Brookfield DV2T Viscometer - Manual (Page 80)

Brookfield Engineering Labs., Inc. Page 80 Manual No. M13-167-B0614
Appendix G - The Brookfield Guardleg
The guard leg was originally designed to protect the spindle during use. The first applications
of the Brookfield Viscometer included hand held operation while measuring fluids in a 55-gallon
drum. It is clear that under those conditions the potential for damage to the spindle was great.
Original construction included a sleeve that protected the spindle from side impact. Early RV
guard legs attached to the dial housing and LV guard legs attached to the bottom of the pivot
cup with a twist and lock mechanism.
The current guard leg is a band of metal in the shape of the letter U with a bracket at the top
that attaches to the pivot cup of a Brookfield Viscometer/Rheometer. Because it must attach
to the pivot cup, the guard leg cannot be used with a Cone/Plate instrument. A guard leg is
supplied with all LV and RV series instruments, but not with the HA or HB series. It’s shape
(shown in Figure G-1 and G-2) is designed to accommodate the spindles of the appropriate
spindle set; therefore, the RV guard leg is wider than the LV due to the large diameter of the
RV #2 spindle. They are not interchangeable.
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 without the guard leg to produce correct 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 identical to those
spindle numbers in the RV spindle set. The HA/HB #1 & #2 have slightly different dimensions
than the corresponding RV spindles. This dimensional difference allows the factors between
the RV and HA/HB #1 & #2 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 re-calibrating a Brookfield Viscometer/Rheometer to any measurement circumstance in