Invar

Minimize thermal changes with "Invar"

Everyone knows that almost all materials will expand when the temperature rises, and shrink when the temperature falls. Often this is not a problem, but in the measurement world, this effect can cause mischief. Nobody likes to measure things with a ruler that is capable of expanding and contracting. It's the same as the old story about the man with two watches – he never knows what time it really is. So wouldn't it be great to have a measuring device that was the same length regardless of the temperature?

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Fortunately, we can get very close to achieving this with Invar. Invar is an extraordinary alloy composed primarily of iron and nickel, along with minor additions of silicon and manganese. The chief advantage of Invar lies in its extremely low coefficient of thermal expansion, which means it undergoes negligible changes in volume with temperature variations. Indeed, its name is derived from the word "invariant," reflecting its stability under shifts in temperature.

To illustrate, consider a 5-foot piece of aluminum, which would expand about 0.024 inches with a 30-degree Fahrenheit temperature increase. A steel piece of the same length would expand approximately 0.017 inches under the same conditions. In stark contrast, a piece of Invar would only expand about 0.001 inches. This remarkable stability is why we've selected Invar for crafting our length standards and scale extension kits.

[Specifically, the thermal coefficient of expansion of the Invar used in our system components is 0.68 x 10^-6 in./in.°F over a range of 0-200°F. In metric units, this is 1.2 x 10^-6 m/m°C.]

Why Brunson's Invar system is so cool!

We know that measurement professionals always try to push the envelope of accuracy as far as practical and/or possible. It also seems like everything works against this goal – things like vibration, air movement, or inherent equipment limitations. We know that any time an obstacle can be minimized, it can be a significant factor in making positive and accurate measurements or alignments. That's why we used Invar as the material for our scale extensions and length standards. Its low coefficient of thermal expansion (see above) makes it the perfect material for the job.

To make your job as easy as possible, we designed a flexible system which allows you to use a variety of standard components to configure a number of different measuring tools. For example, these tools can be used to construct scale extensions of various lengths for optical alignment applications. Various end tips are available to accommodate different types of nominal reference positions. Or the system can be used to construct known and trusted distances between targets for the calibration sequences required by laser trackers, scanners, photogrammetry systems, theodolite systems, etc.

The Three "Cs"

The Brunson Invar System is a simple system of Invar tubing and accessories, designed with three "Cs" in mind: Connection, Calibration, and Certification.

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• Connection: All of our Invar system components are designed to be connected together using standard, removable threaded plugs. Bars of different lengths may be connected together, yielding any desired length. Various terminators such as tooling balls or concave tips, as well as scale holders and target holders may be connected on one or both ends of the bar. The mating surfaces of all components are machined to tight parallelism tolerances to assure you of a precision fit.
  
  
• Calibration: All dimensional components of our Invar system are serialized and accurately measured – and you're provided with a document specifying the precise length of each item by serial number.
  
  
• Certification: All calibrated components are certified as NIST-traceable with included documentation.

Flexibility

Because our Invar System is so flexible, it is well-suited for several different applications:

• Precision scale offsets (extensions)
• Three dimensional measurement system scaling (trackers, photogrammetry, theodolites, etc.)
• Hidden point rods
• Provide known reference lengths for calibration
• Construction of invariant artifacts

Invar, an alloy of iron that expands very little when heated; it contains 64 percent iron and 36 percent nickel. Invar was formerly used for absolute standards of length measurement and is now used for surveying tapes and in watches and various other temperature-sensitive devices. The trademark name was selected by the alloy’s inventor, the Swiss physicist Charles-Édouard Guillaume, to express the invariability of its dimensions when heat is applied.

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