BRINGSVOR AARSTEIN (NO)
NYFORS EBBE (NO)
BRINGSVOR AARSTEIN (NO)
WO1993021516A1 | 1993-10-28 |
FI69372B | 1985-09-30 |
1. | A microwave sensor for measuring of the relative proportions of fluids flowing through a tube (4) , and where a length (9) of the tube (4) makes a part of the sensor (3) together with probes (6,7) which transmit electromagnetic energy into the sensor (3) and receive electromagnetic energy from the sensor (3) ; as the sensor (3) uses resonance at a resonant frequency (fra) below the cutoff frequency (fnm) of the tube (4) , c h a r a c t e r i z e d in that the sensor (3) is provided with at least one radially extending, conducting, internal fin (5) extending along the axis (10) of said tube (4) . |
2. | A microwave sensor as claimed in claim 1, c h a r a c t e r i z e d in that the fin(s) (5) stretch(es) radially along the sensor from the internal wall of the tube (4) to or towards the central axis (10) of said tube. |
3. | A microwave sensor as claimed in one of the claims 1 or 2, c h a r a c t e r i z e d in that the height (H) of the fin(s) (5) in radial direction varies over at least one portion of the fin' s length (L) , and that the total length (L) of the fin(s) is approx. equal to the internal diameter (D) of the tube (4) . |
4. | A microwave sensor as stated in one of the claims 13, c h a r a c t e r i z e d in that a first probe (6) is located centrally to the fin's (5) (the fins') total length (L) and is connected to the tube (4) diametrally to the fin(s) (5) , while the other probe (7) is arranged perpen diculary to the plane in which the fin(s) (5) and the first probe (6) are situated. |
5. | A microwave sensor as stated in one of the claims 14, c h a r a c t e r i z e d in that the fin(s) (5) is (are) mounted within a longitudinal slot (13) in the sensor wall so that it (they) may be taken out through said slot for change/cleaning/maintenance, possibly against a mechanical bias. |
6. | A method for measuring the relative proportions (φ) between two fluids (A,B) situated within a length of tube (4) , as one part of said tube (4) comprises a microwave sensor (3) where microwave energy is fed in via a first probe (6) while microwave energy is received by a second probe (7) , where said measuring is carried out while the sensor (8) is resonant, whereafter the resonant frequency (fm) is determined, the permittivity (εm) is calculated from said resonant frequency, the permittivity (εm) is compared to the permittivity of a known, empirically, calibrated model comprising the same fluids (A,B) , and where the relative proportions (φ) is determined from this comparison, c h a r a c t e r i z e d in that a positive feedback connected amplifier (1) having a frequency dependent ampli¬ fication is used for measuring the resonant frequency, as the amplification, at least at the resonant frequency used, is above the attenuation in a feedback loop (1,2,3), but otherwise is below this value, while the phase shift (Δφ) in the feedback loop (1,2,3) is determined from Δφ = n • 360° where n is an integer above or equal to 1. |
7. | A method according to claim 6, c h a r a c t e r i z e d in that the amplifier (1) comprises at least two series connected amplifier units. |
8. | A method according to claims 6 or 7, c h a r a c t e r i z e d in that the amplification in the feedback loop (1,2,3) is adjusted from a low value until selfoscillations occur and that the frequency of said oscillations is measured during the oscillation process; as it is made sure that the amplification is inversely pro¬ portional to the frequency and thereby ensuring that the lowermost resonant frequency present is used during the measurements. |
9. | A method according to claim 6, 7 or 8, and where the amplifier (1) comprises at least three amplifier stages connected in series, c h a r a c t e r i z e d in that the voltage to at least the first and the last amplifier stage is controlled by a computer (not shown) until resonant oscillations occur. |
10. | A method according to one of the claims 69, c h a r a c t e r i z e d in that the cable length (d) of the conductors (2) used in the feedback loops is adjusted such that a desired value of the resolution is obtained, by determining the resonant frequency. |
11. | A method according to any of the claims 610, used in connection with a mixture of two fluids (A,B) and where the continuous phase of the mixture is electrically conductive, c h a r a c t e r i z e d in that also the conductivity of the mixture is measured between two electrodes (6,7) (pre¬ ferably the same electrodes used by the probes to measure the permittivity) , and that also the temperature of the mixture is measured by a temperature detector (14) ; the measured temperature being used to correct the found conduc¬ tivity value according to empiric values; so that the sen¬ sor's range of measurement is extended to cover all possible relative proportions. |
A B
where VΛ is the volume of the fluid A and VB is the volume of fluid B in a sample of the mixture having the volume Vm=VA+VB. If the fluid A e.g. is water while the fluid B is oil, the expression φA represents the water contents of the mixture. How the permittivity εm of the mixture depends of the proportional relation φ, also depends of how the fluids in question will mix, and accordingly is a specific value depending of the two fluids. As a model for this dependency εrm(φ) either a previously known model known from ref. 1, chapt. 2.4, or an empiric, calibrated model may be used. The value of φ may then later on be calculated from a measured value of Sn-. by use of this model. To find εm a microwave resonator may be used as sensor. Such a sensor has a resonant frequency dependent of the permittivity of the mixture within the sensor. If the resonant frequency is f0 when the sensor is empty and fm when it is filled up with said mixture, the result will according to ref. 1, page 133 be:
From ref. 2: Finish patent FI 69372 it is previously- known to build a microwave resonator in a tube by using such a structure that the resonant frequency is below the cut off frequency, see also ref. l. page 11, for the wave mode in said tube, see also ref. 3 : Collin, R.E., Foundations for Microwave Engineering, McGraw-Hill, 1966, chapt . 3. The microwaves then cannot travel through the tube and accord¬ ingly they will not extend further out in the tube from the sensor. Accordingly the resonator does not need any screen- ing in shape of nets or similar end pieces to obtain a high Q-factor. The present invention, a resonator having an internal radial fin (SFR-sensor) , represents a new method for designing such a microwave resonant sensor. Within a sylindric wave conductor, with other words a circular electric contacting tube, the microwaves may travel according to different wave mode which may be referred to as TEπm or TMnm, see ref. 3, chapt. 3, each having its specific cut-off frequency fnm being dependent of the internal radius α of the tube:
CO ( v TM (3) C, nm 2πα ' nm )'
fC nm [ TE ) 2πα v nm ' (4)
where c is the light velocity in vacuum (3xlO8 m/s) , pnm is zero-crossing number m for the Bessel function of first type and degree n, p'nm is zero-crossing number m for the derived value of the Bessel function of first type and degree n. Table I shows the values of pnm and p'nm for a conven¬ tional cylindric wave guide. The equations (3) and (4) are also valid for a wave guide shaped as a sector of a cylinder. If the sensor angle is 360° the wave guide looks like a cylinder with a radial internal fin extending from the wall and into the central line of the tube, and fastened to the tube. Table II shows the values of pnra and p'nm for such a fin wave guide or wave conductor.
Table I: pnm and p'nra for the wave modes having the lowest cut-off frequencies in a sylindric wave guide.
Table II: pnm and p'nm for wave modes having the lowest cut¬ off frequencies in a wave guide with fin according to the present invention.
From equations (3) and (4) one can find that the limit or cut-off frequency for a wave mode is direct proportional to Pnm or P'nm- From table I it is found that the mode with the lowest cut-off frequency in a cylindric wave guide will be TE11 with p'lx = 1.841 and from table II it is seen that the lowest possible mode within a fin wave guide is TEM1 with p'M1 = 1.1656. Accordingly the lowermost cut-off frequency in a fin wave guide is 37% below that in a sylindric wave guide; or with other words if a fin is connected to the wall in a sylindric wave guide, waves with a frequency 37% lower than the previously minimum frequency may propagate in the part of the wave guide being equipped with such a fin. A microwave resonant mode is based on a TEnm or a TMnm wavemode. The resonator comprises a length L of the wave guide terminated by short-circuited or open end sections, so that the present mode is reflected and accordingly produces a standing wave in the defined part of the wave guide. The wave mode then obtains a third index 1 associated with the length L of the resonator. The resonant frequency for the different modes then will be; see also ref. 1, page 150:
where xnm may have the value of pnm or p'nm. Within a resonator having short-circuited end sections resonant TM-modes with indexes 1=0,1,2, ... and a resonant TE-mode with index 1=1,2,3, ... may be obtained. If the end sections are open, resonant TM-modes having indexes 1=1,2,3, ... and TE-modes with indexes 1=0,1,2, ... may appear. All the resonant modes with an index 1=0 have a resonant frequency independent of the length L of the resonator and identical with the cut-off frequency for the wave mode. If a fin 5 of length L is fastened to the wall in a cylindric tube 4 so that the tube 4 will extend beyond the fin 5 in both ends, a resonator with open ends is obtained. The lowermost resonant mode then will be TE^10 (p'nra=1.165β) with a resonant frequency independent of L and identical with the cut-off frequency for the wave mode TE^1, being below the lowermost cut-off frequency of the tube 4 beyond the fin 5, TE11 (p'nra=l.841) . Therefore the microwaves cannot travel further out in the tube and the mode TE^10 accordingly has a high Q-factor. In a practical embodiment the electro¬ magnetic field close to the open ends of the fin 5 will be disturbed in such a manner that the measured resonant fre¬ quency will be approximately 5% higher than the theoreti¬ cally calculated value. However, this difference is so small that it has no consequences for the above explanation which is based on the theoretical resonant frequencies. This is obtained with a cylindric fin resonator SFR which is an example of the present invention and which may be used to determine the permittivity of the mixture flowing in the tube. The structure is very simple and less intrusive than in corresponding sensors having short-cirvuited ends. In addition to TE5410 the lowest possible resonant modes in an SFR-sensor will be TE5411, TE5412 and TE110. Among these values the resonant frequencies for TE5411 and TE5412 will depend of L. In Fig. 2 the calculated resonant frequencies (accord¬ ing to equation (3)) are shown for those four modes. Accord- ing to Fig. 2 the frequency distance between the resonant frequencies for TE5410 and TE5412 will be large when the length of the resonator or the fin 5 (L) is short, e.g. equal to the internal diameter D of the tube 4. In Fig. 3 the electrical field of resonant mode TEM10, TE5411 and TE110 for a SFR-sensor is shown. The mode TE110 has an electrical field which is zero close to the wall of the tube opposite to the fin 5. If the cables 2 of the metering circuit (see Fig. 4) is connected to the sensor 3 via probes 6 and 7 which are coupled to the electrical field being perpendicular to the wall of the tube 4 of the sensor 3 so that one of the probes 6 opposes the fin 5; all coupling to the mode TE110 is avoided. The mode TE5411 has such a field picture in the longi¬ tudinal direction of the tube that the field maximum occurs at the end of the fin 5 while the field strength is zero at the middle of the fin 5. This will be the case for all resonant modes having an index 1=1; see also ref. 1, page 314; when the ends of the resonator are open. If the probes 6,7 then are arranged so that the distance to both of the ends 11,12 of the fin is equal, such as shown in Fig. 1, coupling is also avoided to mode TE5411. As mode TE5410 in a SFR- sensor is used for measuring of the permittivity of the fluid mixture which is to be measured, there will be large distance to next mode due to the fact that when the probes 6,7 are connected as shown in Fig. 1, coupling to the modes TE5411 and TE110 are avoided. Accordingly the SFR-sensor shown in Fig. 1 is in particular well suited for measuring using TSF-electronics, because the risk that the electronics shall tune to an erroneous resonant frequency easily may be avoided by choosing a correct frequency response of the amplifier (i.e. the amplification as a function of the frequency) . The principle of the TSF-metod for measuring the resonant frequency of a sensor, is shown in Fig. 4, and is also more thoroughly described in ref. l: chapt. 3.4.3, and in ref. 4: "Vainikainen, P.V., Measurement Electronics of Industrial Microwave Resonator Sensors, Thesis for the degree of Doctor of Technology, Helsinki University of Technology, Radio Laboratory, Report S 194, 1991". The measurement is based on an amplifier 1 which is feedback connected by means of cables 2 and the sensor 3. If the amplification in the amplifier 1 at a certain frequency is above the total attenuation in the cables 2 and in the sensor 3, the net positive amplification (expressed in dB) and the signal will be amplified for each new reound through the circuit. Accordingly the circuit will start oscillating at this frequency. The sensor 3 which acts as a band pass filter and therefore this situation will in a practical solution only occur at the reasonant frequencies and very close to these. If amplifiers having an amplification being inversely proportional to the frequency are used, oscilla¬ tions are only possible at the lowermost resonant frequency. This applies in particular to a SFR-sensor having a large frequency distance to the next resonant frequency, also if the coupling to this is stronger than that to the lowermost mode. In addition to the requirement that the net value of the application must be positive, the phase shift in the circuit also must have such a value that the signal obtains identical phase after each round, to maintain the oscil¬ lation. At a certain oscillation frequency the total phase shift accordingly will be:
Δφ = n-360° (6)
where n is an integer. This means that the oscillation usually do not occur exactly at the resonant frequency, but on the closest frequency at which n is an integer and the net amplification is positive. If the resonant frequency is changed as a result of a change in the proportions of the mixture to be measured and accordingly of the permittivity, the oscillation frequency will only change stepwise so that n always will be an integer. This will give a certain loss of accuracy in the measurement. During one round in the TSP-circuit, the phase of the signal passing the sensor the amplifier and the cables, will be shifted. The phase shifting in the amplifier may be con¬ sidered as an additional cable length, and just at the resonant frequency the phase shift in the sensor is zero. The phase requirement then leads to that the total length d of the cables, included the effective -increasement of the length caused by the amplifier, must be an integer multi¬ plied with the wave length in the cables. The distance between frequencies at which oscillations are possible then will be
. _ (n+1) c we c Δf = —2- - = ^7) d.V/ire d.V/ire d,Y/εre
where εrc is the permittivity for the insulation material of the cables. If e.g. d=20 m and εrc=2.2, then Δf=10.1 MHz. The sensitivity for an SFR-sensor 3 having an internal diameter D of 54 mm, and a length L of the fin 5 like 50 mm, has been found to have a mean value approx. 15 MHz/%water. The inaccu¬ racy caused by this phase requirement will then be
Δ% = ± °'5-10'lMHz = ± 0,34% fc (8) va ter 15MHz/ % h wa ter V ' wa ter
In addition to the resonant frequency the phase shift in the sensor will contribute so that the inaccuracy in pracis will be a little below the value calculated according to equation (7) . This result depends on the width of the resonant function, but will be approx. 10%, so that the final result of equation (8) will be ±0.31% water. When the length of cable d is chosen, the width of the resonance function also has to be taken into account so that the distance between the frequencies where the phase shift requirements are met, not will be too large. An acceptable requirement is in practise that Δf in equation (7) has to be less than the width on half power level Δf3dB on the resonance function, see also ref. 1, page 136. Oscillation will then always be possible if the net amplification at the resonant frequency is above 3 dB. During a measuring situation then
Δf < Δf2ZHdBn = ^n- (9)
where Q is the quality factor of the resonance. Q is dependent of the measured permittivity of the mixture, and how strong the coupling between the probes 6 and 7 and the circuit with the sensor 3 are, see also ref. 1, pages 140 and 146. The coupling may be adjusted by an empiric adjustment of the length of the probes. For the above mentioned SFR-sensor the measured Q-value varied from 117 to 39, while the contents of water in oil varied from 0% to 40% and the coupling of the resonant frequency at 0% then was -8 dB. The total attenuation in the circuit is a sum of the attenuation in the cables 2, in the sensor 3 and the attenu¬ ators which in practical solutions always are required to enhance the impedence adaption in such measuring systems. The attenuation in the sensor 3 varies with the attenuation of the mixture which is to be measured. To ensure that the amplification in the circuit always will be positive, the amplifier 1 will i practice comprise two or several amplifier steps connected in series. When the TSF-electronics starts oscillating, the last of the steps in the amplifier will reach saturation and send out the signal comprising many harmonic components. Intermodulation in the last amplifier step then will damage the signal. This may be avoided by regulation of the amplification, e.g. by regulat- ing the voltage to the amplifier. According to the present invention an auxiliary equipment, e.g. a computer is used to regulate the ampli¬ fication while the signal in the circuit is measured. The amplification is increased, and when the oscillations starts the frequency of the signal is measured in a manner known per se. This results in a very broad dynamic range for the meter, which accordingly accept large variations in the attenuation of microwaves in the mixture to be measured. An electronic TSF-circuit has been designed and tested in which three amplifiers of the type MSA-0885 produced by Hewlett-Packard are connected in series. The voltage to the first and last amplifier step is controlled by a computer. The attenuators in the circuit represent together -18 dB, and the TSF-electronics is connected to the above mentioned SFR-sensor, such that d=20 m. This meter has been used in a test where the water contents in oil from the Statfjord field were measured. The tolerance for this meter was better than ±0,4%water. This corresponds reasonably with the number ±0,31%water as uncertainties in the calibration of the model have to be taken into account for s^cK,.^.-) . The details of this solution may vary in many manners within the scope of the present invention. The cross section of the tube and the resonators may be modified in countless different ways as not only circular sylindric cross sections may be used for the sensor, but also polygon 21, oval and irregular shapes, if possible adapted to the cross section of the tube itself. The shape, location and the ends 11,12 of the fin 5 may also be modified. The fin may e.g. be per¬ forated or equipped with holes, and may e.g. have a grid or net structure. It may be rectangular as assumed on Fig. 1, but it may also have a more irregular and profilated shape, not shown separately on the drawings. As an example it may shaped as a semi circular plane. In a similar manner one or both end portions 11,12 of the fin may be round or tilted to give a less abrupt end. The thickness of the fin 5 is not mentioned in particular, as it is a non-critical value, however, it will normally be made as thin as possible without being detrimental to the structural strength of the construction. The fin may also be split into several fin segments which may have the shape of closely arranged segments having equal or different length. In this situation it may talked about several fins or sub-fins with an effec¬ tive total length measured longitudinally to the tube, while each sub-fin may be rather short. Similarily each fin or sub-fin may have a moveout when the exact location along a radial plane is considered, as a certain distance from this radial plane also may be accepted. Due to practical considerations the fin 5 or the fins may be arranged within a slot 13 in the wall of the tube 4 or sensor so that it (they) may be pulled out sidewise of the tube to be changed, cleaned or for maintenance. This is an alternative even if the preferred embodiment will be a fix and stabil construction without such simple possibili¬ ties for this assembling or re-shaping. The fin may also be resilient, e.g. by being suspended in a springy way so that an internal loa'd, e.g. caused by particles in the fluid flow, at certain intervals may force the fin more or less out through the mounting slot and into a sealed slider, while the fin thereafter again will spring back into the sensor. Other modifications may be change of the material in the sensor which may be selected among all known materials for wave guides and cavity resonators. The probes may also be designed in many different ways, but will normally have a coaxial shape with conventional terminals while the length of the probe may be changed as explained above. The number of the probes and the location of same may also vary as the desired frequency response is taken into consideration. Even when a sensor with a radially extending fin is used, it is not necessary to use only the method described to obtain resonance. Resonance may be obtained or the resonant frequency may also be determined according to different and possibly conventional methods, however, the method described is deemed to be a preferred method. This is also the case for using of a feedback connected amplifier, which also only represents a preferred method, as it repre¬ sents an elegant and non expensive solution. However, the present invention also covers a fin resonator used together with other external systems to determine the resonant frequency, as the varied use of at least one fin within the resonator to obtain a resonant frequency below the cut-off frequency of the tube is in itself a new and inventive solution no matter which measuring method that is being used. Accordingly the use of the invention may also be more than measuring the water contents in oil. The invention may be used to determine the proportional relations between two random fluids such as gases, liquids or mixtures of gases and liquids. Nor are self oscillating electronic circuits and adjustable gain quite necessary requirements to obtain measurements with such a fin resonator, but such remidies are thought to be advantageous as they give a large measur¬ ing range and represent a very inexpensive solution. The method may also be varied in many manners. An im- portant matter is that the sensor may be used for measuring of the permittivity and the conductivity at the same time. In a practical solution this may be done as the probes 6,7 also are used to measure the conductivity of the mixture when passing. This will in particular be valuable when the continuous phase of the mixture is electrically conducting, which again will be the case when the continuous phase is water including salt or oil. Electronic circuits may then be connected to measure the conductivity in parallel with the circuits to measure the permittivity. Accordingly one single equipment may be used to measure both the permittivity and the conductivity, and accordingly the measuring range for water in oil will be expanded to cover the complete range from 0% water to 100% water. However, this does not omit the possibility of only measuring the permittivity or only the conductivity. Measuring of the permittivity is best suited when the mixture is non-conducting, while the conductivity measurements are well suited when the mixture is conductive. Accordingly the measuring methods supplements each other. When the conductivity is measured, it may also be of use to measure the temperature of the mixture as the temperature affects the exact value of the conductivity. Such a temperature measurement may be undertaken by a specific detector 14 as assumed on Fig. 1.
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