Measurement Traceability for Labmaster Universal / Labmicrometer Instruments

Pratt & Whitney Labmaster Universal (LMU) / Labmicrometer (LMC) instruments employ a helium–neon (He–Ne) laser interferometer with an air-path optical configuration. The fundamental length reference is the 632.8 nm vacuum wavelength of the He–Ne laser, recognized internationally as a secondary length standard. In practical use, the laser beam traverses air, so the effective wavelength is adjusted according to one of the two methods listed below.

Mastering Method 1 (Recommended)

To establish the internal interferometer scale by the preferred procedure, follow these steps:

• Thermally equalize two NIST-traceable gage blocks and all measured artifacts to a common reference temperature on a thermal soak plate in the controlled-temperature calibration laboratory.

• Mount the first gage block and set a datum by clicking the "1" button in the GageCal software; repeat with the second block and click the "2" button to define the scale factor.

• Assume identical material properties and thermal state between masters and artifacts.

• Perform measurements with the LMU/LMC under the same thermal conditions.

Traceability in this method flows directly from the calibration certificates of the two NIST-traceable gage blocks.

Mastering Method 2 (Environmental Compensation)

For applications requiring explicit refractive-index and thermal-expansion corrections, the following traceable inputs are required:

• One NIST-traceable gage block to establish a reference datum.

• One NIST-traceable air temperature, one NIST-traceable air pressure, and one NIST-traceable air humidity measurement to compute the refractive index and thus the effective laser wavelength in air.

• One or more NIST-traceable material temperature measurements on the artifact to calculate its thermal growth or contraction.

This method yields full traceability to (1) the vacuum wavelength, (2) the gage block datum, and (3) the environmental and material-expansion corrections.

Calibration, Verification, and Compliance

• All master gages (gage blocks, ring gages, etc.) must be periodically calibrated by a NIST-accredited laboratory.

• Use check blocks or ring gauges to verify both the master settings and the LMU/LMC's performance before each measurement campaign.

• Affix a “Calibrate Before Use” sticker to the instrument to satisfy ISO 17025 requirements.

• Enroll in Pratt & Whitney’s Preventive Maintenance program to confirm ongoing laser stability, electronic health, and mechanical integrity.

For additional information or to schedule gage certification and LMU/LMC preventative maintenance, please contact the Pratt & Whitney Service Department at +1 860-286-8181 x230.