Brookfield DV-III Ultra Rheometer - Manual (Page 104)

Brookfield Engineering Laboratories, Inc. Page 104 Manual No. M98-211-E0912
In taking viscosity measurements with the DV-III Ultra Rheometer there are two considerations which
pertain to the low viscosity limit of effective measurement.
1) Viscosity measurements should be accepted within the equivalent % Torque Range from
10% to 100% for any spindle/speed combination.
2) Viscosity measurements should be taken under laminar flow conditions, not under turbulent
flow conditions.
The first consideration has to do with the precision of the instrument. All DV-III Ultra Rheometers
have a full scale range precision of (+/-) 1% of any spindle/speed combination. We discourage taking
readings below 10% of range because the potential viscosity error of (+/-) 1% is a relatively high number
compared to the instrument reading.
The second consideration involves the mechanics of fluid flow. All rheological measurements of
fluid flow properties should be made under laminar flow conditions. Laminar flow is flow wherein all
particle movement is in layers directed by the shearing force. For rotational systems, this means all fluid
movement must be circumferential. When the inertial forces on the fluid become too great, the fluid can
break into turbulent flow wherein the movement of fluid particles becomes random, and the flow can
not be analyzed with standard math models. This turbulence creates a falsely high Rheometer reading,
with the degree of non-linear increase in reading being directly related to the degree of turbulence in
the fluid.
For the following geometries, we have found that an approximate transition point to turbulent flow
occurs:
1) No. 1 LV Spindle: 15 cP at 60 RPM
2) No. 1 RV Spindle: 100 cP at 50 RPM
3) UL Adapter: 0.85 cP at around 70 RPM
4) SC4-18/13R: 1.25 cP at around 240 RPM
Turbulent conditions will exist in these situations whenever the RPM/cP ratio exceeds the values listed
above. The viscosity at which turbulence starts is still at best a guess. Because it is a relationship
between viscous and inertial forces, it can vary dramatically from fluid to fluid. Turbulence starts as a
small deviation or increase in viscosity for a Newtonian fluid and grows quickly. Basically there is no
specific shear that it starts at, only an approximate region of shear depending on the fluid.