Getting the correct measurement is the goal of a measurement process – and it is a process!. Some people think buying the measuring device at a low price is the solution. Getting and using one (or more) involves way more than price! It’s a process in itself, too!
Choosing and using a measurement device to fill a process, engineering or scientific measurement is not a simple task, especially the first time, but it can rise up to hurt production quality (and yield – that means Profits) if not done correctly. Sometimes the obviously simple tasks are not as obvious as everyone believed at the outset.
That means all the first times: the first time for a new measurand, for the first time in a given set of measuring conditions and the first time that you’ve been involved in making a measurement.
The simple and unthinking way is to choose a device based on price or beliefs. People often find a lowest price and buy that way. Not to demean any Purchasing Agent, but a robot could do that; there’s not much thought involved!
It ain’t about initial price; it’s about lifetime price, measuring accurately and reliably! Those, too, mean more than the few words imply.
The present race to lower costs for infrared imaging cameras is an example of high tech devices moving into the realm of low tech workers. Unless someone provides training in how to use the equipment (as some Infrared Thermal Imager makers do), a lot of time & money can be wasted.
So, measurement is about people, too!
The combination of measurement device requirements and people skills have more impact on the choice of device than one thinks.
I’ll give an example based on my own experiences. It’ll take more than one blog post, but I’ll set the scenario here.
Take a “simple” measurement verification task of steel strip width, say with a width of in the range of about 36″ to 60″, or 915 mm to 1525 mm.
Suppose you wanted to measure it to verify that it (that is the cut width – cut by inline trimmers) was correct to ordered width to within a certain measurement tolerance, say ± 1/32″ (0.030″) or ± 0.8 mm.
What could be simpler, eh?
Now, suppose that you wanted to be able to do this, say at least once on every change of strip width run on a 24 hour process, like a continuous pickling line in a steel mill operation that has coils of strip welded end-to-end. There, at least five different people work during the week on the line and tens of different widths are run each day in a typical 19 shift (turn – they hope) per week operation.
The practice worked out by the process line staff team, including the QA manager was: Stop the line within some reasonable distance from the start and end of a strip order, have the inspector walk onto the line, make a measurement, write it down in his notebook, leave the line and the line restarts. The Inspector then keys the data into his computer terminal.
There’s the extra, unquantified variable in the measurement process: people.
Yet, to a person, every “Inspector” claims they can read a good quality tape measure (with a steel tape) to that tolerance or better and thus do the job with it as the measurement device of choice.
Still sounds simple, doesn’t it? I’m pretty sure I could estimate to a half a division on a measurement scale, so would most people with practice and decent, corrected eyesight.
There’s a few problems with the practice from a measurement redundancy perspective, but the line QA manager reasons that over hundreds of stip orders during a week, the errors from single measurements will average out to meet or exceed that required.
The Purchasing Dept. gets orders for a gross of high quality tape measure devices and buys then in one batch; all with the smallest interval between scale marking of 1/16″ (0.062″) or 1.0 mm.
To verify that all tapes work correctly and to “test” the Inspectors’ skills a set of precision fixed length (lengths known to within ± 0.01″ or ± 0.4 mm or better) standard rods that cover the measurement range are purchased and mounted in a large board, each labelled with an identification number.
Each month the Inspectors will test their individual tapes against the unknown standards and the results of the tests reported to the Corporate Measurement Control computer database (Computer Corporation of America’s Model 204).
OK, that’s the setup. After a few months, it wasn’t working as expected:
Widths were out of spec as often as they were in spec. Not a lot, but more than the acceptable tolerance.
So, the tech deptartment were asked to review the situation. I got the job (because all the temperature-measurement problems were fixed and no one else was available). Objective: Find out what’s wrong and fix it!
More to the story soon.
Meanwhile, anyone care to suggest what’s right or what’s wrong with this measurement situation?
For now we end this article with a suggestion based on the Automotive Industry Action Group (AIAG) Measurement Systems Analysis Manual: Gage R&R.
