Holger Venzke

Three-dimensional sensing of rough surfaces by coherence radar

We introduce a three-dimensional sensor designed primarily for rough objects that supplies an accuracy that is limited only by the roughness of the object surface. This differs from conventional optical systems in which the depth accuracy is limited by the aperture. Consequently, our sensor supplies high accuracy with a small aperture, i.e., we can probe narrow crevices and holes. The sensor is based on a Michelson interferometer, with the rough object surface serving as one mirror. The small coherence length of the light source is used. While scanning the object in depth, one can detect the local occurrence of interference within the speckles emerging from the object. We call this method coherence radar.

Interpolation Correlations for Fluid Properties of Humid Air in the Temperature Range 100 °C to 200 °C

This paper provides simple analytical correlations for selected thermodynamic and fluid transport properties for the mixture dry air and water vapor. These correlations are derived from theory as well as from numerical fitting procedures and give expressions for density ϱ, viscosity μ, thermal conductivity k, specific heat cp, and Prandtl number Pr at a working pressure of p=1 bar and for a temperature range from 100 °C to 200 °C. The main purpose is to present a comparatively simple set of equations, as the correlations do not reflect in every case the underlying physical background. Since experimental data are scarce for the properties under investigation, it was in some cases necessary to extrapolate the available correlations to temperatures or water vapor contents where no experimental data could be found. The derived equations are compared with the pure component values for dry air and water vapor and, as far as possible, also for air-water vapor mixtures.

Influence of humidity on hot-wire measurements

When applying hot-wire anemometry to velocity measurements in air, it is standard practice to neglect the effect of humidity. In this paper the influence of the thermodynamic and transport properties of humid air on hot-wire measurements is examined on the basis of the correlations for Nusselt number proposed previously by other researchers. Experimental results at controlled levels of relative humidity between 30% and 90% at , and are reproduced satisfactorily by the theoretical approaches of two of these equations. A corrective term is defined to expand formulae designed for dry air to work in a humid environment. The error in velocity by omitting the influence of humidity is estimated in terms of temperature and relative humidity.

High precision trace humidity measurements with a fibre-coupled diode laser absorption spectrometer at atmospheric pressure

Laser absorption spectroscopy offers the potential for fast and precise trace moisture detection in gases at atmospheric pressure with a small cross sensitivity towards other molecules. We report on the development and calibration of a fibre-coupled tunable diode laser absorption spectrometer operating in the 1 - 100 ppm humidity range. The spectrometer was tested at three European humidity standards laboratories. The performance of the spectrometer was characterized by monitoring constant water vapour concentrations over several hours, yielding a good long-term stability, reproducibility and accuracy. The standard deviations of the measured water vapour concentrations for values above 5 ppm were below ±2%. Comparisons of the instrument with chilled mirror hygrometers demonstrated its fast response time. The dependences of the signal upon the flow rate and temperature are discussed.