Brookfield DV-E Viscometer - Manual (Page 32)

Brookfield Engineering Labs., Inc. Page 32 Manual No. M98-350-J0912
Appendix F - 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.
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. A guard leg is
supplied with all LV and RV series instruments, but not with the HA or HB series. It’s shape
is designed to accommodate the spindles of the appropriate spindle set; therefore, the RV
guard leg is wider than the LV, as shown in Figure F-1, due to the large diameter of the RV
#1 (optional) 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”, Section 3.3.10. It is important to
note that for many viscometer 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 should be known that this type of torque reading will
not convert into a correct centipoise value when using a Brookfield factor if the boundary
conditions are not those specified by Brookfield.