Brookfield DV3T Rheometer - Manual (Page 100)

Brookfield Engineering Labs., Inc. Page 100 Manual No. M13-2100
Special Considerations
In taking viscosity measurements with the DV3T 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 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 precision of the instrument. All DV3T Rheometers have
an accuracy of +/- 1% of the range in use for any standard spindle or cone/plate spindle. (Note that
accuracy values may be higher than 1% when using accessory devices with the DV3T). 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. 2 LV Spindle: 100 cP at 200 RPM
3) No. 1 RV Spindle: 100 cP at 50 RPM
4) UL Adapter: 0.85 cP at 60 RPM
Turbulent conditions will exist in these situations whenever the RPM/cP ratio exceeds the values
listed above.
Effect on accuracy when using accessory devices
The Brookfield rheometer has a stated accuracy of +/- 1% of the range in use. This stated accuracy
applies when the rheometer is used in accordance with the operating instructions detailed in the
instrument instruction manual and the calibration test fluid is used in accordance with the instructions
provided by the fluid supplier (including the critical parameters of temperature control and stated fluid
accuracy). Brookfield’s accuracy statement of +/- 1% of the range in use applies to the Brookfield
rotational rheometer when used with the standard spindles supplied with the instrument, including LV
spindles 1 through 4 (supplied with LV series rheometers), RV spindles 2 through 7 (supplied with
RV series rheometers), and HV series spindles 2 through 7 (supplied with HA series rheometers and
HB series rheometers) in a 600 mL low form Griffin beaker.
Brookfield offers a range of accessories for use with the Brookfield rheometer to accommodate
special measurement circumstances. These accessories, while offering added capability to the user,
also contribute to an expanded measurement tolerance beyond the instrument accuracy of +/- 1% of
the range in use. This expanded measurement tolerance is a function of many parameters including
spindle geometry, accessory alignment accuracy, sample volume requirement, and sample introduction
techniques. The effect of these elements on measurement tolerance must be considered when
verifying the calibration of your Brookfield rheometer. Sample temperature in all test circumstances
is very important, and will also add an additional expanded tolerance depending on the temperature
control system and the calibration verification tests begin with the standard rheometer spindles as
detailed above. Once the calibration of the rheometer itself is confirmed, the expanded tolerance of
the measurement system may be determined using accessory devices. In many cases, this additional
tolerance will be very minimal, but as a general statement, the addition of +/- 1% of the range in use
is reasonable for accessories.