Measurement at Temperatures Other Than 68°F (20°C) - FAQ

 

Frequently Asked Questions

Q: On laser interferometer-based instruments, can parts be measured at a temperature other than 68°F (20°C)?

Yes. Laser interferometer-based instruments can accurately measure parts at temperatures other than 68°F (20°C). When the master and artifact are manufactured from the same material and are stabilized at the same temperature, their thermal expansion is effectively identical. Under these conditions, a two-point calibration using calibrated master sizes allows the instrument to directly display the calibrated size of the artifact without applying a thermal correction to 68°F (20°C).

Because the laser interferometer provides an inherently linear measurement reference based on the wavelength of light, measurement accuracy is maintained throughout the full measuring range rather than only over a limited comparative span. This capability is a significant advantage of laser interferometer-based universal measurement systems.

Q: Why is matching the master and part material important?

Materials expand and contract with temperature according to their coefficient of thermal expansion (CTE). When the master and artifact are produced from the same material and are at the same stabilized temperature, both components expand or contract by essentially the same amount. As a result, thermal effects are inherently cancelled within the measurement process.

This allows accurate dimensional measurements to be obtained even when the environment is not maintained at the standard reference temperature of 68°F (20°C).

Q: How can artifacts that are a different temperature or material than the masters be measured?

The GageCal Temperature and Contact Correction dialog should be used when the artifact differs from the master in either material or temperature. The operator enters the calibrated master size, master material, artifact material, master temperature, artifact temperature, probe contact radius, and probe contact force.

Using this information, the Temperature and Contact Correction function establishes an internal measurement scale that accounts for the artifact’s thermal expansion characteristics, contact penetration behavior, and actual measurement temperature. The instrument then displays the artifact's calibrated size referenced to standard temperature and pressure (STP).

This approach allows accurate measurement of artifacts that differ in material or temperature from the masters while preserving the accuracy advantages of the laser interferometer measurement system. For best results, artifact temperature should remain stable between mastering and measurement.

Laser Interferometer vs. Glass Scale Instruments  

Q: How do laser interferometer-based instruments compare to glass scale-based instruments?

The two technologies differ fundamentally in how length is measured and how accuracy is maintained across the measuring range.

Laser interferometer systems measure displacement directly from the wavelength of light. This approach provides inherently linear displacement measurement, exceptionally high long-range accuracy, minimal hysteresis, and excellent thermal stability. Because the measurement reference is optical rather than a physical encoder scale, position-dependent correction maps are generally unnecessary, full-range two-point calibration is practical, and measurement accuracy is maintained across the entire measuring range. These characteristics make laser interferometer systems particularly well suited for primary dimensional metrology, master gage calibration, long-range precision measurement, and high-accuracy universal measurement systems.

Glass scale systems use precision encoder scales integrated into the machine structure. Accuracy is achieved through encoder interpolation, error mapping, thermal compensation, and software-based correction algorithms. Because the scale itself expands and contracts with temperature, the system must compensate for scale expansion, machine structural effects, and workpiece thermal behavior. As a result, glass scale systems generally rely on position-dependent correction tables, multi-point calibration models, and software compensation to maintain full-range accuracy.

Glass scale systems perform extremely well in many industrial and laboratory applications, particularly in comparative measurement applications where the measuring span is relatively small.

Q: Can glass scale-based instruments also use two-point calibration?

Yes. Many high-quality glass scale-based instruments successfully use two-point calibration methods, particularly over shorter direct-reading ranges.

For shorter measuring ranges, residual scale nonlinearity is typically small relative to the instrument’s accuracy specification. In these applications, two-point calibration can provide excellent performance and practical usability. Pratt & Whitney Measurement Systems offers glass scale-based instruments that employ two-point calibration very effectively within their intended measuring ranges.

The primary distinction is that laser interferometer-based systems maintain inherently linear measurement over much longer ranges and at substantially lower uncertainty levels, whereas glass scale-based systems generally operate over shorter direct-reading ranges and with larger overall accuracy specifications.

As measuring range and accuracy requirements increase, glass scale systems typically rely more heavily on compensation models, calibration mapping, and environmental correction techniques. Laser interferometer systems minimize these requirements because the displacement reference itself remains inherently linear across the full measuring range.

Q: Why are laser interferometer systems often considered 'absolute' measurement systems?

Laser interferometer systems derive displacement directly from an optical wavelength reference rather than from a physical encoder scale.

As a result, the measurement reference remains inherently linear throughout the full measuring range, no position-dependent correction mapping is required, and measurement accuracy depends far less on software compensation models. This architecture has historically made laser interferometer-based systems the preferred solution for the highest levels of dimensional metrology and universal length measurement.