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Unfortunately, when it comes to dimensional measurement, we have to take the environment-particularly temperature-into account. We can’t raid the taxpayers for money to mitigate our working environment so we have to do it on our own, but, with a little bit of care and attention, we can minimize the impact.
One thing in our favor is that we only have a small, physically defined area to deal with, but we have to be more precise than those claiming to know the temperature of the entire planet at any given time.
To get a handle on the problem, we have to understand the basics, so let’s take a look at a few of them.
If the temperature in your laboratory or inspection area increases, the items within it will grow in size. The reverse happens if the temperature drops. To make matters more difficult, different materials will grow or shrink at different rates, the amount of which is referred to as their coefficient of thermal expansion, or CTE when you see it in formulae.
Taking a steel block as an example, for every Celsius degree increase in temperature, the block can grow anywhere from 10.8 to 11.5 millionths of an inch per inch of length, or about 6.3 microinches per degree Fahrenheit. These are approximate values due to variations in the metallurgy that can vary with a batch or ‘pour’ of steel from the mill. Many people work with aluminum or brass that have CTE nearly double that of steel. With that kind of difference in reaction to temperature changes, it’s easy to understand how problems could arise.
If you are making or calibrating gages or gage blocks, even the small differences can have a dramatic impact on everything you do. And it can play an equally important role if you’re measuring machined parts even though your tolerances do not approach those of gages.
The reference temperature for dimensional metrology is 20 C or 68 F. This means that if a measurement was made at these temperatures, it makes no difference what the CTE is of the materials involved in the instruments, gages or items being measured. How close you have to be to the reference temperature will depend on the accuracy you are attempting to achieve in the measurement.
And where did 20 C come from, you might ask? I once read that a long time ago they did a survey of workshop temperatures in Europe and the average temperature was 20 C. I’m always suspicious of round numbers and I suspect it was close enough to that temperature to declare it the winner without too many people getting excited about it.
There’s another basic to be aware of when it comes to temperature. Unlike the climate change folks, we can’t get away with computer software models using iffy estimates combined with some hard numbers to arrive at what appear to be precise answers.
The other basic I refer to is rate of change. Like many parameters in metrology, the more stable you can make the process, the more you can offset, correct, adjust or tweak the numbers or the hardware to get a good answer. If the temperature of what you are measuring or the instruments and masters you are working with are all over the place, your readings will be as well, and you don’t have to be working to millionths of an inch or microns to have it happen to you.
Despite taking these basics into account, there are usually reasonable ways to work with them or around them and obtain realistic measurements. Next time I’ll discuss some of them.
Editor’s note: This is the first part of a two-part series.