Brookfield DV3T Rheometer - Manual (Page 113)

Brookfield Engineering Labs., Inc. Page 113 Manual No. M13-2100
Appendix F - The Brookfield Guardleg
The guard leg was originally designed to protect the spindle during use. The first applications of the
Brookfield Rheometer 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 F-1 and F-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 More Solutions to Sticky Problems. It is important to note that
for many rheometer users, the true viscosity is not as important as a repeatable day to day value. This
repeatable value can be obtained without any special effort for any measurement circumstance. But, it