Brookfield answers your questions.
What's your Viscosity Challenge?
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9/2011 CUSTOMER CHALLENGE: We are using water to verify the calibration of our viscometer. We thought that the viscosity of water is 1cP. Our instrument is reading 1.6cP. Why is the reading so high?BROOKFIELD ANSWERS: Answer from Brookfield: Using water as a fluid to verify the viscosity reading on a rotational viscometer is not recommended by Brookfield. NIST (National Institute of Standards & Technology in Washington DC) defines the viscosity of "distilled" water to be 1cP when measured with a capillary viscometer at 20 degrees Celsius. Brookfield recommends that all customers use a Viscosity Standard Fluid from Brookfield as the requisite item to verify viscometer calibration. Viscosity Standard Fluids have a defined viscosity value which is determined by Brookfield in our Rheology Laboratory using precise temperature control when measuring the viscosity of the fluid. Use of any other material, whether water or the material that you manufacture, is not an acceptable procedure for performing a calibration check on your instrument. There are a range of viscosity values that are available as Viscosity Standard Fluids. Consult your Brookfield Catalog or go to (insert web address) for the possible choices. Submitted by: anonymous |
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5/2011 CUSTOMER CHALLENGE: How does the settling of particles over time affect my viscosity?BROOKFIELD ANSWERS: The viscosity value of a material will change when its composition or structure changes. This change in viscosity is more obvious for liquid with suspended particulates when these particles settle to the bottom. The viscosity readings will change according to the rate of particles settling, which in turn, is dependent of gravity, fluid buoyancy, particulate shape, size and density. Generally, it is common to observe a decreasing in viscosity reading as the particulate settle to the bottom because the molecular size, density, and complexity of the liquid in the upper portion where the spindle is measuring is different than the lower portion. Submitted by: anonymous |
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5/2011 CUSTOMER CHALLENGE: I need to mix my sample in order to test for viscosity. I’m afraid my mixed sample may separate out. Once mixed, how soon should I test viscosity?BROOKFIELD ANSWERS: This is dependent on the nature of the fluid, but to do this reliably, you need to set certain parameters. These parameters will include mixing, temperature, container size and time tested. Thus, for the mixing parameter this should be done with a mechanical mixer for a certain amount of time and not done by hand. Doing this by hand would add too much variability. Then, you would need to decide if a certain temperature needs to be maintained. Container size must be defined to ensure that reliable, accurate viscosity tests are conducted and are repeatable, and a time interval should be defined for testing. A synopsis of a standard operating procedure for this type of mixed material might be:
Remember that inserting a cold spindle into a sample will cause the sample to cool and give an erroneous viscosity reading. Final test parameters are up to the user based on their material. Submitted by: anonymous |
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5/2011 CUSTOMER CHALLENGE: Would the presence of particles/small solids in my fluid affect the viscosity and the viscosity test technique?BROOKFIELD ANSWERS: Yes. When particles are small (<30µ), the fluid’s viscosity will generally increase. The smaller the particle, the bigger the increase (given the same % solids). Particles this small can generally be measured with standard spindles and coaxial cylinder systems but may be a problem with cone/plate systems. Larger solids (fruits/vegetables/small children) in suspension present a different circumstance. In this case, the geometry used must capture the complete mix. Standard spindles or coaxial cylinders generally make the majority of the measurement in the liquid phase of the fluid. In some cases, the solids are pushed away from the geometry. Vane spindles (used at low speeds) can be helpful in this case. Very heavy solids which settle quickly (mine tailings, metallic solids, bu etc.) are another issue. If the settling is slow (i.e., the sample is stable for 5-15 minutes) conventional spindles may be used (assuming that the test procedure is consistent). In extreme cases (i.e. the solids settle in a matter of seconds) a vane type spindle used at high speed may be a useful "difference detector" but will not give accurate viscosity values as turbulence is required to keep the solids in suspension. Submitted by: anonymous |
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5/2011 CUSTOMER CHALLENGE: How does temperature affect viscosity, and are these changes permanent?BROOKFIELD ANSWERS: Increasing temperature will decrease the viscosity for most materials if no chemical reaction, solvent evaporation, or degradation occur. Decreasing the temperature will increase the viscosity. The changes are not permanent, and are said to be thermally reversible or "thermoreversible". Examples:< /br>Silicone liquids have decreasing viscosity upon moderate warming, but regain their initial viscosity upon cooling to their original temperature. The same is true for foods like honey, for example. Glues or adhesives are typically designed to change from liquid upon initial application to solid – the viscosity increases as the chemical reaction occurs, and the change is permanent or irreversible. Chocolates are very sensitive to temperature – heating solid chocolate melts it, and further heating decreases the viscosity. However, overheating may degrade this complex, tasty mixture. Therefore, if that sample is cooled to solidify, and then re-melted, its viscosity may not be the same as that during the first melting, even at the same temperature. Thus, overheating may permanently change the viscosity at a given temperature. Submitted by: anonymous |
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5/22/2008 CUSTOMER CHALLENGE: How Do I Mix Different Viscosities?BROOKFIELD ANSWERS: We know from experience that mixing fluids, even viscosity standard fluids which are 'Newtonian', does not have a convenient formula for combining specific volumes to come up with a known viscosity. It's a matter of trial and error to experimentally determine what works and then using that recipe going forward. Submitted by: Russ |
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5/13/2009 CUSTOMER CHALLENGE: I would like to know if there is a way to give one single viscosity measurement for a non-Newtonian solution (ie. this solution has a viscosity of this) without constraining it a specific shear rate. I was thinking of using the coefficient of thixotrophy (slope of the viscosity-1 verse rate of shear graph) but don't know if there is a better or easier way.BROOKFIELD ANSWERS: There isn't one "universal" measurement that can be used to describe a particular non-Newtonian material's rheology, to the best of my knowledge. However, as long as you clearly identify your experimental procedure, including the Brookfield viscometer's/rheometer's Spring Torque Range, Spindle and Speed, temperature, time before reading, etc., then others may be able to reproduce your results. Submitted by: anonymous |
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3/2/2008 1 CUSTOMER CHALLENGE: "How can I evaluate storage stability?"BROOKFIELD ANSWERS: Storage stability can be evaluated by testing retained samples periodically using an up-down speed or shear rate ramp. The test data provides a "flow curve" showing viscosity vs. shear rate. This signature curve will shift downward over time if the sample becomes"thinner", which implies that suspensions are settling out. If the curve shifts upward, the sample is "thickening", which implies that fluid is possibily escaping or evaporating. In either case, performing the same test each time is important to observe whether the viscosity data is changing. Submitted by: Robert from New Jersey |
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2/26/2008 CUSTOMER CHALLENGE: "How Can I Understand the Rheology of a Fluid?"BROOKFIELD ANSWERS: The preferred method used by Brookfield to understand the rheology of a fluid is to perform an up-down speed or shear rate ramp with the viscometer. The data from the test provides a "flow curve" which characterizes how "shear thinning" the material is. For most materials,viscosity decreases as shear rate increases; this type of behavior is called "pseudo plastic". Customers can do this test themselves or send a sample to us for evaluation. Please call or email if we can be of further assistance with the materials that you are evaluating. Submitted by: Sook from New Jersey |
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4/25/2008 CUSTOMER CHALLENGE: "How do I get consistent readings from Batch to Batch - cooking soups, sauces, beans, gumbo, etc?"BROOKFIELD ANSWERS: The key is to measure viscosity using a spindle and rotational speed that can provide meaningful data. Formulations like gumbos have lots of food chunks; the vane spindle or T-bar spindle should be used running at low rotational speed (usually less than 10rpm). Soups which are homogenous can easily be measured with standard disc spindles between 10 and 100rpm; if there are other ingredients like pieces of vegetables, you may want to consider the vane spindle again. Sauces require the same scrutiny, although the traditional method has more often been the T-bar spindle for thicker formulations like mayonaisse. The method for consistency is to measure each batch and record viscosity readings that must fall within a predetermined range for acceptability. Submitted by: Gardere from New Jersey |
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5/22/2008 CUSTOMER CHALLENGE: How can I better understand the Viscosity of Material Being Pumped?BROOKFIELD ANSWERS: ANSWER: Understanding the viscosity of the material being pumped can help you to properly specify a pump for a given situation. Engineering firms use standard calculations to evaluate pressure head across the pump, volume flow rate, viscosity considerations, etc. If the material is unusual inany respect, it may be beneficial to run a viscosity flow curve on the material to get a precise picture of how it behaves. If we can be of assistance, we have an office in Harlow, Essex in the UK that I can put you in touch with. Submitted by: Roger from the UK |
