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APPENDIX F - THE AMETEK BROOKFIELD GUARD LEG
The guard leg was originally designed to protect the spindle during use. The first applications of the AMETEK
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 are attached to the dial housing and LV guard legs are
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 an AMETEK Brookfield Viscometer/Rheometer. A guard leg is supplied with every LV and RV series
instrument, but not with the HA or HB series. Its 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 the 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 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 AMETEK Brookfield Viscometer 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 to any
measurement circumstance in
"More Solutions to Sticky
Problems". 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 an 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. Our
customers should be aware of its existence, its purpose, and the effect that it may have on data. With this knowledge,
the Viscometer user may make modifications to the recommended method of operation to suit their needs.
M21-400 REV.1.1
D V P L U S D I G I TA L V I S C 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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