Hello, I'm Jim Salsbury with Mitutoyo America Corporation and welcome to the Metrology Training
Lab. In this episode, we are going to introduce you to the type of problems that we see in
dimensional measurement associated with the influence of temperature.
You may have noticed that some of today's more advanced measuring equipment has built-in
temperature compensation systems. These systems are standard on most coordinate measuring
machines, but even this linear height gage has that function.
As you can see in this menu, the software gives you the option, if you want, to enter
material properties as well as temperature information. So what does this do and why
would you use it?
All materials grow and shrink as the temperature changes. You may think these changes are small,
but every measurement is impacted by temperature in some manner. You can't avoid temperature
influences - they are always there. What we all have to do is understand and potentially
reduce the errors from temperature to an acceptable level.
So, first - all dimensions, all sizes, all lengths, all geometric features are defined
at exactly 68 F, which is also equal to exactly 20 C. This is the official legal reference
temperature found in all national and international standards. So, in theory, all your measurements
should be done at exactly 20 C. Of course, that is never possible - you never can be
that exact, and therefore you always have errors from temperature.
In dimensional calibration labs, you may find environmental control at something around
say 18 to 22 C, and if you are just calibrating small micrometers and calipers, the thermal
errors are usually small enough to not cause any concerns. But when you have longer measurements,
or higher accuracy equipment, something like this high accuracy linear height gage, then
temperature can suddenly be a big deal. So let's talk about some good practices to reduce
the impact of temperature.
First of all, you want the temperature to be stable. You want to avoid large doors that
frequently open - particularly large roll up doors. You also want good airflow and not
air blowing on and off right on top of your measurement. Deflect the air away and around
your measurement to let it mix together. Also avoid heat sources close to the measurement,
like lights and computer systems. People are also large sources of heat, so be careful
with large numbers of people, and people coming and going too often. For more critical measurements,
try to set up an area that is separate from the rest of the work going on around you.
You also want to allow your measurement set up to reach equilibrium in temperature - a
process that is often called thermal soak out. If I bring this steel gage block over
to calibrate this linear height, I want the linear height and gage block to sit next to
each other long enough that they both reach the same temperature. In our calibration lab,
the practice is let things sit overnight in the lab.
Now this is an extreme example, but when calibrating gage blocks, we arrange the customer's gage
blocks, which are the white ceramic ones here, one by one, next to our master blocks, these
steel ones. The goal is to have the two blocks be at the same temperature. We also use this
aluminum plate to help with heat transfer.
If you have a stable environment and things thermally soak out properly, you've taken
care of many temperature problems. But what if the average temperature in the room is
not the reference temperature - not at 20 C. That is a bit cold, and many labs run a
few degrees warmer than that. Since that situation is common, and not easily fixed, let's look
at the math so you have a way to estimate the errors to see if that may be a problem
for you.
All materials expand as temperature increases. Different materials expand by different amounts.
You need to know what is called the coefficient of thermal expansion, or CTE, of different
materials to understand the impact.
If you look in the Mitutoyo catalog, you will see the published CTE for our gage blocks.
Our steel blocks expand at 10.8 parts per million per degree C. That means 10.8 microns
per meter or 10.8 microinches or millionths per inch for every 1 degree C of change. At
21 degrees C, this 1 inch gage block will have grown 10.8 millionths of an inch bigger
than at the reference temperature of 20 C.
Mitutoyo ceramic gage blocks have a slightly lower CTE, at 9.3, not 10.8. Mitutoyo actually
designed the ceramic gage blocks to have a CTE close to steel to reduce thermal problems
when they are used.
Like a lot of measuring equipment, this linear height uses a linear encoder, sometimes called
the scale, which is made of glass. In the operation manual for this instrument, you
will find the stated CTE to be 8.1 parts per million per degree C. Just as an example,
let's do a simple calculation to see what happens if we calibrate this linear height
using a 500 mm - or about 20 inch - steel gage block in this typical office temperature
of 22 C.
The equation looks like this:
So is that a problem? Well, if you look up the accuracy
tolerance for this linear height, the 2.7 micron temperature error is about twice the
accuracy specification for this unit. So that means if you calibrate this unit in this environment,
even if the temperature is perfectly stable and the gage block and linear height reach
thermal equilibrium, a perfectly accurate linear height will be found to be significantly
out of tolerance if the room is just 2 degrees off from the reference temperature. A good
unit will look bad because of temperature. When I said that temperature impacts all measurements,
this is what I was talking about.
So what do you do? Well, now you can understand why this unit has built in temperature compensation.
It's purpose is to automatically correct this error. For measuring systems with temperature
compensation, you should use it.
Without temperature compensation, you have two other solutions. First, you could calculate
the error yourself, and correct all your readings. The best solution is to find a better environment
- this gage should really be calibrated in a room that is good to at least 20 ± 0.5
C or better.
Our goal in this episode was to introduce you to temperature effects in dimensional
measurement and to give you a few tools to understand them and improve accuracy. In later
episodes, we'll talk more about other temperature errors and ways to reduce them.
Thank you, I'm Jim Salsbury, and I'll see you next time from the Metrology Training
Lab.