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7. Calibration
The Accubar under goes a rigorous screening and testing at the factory before it is shipped to ensure that it meets the
accuracy specifications over temperature and that it is stable both in zero and span and will continue to be accurate
over time. The drift of the ACCUBAR
®
in the field is typically less than 0.05 mB per six months. This is a small
fraction of the stated accuracy of the Accubar. The stated accuracy of the ACCUBAR
®
is 0.5 mB, although typically
it is much better. Even though it would typically take over 5 years for the ACCUBAR
®
to drift as much as 0.5 mB, it
is recommended that the Accubar, like all precision measurement instruments, be placed on a periodic calibration
schedule.
Factory Calibration
The initial factory calibration typically includes over 600 different pressure temperature points that cover the
complete temperature range from -40 to +60 degrees C and cover the complete pressure range from 600 to 1100 mB.
This ensures that the ACCUBAR
®
meets the specifications over the complete pressure and temperature range. A
calibration verification encompasses nearly 300 points over the complete temperature and pressure range. An
Accubar can be sent back to the factory for a complete calibration over temperature. The Sutron Part number for this
calibration service is: 8700-0005
Metrology Lab Calibration
The ACCUBAR
®
does have the provision for a calibration to be performed by a Metrology Lab. The calibration
coefficients that can be entered by a metrology lab are a scale and an offset parameter. There is not a provision for
any temperature dependent parameters. Most metrology labs would not have the capabilities to provide a calibration
over temperature and pressure. To accurately calibrate an ACCUBAR
®
requires a reference accuracy on the order of
0.1 mB.
To collect data for calibration of the Accubar, the M1 command should be utilize. This will ensure that field offset
or old calibration coefficients do not affect the accuracy of the data collected. The command that is utilized to enter
the Metrology Lab Coefficients is:
aXCosc!
where a is the address character, XC is the extended command to set the
calibration coefficients, o is the offset in mB, s is the scale factor, and c is the
checksum.
The Checksum is the 8 bit sum of the 7 bit ASCII characters (parity is striped) from and including the address
character through the last character of the scale factor. It does not include the checksum string nor its delimiting
polarity sign. The checksum is transmitted as an ASCII string. That means that if the 8-bit sum is 236 then the value
of c is +236. The command to set the offset of the unit at address 0 to 0.0000 and the scale factor to 1.0000 would
be:
0XC+0+1+130!
The calibration coefficients affect the output of the M command, the RS-232 output, the analog output, and the
quadrature output. This command does not affect the output of the M1 command. The data output by the M
command is computed from the following equation:
