Most ceramic engineers and technicians need to make routine viscosity measurements.  Some make such measurements infrequently.  Some supervise others, who make the measurements.  In all cases, there are some "do"s and "don't"s and some procedures which will render measurements meaningless.

This article will focus on the different types of viscometers that are common in ceramic industries.  Next month's issue will then focus on the perils that can ruin measurements made on these types of instruments.

Dynamic versus Kinematic Measurements

Two fundamental types of viscometers are kinematic and dynamic viscometers: 

Kinematic instruments are performed by measuring the time it takes for fluids and suspensions to pass through an orifice as a result of the gravitational head of the sample. 

Dynamic measurements are those in which fluids and suspensions are sheared mechanically independent of the influence of gravitational forces.

       Kinematic Instruments and Measurements

The least expensive viscosity measurement devices are kinematic instruments.  Some of these instruments look like gallon cans with a hole in the bottom.  Some look like small tin cans, each with the requisite hole in the bottom, mounted on long arms so they can easily be dunked into the fluid or suspension to pull samples.  Some are blown glass with bulbs and reservoirs above and below the flow orifice.  In each case, the measurement process is to fill the upper reservoir, and time the passage of a known volume of fluid/suspension through the measuring orifice into the lower reservoir (or back into the tank).

A common use for kinematic viscometers is the measurement of fuel oil viscosities.  When measuring fuel oils, Saybolt Viscometers and Saybolt viscosities are common.  These viscometers consist of an oil bath surrounding a sample chamber.  The orifice in the bottom of the sample chamber is plugged by a cork until the sample fluid has been heated to temperature.  Then, the cork is popped, the stopwatch started, and the time (in seconds) for a known volume to pass through the orifice gives the oil's viscosity.  The heavier and thicker the oil, the higher the number of seconds for the flow to complete, and the more viscous the oil will be. 

There are two common Saybolt orifice sizes.  One gives the viscosity in SSU (Saybolt Seconds Universal) for common, light fuel oils, and the other larger orifice gives the viscosity in SSF (Saybolt Seconds Furol).  Furol stands for "FUel and Road OiLs."  The larger orifice allows 10,000 SSU oils to be measured in shorter, more reasonable times.  The North American Combustion Handbook (North American Mfg. Co., Cleveland, OH) contains a chart that shows the conversions from SSU to SSF and the variations of SSU oil viscosities as a function of temperature.  Rather than measure a ~10,000 SSU oil at room temperature and wait ~10,000 seconds to perform the analysis, or measure it at room temperature with the Furol orifice and wait ~1,000 seconds, most will heat the oil to a higher temperature (e.g., 200+^oF), measure its SSU at that elevated temperature (typically only a few hundred seconds), and then convert (using the chart) the high temperature value to its room temperature viscosity.

ALL kinematic viscometers, however, assume Newtonian rheologies.  Remember:  Newtonian fluids have viscosities that are constant regardless of imposed shear rate.  The peril here is that most ceramic suspensions exhibit non-Newtonian rheologies.  For this reason, kinematic viscosities of ceramic suspensions are generally meaningless and useless.  Can you obtain kinematic measurements on ceramic suspensions?  Sure!!  But what do those measurements mean?  At best, they are used for very simple quality checks by plant personnel.

Even the assumption that all fuel oils are Newtonian is a stretch.  Many high viscosity fuel oils, such as the one suggested in the example (heated to 200+^oF), are non-Newtonian.  Temperature/viscosity charts are based upon Newtonian assumptions.  They, therefore, are not accurate when used with non-Newtonian fluids.  If you check such a chart, you will notice no (i.e., none -- nada) mention of 'shear rates' on such charts.  Since Newtonian fluids have viscosities that are constant and independent of shear rate, shear rates will not be mentioned.

Many times, workers in ceramic production plants will have simple (sometimes hand-made) kinematic devices to 'test' the viscosities of suspensions in production tanks.  They are used to make simple spot checks on production suspension viscosities.  Such devices should not be used for more than 'spot checks' because they really can't tell the kind of information needed to insure rheological consistency from batch to batch.  Don't argue with anyone using such a device.  They will invariably have total faith in their device.  But this type of device will not (cannot) accurately characterize the viscosity behavior of ceramic production suspensions.

Drawbacks to this type of device?  1 -- Each measurement covers a range of low shear rates.  2 -- These have no capabilities to measure dynamic viscosities.  3 -- These have no capabilities to measure non-Newtonian rheologies.  4 -- These can quickly and easily become the viscometer of choice by plant personnel.

Advantages to this type of device?  1 -- They are extremely low cost devices. 

          Dynamic Viscometers and Viscosities

To accurately and fully measure the viscous behavior of ceramic suspensions, you must use a dynamic viscometer.  These take the form of rotating cylinders, concentric cylinders, cones-and-plates, and capillary viscometers.  These types of viscometers vary the shear rates imposed on suspensions, and then measure the shear stresses required to achieve those shear rates.

                    Infinite Sea Viscometers

Some rotational devices are considered to be 'infinite sea' devices.  That is, they have the rotating bob, but no fixed diameter measurement container.  They can be used with a variety of container sizes.  They can be used with 400ml beakers, or 1000+ gallon production tanks. 

It is difficult to precisely control the shear rate in such devices, but generally, when the rpm of the device is doubled, the shear rate is effectively doubled.  It may be difficult to precisely fix the shear rate, but relatively speaking, shear rates can be varied over a wide range.  These infinite sea-type devices cannot achieve truly high shear rates.  They can, however, characterize the rheological nature (shear-thinning/dilatant) of suspensions.  And when these viscometers measure dilatancy (at their relatively low shear rates), beware!!  If they can pick up dilatant properties at their low shear rates, it is very probable that production conditions will also be characterized by dilatant properties.

Over the years, we used this type of viscometer for measuring coal slurry viscosities as well as the viscosities of most ceramic suspensions.  Not only is the cost/benefit ratio of this type of viscometer excellent, portable versions are available which can be used in the plant to replace home-made kinematic viscometers. 

We have also used this type of viscometer to measure gelation behavior of suspensions.  The fact that these viscometers work well at very low shear rates (very low rpms) allows them to be used to monitor gelation as it takes place.  Simply set up the viscometer at a constant low shear rate and monitor viscosity over a period of ten minutes (or so.)

Drawbacks to this type of device?  1 -- Truly high shear rates are not possible.  2 -- Unstable suspensions can settle during measurements.  3 -- Different size measuring containers (beakers, tanks, etc.) alter readings slightly.

Advantages to this type of device?  1 -- They are relatively low cost viscometers.  2 -- They have the capability to measure non-Newtonian character.  3 -- They are fairly sturdy devices.  4 --  They can measure a wide range of viscosities.   5 --  They can be computerized. 

                    Concentric Cylinder and Cone-and-Plate Viscometers

The more expensive viscometers (typically research models) can very precisely measure non-Newtonian viscous behaviors at high shear rates.  As the gap sizes are reduced between cups and bobs, or between cones and plates, and analysis rpms are increased, high shear rate limits of these devices increase.

These viscometers can precisely measure viscosities and characterize rheologies at really high shear rates.  For example, if you have a spray drier slip that you wish to characterize and precisely control, how can you measure its properties under actual atomizer conditions?  Kinematic viscometers can't come close.  Rotational infinite-sea devices can't come close.  But concentric cylinder and cone-and-plate viscometers can achieve shear rates in the desired ranges using high measurement rpms and small gaps.  The sizes of the particles in the slip may, however, limit the maximum imposed shear rates in these instruments.  For example, if all particles are sub-micron sizes, gap sizes can be reduced and imposed shear rates can be especially high.  If the largest particles are up in the standard sieve size range, gap sizes must be relatively large and achievable shear rates will be limited.

Drawbacks to these types of devices?  1 -- These instruments usually are high priced.  2 -- Unstable suspensions can settle in these devices while measurements are being made.  3 -- It is difficult to visually monitor sample fluids while measurements are being made.

Advantages to this type of device?  1 -- These devices are capable of measuring non-Newtonian character at really high shear rates.  2 -- They are capable of measuring a wide range of viscosities.   3 --  They are usually computerized.  4 -- Small sample sizes are characteristic of these devices.  5 -- These make excellent research viscometers.

                    Capillary Viscometers

Another type of viscometer that can measure non-Newtonian behaviors is the capillary viscometer.  These devices pump samples through long, small-diameter capillary tubes.  Viscosities are measured by  monitoring the pressure required to achieve certain flow rates through these capillaries.  Capillary viscometers are not the first type of viscometer that comes to mind to measure non-Newtonian rheologies, but they can make such measurements when operated properly -- when their pumps can achieve wide ranges of pressures and flow rates.

Drawbacks to these types of devices?  1 -- It is impossible to visually monitor sample fluids while measurements are being made.  2 -- Great care must be taken to not clog the capillaries when high shear rates and high pressures are attempted.  Clogging the tubes effectively ruins them and forces their replacement.

Advantages to this type of device?   1 -- They have the capability to measure non-Newtonian character as it would occur in production devices.  If you want to know how a suspension will behave as it passes through an atomizer nozzle, this type of device can demonstrate (if pressures and flow rates can be achieved).   2 -- These are capable of measuring a wide range of viscosities at a wide range of shear rates. 

Conclusions

These are the main types of viscometers that are typically found in ceramic plants and labs.  The most inexpensive of these are most frequently used because their price is right.  Kinematic viscometers, however, cannot characterize non-Newtonian rheologies.  They give only limited information (regardless what their champions may try to say.) 

Moderately-priced, rotational, infinite-sea models find great use as general purpose viscometers.  They can measure wide ranges of viscosities, and they are useful for both Newtonian and non-Newtonian fluids.  They can also be used to measure time-dependent properties such as gelation behavior.  As general purpose instruments, these are highly recommended and well-regarded.

The highest-priced rotational viscometers are best equipped to measure non-Newtonian properties at high shear rates.  These are excellent research viscometers which can also be used for day-to-day measurements.  Usually, however, these viscometers are limited to research measurements and to calibrating less-expensive production instruments.

Part II to Follow

The article in the next issue of the Ceramic Processing E-zine (Part II of this topic) will discuss the perils when making viscosity measurements on these types of devices.

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

www.DingerCeramics.com

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All Rights Reserved.