In-Situ Temperature Calibration Capability for Dimensional Metrology

Abstract

The Dimensional Metrology Group (DMG) at the National Institute of Standards and Technology (NIST) has the capability to perform large range dimensional measurements in a facility called the Tape Tunnel. The Tape Tunnel is equipped with a 60 m long steel bench and a reference interferometer. Various artifacts and instruments, such as tape measures, optical cables, laser trackers, laser distance meters etc. are calibrated against the reference interferometer. The relative uncertainty (Uk=2) in the displacement measurement is 2.4×10-7. A major component of this uncertainty is the uncertainty in measuring the temperature in the Tape Tunnel. There are 14 temperature probes installed along the length of the steel bench; two each at seven equidistant locations. One probe measures the air temperature and the other measures the material temperature (of the steel bench). Historically, calibrating these probes involved removing all the 14 probes and sending them to the NIST Thermodynamic Metrology Group. This process introduced a considerable amount of downtime to the DMG’s measurement capabilities. This also introduced uncertainties due to a) variation in the contact geometry of the material probe with the steel bench during reinstallation, and b) variation in the resistances of the probes’ cables due to pinching and/or elongation. In an attempt to address these issues, a new in-situ temperature calibration system was developed. This paper discusses the system components, an in-situ calibration procedure, the uncertainty sources involved in the calibration process, presents an uncertainty budget, and examines it with a Monte Carlo simulation. This system enables the DMG to perform quicker in-situ temperature calibration, at frequent intervals, with minimal downtime and provides better uncertainties in the dimensional measurements.