The invention also relates to a gas content measuring device that is arranged to measure gas content in a fluid, in which pressure variations occur.

BACKGROUND OF THE INVENTION At present, methods and devices based on ultrasound and density measuring are mainly used for measuring the gas content, and particularly the air content, of a fluid or liquid. In ultrasound measuring, the ultrasound is transmitted through the fluid to be measured and its attenuation is measured.

The attenuation of the ultrasound is a function of the gas content in the fluid, so the more there is gas in the fluid, the more the ultrasound attenuates. In paper industry, the gas content of papermaking pulp or stock is typically measured by means of ultrasound measuring. The quality of the end product, i.e. paper, depends on the quality of the fluid papermaking pulp which is partly defined by its gas content.

Finnish Patent 84,299 discloses a solution, in which the air content of a suspension is determined by measuring the water content of the suspen- sion at two different known pressures. U.S. Patent 4,852,395 also discloses a solution, in which the gas content of a flowing liquid is determined by measur- ing the transmissivity of microwave radiation through the liquid at two different known pressures.

One problem with the gas content measuring based on the ultra- sound attenuation is, however, that the method cannot be applied to suspen- sions containing considerably solid matter. For instance, the gas content measuring of pulp used for paper making can be done only, if the pulp con- sistency is less than 2%. Moreover, it is difficult to apply the method for instal- lations of in-iine type. This is why the ultrasound measuring devices have gen- erally been developed for measuring a sample flow corresponding as closely as possible to pulp. On the other hand, one problem with the density measur- ing is that the measurement is also very sensitive to other components than air, which cause changes in concentration. Air is the most common gas in pa- permaking pulp. In papermaking pulp, typical density values of different sus-

pension components are the following: water = 1, air = 0, wood fiber = 1.3 to 1.5, and fillers = 2.5 to 4.5. These figures show that even a slight decrease in filler amounts in pulp gives the impression that the air content of pulp has in- creased.

The problem with microwave measurings carried out at two known pressures is that pressures employed have to be measured exactly at the point where the microwave measuring is also carried out. The pressure indi- cator is to be accurately calibrated to avoid systematic measuring errors in gas content.

BRIEF DESCRIPTION OF THE INVENTION The aim of the invention is thus to develop a method and an equip- ment for implementing the method to the effect that the above problems can be solved. The method and the measuring device in accordance with the in- vention can also be applied to a suspension with a consistency exceeding 2%.

The solution in accordance with the invention is also insensitive to different densities of various components of a fluid or suspension.

This is achieved with a method described in the preamble, the method being characterized in that measuring is effected with a microwave signal, and at least one variable of the microwave signal is measured after the microwave signal has penetrated through a fluid, and the gas content of the fluid is determined on the basis of changes that pressure variations have caused to said variable of the microwave signal.

A gas content measuring device in accordance with the invention is characterized in that the gas content measuring device comp';ises a micro- wave meter, a transmitting antenna and a receiving antenna, which are ar- ranged to measure at least one variable the microwave signal while the signal penetrates through a fluid, and the gas content measuring device comprises means for determining the gas content in a fluid on the basis of changes that pressure variation has caused to said variable of the microwave signal.

Several advantages are achieved with the method and system of the invention. The solution in accordance with the invention also enables the measuring of gas content in high-consistency suspensions without problems resulting from different densities of the components of the suspension.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying draw- ings, in which Figure 1 illustrates an arrangement for measuring gas content, Figure 2 illustrates a circuitry of a gas content measuring device when a phase of a microwave signal is measured, Figure 3 illustrates phase measurings of a microwave signal in wa- ter to which various quantities of air are added, Figure 4 illustrates the average of a microwave signal phase varia- tion as a function of air content, and Figure 5 illustrates a circuitry of a gas content measuring device when attenuation of a microwave signal is measured.

DETAILED DESCRIPTION OF THE INVENTION The invention is suited for measuring gas content in various fluids, e.g. liquids and suspensions, in which pressure variations occur. The invention is particularly well suited for paper making, without being restricted thereto, however.

The method and typical measuring arrangement of the invention are now examined by means of Figure 1. The arrangement in accordance with Figure 1 comprises a pump 11, a duct 12, a fluid 13, a microwave meter 14, means 15 for determining gas content, a transmitting antenna 16 and a re- ceiving antenna 17. In addition, the arrangement may comprise a pressure in- dicator 18 and a pressure modulator 19. The pump 11 pumps fluid 13, and as a consequence, the fluid flows in the duct 12. The operation of the pump 11 typically causes changes of pressure in the fluid 13. If the fluid 13 contains air, or in a general case, a gaseous substance reacting at least nearly like an ideal gas, the rise in pressure makes the gaseous substance compress. On the other hand, when the pressure is restored to a lower level, the gaseous sub- stance expands back to its former volume. In a typical fluid 13, like a suspen- sion containing wood fibers, no other components compress substantially, apart from gas. Thus the degree of compression indicates the air or gas con- tent of the fluid.

In the solution of the invention, a microwave signal 20 is used for measuring the amount of gas, the microwave signal being transmitted via the

transmitting antenna 16 through the fluid 13 to the receiving antenna 17. The microwave meter 14 measures from the microwave signal 20 some of the fol- lowing variabies: signal phase, travel time, or attenuation while the signal 20 passes through the fluid 1 3.

First is examined mainly the signal travel time, which corresponds, in a general level, to the phase measuring, since changes in phase result from changes in signal travel time. The means 15 determine the gas content of the fluid 13 on the basis of the changes in travel time. If the duct 12 contains gas- free liquid 13, for instance, water, the pressure variations caused by the pump 11 have no effect on the volume of the fluid 13. Then the travel time of the mi- crowave signal remains unchanged in spite of pressure variations. But if the fluid 13 contains gas, volumetric changes in gas produce changes in the travel time of the microwave signal 20. The microwave signal 20 travels faster in gas than in liquid, so the higher the gas content in the fluid 13, the faster the mi- crowave signal passes through the fluid 13. Thus under the effect of high (momentary) pressure in the duct 12, the microwave signal travels a shorter distance in gas, since the gas is compressed, and consequently the signal travels from the transmitting antenna 16 to the receiving antenna 17 slower than at low (momentary) pressure. A change in the microwave travel time also changes a signal phase, and hence the change can be seen as phase changes caused by the pressure. The purpose of the method of the invention is to measure the difference between the longest and shortest travel time, and hence indirectly to deduce the degree of the compression of the gas, and fur- ther, to deduce from the degree of the compression the amount of gas, since a great amount of gas compresses, absolutely measured, more than a small amount. When the absolute level and variation of the pressure are preferably predetermined, and liquids containing no solid particles are measured, the amount of gas can be directly deduced on the basis of the highest and lowest value of the microwave signal travel time.

The signal travel time can be directly measured by measuring the time it takes for the signal to pass through the fluid. However, this kind of pro- cedure is technically difficult to realize. Moreover, since the changes in travel time are important in view of the invention, not the travel time itself, the meas- uring of changes in travel time is advantageously performed by measuring the phase changes of the signal.

The pressure indicator 18 can be utilized in two different ways. The static pressure can be measured with the pressure indicator 18, and from that the actual amount of gas in the process can be determined. The measuring of the amount of gas in accordance with the invention indicates the amount of air at the pressure at which the measuring is carried out. By means of the pres- sure indicator 18 the actual pressure in the process is known, and hence the absolute amount of gas can be calculated in unpressurized state. The dynamic pressure can also be measured with the pressure indicator 18. Since changes in component proportions of the process affect the microwave measuring, it is important that a distinction can be made between the changes resulting from the gases in the process, and the changes resulting from changes in consis- tency or component concentration.

Particularly in suspensions, the travel time, phase and attenuation of the microwave signal also depend on the consistency of the suspension, however. Thus in the solution of the invention, the consistency of the suspen- sion is measured. The consistency can be measured by means of any known method, but most preferably the measuring of consistency is performed with the same microwave meter as the measuring of gas content. Thus the device of the invention is adjusted to measure consistency by means of the micro- wave travel time. Frequency modulation in accordance with the FMCW tech- nique (Frequency Modulated Continuous Wave) is used for measuring. In the FMCW technique, the oscillator frequency is linearly scanned on a broad band like in radar, with a sinusoidal signal at the mixer output. The frequency of this signal indicates the difference in travel time between the reference signal and the received signal, the difference being affected by consistency, for instance.

The consistency typically changes in all liquid and suspension processes slower than, for instance, pressure variation produced by the pump 11. Then the consistency can be measured as a long term average on the basis of the travel time, whereas the gas content is measured, for instance, by the phase measuring based on the travel time in the considerably shorter term (typically less than 1 s) without frequency modulation. In the method of the invention, a separate pressure indicator 18 can be further utilized for essentially measuring the pressure level and the real variation in the travel area of the microwave signal 20. When the exact pressure and its real variation are known, whether the pressure variates considerably or irregularly, it is possible to determine the amount of gas on the basis of the travel time of the microwave signal 20.

Thus, for instance, a correiation between the pressure variation measured with the pressure indicator 18 and the travel time variation (phase variation, at- tenuation variation) measured with the microwave meter 14 can be calculated.

A correlation C(T), in which x(t) is the pressure as a function of time (t) and y(t) is the travel time (phase, attenuation) of the microwave signal as a function of time, is calculated, for example, with the formula C(T) = x(t)y(t - z)dt . By means of the correlation, pressure variations can be distinguished from other factors, such as changes in consistency, affecting the travel time ( phase, at- tenuation).

In the following, the solution in accordance with the invention, based on phase measuring, is examined in greater detail with the help of Fig- ure 2, the solution comprising, with relation to a microwave meter 14, a local oscillator 21, a transmitter amplifier 22, switches 23, a reference channel 24, a receiver amplifier 25 and a mixer 26. Moreover, the figure comprises a trans- mitting antenna 16, a duct 12 and a receiving antenna 17. The oscillator 21 transmits microwave frequency both to the transmitter amplifier 22 and the mixer 26. After the transmitter amplifier 22 has amplified the oscillator 21 sig- nal, it is selected with the switch 23, whether the signal is transmitted to the duct 12 for the actual process measuring, or whether the measuring is per- formed on the reference channel 24, by means of which drifting of the amplifi- ers can be monitored and compensated. The reference channel is typically a transmission line, such as a coaxial cable or a strip line, whose travel time and attenuation remain stable at various temperatures in the long term. After the signal has passed either through the actual process measuring in the parts 16, 12 and 17, or on the reference channel 24, the signal is selected with the switch 23 to the receiver amplifier 25. From the receiver amplifier 25 the signal propagates further to the mixer 26, in which the received signal 29 is multiplied by the original signal from the oscillator 21. If the signals to be multiplied are in phase with each other, the signal indicating the mixer output phase becomes zero. On the other hand, if there is difference in phase, the phase difference signal 27 deviates from zero, and the value of the phase signal 27 depends on the gas content and the consistency. In the gas content measuring, such a frequency is selected advantageously with the oscillator 21 that the output sig- nal 27 of the mixer 26 essentially becomes zero, and as a result, the reference microwave signal 28 and the received microwave signal 29 are at least mo- mentarily in phase with each other. The oscillator 21 is then advantageously

controlled by means of the signal 27. In the solution of the invention this fre- quency is set as the fixed frequency of the microwave signal, and the change in travel time is measured by means of the phase difference between the ref- erence signal 28 indicated by the output signal 27 of the mixer 26 and the re- ceived microwave signal 29. The gas content is advantageously measured by calculating the standard deviation of the variation in phase difference between the mixer 26 and the output signal 27, or the like.

The phases of the reference signal 28 and the received signal 29 can be momentarily equalized for the gas content measuring also without changing the frequency, by using an adjustable delay element 30 between the oscillator 21 and the mixer 26. The delay element 30 then changes the refer- ence signal delay from the oscillator 21 in such a manner that the received signal 29 and the reference signal 28 enter the mixer 26 in the same phase at a desired moment, and consequently the output signal 27 becomes effectively zero. By means of the output signal 27 of the mixer 26, the frequency of the oscillator 21 is advantageously controlled, when selecting a convenient fre- quency, or by means of the output signal 27 the delay element 30 is advanta- geously controlled for zeroing the phase.

Figure 3 shows measuring results. In Figure 3A, the phase signal 27 is the vertical axis and time is the horizontal axis. A graph 31 describes the behaviour of a phase in clean, air-free water. The graph 31 shows that sheer water does not change the phase of the microwave signal 20 under the effect of pressure variation, and only little noise can be seen in the graph. The graph 32 describes the behaviour of a phase of the microwave signal 20 when some air is added to clean water. Pressure variation generated by the pump 11 shows clearly as phase-change oscillation. As regards the graph 33, some more air is added to water, and the amplitude of the phase oscillation is clearly increased with respect to the graph 32. Further, more air is added to water in the situation described by the graph 34, and as a result, the amplitude of the phase change is further increased. Consequently, the amplitude of the phase oscillation is a function of the gas content.

Figure 4 illustrates the measuring of air content in water, in which measuring the standard deviation of the microwave signal phase (vertical axis) is described as a function of the actual air content in water (horizontal axis).

Three different flow rates, 15 I/s, 20 I/s and 25 I/s, have been used in the measuring. The measuring points are marked with squares. In Figure 4, it is

observed that the amplitude of the phase variation and the air content prefera- bly have a simple, linear dependence, independent of the flow rate, which de- pendence can be described by a straight line.

Figure 5 illustrates the measuring of gas content by using attenua- tion as a variable. The solution is fairly similar to the solution in Figure 2. In this case, the microwave signal is connected with a switch 23 in turn to the refer- ence channel 24 and the transmitting antenna 16. The strengths of the refer- ence microwave signal 62 and the received signal 63 are measured in the re- ceiver, and the strengths of the received microwave signal 63 and the refer- ence microwave signal 62 are compared with each other in means 51. The comparison is performed, for instance, by calculating the difference or the re- lationship between the reference signal 62 and the received signal 63. There- after the result obtained by the means 51 is transmitted to the means 15, which determine the gas content on the basis of changes in attenuation, for in- stance, by using the standard deviation or the like.

In the solution of the invention, the pressure variation of the fluid can further be generated by means of a separate pressure modulator 19, in case the pump 11 does not form sufficient pressure pulses. The pressure modulator 19 is, for instance, a diaphragm whose background pressure is variable, a cam or a piston.

The fluid 13 in the solution of the invention is advantageously pa- permaking pulp in the process duct, and hence the method of the invention is used for measuring the gas content in papermaking pulp to improve the quality of paper before the pulp is fed to the paper machine. The fluid 13 may also be a coating paste used for paper coating.

Even though the invention is described in the above with reference to the example in accordance with the accompanying drawings, it is obvious that the invention is not restricted thereto, but it can be modified in a variety of ways within the scope of the inventive idea disclosed in the appended claims.