Brookfield Viscosity Explained
Brookfield viscosity usually refers to a viscosity measurement performed with a Brookfield Viscometer, sometimes refered to as a Brookfield viscosimeter. There are several models of viscometer available from Brookfield but the majority operate in the same manner: the viscometer motor rotates the spindle at a defined speed (measured in rpm) or shear rate and the viscometer measures the resistance to rotation and reports a viscosity value. Various spindle designs can be employed, depending on the nature of the sample and the requirements. Here is a brief introduction to the most popular arrangements:
Dip-in LV Spindles & RV Spindles
Labelled as LV or RV spindle sets, these comprise simple shafts ending in a disk or cylinder. A sample of 400 - 600ml in a suitable container is placed under the viscometer which is then lowered to dip the spindle into the sample up to an immersion mark on the spindle shaft. The dip-in spindle is suitable for comparative testing of the viscosity of free-flowing fluids.
Small Sample Adapter (SSA)
The Small Sample Adapter is a concentric cylinder measuring system, often know as a coaxial or "cup and bob" system. In the small sample adapter a small sample (typical only 5 - 10ml) is sheared between the moving inner cylinder and stationary outer cylinder or chamber. The accessory was, as its name suggests, originally designed for measuring samples of limited quantity however due to its fairly narrow clearances the system operates in a degree of defined shear - a valuable capability of you wish to perform viscosity / shear rate profiles on a sample.
Ultra Low Adapter (ULA)
The ULA is nothing more than a large concentric cylinder designed specifically for bringing lower viscosity liquids into the measuring range of the viscometer.
T-Bar and Helipath
This accessory enables the viscometer to take measurements of soft solids such as pastes, creams and ointments. The Helipath system is a special stand that incorporates a motor that can slowly drive the head of the viscometer down. At the same time the viscometer motor is set to rotate at a low rate (typically 2 - 5 rpm) and the downward motion of the head causes the t-bar spindle to cut a helical path down through the sample. Best practice here is to take several measurements over a period of 30 to 60s and take an average.
So here's the kicker: All the above accessories will deliver a different result when measuring the same sample! So which is the right one? Well that's out of the scope of this article (get some advice) but it is clear that any reported viscosity specification for a material must therefore quote the relevant instrument set-up and measurement conditions.
The most critical parameters that should be included in such a specification include:
- Torque range: In order of increasing torque (and hence viscosity) range: LV, RV, HA or HB. The torque range usually prefixes the model number, for example: RVDVI+ or LVDVII+.
- Spindle e.g. RV2, LV3, T-C, Speed of rotation: 10rpm etc
- Measurement temperature
For many samples the following must also be stipulated:
- Container size and fill level details
- Instrument model: for example DVI+, DVII+ etc
- Measurement time: How long you run the test before taking a reading - critical for thixotropic samples.
- Sample pre-conditioning instructions: Pre-mixing or rest periods.
So a typical spec may look like: Brookfield LVDVII+, 500ml of sample in 600ml beaker, spindle 3 at 20rpm, 25°C, take measurement after 30s.
Here are some typical reported test methods and viscosity specs for a range of products:
- Viscosity of anti-icing fluid, ASTMD2196 Method B: Brookfield LVT Viscometer with Small Sample Adapter SC4-31/13R, 0.3rpm
- Viscosity of resin for plastisols: Brookfield LVT or equivalent, spindle 3 at 12 rpm, temperature 25°, measure viscosity after 60s
- Viscosity of gel-coats: Brookfield RV viscometer, spindle 4 at 4rpm, 25°C.
- Brookfield "thixotropic index" for gelcoats: The ratio of RV4 spindle viscosity at 4rpm to 20rpm at 25°C. Note: "The term "thixotropic index" for this test is now outdated and the test is usually re-labelled "shear-thinning index"
- Aluminum hydroxide gel viscosity for antacid formulations: Brookfield LV spindle 2 at 12rpm
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