OFITE, 11302 Steeplecrest Dr., Houston, TX 77065 USA / Tel: 832-320-7300 / Fax: 713-880-9886 / www.ofite.com
18
Calibration
The HTHP Viscometer is designed to be calibrated with certified calibration
fluids available from OFITE. These fluids are Newtonian silicone oils and are
available in a number of viscosities. The viscosity tables supplied with the
fluids are traceable to the National Institute of Standards (NIST).
The viscometer will retain its calibration as long as the torsion spring Module
is not disassembled and is kept clean and corrosion free. Removing the
spring Cage Assembly from the top of the Coupling as a unit should have
only a minor effect on the mechanical zero.
The ORCADA
®
calibration program should be run prior to running any test on
the unit. It is suggested that a calibration should be performed when:
- The unit has been sitting unused for several weeks.
- The unit has been serviced.
-
Periodically go into the “Calibrate Shear Stress” screen and check the
“Shear Stress Raw” value. If the calibration signal drifts greater than
50% from original setting, recalibrate.
According to API Recommended Practice 10B-2, viscometers being used
for testing well cement should be calibrated quarterly. API Recommended
Practice 13B-1 and 13B-2 specify viscometers being used for drilling fluids
should be checked monthly.
The calibration fluid should be stored at room temperature and away from
direct sunlight. The shelf life of the calibration fluid is 2 years. The calibration
fluid used during the calibration program should be checked for suspended
solids and clarity prior to being stored. Cloudy calibration fluid should be
disposed of properly.
If you intend to re-use the calibration fluid, pour the used fluid into a clean,
sealable bottle for storage. This will prevent the used fluid from contaminating
the new fluid. Then, for the next calibration, pour the used fluid into the test
cell and, if necessary, top off with new fluid. Do not mix used fluid with new.
The ORCADA
®
software has an automated calibration program that will work
with calibration fluids of known viscosity.
1. Turn on the power to the viscometer and let the unit warm up for
approximately 15 minutes.
2. Prepare for a test by following the instructions on page 9.
3. From the main screen in the ORCADA
®
software, choose “Calibrate
Shear Stress” from the “Utilities” menu.
4.
Notice the “Shear Stress Raw” reading, it should display a value of 150 to
300. If the signal reading does not fall within this range, the unit must be
zeroed for calibration.