Most rotational viscometer specifications promise high test accuracy and repeatability. However, an instrument on its own cannot guarantee accurate results. Instrument set-up and using the right test parameters are critical pieces of the puzzle.
Test parameters and turbulence are two key factors to consider when preparing to test with a rotational viscometer. The spindle, speed setting, and temperature of the product are some of the parameters that can impact test results. Additionally, turbulence in the sample’s flow during a test will distort viscosity readings. Knowing how to manage these two factors can significantly contribute to the accuracy of your testing.
The Keys to Accurate Viscosity Testing
The first key to accurate viscosity testing is maintaining consistent test parameters. Following are the parameters that need to be exactly the same for every test you run:
- Temperature of the product
- Amount of sample
- Sample container
- Spindle (use the same spindle – both size and shape)
- Time of test (how long you run the test for)
The second key factor to consider when it comes to accurate testing is turbulence. It is essential to ensure that the flow of the product during testing is smooth, or non-turbulent. You can confirm non-turbulent flow by making sure that the torque reading during your test is at least (and preferably, greater than) 0.1 mNm. For products with lower viscosities, ensuring non-turbulent flow is especially important. You may have to try running your test at several different speeds before you settle on the speed that gives you an ideal torque reading.
Scenarios for Accurate Viscosity Testing
Two common scenarios that utilize viscosity testing are quality control and product development. In each of these situations, there are strategies relating to the two key factors discussed above which you can use to ensure that the testing is accurate.
Scenario 1: Quality Control Testing
When a quality control test shows that the viscosity of a product is outside its usual range, the question arises: is there something wrong with the product, or is the viscometer not working properly? There are steps you can take to answer this question.
First, check that the flow is non-turbulent, and that the test parameters are exactly the same as the tests you’ve run on previous batches. If your instrument is capable of saving test parameters for future use, do so. If not, keep a list beside the instrument detailing the test parameters that are to be used for each product your company tests.
A temperature control accessory (sometimes called a thermostatic accessory) may be necessary if the product is hotter or colder than room temperature during testing. This accessory will keep the product at a consistent temperature for each test you run. In the unlikely instance that the product is at room temperature during testing, but you notice that there is a significant change in viscosity with only a degree or two difference in temperature, you might also want to consider using a temperature control accessory to maintain precisely consistent parameters.
If the test parameters were consistent with previous tests, you can do either or both of the following, depending on your instrument’s capabilities. First, if the viscometer has the capability, run a zero setting. The zero setting simulates a test without the spindle attached. Think of it almost like a “re-set” button. Once that’s done, test the product again.
If you’re still getting an unusual result, follow the viscometer’s calibration procedure, using a standard calibration oil.
Once you’ve ruled out an instrument fault, you can be assured that an aberration in the test result is due to the product itself, and take the appropriate action to resolve the situation.
Scenario 2: Product Development
As with quality control testing, it’s also important to ensure non-turbulent flow and consistent test parameters during testing in the product development stage. Doing so allows you to monitor the effects of formulation adjustments to your product. If the flow was smooth and the test parameters were consistent with previous tests, you’ll know that any changes to the product’s viscosity were a result of changes to the formula.
When you’re doing R&D (research and development), you don’t always know what the test parameters ought to be. You may need to determine your own test parameters for use during product development. If so, here are some things to consider for each of the parameters:
- Temperature: Changes in temperature always affect viscosity. You may want to consider testing your product in development at the same temperature you expect to be testing at when you do quality control checks in the production line.
- Amount of sample and sample container: These parameters will depend on what spindle you choose. Each spindle has its own requirements for the amount of sample needed and what kind of sample container to use. Some spindles have special cups or tubes that you can get to hold the sample, which allow for an even more accurate reading.
- Spindle: What type of spindle you choose (coaxial cylinder, anchor, bob, disc, etc.) depends on your product. Most spindles, whatever the type, come in different sizes, and each of these sizes corresponds to a particular viscosity range. Sometimes your product might fit in the viscosity range of more than one spindle size. In this case, choose the spindle size that gets you a non-turbulent reading (a torque measurement above 0.1 mNm) at a moderate speed (not at the top or bottom end of your instrument’s speed range).
- Speed: Choose a speed that gives you a torque reading above 0.1 mNm. If your product doesn’t flow when no forces are acting on it (think ketchup, mayonnaise, mashed potatoes, etc.) make sure that the speed isn’t so fast that it’s creating a hole in the product surrounding the spindle. If you can’t do that, try a different spindle size and/or type.
- Time of test: Unless you have an application where it would be important to know how the product reacts after the spindle has been turning for a specific amount of time (such as paint, which decreases in viscosity the longer it is stirred), you only need to run your test for a minute or two, at most. Anywhere from 20 seconds to 2 minutes would be a good range to consider if time is not a factor in your results. Once you’ve chosen a time limit for your test, just make sure you run every test thereafter for the same amount of time.
Accurate viscosity testing isn’t as tricky to do as it might sound. If you clearly difine the test parameter for each product, most instruments will give repeatable results. Some viscometers let you save test parameters on the instrument, so it’s even easier to prepare and run a test. Even if your instrument doesn’t have this capability, most viscometers are still pretty user-friendly, and it shouldn’t be too difficult to manually input the parameters for each test.
'Til next time!
P.S. If you need help choosing the right instrument, spindles, or parameters for your test, contact us and tell us about your product and testing needs.
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