DH Instruments FPG8601, Руководство по эксплуатации и обслуживанию

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7. PISTON-CYLINDER MAINTENANCE
Page 119 © 2007 DH Instruments, a Fluke Company
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Proper adjustment of the needle valve is required to continue. Improper adjustment will
result in apparent non repeatability and lead to an incorrect effective area determination.
7.8.4 TAKING DATA (FOR EACH PRESSURE)
Prior to taking data make sure that the piston gauge piston is floating and rotating within
the proper limits. Use the automatic rotation keys as required on a PG7000 to rotate the
piston and masses.
Always take data at approximately the same piston rotation rate. When using a PG7000,
a rotation rate of 5 RPM or less is recommended.
Use the manual data acquisition average toolbar (see Section 6.4.2) option to average
the pressure for the duration of at least 1 piston rotation (15s).
Allow the piston gauge piston to stop rotating. Restart the rotation and repeat step
above. At least 3 independent averages should be taken at each test pressure. Zeroing
and verifying the internal calibration of the FPG is supported but typically not necessary.
7.8.5 CALCULATING THE EFFECTIVE AREA
Determine the difference in pressure between the piston gauge and the FPG (PG-FPG).
Convert the difference to mPa.
Perform a standard linear regression on the pressure difference [mPa] relative to the
FPG pressure [kPa]. Do not force the regression through zero.
Take the slope from the regression and apply it to the following formula to determine the
correction to the effective area of the piston-cylinder.
( )
m A A
test e
6
10 1
− + =
The 10
-6
value is a unit conversion factor used to convert kPa to mPa. If other pressure
units are used, make sure that the slope value applied to the effective area is
dimensionless. If the difference between the FPG and PG is expressed in kPa and the FPG
pressure is expressed in kPa, there is no need for a conversion factor. When mPa and kPa
are used, the slope is a PPM value that gives a direct indication of how well the
instruments agree.
Table 41.
Effective Area Calculation Variables
VARIABLE DEFINITION
Ae Calculated effective area m
2
A
test
The effective area of the FPG piston in m
2
. This value is
available in the FPG calibration setup (see Section 6.6.3).
m Calculated slope determined from the linear regression.
When the FPG pressure is in kPa and the pressure
difference is in mPa, the value has the unit (mPa/kPa).