Brookfield DV2T Viscometer - Manual (Page 65)

Brookfield Engineering Labs., Inc. Page 65 Manual No. M13-167-B0614
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 DV2T Viscometers
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
DV2T). 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 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 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 viscometer has a stated accuracy of +/- 1% of the range in use. This stated
accuracy applies when the viscometer 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 viscometer when used with the standard
spindles supplied with the instrument, including LV spindles 1 through 4 (supplied with LV
series viscometers), RV spindles 2 through 7 (supplied with RV series viscometers), and HV
series spindles 2 through 7 (supplied with HA series viscometers and HB series viscometers)
in a 600 mL low form Griffin beaker.
Brookfield offers a range of accessories for use with the Brookfield viscometer 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
viscometer. 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 viscometer spindles as detailed above.
Once the calibration of the viscometer 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.