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Thermal effects can
affect your results.
It's consistency of temperature,
not the actual temperature, that is important.
Thermal errors can stack
up.
Consistent
temperature is more important than the actual temperature
For measurement
uncertainty purposes, you want to assure that linear expansion dimensional
errors attributable to temperature variation are minimized - less than 10% of
your intended accuracy.
Thermal Expansion Coefficient -
The thermal expansion coefficient (CTE) of tool steel is added to the
measurement uncertainty calculation where relevant. The Testing Laboratory
considers consistency in temperature most important. This policy was derived
from MIL-STD-120, which states: "Whenever precision measurements are to be made,
the temperature should constantly be kept as near to 68 degrees as
possible. Since most gages and measuring instruments are usually made of
steel …. the requirement that the temperature remain constant is more
important than the actual temperature."
Based on the above statement in
bold, our laboratory tracked the temperature with its computerized
temperature control system over a period of a month in order to determine the
amount of deviation from 68 degrees. The amount of this deviation is used to
calculate the Linear Expansion per unit length per degree Fahrenheit. This
amount is used in the calculation of relevant measurement uncertainties.
For steel, the coefficient we
used was 0.000006″ per degree of temperature change. (That's six millionths of
an inch per degree F.)
For copper and copper alloys, we
used 0.000009″ per degree of temperature change. (That's nine millionths of an
inch per degree F.)
For aluminum, the figure we used
was 0.000013 " per degree of temperature change. (That's thirteen millionths of
an inch per degree F.)
While room air conditioning is
important, don't forget that handling gages can affect your measurement system,
too.
Chart From
Kennedy and Andrews Inspection and Gaging
Note that gaging can pick
up operators' body heat and that temperature errors can thus stack up …
Photo credit:http://www.commodorecomputerclub.com/images/020411/IMG_1868.jpg
Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which originally appeared here.
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Re: Three Considerations - Gaging Inspection Lab
