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VISCOSITY FORUM: Ask the experts

How do you test construction materials for “flowability”?

Measuring Viscosity? Kinematic or Dynamic Method. How to Choose?

11/21/13 at 11am EST

An Interview With Pat Maggi

Capillary and rotational viscometers are used around the world to measure viscosity. Why are there two different methods? Does each have certain advantages that recommend its use for certain categories of materials? This interview will review various test methods that encompass one or the other technique and provide guidance on how best to proceed when in doubt on the choice. read more

How do you test construction materials for “flowability”?

How do you test construction materials for “flowability”?

11/6/13 at 11am EST

Dr. Guy Rosenthal Interview

How do you test construction materials for “flowability”? For example, when you open a tub of joint compound, does it have the right consistency? Do you know right away when you stick your trowel in? Or is the proof of the pudding in how it spreads? read more

Life Below the Yield

Viscometer vs. Texture Analyzer

10/23/12

An Interview with Ross Clark

When does a viscometer make sense and when does a texture analyzer make sense? read more

 

BROOKFIELD RESPONDS: Q+A

03/2014

CUSTOMER CHALLENGE:We have a Brookfield Digital Viscometer DV2+. We have been recording a lot of under torque (<10%) readings on our stability samples within the last few months. We perform daily system calibration checks on the viscometer with a Brookfield viscosity standard fluid. Both are performed using a T-bar spindle. When the operator was questioned about the measurement data for our samples, we noted that the torque values are lower during upward motion of the Helipath. If we remove entrapped air from the sample and the spindle is placed in a different location, the torque measurement will increase above 10%. Upon inspection of the instrument, we found that the T-bar has a slight bend that was not visible by sight observation, but could be noticed when the bar was laid on a flat surface.
We have removed the bent T-bar from service. Would this have given us these low torque results? Also I was unsure if improper equilibration of our sample could contribute to these low torque results. We just received this instrument back from its annual servicing by Brookfield. This low torque was already occurring before this service and is happening again now after it has been returned. I am aware that to improve torque we could increase out rpm, however I am limited to our testing specifications. So I was wondering what other items I could look into that could cause our lower torque readings.

BROOKFIELD ANSWERS:

A torque reading of >10% is concerning because the relative error for the viscosity measurement is a potentially large number. For this reason, we advise customers not to use readings below 10% on the torque scale.

More importantly, when you are running the calibration check on the viscometer, you cannot use a T-bar spindle. This must be done with a disc spindle and the fluid at 25C. Here is a link to a video on our web site that explains this:

http://www.brookfieldengineering.com/support
/videos.asp

Look at the procedure for Standard Disc Spindle Calibration.

When using the Helipath and T-bar spindle, best practice is to take data in a downward direction only. Make sure that the torque reading has stabilized, especially when running at low speed > 1rpm.

Some suggestions for improving your test method all necessitate a change in the procedure. Rotating the spindle at a higher speed is the easiest and most straightforward. Changing to a viscometer model with higher toque capacity is the next choice. The final consideration is different spindle geometry, such as cone plate, that could produce a more stable reading.

Brookfield can test samples in our Rheology Lab and provide alternative methods upon request.

02/2014

CUSTOMER CHALLENGE: We produce certain thickeners in cleaning and personal care products. We want to purchase a new viscometer (although local rep suggests a rheometer). In our understanding, the rheometer can perform the function of the viscometer (and more), the downside is the inability to match the standard industry conditions for QA/QC purposes. Is this correct in general terms? Any other difference or disadvantage to consider? Finally, please advise which model of rheometer with accessories is most suitable for our application (personal care and cleaning products such as shampoos, conditioners, gels, etc).

BROOKFIELD ANSWERS:

Our customers generally find the R/S Rheometer provides several major benefits including rugged construction which lengthens service time, controlled stress as well as controlled rate operation which enhances rheological evaluation, and controlled stress operation for the direct measurement of yield stress. The direct measurement of yield stress can be a very important rheological parameter in many personal care products including shampoos, body lotions, and creams.

The new Brookfield DV2T and DV3T instruments offer enhanced operation over the former models, however, not quite as much rheological characterization as the R/S Rheometer. You might consider these instruments as a good bridge between the two worlds.

Your decision may ultimately be determined by the answer to this question: “Is it more important to have readings which are comparable to multiple locations, or to perform the best rheological analysis possible?” You may wish to consult with your associates to determine just how much comparative measurements will be required.

01/2014

CUSTOMER CHALLENGE: We have an old RVT Dial Viscometer that works great. The majority of the samples are around 10000-15000 cP when measuring 3 viscosity data points at speeds 5, 10 and 20 RPM (torque is between 20% and 90%). My challenge is determining if the sample is Newtonian or non-Newtonian with just 3 data points. How much difference should there be between the viscosity values at each speed to consider a material non-Newtonian? I realize the answer may not be clear cut, so just an “in general” idea would help.

BROOKFIELD ANSWERS:

An order of magnitude difference in speed would be ideal, say 5 to 50 rpm, to test for non-Newtonian behavior. Using 5 to 20 rpm is still good enough to get a reasonable picture of the viscosity behavior. Variance in viscosity of a few % would be acceptable to conclude that the material is either “Newtonian” or slightly “non-Newtonian”. Use < 5% as a first pass guide. Also, check whether the readings are repeatable by running your test 2 times in a row. Contact Brookfield to review the results for clarification if not certain.

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VISCOSITY OF WATER TABLE

View the viscosity of water at temperatures between 0 - 100° C.

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VISCOSITY OF MATERIALS TABLE

Instant view of some dynamic viscosities of Newtonian fluids.

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VISCOSITY CALCULATOR

The cup to viscosity conversion engine assumes fluid is Newtonian.

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YIELD STRESS VALUES

Examples of values for typical food and personal care products

See the Table
 
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