The Terminology of Testing

PLC Probe

In an 1891 lecture, the physicist Lord Kelvin stated, “When you can measure what you are speaking about, and express it in numbers, you know something about it: but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely in your thoughts, advanced the state of science.”

The message from Lord Kelvin today for those in the converting and flexible packaging industries is that they are not fully knowledgeable about many items unless they attach numbers to them.

The coating weight of an adhesive or coating is a good example to consider. If we apply a heat-seal coating to a polyester film, we are depositing a specific amount of the material onto the film. The coating weight might be high or low, but what do those terms actually mean? Is 3 lbs/ream a high coating weight or a low coating weight? Obviously, the answer to this question depends on the type of heat-seal coating.

For a given heat-seal coating, the terms high and low have only a relative meaning. The numerical term, 3 lbs/ream, has a very distinct meaning. We can understand this value and take some action with it such as discuss it with other people. The action we can take is to increase or decrease it by a desired level.

We arrive at the value of 3 lbs/ream for our hypothetical heat-seal coating by running an actual test to measure the amount of coating on the surface. Usually this involves weighing a given amount of coated substrate, removing the coating from the substrate without changing the substrate in any other way, and weighing the now uncoated substrate again. The difference between the coated substrate and the same piece of uncoated substrate allows calculation of the coating weight.

Having generated a number by a test such as coating weight measurement, the second item is to attach the correct units to the number. In this case, we are assuming the test used parameters such that the measured value was pounds per ream. Other units are possible when measuring coating weights, such as grams per square meter to mention only one. Changing from one set of units to another requires some relatively simple calculations. The important point is that one must always use the units with the number. Using only the number has no meaning and can cause confusion when trying to make comparisons or reporting the value to someone else.

Besides the units of the measurement, a number of points are important concerning the actual number itself. Perhaps the first is accuracy. By definition, this is the exact conformation of the measurement to the actual fact. In other words, something accurate is errorless. A related term is precision. A dictionary tells us that precision is the exactness with which a number is specified. Replication is the act of making more than one measurement of the same test. Another important term is standard deviation, which is the measure of variation within a dispersion of measurements.

Understanding significant figures is equally important. Significant figures are the number of digits that correspond to the accuracy of the measuring device. When measuring coating weights of adhesives and coatings, the number of significant figures probably is only two decimal places. In the case of the temperature of a nip roller, the number of significant figures probably is only one decimal place.

Finally, let us mention calibration briefly. To measure something like the nip roll temperature in a laminating process, we might use a device that contacts the roll and indicates the temperature. When doing such a test, the user of the device must be certain it accurately measures the temperature over the range of values in which the temperature falls. Calibration, therefore, requires an independent test of items at known temperatures using the testing device to ensure its accuracy.

Readers who wish to learn more about any terms in this discussion should consult the various books and other information that is readily available concerning product and process control. Unfortunately, making and reporting a measurement is not always as easy as it might appear initially.



David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at dbentley@unm.edu.


To read more of David J. Bentley’s PLC Probe columns, visit our PLC Probe Archives.



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