PPC2 AF OPERATION AND MAINTENANCE MANUAL
© 1998 DH Instruments, Inc. Page 102
AUTOZERO BY COMPARISON AT ATMOSPHERE ERRORS
1. For autozero to proceed when "seeking autozero", stability of ± 0.000 15 psi
(1 Pa)/second as measured by the Lo reference pressure transducer must be met.
If the stability test is not met within 60 seconds, then "Autozero error: ATM,
stability timeout" will be displayed. Press ESCAPE or ENTER to return to the
autozero menu. Correct the source of instability and retry.
2. If the value of PA(z)net resulting from the current autozero routine exceeds ± 0.15 %
of full scale of the Hi reference pressure transducer (1.5 psi, 100 kPa), the PA(z)Net
value determined will be displayed with the message, "ERROR: data rejected".
A PA(z)net greater than ± 0.15 % F.S. is assumed to indicate improper autozero
conditions, faulty calibration of the Lo and/or Hi reference transducer or damage to a
reference transducer. Press ENTER or ESCAPE from this error to return to the
autozero menu.
3. If the value of PA(z)net resulting from the current autozero routine exceeds ± 0.015 %
but is less than ± 0.15 % of full scale of the Hi reference pressure transducer (0.15,
10 kPa to 1.5 psi, 100 kPa), the PA(z)Net value determined will be displayed with the
message, "WARNING: high value". A PA(z)net greater than 0.015 % F.S. may indicate
improper autozero conditions, faulty calibration of the Lo and/or Hi reference
transducer or damage to a reference transducer. Press ENTER to proceed to the
"New PAzNet/ Old PAzNet" screen or ESCAPE to return to the autozero menu.
The autozero by comparison at atmosphere routine is always run in range H3 but the results
are used to readjust PA(z)net for all the ranges of the Hi pressure reference transducer.
5.2.2 LO REF (BY VACUUM)
PURPOSE
To automatically rezero (offset) the Lo reference pressure transducer (see Section 3.4.4.1).
PRINCIPLE
The autozero routine for the Lo reference pressure transducer uses the pressure present at a
specific point in the rate of change of pressure curve of the low pressure reference transducer
as it goes down from atmosphere in response to the application of vacuum as a repeatable
reference point.
The Lo reference transducer is isolated at atmospheric pressure from the AUTOZERO port by
a solenoid valve (see Section 1.2.7). A vacuum is applied to the AUTOZERO port. The Lo
reference transducer autozero isolation valve is opened applying the vacuum to the Lo
reference transducer. The transducer's response to the application of vacuum under these
controlled conditions is a repeatable rate of change of pressure curve. In subsequent
applications of vacuum, at the same rate of change of pressure point, the same pressure is
applied. This point is not assumed to be accurate in the absolute sense; it is a repeatable
baseline against which to measure zero drift over time.
The pressure indicated by the Lo reference transducer at the repeatable pressure point is
recorded. The value recorded [PA(z)raw], less the "natural" reading at that point [PA(z)tare],
represents the zero drift since the last calibration of the Lo reference transducer [PA(z)net].
The "natural" reading [PA(z)tare], is the pressure indicated by the Lo reference transducer at
the repeatable pressure point just after the Lo reference transducer has been calibrated.
To Next Page
Loading...