There are 30 samples to run for viscosity analysis and the boss wants the results by 5 p.m. Without having to think twice, the lab tech knows that this is a nearimpossible task, given that each sample can take up to 15 minutes, due to the temperature conditioning that’s required and the messy cleanup afterward. The only possibility is to work through breaks and lunch and hope that nothing goes wrong. This is the dilemma for any lab tech in a business that has weathered the economic recession, been reluctant to hire, and loaded additional responsibilities on existing staff.
Saving time on each test in order to do more is the mantra of all analytical labs today, whether in R&D or QC. What easy-to-implement things in viscosity testing could make an improvement over current practice, enhance the results, and actually reduce the time a lab tech must spend running the test? This article reviews advances in viscosity measurement instrumentation and shows how lab managers can use these to increase productivity.
Viscosity tests are most often performed on samples in a standard 600mL beaker with the traditional disc spindle. This is a lot of sample material, perhaps more than is really necessary to run a meaningful test. Add to that the requirement for temperature control, which is becoming a recommended best practice, and you can see the test minutes per sample piling up. Consider the possibility of reducing the sample size to save on both test time and cost of material.
The Small Sample Adapter provides a ready means of reducing sample size to less than 20mL. As shown in Figure 1, the sample goes into a cylindrical chamber, which fits inside a water jacket that is connected to a circulating water bath. The obvious advantage is that it takes significantly less time to reach temperature equilibrium. This automatically saves precious minutes when running a viscosity test.
Another potential option is to use the disposable chamber for the Small Sample Adapter, which can save time on cleanup. This is a practical choice for samples that are either highly viscous or make the sample chamber difficult to clean. Compare the small cost of the disposable chamber against the labor time and expense of cleaning the 600mL beaker, and you may have taken another important step toward increased efficiency in viscosity testing.
This need was mentioned above. Some materials are highly temperature sensitive, so it’s best to make sure that sample, chamber, and spindle are at equilibrium temperature. But how long do you wait for equilibrium? Experience with your material usually establishes this time period, which can take from several minutes up to an hour. Select viscometers in today’s market actually integrate the temperature control function as part of the built-in test capability. Otherwise, use of a benchtop timer is required. The inherent advantage of having this control capability in the viscometer is that the test will commence automatically as soon as the required temperature set point is achieved.
Connecting the spindle
The long-established method for connecting the spindle is to screw it onto the viscometer. One slight catch is that the direction of rotation is the opposite of what you might think. This in and of itself sometimes causes problems for the technician, who momentarily forgets to rotate the spindle clockwise as he or she looks down on the instrument. Potential damage, not to mention momentary frustration, can result.
Another detail of proper handling of the disc spindle is that it has to be cocked at an angle during immersion so that the disc does not trap air on its bottom side (Figure 2). The consequence of having air bubbles beneath the disc is the creation of artificially high viscosity readings. This naturally supports use of the Small Sample Adapter, which prevents this type situation from occuring.
So regardless of spindle choice, there are new connection mechanisms for attaching the spindle that may save time and minimize potential damage. For existing viscometers, there is an inexpensive device called Quick Connect that can be attached to any spindle (Figure 3). When subsequently connected to the viscometer, the sleeve on the Quick Connect is raised to remove the spindle. The same movement of the sleeve allows the technician to reattach the spindle.
An alternative mechanism is called EZ-Lock, and operates similarly to Quick Connect. The difference is that the mechanism is built into the instrument. The advantage of EZ-Lock is that it provides inherent protection to the sensitive suspension system inside the instrument.
Accurate test time
Once the test setup is ready, how long do you allow the spindle to rotate before capturing the viscosity reading? Most test methods specify a time interval, which ensures that the spindle has rotated at least five complete revolutions. This allows the measurement system within the instrument to stabilize and guarantees better repeatability of the viscosity reading. At 10 rpm, the technician waits 30 seconds before recording the reading, while at 1 rpm the test takes 5 minutes.
But what if the technician becomes distracted and forgets to set a timer? Can this affect the final reading? If the material is time-sensitive, the answer is yes; most likely, the reading will be lower than expected. If there is no time sensitivity, then there is nothing to worry about.
Is there a foolproof way to be sure that test time is managed correctly? One practical approach is to use the built-in time clock that resides in today’s continuoussensing digital viscometers. As stated above, viscosity tests require that the spindle rotate for a certain period of time or for a fixed number of revolutions. The time spent waiting for the spindle to complete its work can be wasteful, especially if the tech has to attend the instrument for several minutes. The adventurous tech who attempts to get other tasks moving during this lull sometimes misses the appointed time for completion of the test, and writes down whatever value is displayed upon his or her return to the instrument. How much easier it is to let the embedded clock in today’s new viscometers automatically stop the spindle rotation when the time is up, and record the final viscosity reading.
Perhaps this simple, but effective, procedure can save 1 to 2 minutes per viscosity test. For a lab that needs to run 30 tests per day, the time saved can be 30 minutes to an hour.
Documenting the viscosity reading
Many technicians write down the viscosity reading at the conclusion of the test. There is always the possibility that the number is copied incorrectly. Wouldn’t it be a lot simpler to have the viscometer send the data to a PC or printer? In R&D, availability of a PC is seldom an issue. For QC, the use of PCs is normally limited to a lab environment. On the production floor, use of a printer makes more sense. In either case, choice of the proper digital viscometer permits automatic transmission of data to an external device or network. Today’s digital instruments are designed with this capability and can save valuable seconds with each test, not to mention eliminating potential copying mistakes due to human error.
Delivering the results on time
The above recommendations will save enough time to complete the initial assignment of getting 30 samples tested within a day. Equally important, the results will be correct. One side benefit not mentioned before is the spare time that can be used to deal with other tasks while the viscometer is running. Since each test executes automatically without further operator involvment, the tech has newly found free time to get other things done.
Advanced viscosity testing
Single-point viscosity tests are commonplace. This means using one spindle at a defined speed to run the test. Hopefully, temperature control and time of spindle rotation are considered and taken care of in the test method. If the viscosity reading falls between two established values, then the test passes and the material is deemed acceptable.
The ability to take two or more viscosity readings on the same sample, but at different rotational speeds, offers a better methodology to evaluate materials for shear thinning behavior. Most fluids and semisolid materials are pseudoplastic to some extent, and the ability to measure this property may provide more meaningful data for QC. Programming a viscometer to perform this type of test automatically is possible with the advanced viscometers available in today’s market.
Use of math models to analyze the data from this more detailed test gives potentially new useful information not possible with the single-point test. The chocolate industry, for example, uses the Casson equation to compute a consistency index and yield stress value (Figure 4). The former value characterizes the degree of pseudoplasticity while the latter value quantifies the ability of the chocolate to hold its shape.
Proper selection and use of viscosity instrumentation can bring valuable time savings to test programs in today’s busy labs. Increased data throughput, coupled with detailed analysis, is a new horizon that has potential to offer enhanced quality control. Take the time to talk with viscometer manufacturers before making your next investment and map out the test strategy that will keep your company ahead of the pack in viscosity testing.