will not be accounted for.
3. User-Adjusted Zero: This method allows the user to enter any pressure
value when a known reference is applied (for example, the local barometric
pressure reading). The manometer will compare its actual measured value
with the entered value, and calculate a new zero reference based on the offset.