DH Instruments PPC2 AF - Hi Ref (by Comp at Atm)

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PPC2 AF OPERATION AND MAINTENANCE MANUAL

 OPERATION

To access the autozero, press 1AutoZ under 4Internal of the

SPECIAL menu. The display is:

Autozero

1View 2Edit 3Run

Selecting 1View allows the currently used values of PA(z)net and PA(z)tare (see Principle above) with the

date that the PA(z)net was determined to be viewed. Values for each range are stepped through by

pressing ENTER repeatedly. (See Section 5.2.3)

Selecting 2Edit allows the value of PA(z) net to be changed. (See Section 5.2.3)

Selecting 3Run leads to running the autozero sequence. (See

Sections 5.2.1 and 5.2.2) The display is:

Autozero ref sensor

1Hi 2Lo

Select 1Hi to run the autozero routine for the Hi (1 000 psia) reference transducer (see Section 5.2.1 Hi ref

by comp at atm).

Select 2Lo to run the autozero routine for the Lo (50 psia) reference transducer (see Section 5.2.2 Lo ref by vacuum).

Access for editing autozero values independently of the automated autozero routines is provided in

2Cal of 4Internal under the SPECIAL menu. Running the PA(z)tare autozero routine or editing and

activating PA(z)tare values causes the value of PA(z) net to go to zero. (See Section 5.4.4.5)

5.2.1 HI REF (BY COMP AT ATM)

 PURPOSE

To automatically rezero (offset) the Hi reference pressure transducer (see Section 3.4.4.1).

 PRINCIPLE

The autozero routine for the Hi reference pressure transducer uses the Lo reference pressure

transducer as a stable reference value against which to measure zero drift. This is valid since

the reference transducer zero drift is proportional to the transducer full scale and the full scale

of the Hi reference transducer is 20 times greater than the full scale of the Lo reference

transducer.

The Hi and Lo reference transducer readings are compared at atmospheric pressure and the

difference between the two is measured. The difference found [PA(z)raw], less the "natural"

difference [PA(z)tare], represents the zero drift since the last calibration of the Hi reference

transducer [PA(z)net]. The "natural" difference [PA(z)tare] is the difference found between

the two reference pressure transducers just after they have been calibrated.