Effective humidity in length measurements: comparison of three approaches

Abstract

Humidity is one of the key atmospheric parameters influencing the refractive index of air. Consequently, humidity influences all length measurements whose scale is derived from the speed of light. In this work, we present two laser spectrometers developed for determining the average humidity of air over a long measurement path where local variations may be difficult to measure using conventional sensors. Further, these laser-based systems allow, in principle, good spatial overlap with the beams used in dimensional measurements. The developed systems were compared to each other and to traceable reference sensors during a 65 h measurement campaign. The performance of the systems was investigated under three different conditions: steady state, humidity transient and temperature transient. Both systems were separately tested in outdoor environment at distances up to several hundreds of metres. The measurement results demonstrate that the systems are able to measure the relative humidity below the 4% uncertainty level both in indoor and in outdoor environment.

Spectroscopicallyin situtraceable heterodyne frequency-scanning interferometry for distances up to 50 m

“…A calibration by interferometric comparison over several tens of meters as performed in section 4 delivers satisfactory results as well, in particular since the overall measurement uncertainty of such a macroscopic distance measurement will in practice be governed by the uncertainty of the index of refraction, being at best in the order of a few parts in 10 −7 (see e.g. [24,25]).…”

High-precision diode-laser-based temperature measurement for air refractive index compensation

“…The humidity measurement utilizes the same modulated and normalized absorption technique as the temperature measurement setup, but instead of measuring the ratio of two absorption peaks, only one water transition and baseline determination are required for determining the number density of water. A detailed description of the humidity measurement configuration and results is given in [21].…”

Air index compensated interferometer as a prospective novel primary standard for baseline calibrations

“…In sunny weather the temperature gradients even on short scales easily exceeds 1 K which leads to a length uncertainty of more than 10 −6 l if they cannot be determined correctly [9]. One approach to resolve this problem is based on spectroscopic temperature and humidity measurements [10,11]. The effective temperature can be measured by the velocity of sound in air, however, this method is not applicable for long distances outdoor [12].…”

“…In moist air the humidity term of the equation ( 4) prevents the cancellation of the air parameters in the factor A, so this parameter depends on temperature, pressure, and humidity. A solution for this problem is to measure only the partial pressure of water vapour p w either conventionally or optically by intrinsic methods [11]. The length l can then be calculated from l 1 , l 2 , and p w [14].…”