Ebbe Nyfors

The complex dielectric constant of snow at microwave frequencies

The complex dielectric constant of snow has been measured at microwave frequencies. New and old snow at different stages of metamorphosis have been studied. The results indicate that the complex dielectric constant is practically independent of the structure of snow. For dry snow, the dielectric constant is determined by the density. For wet snow, the imaginary part and the increase of the real part due to liquid water have the same volumetric wetness dependence. The frequency dependence of the complex dielectric constant of wet snow is the same as that of water. A nomograph for determining the density and wetness of wet snow from its dielectric constant is given. A snow sensor for field measurement of the dielectric constant has been developed. It can be used for determining the density and the wetness of snow by a single measurement.

Radiowave sensor for measuring the properties of dielectric sheets: Application to veneer moisture content and mass per unit area measurement

A radiowave sensor for measuring the properties of thin dielectric sheets has been constructed. The sensor is an electromagnetic stripline resonator with two center conductors. The sensor is non-contacting, tolerates moderate perpendicular movements of the measurement object, and has high sensitivity even for thin sheets. Also, an array of several adjacent sensors and a fast measuring equipment have been developed for the real-time mapping of wide sheets on a fast transporter. The sensor has been applied to simultaneous measurement of the moisture content and mass per unit area of veneer sheets, both undried (green veneer) and dried. The resonance frequency of the sensors used was about 350 MHz. Results obtained in the laboratory and in the factory are reported.

Microwave Humidity Sensor for Difficult Environmental Conditions

The paper describes an electromagnetic humidity sensor based on a resonance perturbation technique. The sensor is capable of measuring humidity with a wide dynamic range in industrial environment and is insensitive for dust and dirt. The all-metal construction makes the sensor immune to damage by high temperatures. Measurement set-up for a resonance sensor and a construction of a microwave moisture meter are presented. Measurement results with the sensor mounted in a veneer dryer are reported.

Sensor for the Measurement of Pressure

Preliminary tests for a sensor for measuring pressure as a function of location were performed. The sensor is a stripline with electrothermomechanical film as a substrate. The film consists of plastic with air cells. When pressure is applied on the film, the dimensions of the air cells change. When the film is used as substrate in a stripline the pressure changes the impedance of the stripline, which can be detected with a pulse or a voltage step transmitted in the line. The reflection from the impedance discontinuity gives the amplitude and the location of the pressure.

Microwave Sensor for Snowpack Wetness and Density Profile Measurement

A microwave sensor for fast field measurements of snowpack wetness and density profiles is described. The sensor is an open resonator that can be pushed into snow. The resonant frequency (around 1 GHz) and the Q factor are automatically measured as a function of depth and converted to wetness and density values.

Dual-mode stripline resonator array for fast error compensated moisture mapping of paper web

A mechanically simple and therefore inexpensive sensor array for fast mapping of the water content of wet paper has been developed. The sensors are UHF stripline resonators, which have two degenerate resonance modes, even and odd. The difference of these two frequencies can be used to achieve a high accuracy, because the resonance frequency of the odd mode is not affected by moisture changes in the wet paper, and can therefore be used for error compensation. Because of the electronic scanning, the measurement is very fast and it makes almost real-time water content profiling possible.<<ETX>>

Sensor for Measuring the Mass Per Unit Area of a Dielectric Layer: Results of using an Array of Sensors in a Particle Board Factory

In a particle board factory it is important to know the thickness of the layer of wooden particles before pressing. A suitable resonant stripline sensor for the measurement of the mass per unit area was presented in the EuMC last year [1]. For the practical measurements, a sensor array and the necessary measurement equipment have been developed. With this array the 1.8 m broad particle cake can be measured twice a second while it moves past the array. Several transverse profiles of the particle cake were measured in a particl e board factory, both with the sensor array, and by cutting and weighing the pressed board, and the results were compared.

High-Resolution Portable Impulse Radar

This paper describes an impulse (short pulse) radar with high distance resolution. The radar is intended for short-range (< 2 m) detection of subsurface objects or boundaries. The primary application is detection of rot in 1iving trees and power or telephone 1ine poles but many applications can also be found for example from construction industry. The length of the impulse of this radar can be selected between 0.2 to 0.35 nano-seconds corresponding to a distance resolution of 3-5.3 cm in air. The reflected waveform is measured with a sampling type receiver. The waveform is digitized, processed in a microcomputer with powerful signal processing capabilities and displayed on an intensity display as a two-dimensional image. The equipment consists of a high-frequency hand-held unit and a control unit (see Figure 1). The equipment is portable with light weight and low power consumption.

Fast Moisture Profile Mapping of a Wet Paper Web with a Dual-Mode Resonator Array

A sensor array for measuring water mass per unit area of wet paper web has been constructed. The sensors are stripline resonators with two center conductors and two resonance modes, even and odd. By measuring the resonance frequency difference of these modes several disturbing factors can be partly compensated, for example air humidity and temperature, sensor temperature and vertical location of the paper web in the sensor. An overall water mass per unit area measurement accuracy of 0.5 % has been estimated for the array.

Sensor for Measuring the Mass Per Unit Area of a Dielectric Layer

A sensor for non-contact measurements of the thickness or mass per unit area of a dielectric layer has been developed. It has been constructed for measuring a cake of wooden particles on a steel band in the particle board industry. The sensor is suspended at a distance above the layer of particles to form a strip line resonator together with the steel band. The resonant frequency and the quality factor are measured. The resonant frequency correlates well with the mass per unit area and the quality factor can be used to deduce the mean wetness in the layer. The sensor is insensitive to moderate variations in the distance between the sensor and the steel band.