Transient heating in fixed length optical cavities for use as temperature and pressure standards

Area of Science:

• Metrology
• Optical Physics
• Thermodynamics

Background:

• Optical refractometry offers direct pressure and temperature measurement from gas properties.
• The NIST fixed-length optical cavity (FLOC) refractometer was previously assessed as a pressure standard.

Purpose of the Study:

• Evaluate the NIST refractometer's feasibility as a primary temperature standard.
• Address challenges in achieving temperature uniformity within the optical cavity during gas-filled operation.

Main Methods:

• Comparison of computer modeling with laboratory measurements to validate thermal behavior predictions.
• Assessment of temperature uniformity across the refractometer, requiring < 0.5 mK for accuracy.

Main Results:

• Validated computer model accurately predicts thermal behavior and measurement uncertainty.
• Achieved temperature equivalence between glass elements and copper chamber within 0.5 mK after 3000 s equilibration (1 kPa to 100 kPa).

Conclusions:

• The NIST refractometer can determine gas temperature with an uncertainty (k=1) of 0.5 mK.
• The findings support the refractometer's potential as a primary temperature standard.