Effective humidity in length measurements: comparison of three approaches

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.