DH Instruments PPC2 AF - Autozero of Reference Transducers

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5. MAINTENANCE, ADJUSTMENTS AND CALIBRATION

5.2 AUTOZERO OF REFERENCE TRANSDUCERS

 PURPOSE

To change the offset (zero) of the Lo and Hi reference pressure transducers relative to a reference value.

 PRINCIPLE

The main component of drift over time of the PPC2 AF reference pressure transducers is zero drift or

offset, independent of span. The autozero routines (AutoZ) allow the Lo and Hi reference transducers to

be rezeroed relative to a reference between recalibrations. This allows the measurement accuracy

specification to be maintained with less frequent full calibrations.

The autozero routines use changes over time in the difference between a reference transducer and a

stable zero reference to correct the reference transducer for zero drift. The "natural" reference transducer

disagreement with the zero reference [PA(z)tare] is determined at the time the reference transducer is

calibrated. Between calibrations, the autozero routine measures the current disagreement between the

reference transducer and its zero reference [PA(z)raw]. The difference between the current disagreement

and the "natural" disagreement [PA(z)net = PA(z)raw - PA(z)tare] represents the reference transducer’s

zero drift and is added to PA to compensate for zero drift(see Section 5.4.1 PRINCIPLE, PA/PM Coefficients

The Lo reference transducer (50 psia) is autozeroed relative to a vacuum applied to the PPC2 AF

AUTOZERO port. The Hi reference transducer (1 000 psia) is autozeroed at atmospheric pressure

relative to the Lo reference transducer.

The AutoZ function supports the automated routines used to apply the reference zero pressure to the

reference transducer and performs the calculations and data handling necessary to update and maintain

the autozero values.

TO ASSURE MAINTENANCE OF PPC2 AF REFERENCE TRANSDUCER MEASUREMENT

SPECIFICATIONS, IT IS RECOMMENDED THAT THE AUTOZERO ROUTINES BE EXECUTED

AT LEAST EVERY SIXTY DAYS OR WHEN THE SYSTEM HAS BEEN EXPOSED TO

TEMPERATURE CHANGES

EXCEEDING ± 20 ºC (36 ºF).

PA(z) Calculations:

PA(z) =

Reading of reference transducer

at zero reference pressure with

no AutoZ value applied

- Zero reference value

PA(z)tare = PA(z) determined just after

calibration.

PA(z)raw = PA(z) determined between

calibrations

PA(z)net = PA(z)raw - PA(z)tare

In regular operation, PA(z) net is subtracted from PA:

Reference transducer

pressure

= (Uncorrected pressure • PM) + (PA - Pa(z)net)

(See Section 5.4.1 Principle, PA/PM Coefficients)