Rotational viscometers are the most widely used tool in today's QC labs for making viscosity measurements. Many small changes, like reduction in sample size and shortening the viscosity test time, can lead to significant increase in QC lab productivity.
Rapid turnaround time in viscosity testing is fundamental to Quality Control’s role in improving product consistency and minimizing batch rework or outright rejection. There are four factors to consider as lab manager that can reduce test time and/or improve the usefulness of test results:
1. Reduce sample size to the minimum necessary
2. Control sample temperature as quickly as possible
3. Reduce the time required to run the viscosity test procedure
4. Automate the entire test if feasible
SAMPLE SIZE: QC viscosity test methods have been in existence for ages. Many do not get reviewed to see if they still make sense or are being done in the most practical way. Grab samples from the production floor may be up to a half-liter or more in size, when perhaps much less will do. The test apparatus used to make the viscosity measurement can be selected to work specifically with small sample sizes. Disposable chambers for use with accessories like Small Sample Adapter (see Figure 1) require 16mL or less, depending on spindle used, and can be discarded after the test is run.
TEMPERATURE CONTROL: One reason for variability in viscosity test results is absence of temperature control when making the measurement. All fluids and semi-solids are temperature dependent. R&D should provide an assessment of the viscosity temperature profile and decide whether temperature control should be practiced during the QC test. If yes, then the choice of small sample size above makes even more sense. The time needed to bring 16mL of sample to temperature is relatively small compared to a half-liter. The Small Sample Adapter comes with a water jacket that brings the sample to temperature equilibrium in a matter of minutes.
VISCOSITY TEST TIME: Once the spindle is immersed in the test sample and temperature equilibrium is achieved, the test may last a few minutes. Regardless of the rotational speed(s) used, there is the possibility that the time of rotation before the viscosity measurement is recorded is longer than necessary. What guideline should you use? Once the viscosity reading stabilizes, this is a good indication that a valid reading has been obtained. QC should report this observation back to R&D for consideration in shortening the test. Precious seconds, if not minutes, can be saved by using good judgement.
AUTOMATING THE VISCOSITY TEST: Today’s benchtop viscometers have the capability to run tests automatically. The instrument shown in Figure 1 can monitor sample temperature until equilibrium is achieved, then start the spindle rotating for a defined time interval, and finally record the measured viscosity value. Data capture can be automated as well if the instrument is connected to a printer or PC.
Busy QC labs that run dozens of tests every day look to economize whenever possible. Saving a half minute on every test can bring significant returns toward boosting lab throughput. The QC Lab Manager can easily make this calculation once the viscosity test procedure has been reviewed and analyzed.
Small changes in viscosity test procedure for QC Lab operation can lead to big improvements. Time saved per test brings immediate return to the bottom line. The investment in the equipment shown in Figure 1 is minor compared to the financial savings that result from reduced test time. Perhaps it’s time to reconsider your viscosity test method and economize.