In this post, I want to talk about the proper way to express test results on certificates of analysis. To explain this, I am going to talk about the concept of significant figures (or significant digits). What are significant figures?
According to Wikipedia:
“The significant figures of a number are digits that carry meaning contributing to its measurement resolution.”
Confused? Let me explain.
When I first started hearing about the ISO 9000 quality system in the 1990’s there was a lot of resistance to it. A lot companies were unhappy about being pressured or required to adopt the ISO 9000 system by their customers. It was not looked at as being a benefit to their businesses; it was looked at as a giant hoop that you had to jump through in order to be able to get business opportunities from some larger companies. I never agreed with this viewpoint. I saw the benefits of at least some parts of the ISO 9000 standard pretty early on.
Since the 1990’s, the ISO 9000 standard has gone mainstream. Numerous industries have adopted it. No one more vigorously than the auto industry which has their own version which they call TS 16949 (basically ISO 9000 plus some specific auto industry requirements).
However, since then, some cracks have developed that are causing the ISO 9000 system to not only become just a hoop that companies have to jump through, but a series of flaming hoops. Some changes need to be made in order for this system to achieve its goals and actually work for the companies trying to adopt it.
Almost everyone by now has at least some familiarity with BPA in plastic, especially if you have kids. For those of you that are not familiar, here is the media narrative on BPA. BPA is an additive that is used in plastic materials. This additive causes horrible health problems but the evil chemical industry won’t remove it and the FDA must be being bought off by the same evil chemical industry to continue to allow its use. In fact the whole obesity epidemic might be the fault of BPA and thus the evil chemical industry.
Don’t believe that that is the narrative? Check out this Newsweek article.
Now, let’s talk about the reality of BPA.
If you are unaware, there are two sets of test methods that are currently in use for many of the common tests that we see reported on data sheets and certificates of analysis. This has caused a lot of confusion.
To add to the confusion, I have seen many data sheets and certificates of analysis in which data is reported incorrectly. None more than izod impact. There is currently an ASTM method and an ISO method for testing izod impact. Although some data sheets claim that they are, the two test methods used for izod impact are definitely not equivalent. They are two different test methods and there is no way of converting between the two.
As I mentioned in a recent post, after a number of years of searching, I finally found a copy of the Plastics Materials Digest – International Plastics Selector. Before the internet, when I needed to find a material or look up properties, this is how I did it.
The books contain thousands of property data sheets as well an index that allows you to look up materials by trade name and even physical properties.These books were very good. If you needed a polypropylene with tensile strength over 3600 psi, you could find it in the index and then it would refer you to the page where you could find the full data sheet.
The copy that I found happened to be from 1988, the same year that I started in the industry.
I have had a little time to look through the book now and I have a few observations.