In the Part I of this topic, we covered the different types of viscometers and viscosity measurements that are routinely available in the ceramic process industries.  In Part II, we will mention special capabilities and warn of specific perils that can befall those who routinely make viscosity and rheology measurements with each of these different types of instruments.

Kinematic Viscometers

These viscometers should NOT be used for viscosity measurements of any types of ceramic suspensions.

As sure as I make this recommendation, someone will say, "But we use them all the time!  Successfully!"  I simply don't believe they are of any value and I recommend they not be used.  At all!  At best, they are a crude, quick test of viscosity that requires some practice to gain consistency.  Kinematic viscosities cannot measure dynamic viscosity.  They cannot measure rheologies.  They are based on the assumption that the fluid being measured is Newtonian -- which ceramic suspensions are not.  As far as I'm concerned, they have no place in a ceramic lab or production floor.

There are always those who can do anything and everything with nothing.  They can measure all sorts of rheological properties with a crude, homemade kinematic viscometer (and save the company big bucks in the process.)  They can look at the flame in the kiln and adjust firing temperatures by eye (translation:  "You don't need a fancy controller -- you only need me and my calibrated eyeballs.")  They can run and control the plant perfectly fine by themselves without requiring any computerized procedures like Predictive Process Control (PPC).  Etc. 

I don't recommend this type of 'viscometer' for any ceramic facility.  If you use one, you're on your own.  If you have a technician or engineer who uses one and swears by it -- you decide if it is worth the hassle to change.  The decision is whether to trust one person to make accurately readings consistently without a dynamic viscometer, or to use a dynamic viscometer to measure some real, useful data. 

Capillary Viscometers

The capillary tubes in these viscometers tend to be really fine diameters.  As such they are not really made for ceramic suspensions.  If you have such a viscometer, you can certainly try to use it for ceramic suspensions -- but be careful.

          1.  Beware of Dilatant Blockages

Beware!  If you push particles through the tubes at too high velocities, dilatant blockages can occur, and you can block the tube (permanently).  A blocked tube is a ruined tube.  It will cost money to throw the old one away and replace it with a new one. 

Successful utilization of this type of viscometer also depends on the type, capacity, and capabilities of its pump.  Ceramic suspensions may not be compatible with the system.  Unwanted abrasion can occur in the pump and in the capillary tubes.  It will be difficult to clean the tubes after analysis as well.  Lots of little problems will present themselves.

Capillary viscometers, however, do have the capability to measure rheological properties.  The flow rate must be varied to obtain at least two different shear rates.  When this type of variation is imposed, rheological properties can be measured. 

         2.  A Plus -- This Method is Most Similar to the Production Environment

Fundamentally, this is a good way to measure viscosities and rheologies.  Push the suspension through a tube at at least two different shear rates, and you will have rheological data.  This also is about as close as you can come to having true processing conditions during an analysis:  pumping fluid through a small diameter tube is similar to pumping suspensions through pipes, valves, atomizers, etc. 

          3.  Proceed Slowly Near or Past the Onset of Dilatancy

Be careful, when increasing shear rates.  If the data indicates that you are near to the onset of dilatancy, or that you have passed that point, don't increase flow velocities much more because extreme dilatancy and dilatant blockages are not too far removed.  Both can ruin or seriously diminish the capabilities of these viscometers.

          4.  You Cannot See Inside the Metal Tubes
          5.  Bingham Measurements Indicate Dilatant Blockages

Another problem with this method is that the tubes are metal, so you can't see what's happening during the tests.  The second point listed is a related problem.  Experience suggests that many measurements that indicate Bingham rheologies are not correct.  A true Bingham suspension is one that starts at low shear rate with a high viscosity and that finishes with viscosities asymptotically approaching a lower limit at high shear rates.  All real suspensions will go dilatant at high shear rates.  Bingham suspensions do not.  When measurements appear to be Bingham, it usually means a dilatant blockage has occurred and the measurements are indicative of the force required to slide the blockage along the walls of the viscometer. 

This phenomenon can also occur in cone-and-plate viscometers and cup-and-bob viscometers.  When measuring the force of a sliding blockage, the friction increases as the shear rate increases.  The results will be identical to what one would expect from a true Bingham suspension.  [True Bingham suspensions probably don't exist, but they are simple mathematically and therefore desirable and useful.]

If one could see into the tube to determine whether the suspension is actually flowing freely, or if a blockage has formed and the blockage is sliding, this problem could be solved.  But it is not possible to see what is happening inside a capillary viscometer as the suspension is 'flowing' through the tube. 

            If You Have A Capillary Viscometer ....

If you need a rheology measurement and you have a capillary viscometer, you might want to try it.  Read the directions carefully and pay special attention to warnings and recommendations for maximum particle sizes.  If you find no recommendations about particle sizes, or no mention at all about particles, simply assume the instrument was designed to perform with particle-free fluids.  In such cases, don't try it.

But if you have an instrument with larger tubes, or one that routine is used on particulate suspensions, feel free to make rheology measurements by varying the speed of flow to vary the applied shear rate.  Without varying shear rates, rheologies CANNOT be measured.

Cup-and-Bob Viscometers

          1.  Excellent for Measuring Rheologies of Suspensions

These are excellent viscometers for measuring rheologies and viscosities.  Many are limited to shear rates that are lower than are experienced in the plant.  On the other hand, these viscometers usually can achieve higher shear rates than the 'infinite sea' type viscometers.  They can also tightly control the shear rates to very uniform values over wide ranges of shear rates.

          2.  You Cannot See Within the Fluid or Between the Cup and the Bob
          3.  Bingham Measurements Indicate Dilatant Blockages

The problem, mentioned above, that rheologies appear Bingham at high shear rates also applies here.  Refer to the discussion above on this point.

We routinely attempted to monitor the edges of the suspension where exposed at the top of the cup/bob gap.  If this went from wet-looking on the shear-thinning side of the curve, through an abrupt change to a hard, dry-looking solid system past the onset of dilatancy, we assumed (1) that the suspension had locked up in a dilatant blockage, (2) that it was no longer being sheared, and (3) that it was merely sliding along the surfaces of the cup and the bob. 

We have seen suspensions pumped through narrow tubes at higher and higher flow velocities until all flow stopped.  Those tubes then had to be thrown away and replaced.  Cut open a tube like that and you will see that it is full of particles which are locked in place.

When blockages begin to form in cup-and-bob viscometers, flow rates and applied stresses are not usually sufficient to create immovable blockages.  But blockages can occur during viscosity measurements.  When they do, erroneous readings are produced.  Just like with capillary viscometers, it is not possible to see what is happening to the suspension being sheared.  From the outside, one can only assume it is being sheared when the viscometer says it is being sheared.  If shear stops due to a blockage, however, the viscometer won't be able to detect it and will continue merrily on its way taking measurements as if nothing adverse has happened. 

What actually is the viscosity of an immovable dilatant blockage?  That's a trick question -- a dilatant blockage is a solid which has no viscous properties.

Most viscometers of this type control rates of revolution (rpm) and measure resulting shear stresses.  They can't monitor whether the fluids or suspensions are actually being sheared, or whether the shearing has stopped.  In simple fluids (for which most viscometers were designed), shear won't ever stop because blockages cannot occur.  There is nothing in a simple fluid which can form a blockage. 

Suspensions, however, are different!  They contain particles, and those particles can interfere and eventually form blockages.  This is totally unusual and foreign for someone who has spent all of their professional life measuring viscosities and rheologies of simple fluids.  It is an everyday occurrence for ceramists, however.

           4.  Profiled Cups and Bobs

To prevent test suspensions from sliding, profiled cups and bobs are available.  Profiled cups and bobs have long grooves down their lengths.  When using these, one must assume that instrument measurements are then corrected properly for the new surface profiles.  Corrections are within the instrument manufacturers' control and responsibility.

          5.  High Shear Rates vs Largest Particle Sizes

There are two problems that run counter to each other within these viscometers.  The highest shear rates, and the most accurate/constant shear rates, occur when the gap size is minimized.  When suspensions contain relatively large particles, gap sizes must be relatively large.  The accurate measurement of suspensions containing relatively large particles at really high shear rates, therefore, is difficult to achieve.

Rpm values are limited on all such viscometers.  The highest shear rates measurable on a particular instrument are functions of the maximum rpm of the instrument and the minimum gap size.  The smaller gap sizes also produce very uniform shear rates at any point between the cup and the bob.  When particulate suspensions are measured, however, gap sizes must remain large to accommodate the largest particles.  The highest shear rate at which a simple fluid can be measured is usually higher than the highest shear rate at which a suspension can be measured.

The result of these two problems is that it may not be possible to achieve the instrument's maximum shear rate when measuring viscous properties of particulate suspensions.

          If You Have a Cup-and-Bob Viscometer ....

These are great viscometers for measuring rheological properties of suspensions.  I recommend them highly.

Cone-and-Plate Viscometers

          1.  Excellent for Measuring Rheologies of Suspensions

These, too, are excellent viscometers for measuring rheologies and viscosities of suspensions.  Cone and plate viscometers can achieve some of the highest shear rates possible in viscometers, so if high shear behaviors are of interest, here's the instrument for you. 

          2.  Cannot See Within the Fluid or Between the Cone and Plate
          3.  Bingham Measurements Indicate Dilatant Blockages

The problem, mentioned above, that rheologies appear Bingham at high shear rates applies to cone-and-plate viscometers as well.  See the discussions above on this subject. 

           4.  High Shear Rates vs Largest Particle Sizes

This problem applies equally to cone-and-plate and cup-and-bob viscometers.  The highest shear rates, and the most accurate/constant shear rates occur when the cone angle is largest and the angle within the conical gap is smallest.  Once again, this conflicts with particles in suspensions.  Unlike cup-and-bob viscometers, cone angles can be very large, gap angles can be very small, and the cone-and-plate viscometers can still operate well -- even with large particles.  Why?  Although proper geometry for measurements requires the tip of the cone to be very close to the plate, the tip can be backed away without much consequence.  When the tip has been backed away to accommodate the large particles, that fact may not show in the measurements and the results will appear to be normal.   

To prevent particles from interfering with cone and plate rotations, some cones are "truncated."  This means the tip of the cone has been flattened so the cone is actually a disk with tapered edges.  The outer conical part of this "cone" should be the same distance from the plate as it would be if the cone had not been truncated.  But this produces errors in the calculation of the results.  When equations, which assume a true cone, are used for truncated cones, errors will result.

Some have gone as far as to exchange the cone for a plate.  In this case, the instrument becomes a plate-and-plate viscometer.  The revolving cone is replaced by a revolving plate.  The gap size, once again, can be adjusted to accommodate the particles.  The equations, once again, may or may not accommodate this variation. 

Measurements of extremely high shear rates are possible using cone-and-plate viscometers.  But the variations required to accommodate suspensions may prevent the achievement of such extreme conditions with particulate suspensions.

          5.  High Prices

This is not an operational drawback, but cone-and-plate viscometers are among the most expensive of the instruments capable of measuring rheological properties.  These types of viscometers are typically used for research.  If they're available in a lab, they can certainly be used for daily process control.  ....but would you want to use a research grade viscometer for daily process control measurements?

          If You Have a Cone-and-Plate Viscometer

These are great viscometers for measuring rheological properties of suspensions -- especially in research labs.  Personally, I would not use one of these for day-to-day process control measurements.  I'd purchase an 'infinite sea' type of viscometer for that.

Infinite Sea Viscometers

This type of viscometer was saved for last, because this is the viscometer I'd recommend for daily use in any ceramic processing plant and/or lab.

         1.  Excellent for Measuring Rheologies of Suspensions

Although these viscometers typically don't cover a really wide range of shear rates, they cover wide enough shear rate ranges to measure rheological properties of most ceramic suspensions.  Some of these instruments can only reach 100 rpm.  Some go to 250 rpm.  But these values are high enough, and there are enough possible rpm settings within the working range to measure shear-thinning as well as dilatant properties of suspensions. 

          2.  The Size of the Measuring Container Should Be Fixed

The various rpm and bob configurations will produce different readings when container sizes vary from measurement to measurement.  We typically use a metal milkshake mixer cup as both mixing and measuring containers.  We use milkshake mixer cups to measure lab samples of suspensions, so this allows the same container to be used alternately for mixing and for measuring viscosity.  With a standardized measuring cup size like this, day to day measurements will be consistent.

          3.  Portable Units of This Type are Available

Not only can computerized instruments of this type be installed in process control labs, portable (hand-held) units of this same type are also available for use in the plant.  The portable units used in the plant make comparable measurements to the stationary units used in the lab.

          4.  Shear Rate Capabilities Are High Enough to Measure Dilatancy

Make no mistake.  Infinite-sea viscometers are NOT high shear viscometers, but they can be used to measure dilatancy and the onset of dilatancy.  Since they are limited to relatively low shear rates, when dilatancy is indicated on the viscosity measurements, dilatancy will almost certainly be present out in the plant.

          5.  Remember -- Rheological Measurements Require Multiple Shear Rates

A single measurement at a single shear rate does not say anything about rheological properties.  It only describes a single viscosity at a single shear rate.  But two measurements at two different shear rates measures rheological properties.

These viscometers cover a wide range of shear rates (measurement rpm), so you can take advantage of that fact.  When the viscometer is computerized, like many are, an automatic program to measure rheological properties over a wide range of shear rates can be set up to be automatically recorded.

          6.  Thixotropies Can Be Measured On These As Well

Thixotropic properties are time-dependent properties.  Such measurements are fully within the capabilities of this type of instrument.  Set up the viscometer to measure viscous properties at constant rpm for any length of time.  Put the sample in the milkshake mixer container;  zap it for a minute on the milkshake mixer to remove all gel structure;  and then immerse the bob in the freshly stirred suspension and measure the viscosity at a very low rpm for 10 or 20 minutes to see the build up of viscosity.

          7.  Remember the Drop of Water at the Spindle/Suspension Interface

Don't forget to put a drop of water on the surface of the suspension where the spindle penetrates the surface.  This will have minimal effect on the measured suspension viscosity, but it will prevent a large scummy patch from forming around the spindle that can ruin measurements of long duration.  Without the drop of water, the dry patch attached to the spindle at the suspension surface will grow in size and spin about the surface with the spindle.  As this patch grows, it will apply more and more drag on the spindle and ruin the measurement.  With the drop of water, no dry patch will form.

          If You Have an Infinite-Sea Viscometer .....

We recommend these to all ceramic plants.  The price is right for use in both the lab and the plant.  In fact, most plants that use them own several.  The computerized versions are in the lab measuring test samples, and the non-computerized versions are out in the plant measuring process slips.

Summary

Each different type of viscometer has its own peculiar quirks that must be accommodated.  I encourage each person responsible for care and feeding of a viscometer to read its manual carefully to determine the extent of its capabilities.  In many cases, I would be the last one to recommend reading a manual, but my experience with viscometers is that someone must be responsible for them and also be familiar with all of their quirks and special requirements.

In order of preference, for a ceramic process company, I'd recommend:

1.  Infinite-Sea Viscometer
2.  Cup-and-Bob Viscometer
3.  Cone-and-Plate Viscometer

I don't recommend kinematic viscometers at all, and I wouldn't recommend the purchase of a capillary viscometer for routine measurements.  If a capillary viscometer happened to be available on site, I'd try to determine what its capabilities are -- and then maybe, try to use it.

Each type of viscometer has its own peculiar perils and quirks.  Many of these are mentioned above.  The best way to proceed is to name a "Viscosity Expert" within your company.  Make him/her learn all of the special details and quirks of the viscometer, and send every employee with viscosity questions to him/her.  That way, you will get your best results while staying away from any pitfalls that may be lurking.

Miscellany

Suggested topics for future issues of this E-zine .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...

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Copyright © 2006  Dennis R Dinger

103 Augusta Rd, Clemson, SC 29631   (864) 654-5731

consulting@DingerCeramics.com

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All Rights Reserved.