Brookfield DV-I Prime - Manual (Page 50)

Brookfield Engineering Laboratories, Inc. Page 50 Manual No. M/07-022-D0613
When taking viscosity measurements with the DV-I PRIME Viscometer, there are two
considerations, which pertain to the low viscosity limit of effective measurement.
1. Viscosity measurements should be taken within the equivalent % Torque Range from 10%
to 100% for any combination of spindle/speed rotation.
2. Viscosity measurements should be taken under laminar flow conditions, not under turbulent
flow conditions.
The first consideration has to do with the accuracy of the instrument. All DV-I PRIME Viscometers
have a Full Scale Range allowable error of (+/-) 1% of any spindle/speed in use. 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 cannot be analyzed with standard math models. This turbulence
creates a falsely high viscometer 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 to the onset of turbulent
flow occurs in the following situation:
1) No. 1 LV Spindle: 15 cP at 60 RPM
2) No. 1 RV Spindle: 100 cP at 50 RPM (optional spindle available from Brookfield)
3) UL Adapter: 0.85 cP at 60 RPM
Turbulent conditions may 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, and 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.