Microwave Moisture Meters tor the Paper and Pulp Industry * Kjell

Lindberg and

t Ulf Ternstrom

INTRODUCTION Periodically microwave methods for measuring moisture content of various materials appear in the literature. Commercially available microwave moisture meters for practical use are uncommon and the success of those that exist appears to be scant. Almost without exception, the proposed and existing microwave meters are using absorption of microwaves as a measure of moisture content. This is analogous to using electric conductivity at lower frequencies and transmission or reflection of infra-red radiation at higher frequencies. Very few designers seem to have paid any attention to purely dielectric measurements using phase shift or changes in the resonant frequency of a microwave resonant circuit. We believe that the preference for absorption or attenuation measurements has two main reasons: (a) at first sight attenuation measurements may have some nice technical advantages, and (b) it is much easier to put together an attenuation measuring system mostly out of standard laboratory components, than tackling the problem of designing a rugged on-line system to measure the dielectric constant of a material at microwave frequencies. In 1964, research and development on microwave moisture meters was started at the Microwave Department at the Royal Institute of Technology in Stockholm. After some introductory experiments with attenuation measurements all the efforts were concentrated on a purely dielectric method of determining water content. It was applied on paper, pulp, textiles, margarine and other materials for which it is important to know the water content during production*. The Microwave Department had a great deal of experience from using microwave resonators as research tools for studying the dielectric properties of various materials. There was a group at the Department, specialising in the field of microwave solid state electronics. In close co-operation with, among others, the Swedish paper industry, various types of moisture meters were developed and found to be useful tools for process measurement. In 1968 the first meters became commercially available through Skandinaviska Processinstrument AB (Scanpro Ltd.) in Stockholm.

MICROWAVES A common definition of microwaves is that they are electromagnetic waves of wavelengths between 1 and 0·001 m, approximately. This corresponds to frequencies between 300 and 300,000 MHz. In this band you find the frequencies used in radar stations, in satellites or ground-based microwave links relaying TV programmes and in electronic ovens heating pre-cooked dishes. Most of the moisture nleteJ S we will discuss operate at frequencies between 800 and 4,000 MHz. But we have also applied microwave principles to the design of meters operating at frequencies below 100 MHz. Cavity resonators are em-

• Microwave Department, Royal Institute of Technology, Stockholm t Skandinaviska Processinstrument AB (Scanpro Ltd.), Stockholm Paper presented at the International Symposium· on 'Moisture Measurement and Control', of the North West Kent Section of the Institute, Maidstone, Kent,-September 1969.

MEASUREMENT AND CONTROL, Vol3, March, 1970

ployed instead of coil-and-capacitor resonant circuits; the concept of electro-magnetic fields is used rather than currents and voltages etc. An almost necessary requirement for the design of simple, rugged and dependable industrial devices of this kind is that solid-state techniques are used throughout. The klystrons, which until recently were used to generate microwave low power signals, require high and accurately stabilised signals, generated in bulky power supplies. The klystrons themselves have fairly large dimensions and a rather short life-time. Modern solid-state microwave sources are very small, operate at some tens of volts and have a very long life. Our moisture meters employ such microwave solid-state sources, especially designed for the meters. Transistors and integrated circuits are used throughout the signal-handling electronics.

MICROWAVES AND MOISTURE Physical Considerations Most methods of measuring moisture content in for instance a paper web, use the losses and / or the dielectric constant of the paper as a nleasure of water content. The concept of losses then includes electric conductivity at low frequencies as well as absorption of waves in the microwave and infra-red frequency regions. Apart from most other methods, our microwave method is based on determination of the dielectric constant without any influence at all from the losses of the material. This is often advantageous because the losses can vary more than the dielectric constant when the composition of the material is changed, pH, amount of filler, salt content, etc. are examples of composition factors which may influence the losses appreciably. One example concerns the dependence on pH. A meter with a capacitive sensor may change its calibration with as much as 20 to 30 per cent reI. when pH increases from 4 to 6. A corresponding step exists in the conductivity curve. The calibration of the microwave meter which is independent of the losses is also independent of pH (cf Figure 15). These facts indicate that the pH sensitivity of the capacitive meter should be due to conductivity variations. At microwaves and infra-red frequencies the absorption depends on the composition of the material in a more favourable manner than at lower frequencies. But even if the most important differences between for instance, microwave attenuation and dielectric constant measurements are of technical nature, the dielectric constant is in most cases a more dependable measure of the water content than the absorption. Technical Considerations Modern capacitive moisture meters working at relatively high frequencies often use resonance techniques to determine the capacitance variations caused by the wet material. Our meters employ microwave resonators as sensing elements, and it is therefore natural to introduce the concept of Q-value when discussing the radio-frequency and micro-wave resonant circuits.

* B. AGDUR, Acta IMEKO

1967, p.411

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High Q-value

Fig. 3.

Frequency

Fig. 1.

Resonance curves for circuits of different Q-values

Resonance curves of different Q-values are illustrated in Figure 1, if you employ an off-resonance technique you use the slope of the resonance curve as a kind of frequency to amplitude discriminator. Especially if you are trying to include very high water contents in your range of measurement, the resonance curve gradually becomes broader when the water content is increased. Your calibration curve for the equipment will thus include the varying Q-value and by that the losses or the absorption. 'Another method is to use an on-resonance technique. This means that in measuring the shift of the resonance frequency caused by the wet material, you always use the displacement of the very peak of the resonance curve. That is the method used in our moisture meters. To get the best resolution and accuracy, the Q-value and the sensitivity to changes in the dielectric constant should be high. A high Q-value makes an accurate determination of the peak of the resonance curve possible. A high sensitivity makes the measurement result less sensitive to the temperature of the sensor, to stray capacitances, equipment stability etc. There are, however, practical restrictions on the Q-value and the sensitivity. The unloaded Q-value of a measuring capacitor can hardly exceed 100; as a rule it is much lower. Especially when the material has high losses, like for instance a wet paper web or machine felt, a heavy loading of the sensor in order to get a good sensitivity may decrease the Q-value to such a low level, that the influence from the losses on the resonant fr~quency becomes too large. The above difficulties are overcome by the use of microwave resonators. They may be designed for unloaded Q-values of many thousands; 500 is a typical value used for heavily loaded moisture sensors, and we will see in a later section how special microwave design features make accurate measurements of small frequency shifts possible, thus eliminating the need for dangerously large frequency shifts in order to get sufficient accuracy.

Fig. 2.

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One-sided sensor for water content of paper machine felts

Two-sided sensor for moisture content of paper webs (for experimental paper machine)

Accuracy and Resolution Especially when measuring moisture content it is important not to mix up accuracy and resolution. At 2,000 MHz resonance frequency and 20 MHz frequency deviation the peak of the resonance curve may readily be determined with a resolution better than 20 kHz thanks to the high Q-values used. That gives a minimum resolution of one part in one thousand. This as a rule is more than is needed but may be useful in certain applications. Accuracy depends, however, also on other factors. One is how properties of the material other than the water content influence the quantity which is chosen as a measure of the water content. Another factor is how well this quantity is measured with regard to resolution and stability of the equipment itself. Concerning the first component of accuracy we may state, that the microwave meter is about as dependent on the basis weight and probably on the temperature of the material to be measured as are, for instance, conventional, high standard capacitive meters. It is dependent too, on the composition of the material, but it is believed, to a much smaller extent than ordinary meters, as we have mentioned earlier. Maybe

Fig. 4.

Sensor for water content of materials flowing in tubes MEASUREMENT AND CONTROL, Vol 3, March, 1970

Lindberg and Ternstrom

Fig. 6. Central unit of portable 3-channel moisture meter for paper with low and high moisture content and for machine felts

Fig. 5.

Sensor for water content of bulk materials

the most striking characteristic is that high water contents can be measured with high accuracy. Advantages like these are consequences of the physical and technical properties of microwaves and microwave circuits as discussed above. Now we will see how microwave design technique can be employed to give high quality to the second component of accuracy; how well the dielectric constant is determined.

mE SENSORS The sensing element of the moisture meters is a microwave resonator whose geometrical form may be adapted to the requirements of the process. Figures 2 to 5 show sensors for moving webs, for substances flowing in tubes and for bulk materials. The change of resonant frequency caused by the wet material depends on the dielectric constant and the amount of material in the sensor. If the basis weight of the paper web, the weight of the bulk material, the density of the substance flowing in a tube, etc. is known or constant, then the meter may be adjusted to read per cent moisture directly. In this respect it does not differ from conventional meters. A change of the geometrical dimensions of a resonator also changes its resonant frequency. Thus temperature variations or direct mechanical deformations can cause frequency changes which could be mistaken for moisture variations. This is analogous to the situation for a measuring condensor. To eliminate this effect a unique property of microwave resonators is used. A resonator has an infinite number of discrete resonant frequencies, each belonging to a certain mode of oscillation having a certain field configuration. 'A measuring mode is selected which has a field configuration such that the wet material in the resonator affects the resonant frequency belonging to the mode significantlY. Then a reference mode is chosen which has a field configuration such that the corresponding resonant frequency is independent of the wet material. Moreover, the resonator is designed so that both modes have the same sensitivity to small deformations of the resonator. The difference of the two resonant frequencies is the quantity to be measured. The sensors thus become temperature compensated. This design principle also makes the two-sided sensors to be mentioned below insensitive to moderate variations of the measuring gap. Practical experience shows that the meters have an excellent long-term stability. The principal reasons for this are beMEASUREMENT AND CONTROL, Vol 3, March, 1970

lieved to be, 1) the moisture determination is based on a pure frequency measurement, the amplitude of the measuring signals being of no importance to the result and, 2) the compensation of the sensor just described is very efficient. The sensors for paper, pulp and textile webs are of two principal types. One is one-sided and has to touch the web, Figure 2. The other type is divided into two halves, one on each side of the web, and does not touch the web, Figure 3. The one-sided sensor is used in portable meters and when easy traversing of machine-mounted sensors across the web is required. The two-sided type is suitable when the web is easily damaged by a contacting sensor, for instance when the paper is coated on both sides or has a large water content at a low basis weight. The two-sided type is especially easy to calibrate because the position of the sample in the 5 to 10 mm wide gap between the sensor halves does not influence the measurement. Very sensitive qualities are easily stabilized against any contact with the sensor through air cushions generated in the sensor. All the microwave circuitry and some of the signal-handling circuit are built into the sensor housing. The amplitude of the measuring signal leaving the sensor is of no importance to the result, if it only exceeds a minimum value. This gives a high security against interference. The measuring cable does not belong to the measuring circuit and its length may be chosen deliberately. The equipments we will discuss in the following employ the one- and two-sided types of sensors just discussed. At the presentation of the paper we hope to be able to report on the results from current tests of meters for bulk materials, notably for entire pulp bales.

THE MEASURING SYSTEM The microwave system of measuring water content consists of a number of building-blocks. Besides the various types of sensors there are portable and panel-mounted central units, presentation units, mounting brackets and traversing gears, control units, etc. A special component of the system is a fast electronic multichannel switch that may be built into the central unit. In practice, it makes several independent meters out of one central unit and a number of sensors. The reduction in cost is considerable even for a two-channel equipment. Figure 6 shows a portable moisture meter for simultaneous recording in three channels. It is equipped with one measuring head for paper with low moisture content, one head for wet paper and one for the press felts. A one-channel meter makes 200 complete measurements a second. This means that on a nlachine running at a speed of 600 m a minute, a new measurement is made every 5th cm of the web. The corresponding signal is available as a step-curve which may be recorded on an oscilloscope or a high-speed recorder. This makes detailed process studies possible for instance in the press section. Most types of recorders, slave instruments, regulators etc. T35

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