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APPENDIX G - THE AMETEK BROOKFIELD GUARD LEG
The guard leg was originally designed to protect the spindle during use. The first applications of the AMETEK
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 AMETEK 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) 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 AMETEK 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 avail-
able. In practice, a smaller vessel may be used, and the guard leg is removed. The AMETEK 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. AMETEK Brookfield has outlined a method for re-calibrating a AMETEK 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 should be known that this
type of torque reading will not convert into a correct centipoise value when using a AMETEK Brookfield factor if the
boundary conditions are not those specified by AMETEK Brookfield.
The guard leg is a part of the calibration check of the AMETEK Brookfield LV and RV series Viscometer/ Rheometer.
Our customers should be aware of its existence, its purpose, and the effect that it may have on data. With this
knowledge, the rheometer user may make modifications to the recommended method of operation to suit their
needs.
M19-2101 REVISION 2.0
D V N E X T R H E O M E T E R - O P E R AT I O N A L M A N U A L
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