I used to think that liquids were pretty simple things. Is that what you think too?
Not so fast. Let’s take a look at three properties of liquids and see if we change our view.
Here at CSC, we work with Consistency, Surface Tension and Viscosity of liquids.
As an indicator of Consistency, sometimes defined as thickness or runniness, we use a Bostwick Consistometer. Producers of ketchup, soups, sauces, paints, liquid mixes and the like use the property of Consistency as a check of quality on their incoming and finished product.
Surface Tension, the property that resists external forces at the surface, is a characteristic we measure with duNouy Ring Tensiometers. Research and Development projects for coatings, adhesives and surfactants use these measurements. Surface tension changes are a basic in monitoring processes such as parts cleaning and electroplating.
Viscosity, the measurement of the resistance of a liquid to flowing, is the third quality of liquids that we work with. This characteristic is important in many food products, particularly where flow is important. For example, with pancake syrup, the effectiveness of petroleum compounds are dependent on Viscosity as well as the design characteristics of liquid handling systems.
We are constantly asked how these properties relate to one another. There are very few factors that are associated between these properties. First, there is the general impression of thickness – a liquid with higher viscosity usually means a comparatively higher consistency, but not necessarily a higher surface tension. As temperature increases all three properties become lower in an inverse relationship to temperature.
Other than these associations, across the board correlations do not exist.
This is due to what is behind each of the properties. Consistency is a measure of how a liquid material reacts to gravity. Surface Tension, the resistance of the surface to external forces is a result of the attraction of the molecules and the level of hydrogen bonding. Viscosity’s resistance to flow is a combination of the size of the molecules and the shape of the molecules.
Because of these differences, correlation or conversion of readings between the properties of Consistency, Surface Tension and Viscosity is not possible except between liquids of very similar molecular structure, shape and attraction. Some studies of selected alkanes have shown linear relationship between the log of Surface Tension and the reciprocal of Viscosity, but for a narrow range of molecular structure.
As a result we must give the answer – there is no general correlation of Viscosity to Consistency, or Viscosity to Surface Tension, or Surface Tension to Consistency.
The measurements of Consistency and Surface Tension are straightforward and only need a fixed temperature to assure consistent readings from test to test. Check out the videos.
Surface Tension Test
Measuring Viscosity adds several complications. First, there are several types reactions to the movement of liquids. Then there is the relationship of the force used to the speed of movement.
Viscosity, as the resistance to flow has two critical aspects. The first is a measurement of the force needed to move the layers of a liquid against each other. The second complication arises from the speed at which you move the layers.
Two new terms come into play when dealing with Viscosity. The force needed to move a liquid is known as shear stress and the speed of movement is known as shear rate. The ratio of shear stress to shear rate (shear stress/ shear rate) is the basis of the term viscosity.
Adding these two factors (force and speed) begins to complicate the development of Viscosity measurement. In addition, There are different kinds of liquids. There are fluids in which the viscosity stays constant at any velocity or shear rate, fluids in which viscosity declines with velocity, yet another ( you may have guessed) where viscosity increases with shear rate, others include liquids which show a change in viscosity over time at a given rate and fluids in which viscosity changes with speed but does not even start to move until a specific levelof force is applied.
The first type is known as a Newtonian fluid: water, hydrocarbons, mineral oils, syrup and resins are examples. The Viscosity of Newtonian fluids does not change with changes in speed/velocity.
The second type belongs to a category known as Non-Newtonian, and there are several types of Non-Newtonian fluids in this category some examples include.
The group of fluids for which an increase in speed lowers Viscosity is known as Shear Thinning or Pseudoplastic; ketchup, paint, nail polish, and polymer solutions are examples.
Next are liquids that increase in Viscosity as the speed increases, called Dilatant (almost like a playboy - Dilettante) or Shear Thickening; silica, glycol,cornstarch solutions and some slurries are included here.
Liquids that experience a changes in viscosity as a constant stress is applied to them over time are known as Thixotropic, if the viscosity decrease or Rheopectic if viscosity increases. Examples are inks, Clays and drilling muds.
Finally, there are fluids that require a certain stress is needed before they flow, called Plastic. This type includes gels, latex paints, lotions, and toothpaste. (Just to further complicate matters, a fluid can be both plastic and another type of non-Newtonian fluid, often pseudoplastic or thixotropic.)
Viscosity measurement = simplicity personified, right?
These details of the basic two-phase measurement requirement (stress and velocity) for Viscosity and the complex actions of the different fluid categories hopefully illustrate that the lack of direct correlations of Viscosity to Surface Tension and Consistency is based on the fundamental differences of the these three properties of a liquid.
I hope this helped shine light on the reasons that there are no general correlations among these three properties, and that the requirements for instruments to measure them are significantly different.
Now you may understand why I am continuously baffled by this test equipment undertaking.
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P.P.S. If you like more information on instruments that measure Consistency, Surface Tension or Viscosity clicks on the appropriate icon here.
By Art Gatenby
Since I joined CSC Scientific in July 2013, I’ve been on a steep learning curve. You see, I’ve never been much of a science person. There was even one particular chemistry class in high school used to give me migraine headaches - routinely.
Having no real background in the sciences means that I’ve had a lot to learn since I joined a company that sells scientific testing equipment. Maybe you’re nodding in sympathy with me right now. Many of our clients – that is, the people doing the purchasing on behalf of their company – are not scientists and don’t have a thorough knowledge of the scientific principles behind the products they’re told to buy.
The good news is that you don’t have to be a scientific expert to make an informed decision about our products. A basic understanding of the scientific principles behind an instrument can lead you to ask the right questions to help you choose the instrument that will fit your testing needs. To that end, I’ve started to create a video series called “The Basics”.
This series is comprised of short videos that explain the basic concepts around such subjects as viscosity, Karl Fischer titration, and surface tension. I hope they’ll be both fun and informative, and would love to hear your feedback and requests for future episodes.
I’m starting the series with the topic I’ve focused on most since my arrival at CSC Scientific: Viscosity.
Viscosity is a notoriously tricky subject, but one that I hope will be made a little clearer by this entertaining little video:
(I pop up into a bigger frame when you click on me.)
Hope you enjoyed the video! Leave a comment below and let me know what would make it better, or what topic you'd like me to address next.
Till next time,
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Sieve testing, as I have stated many times, is the Cinderella of particle size analysis because it delivers more value than expected from something that’s so easy to use and relatively inexpensive. However, the problem with standard sieving techniques using wire mesh sieves is that they begin to exhibit accuracy problems in the lower micron sizes.
Standard sieves are advertised with openings as low as 30m (microns). At these small sizes, tests usually end with poor and inconsistent results. Inherent variability in wire mesh causes wide relative difference in aperture sizes at these small sizes.
This is the area where alternate techniques such as laser diffraction become the accepted way to get accurate particle size analysis. These techniques, however, are significantly more costly and are technically more difficult to use.
There is another culprit contributing to the difficulty of getting good results for particle size analysis in the lower micron ranges.
However, Cinderella and Prince Charming have a couple of tricks up their sleeves.
The process of electroforming can produce sieves with apertures in the 2 to 3 micron range with acceptable variation between opening sizes – a far cry from woven wire mesh.
Their second trick is the development of the vacuum siever. This design places a rotating vacuum below a sieve that moves the sample over the sieve surface while pulling the particle through the openings. These vacuum sievers can be effective down to the 20m levels. One drawback of these vacuum-sieving instruments is that they will only process one sieve at a time.
Cinderella and the Prince have one more trick, which is the Sonic Sifter. This instrument can process 3 to 5 sieves at a time. The Sonic Sifter applies a vertical, oscillating column of air to the sieves; the air column oscillates 60 times per second to present the particles to the sieve openings. This, along with a mechanical pulse, shears agglomerates and reorients the particle in the air column. The Sonic Sifter is effective in the separation of 3m particles and can complete a test in as little as 10 seconds. Check it out.
Costs for the different techniques of particle size analysis begin at about $1,000 for a basic shaker and 5 non-certified sieves. The Sonic Sifter prices are at the level of $7,000 without sieves. Vacuum sievers cost approximately $9,000. Finally, the Laser Diffraction equipment begins in the $25,000 to $30,000 level and frequently costs in excess of $50,000.
If it seems that the design of reliable particle size analysis in the low micron range is esoteric you are thinking correctly. The determination of the best method is a combination of the nature of the material, the accuracy and repeatability of the results needed, the budget available, and the technical ability of the operators.
The range of equipment and techniques available should provide an alternative that will work in most situations.
As usual, even analyses that should be easy turn out to disconcert and perplex me.
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By Art Gatenby
Can you do a fast moisture test? The answer is, “maybe”.
The direct methods of Loss on Drying and Karl Fischer have the benefit that between them they can get a good moisture content result on almost any product or material.
But these tests usually take several minutes.
When you need the moisture content in a truck load of grain while the load is being dumped, or when you need to check several hundred bags of coffee at an auction, or when you need to get a moisture gradient in a pile of corn, a faster test is crucial.
Can you do it?
The short answer is, “yes”. There are, however certain caveats to this answer.
The techniques available for getting a faster moisture reading are based on the presence of an electrical/electronic signature of moisture in a material or substance. These techniques measure change in resistance, conductivity, capacitance or RF power absorption as an indication of change in moisture. In products where changes in any of these characteristics can be see with a change in moisture, the resulting levels will reflect a relative amount of moisture.
To get actual moisture from a range of these electrical readings, a comparison has to be made to results from a direct method of moisture determination (such as Loss on Drying). These comparisons result in what is called a calibration curve, which is a plot of direct method vs. electrical readings.
For any of these indirect techniques used to get a fast moisture reading, the calibrations represent a curse.
This curse is manifest first in that different substances have different relationships between moisture and electrical signal. For example, a calibration curve for corn is different than a calibration curve for soya. Each provides accurate measurements. However if you mix soya and corn, let’s say 40% corn and 60% soya, a new calibration will have to be done for this mix. Furthermore, if the mix proportions change, the calibration curve will be different.
The second part of the curse is that as moisture levels increase a state of relative saturation occurs. At this point, the change in electrical signal becomes small in relation to the change in moisture and an accurate, repeatable reading cannot be made.
Finally there are some materials that do not have an inherent relationship between electrical characteristic and moisture change, and these techniques cannot be used.
In conclusion, the answer is “yes” to getting a fast moisture measurement if your product exhibits changes in electrical/electronic characteristics with a change in moisture and it is lower than the cut of relative moisture saturation.
Check out our range of indirect moisture measurement products to see what might be available as a fast test for your material.
Everywhere I turn, what seems simple (fast moisture test) becomes indecipherable.
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by Art Gatenby
Automatic digital tensiometers are expensive - three to four times more so than a high-precision manual tensiometer. We hope to clearly depict when an automatic digital tensiometer is not merely nice to have, but essential.
duNouy Ring and Wilhelmy Plate Techniques.
The duNouy Ring method is based on a technique developed by P. Lecomite duNouy and popularized in a paper published in 1925. In this technique a platinum ring is first submerged below the surface of a liquid. The ring is then brought up through the surface. The force necessary to do this and break the meniscus formed at the surface of the fluid is measured. This force is transformed into surface tension terms, usually dynes per cm.
The duNouy method has been used for measuring a wide range of products. It can be used for very low surface tensions, and any surface tension up to a high of 90 dynes/cm. Traditional torsion balance tensiometers are still in wide use because of their inherent precision and stability.
The Wilhelmy plate method is based on the force applied by a liquid to pull on a material immersed in that liquid. The higher the surface tension the greater the force. Wilhelmy plates are well suited for high surface tension liquids and can be used to measure changes in surface tension over time. Tensiometers based upon electronic balances are often used for Wilhelmy Plates applications. These usually provide a digital readout but have limited capability for time-based surface tension analysis.
If your requirements call for measuring only surface tension, the more basic duNouy ring tensiometer is probably the best choice. When you have very thick, high viscosity requirements, a Wilhelm Plate instrument will work.
When conducting basic surface tension tests, in normal ranges, we recommend the duNouy Ring Tensiometer.
Requirements that Point to the Need for an Automatic Tensiometer
If you have requirements for measuring surface tension which change over time, such as measuring the reaction times of surfactant, the Automatic Tensiometer is a good solution.
An automatic tensiometer has unique capabilities for determining Lamella Length, which is the amount of stretch in a liquid between the development of maximum force and the total release of a duNouy ring.
Measurements of changes in apparent surface tension or wettability of different substrates are enhanced with automated techniques.
Most automatic tensiometers have attachments that will control the temperature of the sample.
Most automatic tensiometers also perform routine duNouy Ring and Wilhelmy plate tests. These instruments also record historical results, perform statistical analyses of multiple tests, and plot test readings.
When Do You Need An Automated Solution?
A number of testing requirements clearly point to the need for an automatic tensiometer.
- Time based surface tension tests
- Measurement of Lamella Length
- Wettability analysis
- Temperature controlled samples and/or
- Continuous recording, plotting and retention of all test results
If these are part of your analysis requirements, an automatic Tensiometer is the only practical answer. The ability to conduct your routine duNouy and Wilhelmy Method tests comes as a free added benefit.
We hope this has provided some guidance for the answer to When Do I Need An Automatic Tensiometer?
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The wide range of issues involved with selection and use of laboratory test equipment frequently baffle but always interest me.
Hope we help.
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Break the Curse of Particle Size Calculations.
Every now and then I’ve had to calculate the results of a sieve test: I’d get the sieves and sample loaded on the shaker, run the shaker, then realize I’d have to start over because I forgot to get the empty weight of each sieve. Or I’d have to carefully brush out the sample onto a balance.
Of course, I also had to clean each sieve.
By the time I got every thing right and recorded I’d been interrupted and got the number wrong. A study in boredom and frustration.
If you’ve ever had to do a sieve test and calculate residuals and record the results, I think you might relate to this short video.
If you do sieving, I hope this touches a nerve and demonstrates a stress-free alternative. If that’s the case, please share this with your associates.
In my usual state of somewhere between total befuddlement and partial understanding, I’m still working on the puzzles of measurement.
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Karl Fischer Titration and Loss on Drying (LOD) are both methods for determining moisture content in a product.
That’s where the similarities end, though. Here’s the difference between Karl Fischer and Loss on Drying:
What Moisture Does It Measure?
Karl Fischer Titration is a method measures only the water content (i.e. it's water-specific) in a product sample.
Loss on Drying, on the other hand, measures the total change in weight of a material as a result of drying. For some products, components such as alcohol or fat evaporate with the water. Therefore, the LOD method measures both the water and volatile impurities such as those mentioned previously.
How Do They Get at the Moisture?
Karl Fischer titration is a chemical method. It involves adding a reagent to the sample to cause a reaction that converts the water in a product to a non-conductive chemical.
Loss on Drying compares the weight of a product before and after it is dried. This difference in weight is taken as the percentage of moisture in the product.
How Do You Choose the Best Method?
Choosing the right method of moisture determination can be tricky, and there is no one-size-fits-all answer to this question. However, there are a few things that would lead you to favor Karl Fischer titration over the more commonly used LOD method.
Does your product contain a high level of constituents, which can distort the moisture reading? (In other words, does it contain more than trace amounts of those “volatile impurities” mentioned earlier, enough to significantly alter the results?)
Does your product have a very low moisture content?
- If “yes”, Karl Fischer frequently provides more consistent results.
Loss-on drying is the most commonly used of the methods. In order to help answer the question "Will Loss-On Drying Work for Me?" we have created a chart that rates materials/products based on how effective the LOD method will be for determining moisture content.
Loss-on Drying Effectiveness for Common Applications:
I hope this has been helpful to you. Please share your opinions and/or questions in the comments section below!
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How Can I Measure Viscosity? The answer, like most answers in the measurement world, is it depends. To find out what it depends on we'll start with defining viscosity.
What is Viscosity?
Viscosity is the mechanical friction between molecules in motion, and the resistance to deformation because of mutual attraction of the molecules (in other words, resistance to flow).
There are two types of viscosity.
1. Dynamic viscosity, also known as absolute viscosity, is the tangential force per unit area required to move one horizontal plane with respect to another horizontal plane at a unit velocity when maintained a unit distance apart by the fluid.
Kinematic viscosity is the ratio is the ratio of absolute/dynamic viscosity to density.
Ways to Measure Viscosity
Before the begining of the 20th Century, in the 1800's, the first measurements of vicsosity were made using capillary tube methods. Start with that the following is a brief summary of the different techniques/instruments that have been developed and are in use today.
The earliest methods for measuring viscosity were based on using capillary tubes and measuring the time it took for a volume of liquid to pass through the length of the tube. These developments were in place before the turn of the 20th century and are known as Ostwald or Ubbelohde viscometers.
Similar to this method is the Zahn Cup, which is a small container with a handle and a small hole in the bottom. The time it takes to empty the cup through the hole is correlated to viscosity. The Zahn cup is often used in the paint industry.
Falling Sphere Viscometer
Another technique is the Falling Sphere Viscometer, in which a sphere of known density is dropped into the fluid sample and the time it takes for the sphere to fall to a specified point is recorded. This method has been used on ships to monitor the quality of the fuel going into the ship’s engine. A similar product is the Falling Piston Viscometer.
Vibrational Viscometers measure the damping of an oscillating electromechanical resonator immersed in a fluid. This technique is often used in-process to give continuous readings in a product stream, batch vessel, or in other process applications.
The rotational viscometer measures the torque required to turn an object in a fluid as a function of that fluid’s viscosity. This method is frequently used in quality control and production laboratories.
The simple liquids react in a simple way to speed of flow and time and are called pure or Newtonian fluids. These pure fluids include things like water and milk.
However, there is a class of non Newtonian fluids that react in very differnt ways. The techniques for measuring these viscosities require complex testing methods and which may use other types of instruments.
I hope this has helped to explain some of the ways viscosity is measured. Let us know what questions you have by leaving a comment, or sending us an email at firstname.lastname@example.org.
Until next time, I remain bewildered as ever.
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By Art Gatenby
Jim’s life was dull. So, so dull. He spent his days surrounded by piles of sieves – his glamorous job was to calculate the ratio of particles left in the sieves of each stack after a sample of his company’s product was run through the stack using a sieve shaker. It was a job much like the one his cousin Hiram had, although he seemed to remember hearing that Hiram had found an easier way to do it.
Jim hated his work. He had to painstakingly brush out the contents of each sieve onto a balance and note the weight, hoping that the total would add up to the original sample weight so that he’d know his ratio calculations were correct.
They seldom were. He often came close, but those pesky sieves insisted on keeping some of the product held tightly in their mesh nets no matter how long or hard he brushed them. Then there were the calculations – those endless strings of numbers that set his head spinning.
Jim was a smart man – he’d never have gotten the job otherwise – but after hours upon hours of staring at numbers on a paper, some of them would start to jump around. It was understandable to everyone, except his boss.
“Particle size analysis is the key to our manufacturing process,” his boss had said. “We can’t afford to get it wrong.”
His day was going much like any other. Monotony. Struggle. Confusion. Jim thought he felt a headache coming on.
But all that changed when his phone came to life…
We get the same two questions almost every week about the CSC Bostwick Consistometer:
1) What angle do I use to set it up?
2) How do I level the Consistometer?
The principle of the CSC Bostwick Consistometer is based on the slump cone. In this procedure, a cone is filled with the material to be tested. It is then set on a level surface with the open area facing downward. The cone is pulled away and after a fixed time, the amount that the material slumped is measured. The thicker the material, the less slumping occurs.
In the CSC Bostwick Consistometer, the area behind the closed gate is filled with the material to be tested. This cube of material is analogous to the material inside the cone. When the gate is released, it opens one side of the cube, allowing the material to flow down the trough – similar to the slumping action when the cone is lifted. The amount of analogous slump is measure by the distance the material flows in a given length of time.
To make this work consistently from test to test, the CSC Bostwick Consistometer needs to be perfectly level. This can usually be achieved using the circular level mounted on the Bostwick and adjusting the leveling screws. Occasionally the front of the Consistometer gets bent. In those cases the angle of the consistometer should be measured by placing a small level in the bottom to the trough.
To review, the answers to the questions raised at the beginning of this article:
1) The CSC Bostwick Consistometer should be level, not at any angle.
2) The Bostwick is leveled using the circular level on the front (adjusting the two leveling screws as needed) or with a level in the trough.
The consistency measurement is made by reading the number on the trough where the outermost edge of the sample has spread to at the pre-established time.
I hope that this has helped to clarify the set-up of the CSC Bostwick Consistometer.
Please share this with anyone you think would find it useful.
As always, a slightly disoriented,
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P.P.S. We have a video - "How to Run a Bostwick Consistometer Test"