Brookfield DV-III Rheometer - Manual (Page 48)

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3) Total allowable error is (122.57 + 500) cP = ( + /-) 622.57 cP.
4) Therefore, any viscosity reading between 11,634.4 and 12,879.6 cP indicates that the Rheometer
is operating correctly. Any reading outside these limits may indicate a Rheometer problem.
Contact the Brookfield technical sales department or your local Brookfield dealer/distributor
with test results to determine the nature of the problem.
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 1) 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 #1 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. 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 recalibrating a Brookfield
Viscometer/Rheometer to any measurement circumstance in More Solutions to Sticky Problems,