Model 35 Viscometer Instruction Manual
208878
Revision N, February 2013
23
6
Instrument Calibration Check
A calibration check only verifies the instrument’s correct
mechanical operation — its torsion springs, bearings, and shaft.
Changing the rotors and bobs only reconfigures the geometry of
the shear gap. These changes do not affect the torsion springs,
bearings, or shaft. Therefore, calibration is not required when
changing rotors or bobs.
Periodically, the Model 35 viscometer should be checked for proper calibration. If
the measurements do not meet the specified accuracy, then the viscometer should
be calibrated or repaired. For continuous accurate measurements, the instrument
must be properly calibrated.
In accordance with API 13B-1 and 13B-2, Fann recommends
calibrating the Model 35 before it is placed in service and at least
monthly while it is in service. However, calibration frequency
depends on your usage and laboratory quality assurance
program.
The calibration is checked by applying known torques to the bob shaft. For any
applied torque, within the torque range of the spring, there should be a specific dial
reading (plus or minus a small tolerance). There are two methods of calibration
check — 1) dead weight calibration check, and 2) standard fluid calibration check.
If the spring requires adjustment, the proper setting can be easily verified.
The standard fluid calibration check verifies that the complete instrument is
operating properly. This calibration method will identify a bent bob shaft, rotor
eccentricity, and/or runout of the rotor or bob more effectively than the dead weight
method.