
Model 35 Viscometer Instruction Manual
208878
Revision N, February 2013
28
7
Test Analysis
This section explains how to obtain the test results, which includes reading values
from the viscometer and performing calculations.
The reference for measuring viscosity and performing calculations is API
Recommended Practice for Field Testing Water Based Drilling Fluids, API RP
13B-1
/
ISO 10414-1.
7.1
Newtonian Viscosity Calculation
The Newtonian viscosity in centipoise (cP) may be read directly from the dial when
the viscometer speed is 300 rpm and the rotor-bob-torsion spring combination is
R1-B1-F1. Other springs may be used provided that the dial reading is multiplied
by the "f" factor (spring constant) to calculate the viscosity.
To calculate Newtonian viscosities in centipoise with the Fann viscometer, use the
following equation:
ƞ
N
= S x
θ
x f x C
Equation 7-1
where,
S is the speed factor (Refer to Table 7-2)
θ
is the dial reading
f is the spring factor (Refer to Table 7-3)
C is the rotor-bob factor (Refer to Table 7-1)
ƞ
N
is the Newtonian viscosity (cP)
Example: Using an R2-B1 combination at a speed of 600 rpm with an F5.0 spring,
and a dial deflects to 189, the viscosity is
ƞ
N
= 0.5 x 189 x 5 x 0.315 = 149 cP
Rotor-bob-torsion spring combinations with the larger gaps are
likely to give results that differ from these figures. For best
accuracy, calibrate with a standard fluid having a viscosity near
the range of interest and using the R-B-F combination to be used
in the test.