7. MAINTENANCE OF TRACEABILITY AND RECALIBRATION
Page 167
© 2007 DH Instruments, a Fluke Company
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7.1
PRINCIPLES OF GFS2102 TRACEABILITY MAINTENANCE
The GFS2102 system is designed to provide state of the art measurement uncertainty specifications with
a very high degree of confidence over time while minimizing maintenance requirements.
The maintenance of traceability and associated recalibration of the GFS2102 system requires
metrological maintenance procedures that are performed locally at regular intervals (see Sections 7.2 and
7.3) and the regular recalibration of certain system elements (see Sections 7.4 and above).
7.2
[PC TIMER CALIBRATION]
The
GFS Tools [Tools][PC Timer Calibration]
menu choice is provided to adjust and/or compare the
internal clock of the host PC to the calibrated clock of the LCM (see Figure 108). The internal clock of the
host PC is used to integrate flow from a DUT to obtain a mass. For this reason, it is important for the PC
clock and LCM clock to agree. However, perfect agreement is not critical. Flow is inherently an average
quantity. Integrating a flow that updates ever second with a time that has error off ±5 ms will not yield a
significant flow integration error.
Use the displayed
Time Calibration
form to perform the comparison. Press
[Start]
to begin the process.
The LCM and PC clock comparison information displays on the screen while the test is active. Allow the
clocks to run for at least one hour then press
[Stop]
. A predicted span correction is provided. Press
[Apply]
to use this new span correction to adjust the PC clock. Repeat the process as many times as desired then
close the form to return to the main program.
7.3
MOLBOX AUTOZ AND TARE OPERATIONS
Although not sold directly as part of the GFS system, the molbox is considered a necessary and integral
piece of the system. In order to maintain the best performance of the GFS over time when used with a
molbloc/molbox, the molbox must have its AutoZ and Tare functions utilized periodically. Refer to the
molbox Operation and Maintenance Manual for the proper operation of the AutoZ and Tare functions.
Any molbloc-L low flow (< 500 sccm) test definition in GFS Tools should include a molbox Tare before
each flow point, and any test higher than that should include a molbox Tare before the first flow point. If
molbloc-S is in regular use with the GFS, the AutoZ function should be manually performed monthly.
7.4
REFERENCE MASS RECALIBRATION
The reference mass is recalibrated by redetermining the true mass value. It is recommended that the
reference mass be recalibrated at the end of the first and second years of operation. Then, based on the
observed stability, a longer calibration interval may be assigned. Though other organizations may be
able to perform these calibrations, it is recommended that a DHI Calibration Service be used, if possible.
The reference mass, all three mounting legs, and all three leg screws MUST be calibrated as one mass.
Any replacement parts will require the recalibration of the assembly as one mass.