A microwave resonance dew-point hygrometer

Abstract

We report the first measurements of a quasi-spherical microwave resonator used as a dew-point hygrometer. In conventional dew-point hygrometers, the condensation of water from humid gas flowing over a mirror is detected optically, and the mirror surface is then temperature-controlled to yield a stable condensed layer. In our experiments we flowed moist air from a humidity generator through a quasi-spherical resonator and detected the onset of condensation by measuring the frequency ratio of selected microwave modes. We verified the basic operation of the device over the dew-point range 9.5–13.5 °C by comparison with calibrated chilled-mirror hygrometers. These tests indicate that the microwave method may allow a quantitative estimation of the volume and thickness of the water layer which is condensed on the inner surface of the resonator. The experiments reported here are preliminary due to the limited time available for the work, but show the potential of the method for detecting not only water but a variety of other liquid or solid condensates. The robust all-metal construction should make the device appropriate for use in industrial applications over a wide range of temperatures and pressures.

Progress towards the determination of thermodynamic temperature with ultra-low uncertainty

“…below the temperature of the critical point of the measurement gases, significant perturbations may be induced from pre-condensation. These phenomena are not currently amenable to accurate modelling but their occurrence may be revealed by selective comparison of TM or TE microwave modes with a method previously exploited for microwave hygrometry [18]. At the highest temperatures, maintaining the gas purity within acceptable limits represents a critical issue, as does introducing the acoustic and microwave signals from transducers maintained at ambient temperature.…”

Accurate Characterization of High-$Q$ Microwave Resonances for Metrology Applications

“…The frequency term at the numerator allows for modeling of asymmetrical (skewed) resonances. In case of n degenerate modes, the superposition principle can be used, thus fitting each mode with (12) and summing them up. In a real resonator, non-zero background transmission is always present due to cross-talk, to the tails of neighbor resonance modes and to the effect of cables and connectors in case of uncalibrated or partiallycalibrated measurements.…”

“…Beyond this, they have found widespread application as highly-sensitive sensors in industry (e.g., for moisture and displacement sensing applications [1]- [4]), scientific research and metrology [5]- [9]. In fact, among all physical quantities, frequency and time can be measured with exceptional accuracy, thus making microwave resonances one of the most interesting methods for measuring temperature [10], [11], pressure [8] and humidity [12], [13], for accurately characterizing permittivity, permeability, mass, density and water content for a variety of materials [9], [14] and for detecting foreign particles [15], since the resonance frequency of a microwave resonator depends on all these physical parameters. Moreover, high sensitivity, high speed, relatively low cost, high adaptability and flexibility and their non-destructive, non-contact nature are the main advantages of resonance-based measurements, together with their ability to take precise measurements within a small area.…”

Dew Point Calibration System Using a Quartz Crystal Sensor with a Differential Frequency Method

“…Dew point hygrometers have been used as a standard for measuring the humidity of gases and are used in many laboratories as the reference calibration standard. Many different dew point hygrometers based on chilled mirrors [6], surface acoustic waves [7], optical fibers [8], microwave resonators [9], CMOS technology with capacitive detection of condensed water, [10] etc., have been reported.…”