How To Measure in Artificial Atmospheric Pressure Environments with Laser Trackers

March 1, 2007
/ Print / Reprints /
ShareMore
/ Text Size+
Just how fast is that laser beam traveling? The answer may surprise you.

The speed of light is 186,000 miles per second in a vacuum. Fortunately, we don’t live in a vacuum. However, temperature and air pressure (atmospheric pressure) have a direct effect on the density of air and consequently the speed of light. Change in the speed of light, or “index of refraction,” is a value that must be considered for precision measurements with a laser tracker system.

Laser tracker systems are equipped with some method for measuring the ambient temperature and air pressure. Slow changes in the temperature and pressure are typically measured and compensated for automatically. Sudden changes in temperature are usually noticed by an operator because they can be felt. But sudden changes in ambient atmospheric pressure are often unnoticed by an operator, yet can have as dramatic an effect on measurement accuracy as sudden temperature changes.

In an open building, air pressure will change no more quickly than the outside weather changes. But in a closed building there are sometimes areas with “artificial” atmospheric pressures. Clean-rooms usually maintain a “positive pressure” so that when a door is opened, air will flow out of, rather than into the room, preventing dust and other contaminants from entering the area. Composite shops and paint shops frequently maintain a “negative pressure” so that when a door is opened, vapors and fumes will remain inside the room. Any place that is well sealed has the potential of having an artificial atmospheric pressure. If the air-conditioning system is out-of-balance between the return-air and the blower, an artificial pressure environment is possible. When opening a door, if you hear a little “hiss,” then there is probably artificial pressure. Many metrology labs can have artificial positive pressure during the summer months, because labs are often built in the middle of the plant and the air-conditioning system works hard to cool the room.

The problem is not the artificial pressure, but rather the instant change in air pressure when a door opens. The change in pressure and the change in the index of refraction are instantaneous. If measurements are being taken with the tracker at that moment, they will be significantly affected.

There is no perfect solution to this problem. A skilled operator must be aware of the potential problem and work to prevent his measurements from containing errors that are caused by sudden changes in atmospheric pressure. Measuring a point again if sudden pressure changes are noted is a good practice.

By Kevin Barron

Leica Geosystems (Metrology Division)

Did you enjoy this article? Click here to subscribe to Quality Magazine. 

You must login or register in order to post a comment.

Multimedia

Videos

Podcasts

Charles J. Hellier has been active in the technology of nondestructive testing and related quality and inspection fields since 1957. Here he talks with Quality's managing editor, Michelle Bangert, about the importance of training.
More Podcasts

Quality Magazine

CoverImage

2014 October

Check out the October 2014 edition of Quality Magazine for features!

Table Of Contents Subscribe

The Skills Gap

What is the key to solving the so-called skills gap in the quality industry?
View Results Poll Archive

Clear Seas Research

qcast_ClearSeas_logo.gifWith access to over one million professionals and more than 60 industry-specific publications,Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

STAY CONNECTED

facebook_40.png twitter_40px.png  youtube_40px.pnglinkedin_40px.png  

eNewsletters