Brookfield DV-II+ Programmable Viscometer - Manual (Page 59)

Brookfield Engineering Labs., Inc. Page 59 Manual No. M/97-164-F1 102
The calibration of the Brookfield Viscometer/Rheometer is determined using a 600 ml Low Form Griffin
Beaker. The calibration of LV and RV series instruments includes the guard leg. The beaker wall (for
HA/HB instruments) or the guard leg (for LV/RV instruments) define what is called the "outer boundary"
of the measurement. The spindle factors for the LV, RV, and HA/HB spindles were developed with the
above boundary conditions. The spindle factors are used to convert the instrument torque (expressed as
the dial reading or %Torque value) into centipoise. Theoretically, if measurements are made with
different boundary conditions, e.g., without the guard leg or in a container other than 600 ml beaker, then
the spindle factors found on the Factor Finder cannot be used to accurately calculate an absolute
viscosity. Changing the boundary conditions does not change the viscosity of the fluid, but it does change
how the instrument torque is converted to centipoise. Without changing the spindle factor to suit the new
boundary conditions, the calculation from instrument torque to viscosity will be incorrect.
Practically speaking, the guard
leg has the greatest effect when
used with the #1 & #2 spindles
of the LV and RV spindle sets
(Note: RV/HA/HB #1 spindle is
not included in standard spindle
set). Any other LV (#3 & #4) or
RV (#3 - #7) spindle can be used
in a 600 ml beaker with or with-
out the guard leg to produce cor-
rect results. The HA and HB
series Viscometers/Rheometers
are not supplied with guard legs
in order to reduce the potential
problems when measuring high
viscosity materials. HA/HB
spindles #3 through #7 are iden-
tical to those spindle numbers in
the RV spindle set. The HA/HB
#1 & #2 have slightly different
dimensions than the correspond-
ing RV spindles. This dimen-
sional difference allows the fac-
tors between the RV and HA/
HB #1 spindles to follow
the same ratios as the instrument torque even though the boundary conditions are different.
The recommended procedures of using a 600 ml beaker and the guard leg are difficult for some customers
to follow. The guard leg is one more item to clean. In some applications the 500 ml of test fluid required
to immerse the spindles in a 600 ml beaker is not available. In practice, a smaller vessel may be used
and the guard leg is removed. The Brookfield Viscometer/Rheometer will produce an accurate and
repeatable torque reading under any measurement circumstance. However, the conversion of this torque
reading to centipoise will only be correct if the factor used was developed for those specific conditions.
Brookfield has outlined a method for recalibrating a Brookfield Viscometer/Rheometer to any measure-
ment circumstance in More Solutions to Sticky Problems, Section 3.3.10. It is important to note that for
RV Guardleg LV Guardleg