Fourth-generation wire micrometer that rivals best in the world

Researchers at the National Institute of Standards and Technology (NIST) have developed a dramatically improved laser-based device that measures the diameter of fine-gauge wires, fibers, and other objects only about three times the thickness of a human hair.

Known as a laser micrometer, the device’s accuracy is comparable to its state-of-the-art counterparts but is inexpensive, simple and easy to operate.

NIST scientists John Stoop and Ted Dieron reported their findings on the December 15, 2020, metrology issue.

The new micrometer uses an advanced laser displacement interferometer, which relies on light to measure the thickness of objects placed between two metal contacts. With the new system, researchers can measure the diameter of any object less than 50 millimeters wide, including fine-gauge wire and fiber, with an uncertainty of only 2 nanometers. This is twice the accuracy of previous laser micrometers developed at NIST.

Stoop and Dorian made the new micrometer almost completely out of the invar, a nickel-iron alloy known for its thermal stability. This means that the material does not react to small changes in temperature, resisting expansion or contraction. As a result, the measuring device is less likely to error than other state-of-the-art instruments.

In fact, the improvement “puts the new NIST micrometer at a level equal to the world’s best,” Stop said. In addition, the NIST micrometer is less expensive and simpler. For example, because the NIST instrument is not automated like other state-of-the-art instruments, it is inexpensive to build, simple in design and easy to keep under tight statistical control.

“It’s always a challenge to achieve the best in world performance without breaking the bank,” Stop said.

Manufacturers are working much thinner with fiber and wiring than they did a decade ago for optical communications and on-chip electrical networks. This has led to the need for a laser micrometer that can measure small diameters with high accuracy and establish standard-diameter “master” fibers that can be used as a reference to assess the diameter of other fibers Can.

At the other end of the scale, there is an increasing need to measure the size of large, centimeter-diameter pressure pistons and cylinder gauges, which NIST micrometers can also perform. Because the piston pressure is proportional to its area, small errors in measuring the diameter of the piston can also produce significant errors in the pressure calculation.

Measuring the diameter of thin fibers and wires is a delicate operation because these objects can move or change their shape relatively easily. If those distortions are not noted, they can cause a significant error in the measured size.

To account for the deformation, the NIST researchers designed their micrometers so that they could vary the force applied by the contacts holding them in place.

By measuring the variation in the diameter of the object when different contact forces were applied, the researchers were able to extrapolate the diameter, when no force is applied to the object, the undeveloped diameter.

The new design allows researchers to operate the equipment remotely, eliminating the possibility of introducing heat into the system through human interaction. Researchers also suggested carbide contacts as a more stable way to hold the measured object. All these improvements increased the accuracy of the device.

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