WO2002040948A1

WO2002040948A1 - Flow velocity meter

Info

Publication number: WO2002040948A1
Application number: PCT/SE2001/002541
Authority: WO
Inventors: Per-åke Strömberg
Original assignee: Stroemberg Per Aake
Priority date: 2000-11-15
Filing date: 2001-11-15
Publication date: 2002-05-23
Also published as: SE0004219D0; SE517922C2; SE0004219L; AU2002214518A1; EP1342057A1

Abstract

In a flow velocity meter of ultrasonic type sound signals are transmitted in a ring-around-process between two ultrasonic transmitters/detectors (G1, G2) placed opposite each other, having their transmission /receiving direction located in an angle to the flow direction (3) of a fluid. In the transmitting operation the polarities of the transmitted sound pulses are changed to that the currently active one of the transmitters/receivers is controlled to transmit the sound signals with positive and negative polarities chosen to reduce the influence of sound signals reflected by the transmitters/receivers on the direct sound signals that are received by the other one of the transmitters/receivers. The transmission of the signals is activated by a pulse transmission unit (27) so that for example each sound signal contains two pulses transmitted successively. The polarities are controlled by a multiplexer (13) that receives a control signal from a control unit (25). A polarity change is also made for received signals by means of another multiplexer (11) so that the received signals always have the same polarity. It facilitates the detection of a particular position in the received signals such as a suitably chosen zero passing using a zero point detector (21).

Description

FLOW VELOCITY METER TECHNICAL FIELD

The present invention relates to a flow velocity meter. BACKGROUND OF THE INVENTION AND PRIOR ART 5 For measuring the flow velocity of a fluid sound signals can be emitted in the fluid since the propagation speed of the signals in the fluid is influenced by the flow velocity of the fluid. For measuring, the run time of the signals from a fixed point to another point can then be determined and from the measured run time a value of the flow velocity of the fluid can be derived. Then it can be necessary to know the velocity of sound in the fluid which velocity o however is dependent on temperature.

The sound velocity of for example water is about 1500 m/s and the flow velocities which can be of interest to measure can in many cases be for example from about 50 m/s down to 10 mm/s. It is particularly difficult to measure the low velocities since for them the influence on the run times of the sound signals is smallest. It can be a requirement that also s the values measured for the low flow velocities should have a fault less than 5 %. In order to achieve it a method is used, see the U.S. patent cited below, that is called "sing-around" or "ring-around" and that comprises that the resolution in -the -measurement is increased by emitting the sound signal between two fixedly located sound emitters/detectors a large number of times in a first direction at successive times that are repeated with a relatively high o frequency and after that in the opposite direction the same number of times. Values of the sums of the run times for the signals in each direction are determined. From these two total times the flow velocity can be calculated, provided that in addition the distance between sound source and sound detector is known and further the time delay is known that is obtained for each run before the received signal has been detected. 5 In U.S. patent 5,493,916 and the published European patent application 0 591 368, inventor N. Bignell, such a measuring method is disclosed. Ultrasound pulses having a frequency less than 200 kHz are issued between two emitters/detectors placed in the flow path of the fluid and the run times are determined both for pulses moving with the fluid, downstream, and pulses that move in a direction opposite the flow direction of the fluid, 0 upstream. Error sources because of plural activated acoustic modes are eliminated by a special use of polarity of emitted pulses, such as typically p,p,p,n,p,p,p,n,p,.... The different polarities are obtained by a rotation of phase when emitting the sound signal.

A problem that can be obtained when using such measuring methods is caused by the fact that the emitted sound signal is reflected between the two transceivers. Thereby, reflected 5 sound signals are mixed with the directly emitted sound signals and thereby affect the shape thereof, such as rotate the phase of the signals to be detected. The detection of received signals is affected and results in time errors which are not the type systematic errors, which can be predicted.

SUMMARY OF THE INVENTION o It is an object of the present invention to provide a flow velocity meter that has a reliable, simple structure and that has a good accuracy.

In a flow velocity meter of ultra sound type, trains or sequences of sound signals are emitted, each sound signal advantageously being in the shape of a short sequence of pulses, between two ultra sound emitters/detectors located opposite each other so that they pass in an angle in relation to the flow velocity of a fluid. A sing-around or ring- around procedure according to the description above is used comprising a sequence including a large number of 5 sound signals emitted successively from one of the emitters/detectors to the other one and thereafter a sequence including the same number of signals emitted from the other emitter/detector. In the emission the polarity of the emitted signals are changed or switched, so that the currently emitting one of the emitters/receivers is controlled to emit sound signals having positive and negative polarities selected to reduce the influence of signals reflected by 0 the emitters/detectors on direct signals received by the other one of the emitter/receivers. Then generally, the same number of negative sound signals can be emitted successively that exist in a subsequence emitted directly before, comprising only positive sound signals. An inversion of polarity or phase is thus made at the emitter but is also advantageously performed at the receiver in order that the received signals should always have the same polarity and s thereby substantially the same shape. It facilitates the detection of a particular position in the received signals such as a particularly selected zero crossing. The detection of the particular position can be facilitated by using a procedure comprising a preset time period after emission of a signal, before the end of which no detection is to be made. After the preset time period the received signal must pass a threshold level after which finally the first zero detection is o made. The same procedure including a threshold level that is to be passed after the preset time period can be used also for the next issued sound signal. For the following sound signals reflections have influence and for them only the measured time period between emission and detection is used to determine the time after which the detection of zero crossing is to be made. In that case the threshold level is not considered. This can be described in the way that 5 after measuring for the first sound signal/signals the preset time period is changed to a value that is determined from the value determined in that measurement of the run times of the sound signals between emitter and detector.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice 0 of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the invention are set forth with particularly in the appended 5 claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which:

- Fig. 1 is a schematic picture of a flow velocity meter comprising two ultra sound heads o placed for measuring the velocity of a fluid flowing in a pipe and the connection of the heads to a control unit,

- Fig. 2 is a diagram showing the shape of emitted sound pulses, - Fig. 3 is a block diagram of the different steps that must be executed by different components in the flow velocity meter,

- Fig. 4 is a diagram in the shape of a display image showing a detected light signal obtained directly from a sound transmitter,

5 - Figs. 5 - 7 are diagrams similar to Fig. 4 that show a detected sound signal obtained directly from a sound transmitter together with superposed reflected signals. DESCRIPTION OF A PREFERRED EMBODIMENT

In Fig. 1 a cylindric pipe or tube 1 is shown in which a fluid moves or flows in the direction of the arrow 3 with a propagation or a flow velocity v. A first sound emitter and o detector or transmitter and receiver, in the following only called transceiver, Gl is arranged to include a flat surface having a normal forming an oblique angle φ to the flow direction, i.e. to the direction of the arrow 3. The flat surface of the transceiver forms a bottom of a short cylindric tubular piece 5 that starts from or connects to the tube 1 and forms the same angle φ in relation thereto. The tubular piece 5 can be so short that an edge portion of the flat surface s of the transceiver Gl is located quite at the path of flow inside the tube 1. The tubular piece thus has the shape of a short cylinder, the bottom of which is located in a straight angle to the axis of the cylindrical shape and the top part of which is obliquely cut with a profile at the connection to the tube. Furthermore, Gl is located having its normal in the direction of flow so that its flat surface is affected by a negative dynamic pressure from the fluid. A second, o identical transceiver G2 is designed and placed in the same way having its flat surface at the bottom of a second short tubular piece 7 that has the same shape and the same cylinder axis as the first tubular piece 5. The flat surfaces of the transceivers Gl, G2 are thus located opposite and parallel to each other. The second transceiver G2 is affected by a positive dynamic pressure or flow pressure since the normal of its flat surface has a component opposite the 5 direction of flow. The two short tubular pieces 5, 7 can be taken as parts of a cylindrical tube that intersects the tube 1 in the angle φ. This angle φ should, in order to achieve a good accuracy, be as small as possible. A too small angle can, however, in some cases give too great flow losses in the measurement device and thus a preferred range of the angle can be 5 - 60°. 0 The flat surfaces of the transceivers Gl, G2 that are in contact with the flowing fluid can for a suitable electrical connection be made to emit sound waves and to detect sound waves incoming to the surfaces. The sound waves emitted from one of the transceivers can then be detected by the opposite transceiver. The sound waves obtain different run times between the transceivers depending on the flow velocity v of the fluid in the tube 1. Each 5 transceiver has two electrical terminals, a plus terminal and a minus terminal. When providing an electrical activating voltage on the plus terminal, pulses or sound signals are obtained from the transceiver having a certain polarity but if the same voltage is provided to the minus terminal of the transceiver, pulses or sound signals having the opposite polarity are delivered. In the corresponding way it is true that if the transceivers work as detectors and they detect a 0 received signal having a certain polarity, on the plus and minus terminal electrical signals are provided that are the inverted images of each other. The terminals of the transceivers are by two electrical lines connected to a first controllable multiplexer 11 for receiving/detecting, a second controllable multiplexer 12 for transmitting and third and forth controllable multiplexers 13, 14 which through resistors R3, R4 are connected to a fixed reference voltage, for example ground. Each of the multiplexers has two control input terminals and the multiplexers 11 and 13 are controlled by two control signals Al and Bl and the multiplexers 5 12 and 14 are controlled by two control signals A2 and B2.

The receiving multiplexer 11 is through a filter 15, an amplifier 17, a pre-trigger unit 19 and a zero detector 21 connected to an input terminal of a control unit 25 that can be some type of microprocessor and that also has output terminals coupled to the control inputs of the multiplexers 11 - 14 for controlling them using the control signals Al, Bl and A2, B2 o respectively. The control unit 25 can on another output terminal provide a signal for emitting sound waves. This signal operates, through a gate 31, a pulse emitting unit or signal generator 25 that when receiving the signal provides a suitable output voltage to the emitting multiplexer 12, from which the output voltage is then transferred to the transceiver Gl, G2 currently selected by the control unit 25 and is given a selected polarity. 5 The control unit 25 is connected to or comprises two timers or timing circuits 29.1,

29.2.

When the pulse emitting unit 27 receives a signal from the control unit 25, it provides, through the multiplexer 12, the currently selected transceiver Gl, G2 and the selected terminal thereof, the plus or minus terminal, so that it changes its dimension or shape a little and o thereby, from its flat surface, emits a sound signal of the type shown in Fig. 2. The sound signal comprises in the case shown two successive pulses, that correspond to two pulses of a longer pulse train that is obtained when the sound transceiver is activated to change its dimensions with the eigenfrequency thereof. The sound transceivers Gl, G2 can preferably be designed to have eigenfrequencies of a magnitude of the order of 1 - 4 MHz. For for example 5 the eigenfrequency of 1 MHz the time period is 1 μs, that results in a length of time between the start of the first pulse and the end of the second pulse that corresponds to 1 1/2 μs, generally 1 1/2 time period, provided that the pulses take half of each period. If the activating signal is delivered on the second terminal of the transceivers, the inverted signal of the signal according to Fig. 2 is obtained, i.e. a signal having the opposite, in this case a negative, 0 polarity, in the case where the signal according to Fig. 2 is supposed to have a positive polarity. Generally each sound signal can comprise small number of rectangular pulses having a frequency substantially corresponding to the eigenfrequency of the transmitters/receivers, in particularly a number smaller than five, such as two or three pulses and preferably two pulses.

From measurements of the run times of sound signals that are propagated between the 5 transceivers Gl, G2 a measure of the flow velocity of the fluid can be obtained. Thus the time period ^ required for a sound signal to propagate from the transceiver Gl to G2, i.e. upstream, and the time t₂ for a sound signal to propagate from the transceiver G2 to Gl, i.e. down stream, are measured.

A sound signal emitted from one of the transceivers passes partly through fluid moving 0 obliquely in relation to the propagation direction of the sound signal, partly through fluid that can be considered to be rather still.

The path through moving fluid has a length l_j and is approximately given by - X sxn φ- where d is the diameter of the tube 1. The path through still-standing fluid has a respective length 1₂ approximately given by

1₂ ' tan φ

5 where d₂ is the diameter of the surface of the transceivers Gl, G2 and of the tubular pieces 5, 7.

The run times of the sound signals are then

t = ¹1 ^{J~ 1}2 c + v cos φ

2 ₂ = v cos φ o where c is the velocity of sound in the fluid. From these formulae a formula of the velocity v of the fluid can be derived.

When emitting sound signals the first transceiver Gl starts by emitting a signal. It is received by the second transceiver G2 and at detecting the signal, and as will be described in more detail hereinafter, at detection of a particular position in the received signal, the first s transceiver again emits a signal. This procedure is then repeated a large number of times in order to obtain a suitable accuracy of the measurement. If it is repeated N times and the summed measured time for all sound signals that are emitted from the first transceiver Gl and detected by the second detector G2 is t_ml and the corresponding time for all sound signals emitted in the opposite direction is t^ and t_f is the time delay existing between the time when o the particular position in a received signal is actually received by the detecting transceiver and the time when, after this particular position in a received signal has been detected by the opposite detector, a new signal is emitted, it is true that

t_{2 N} t_f

5 It can be demonstrated that the flow velocity is then with good accuracy given by

v = K- ( - i ) tml _ t *-m2 _ -

N N where K is a constant depending on the geometric shape of the tube, among other things the angle φ according to the discussion above. It can be observed that this formula of the flow velocity is independent of temperature, this being true provided that the flow velocity v is 0 small in relation to the sound velocity c in the fluid. The different steps that are used when measuring the velocity of the fluid will now be described with reference to the flow diagram of Fig. 3.

The measurements are initiated in a first step 101. Then a delay is programmed that is to be used before pre-triggering, see the discussion below, can be performed. This delay is 5 included in order that the flow velocity meter will not trigger on disturbances or interference when transmission of sound signals is performed.

In the next block 103 the time measurement is started by emitting an ultra sound signal in a direction with the flow, i.e. from the transceiver Gl . The ultra sound signal is emitted as two identical rectangular pulses as has been described above with reference to Fig. 2, having o a polarity selected according to the description below. At the same time as the start of the first pulse in the sound signal the time measurement is started by a signal being sent from the control unit 25 to a timer 29.1. At the same time another timer 29.2 is started for a programmed delay of about 10 μs counted from the start of the first pulse, before any detection of the signal is to be made. The number of clock pulses that corresponds to the first s programmed delay can be stored in a memory, the content of which is copied to a delay memory in the control unit 25.

In the next block 105 the signal from Gl emitted with the flow is received by the transceiver G2, and is by the receiving transceiver G2 converted to an electric signal that through the receiving multiplexer 12 is first conducted to the filter 15 to be band pass filtered o therein. The filter 15 has a pass band that is located around the frequency of the ultra sound signal. The filtered signal is then conducted to the amplifier 17 and is amplified therein. In addition a constant voltage of about 1.5 V is added to the signal in the amplifier in order to give a simple detection of zero crossings. It is further awaited that the timer 29.2 that has been started for delaying the detection, will provide a signal that the set time has elapsed. 5 When such a signal is obtained, the timer is reset and the pre-triggering is executed, that includes that the control unit 25 now awaits that the signal will become lower than some positive threshold level, for example 0.75 V, or if the threshold level is negative, that the signal becomes larger than it. That the signal has become lower than or larger than the threshold level respectively means that a wanted ultra sound signal now has been received. 0 Then, detection of the fact that the signal passes the zero level, the zero detection, can be executed. The indicated voltage levels, the zero level and the pre-triggering voltage, are taken in relation to the applied fixed DC voltage.

For zero crossing detection, from the zero detector 21 a pulse is issued that directly triggers emission of a new pulse through the gate 31. The zero detection pulse is conducted 5 directly through the gate 31 to the pulse emission unit 21 for activating it so that it through the pulse transmission multiplexer provides a suitable signal to the transceiver Gl to again emit a sound signal. In detecting the zero crossing, when the pulse provided by the zero detector 21 is used, the previously reset timer 29.2 for the program delay is restarted.

For the now second emitted sound signal the same procedure is repeated in the detection 0 for the corresponding received sound signal as that has been used for the first sound signal. In the detection of the second received sound signal also the timer 29.1 is read that was started when activating the pulse emission unit 27 for the first emission of a sound signal. For sound signals emitted thereafter the detection is disturbed by reflections of earlier sound signals. Therefore, for the following received sound signals the condition is not used that they are to pass a threshold level. Instead only a time period is used that starts at the emission of the sound signal and that has to elapse before the zero detection is made. This time period must 5 be very well determined and its length is obtained from the performed reading of the timer that defines the run time of the two first sound signals. This time period derived from the measurements can be stored in a particular register in the processor 25 and is thus in the following used as the read-out delay. Otherwise, the same procedure is repeated for following sound signals. A large number N signals are emitted where N is selected to provide the o desired accuracy in the measurement. The process is interrupted by the microprocessor 25, switching off, after receiving N zero detection pulses, the direct transmission of zero detection pulses through the gate 31 from the zero point detector 21 to the pulse emission unit 27, by providing a suitable control signal to the gate. In the next block 107 the timer 29.1 is read and the read number of clock pulses is stored in a memory for t^ . Thereafter, in the next block s 109 the same measurement process is repeated as in the blocks 105 and 107 but for sound pulses emitted upstreams, i.e. emitted by the transceiver G2 and detected by the transceiver Gl. After N sound signals having being emitted and detected, the obtained number of clock pulses is stored in a memory for t^.

The number of sound signals N, for which the measurement is made, can for example o be selected, so that the difference between the total times t-^, t^ measured for upstream signals and downstream signals has reached a predetermined value. It can be difficult to predict this and then it is tested, after having obtained the measured numbers, whether the difference is equal to or larger than such a predetermined value. If this condition is satisfied, the measurement is terminated in a block 111. In it the flow velocity is calculated using the 5 formula mentioned above. If the difference is smaller than the predetermined value, the whole measuring process can be repeated so that instead total times t^, t^ for 2N sound signals are obtained. Then it is again tested whether the difference now is sufficient, after which another measurement can be performed if necessary.

For the high frequencies that are used when emitting the measuring sound signals, of the 0 magnitude of order 1 MHz and more, and by the fact that for example only two pulses exist in each individual sound signal, substantially only one single acoustic mode is activated in the emission of the sound signals. If more modes are activated, such as is the case in the process according to the U.S. patent and the European patent application cited above, when lower frequencies are used, this gives problems in the detection, since different modes have different 5 propagation velocities. However, also for high frequencies reflections occur against the walls in the measurement space in which the sound signals are propagated, primarily against the opposite detector surfaces. This fact results in that to each detector surface, in addition to the direct sound signal, also other reflected sound signals simultaneously arrive which have lower amplitudes. The detector thereby receives a composite signal in which the largest component 0 is the direct signal. This means that the time of receiving the direct signals becomes indefinite. Generally the time is determined as the time when the received signal passes a particularly selected level. This time will then have an error due to the reflected components in the received signal.

In Fig. 5 is illustrated how a sound signal emitted according to the description above can look at the surface of the receiving detector. At 41 a pre- trigger level is shown that can be used and at 43 a zero crossing that is used by the zero point detector 21 for determining the 5 time when receiving the sound signal.

In order to reduce or eliminate the influence of reflected sound signals in the measurement, the polarity of the emitted signals can be changed according to a predetermined, specially selected advantageous pattern. The polarities of the emitted sound signals are then to be changed so that for the reflections in the measuring process, the numbers of positive and ιo negative reflections of different orders become approximately equal, considering only reflections against the opposite transceiver surfaces. In Figs. 6 - 8 received signal shapes are illustrated having reflected components of different polarities. In Fig. 6 thus the sound signal at the detector is shown that is composed of both the direct signal that is assumed to have a positive polarity and a sound signal that has been reflected once and also has positive polarity, is and a sound signal that has been reflected twice and also has a positive polarity. In Fig. 7 the sound signal is shown when the two reflected signals have a negative polarity, whereas the sound signal according to Fig. 8 is obtained in the case where the signal that has been reflected once has a positive polarity and the twice reflected signal has a negative polarity.

The activating electric signal emitted by the pulse emission unit 27 and the electric

₂o signal for which zero point detection is made in the detector 21 always have the same polarity, this being obtained by switching the poles in the transmission multiplexer 12 and the receiving multiplexer 11. If G1 + denotes the positive terminal of the transceiver Gl, Gl- the negative terminal thereof, G2+ the positive terminal of the transceiver G2 and G2- the negative

- terminal thereof, the following is true for the control signals Al, Bl and A2, B2 respectively

₂5 to the multiplexers 11, 13 and 12, 14:

In the columns having a transceiver, terminal at the top, in the spaces below the multiplexer is indicated, the input terminal of which is connected to this transceiver terminal.

For a positive downstream signal the control signals Al, Bl and A2, B2 should have the values 0, 0 and for a negative downstream signal the values 1, 0. For a positive upstream signal the control signals Al, Bl and A2, B2 should have the values 0, 1 and for a negative 5 downstream signal the values 1, 1.

The fact that the activating electric signal and the electric signal for which the zero point detection is performed always have the same polarity, results in the fact that the signal for which a zero crossing is to be detected always has substantially the same shape, independently of the fact whether the actually emitted sound signal has a positive or negative polarity, and is o only distorted in different ways depending on the reflections in the measurement space. The actually detected times for the zero crossings, as illustrated in Figs.5-8, have thus deviations from the ideal times which would be obtained in the case where the sound signals are emitted with so large time intervals that the reflections do not effect the sound signals at the detector surface. The sum of these deviations δ_1; δ₂, δ , δ₄, ... can for a suitable choice of the s polarities of the emitted sound signals be made to be very close to zero, i.e. so that the errors balance each other when summing them. This fact results in that the errors due to the reflections do not influence the sum of the measured run times for the sound signals in each direction.

A suitable sequence of signals is formed by 3 positive, 3 negative, 1 positive, 1 o negative, 3 positive, 3 negative, 1 positive, 1 negative, 3 positive, 3 negative, ... etc. or in short ppp nnnpnpppnnnpnppp nnn ..., where the letters p, n denote a signal having a positive and a negative polarity respectively. For such a sequence, from reflections against one surface, reflections against two surfaces, reflections against three surfaces etc. half the reflections will be positive and half the reflections will be negative. 5 The signals and their reflections are schematically illustrated below:

p p p n n n p n p p p n n n p n p p p n n n p n p p p n n n p n . * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

+ - - + + - - + + - - + + - - 0 - + + - - + + - - + + - - + +

- + - + - + - + - + - + - +

+ + - - + + - - + + - - +

- - + + - - + + - - + + -

5 At the top of the schematic the preferred sequence is shown using the letter notations p, n for the polarities according to the discussion above, the time axis extending with increasing values to the right. The asterisks below the letters denote the times when the sound signal is emitted. In the next row, for every second signal is indicated how reflected signals of the first order, i.e. sound signals that have been reflected twice, influence in the detection of the sound 0 signal, the polarity of which is indicated straight above, the directly emitted sound signals for this row all being emitted in a first direction. The influence is indicated by a plus sign, + , if the sound signal and the reflected signal have the same polarity and by a minus sign, -, if they have different polarities. In the next row the same thing is indicated for the sound signals skipped in the first row. In the two rows below them is indicated how the reflected signals of 5 the second order influence in the detection of the directly emitted sound signals, the first of these two rows showing the state for every second sound signal and the second row for the remaining sound signals. In the fifth and sixth rows the same thing is shown for reflected signals of the third order. Generally, a reflected signal of the n:th order has been submitted to 2n reflections. o Thus, for the segment of sound signals that are shown in the schematic above, for reflections of the first order, for sound signals having odd order numbers, there are 7 reflections having a positive influence and 8 reflections having a negative influence. 8 reflections exist of the same order having a positive influence and 7 reflections having a negative influence for sound signals having even order numbers. Furthermore, for sound 5 signals having odd order numbers, 7 reflections of the second order exist that have a positive influence and 7 having a negative influence, and the same fact is true for sound signals having even order numbers. For sound signals having odd order numbers, 7 reflections of the third order exist that have a positive influence and 6 reflections that have a negative influence, and for sound signals having even order numbers the corresponding numbers are 6 and 7. The o same condition, comprising only a difference at most being equal to one, is obtained for longer sequences.

If the errors above are added, the same number of positive and negative errors is obtained. For the fourth reflections the polarity will always be the same and in order to compensate also therefor, the pattern of the polarities can be further adjusted, but since in this 5 case the fourth reflections will not significantly affect the result (the fourth reflection arrives from 13.5 period before and during this time period it has been attenuated) this reflection thus does not have to be adjusted to obtain as many positive as negative errors from the fourth reflections and higher. If it would be desirable to adjust for further levels, the sequence pppnnnpn can be seen as a packet that then in a similar way has its polarities switched in 0 order to eliminate errors for higher order reflections.

pp = positive packet (pppnnnpn) np = negative packet (nnnpppnp)

Then the series can be expanded by emitting the following series:

pp,pp,np,np,pp,pp,np,np,

5 In the process according to the description above, as has already been stated, as many positive as negative errors are obtained. It appears that this gives a high accuracy in the determination of the total time if the following condition is fulfilled.

The reflected signal is relatively small compared to the direct one and therefore the largest influence one the time for the zero crossing is derived from the orthogonal portion of the reflection. This fact is realized if one considers the direct signal and the reflected signals as two sinus waves which in the region for zero detection are added to each other and which have a phase offset θ. Then the orthogonal portion of the reflected signal is equal to (amplitude of reflected signal) * sin θ.

While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous additional advantages, modifications and changes will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described here- in. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention.

Claims

1. A method of determining the velocity of a fluid, comprising two sound emitters/receivers between which sequences of sound signals are emitted to propagate in the fluid and then to be received, the sound signals being emitted with positive or negative

5 polarities and the total time for the propagation of the time signals between the sound emitters/receivers being determined and therefrom a measurement of the velocity being calculated, characterized in that the sequence of sound signals is transmitted from one of the sound emitters/receivers with positive and negative polarities of the sound signals in the sequence selected to reduce the influence of sound signals reflected against the sound o emitters/receivers on direct sound signals received by the other one of the sound emitters/receivers .

2. A method according to claim 1, characterized in that the polarities are selected so that all the received sound signals, totally or taken as a group, are influenced by substantially the same number of reflected sound signals of a positive and of a negative polarity. s

3. A method according to claim 1, characterized in that the polarities of sound signals in the emitted sequence are selected so that in groups of sound signals emitted after each other, in each group first a number of sound signals having a first polarity are emitted and thereafter the same number of sound signals having a second polarity opposite the first one.

4. A method according to claim 3, characterized in that the groups comprise first o groups and second groups alternating with each other, in the first groups three sound signals being emitted having the first polarity and three sound signals having the second polarity and in the second groups one sound signal being emitted having the first polarity and one sound signal having the second polarity.

5. A method according to claim 1, characterized in that in receiving the sound signals a 5 detection of a predetermined position in the sound signal is performed, and that before detection, for sound signals having a predetermined polarity, an inversion of the sound signals with respect to their polarity or phase being made, whereby the detection is always made of the predetermined position in sound signals having the same polarity that is opposite the predetermined polarity. o

6. A method according to claim 1, characterized in that in receiving the sound signals a detection of a predetermined decision in the sound signals is performed, the detection for each emitted sound signal being made after a set time period counted from the time of the emission of the sound signal.

7. A method according to claim 6, characterized in that the set time period for the first 5 emitted sound signal is a preset time period and that the detection of a predetermined position in the corresponding received sound signal is not performed after this preset time period until the received sound signal has become lower than a positive threshold level or larger than a negative threshold level.

8. A method according to claim 6, characterized in that the set time period for emitted 0 sound signals is determined from measuring run times for previously emitted sound signals.

9. A method according to claim 1, characterized in that the sound emitters/receivers are arranged to emit/receive sound signals in a direction forming an angle to the flow velocity of the fluid, the angle being an acute angle, in particular an angle between 5 and 60°.

10. A method according to claim 1, characterized in that in emitting sound signals from the sound emitters/receivers they are activated to emit sound signals having a frequency substantially corresponding to the eigenfrequency of the sound emitters/receivers. 5

11. A method according to claim 10, characterized in that in emitting sound signals from the sound emitters/receivers they are activated to emit for each sound signal a small number of pulses having a frequency substantially corresponding to the eigenfrequency of the sound emitters/receivers, in particular two or three pulses and preferably two pulses.

12. A device for determining the velocity of a fluid, comprising two sound o emitters/ receivers, between which sequences of sound signals are emitted to propagate in the fluid and then to be received, the sound signals being emitted with positive or negative polarities, and further comprising measuring means connected to the sound emitters/receivers for determining the total time for propagation of the sound signals between the sound emitters/receivers and calculating means connected to the measuring means for calculating 5 from the total determined time a measure of the velocity, characterized by control means connected to the sound emitters/receivers and arranged to provide control signals to them so that one of the sound emitters/receivers is controlled to emit the sequence of sound signals therefrom with positive and negative polarities in the sound signals selected to reduce the influence of sound signals reflected by the sound emitters/receivers on direct sound signals o received by the other one of the sound emitters/receivers.

13. A device according to claim 12, characterized in that the control means are arranged to provide control signals so that the polarities of the sound signals emitted from said one of the sound emitters/receivers are such that all the received sound signals, totally or taken as a group, are affected by substantially the same number of reflected sound signals 5 having a positive polarity as by reflected signals having a negative polarity.

14. A device according to claim 12, characterized in that the control means are arranged to provide control signals, so that the polarities of the sound signals emitted from said one of the sound emitters/receivers are such that in groups of sound signals emitted after each other, in each group first a number of sound signals having a first polarity are emitted o and thereafter the same number of sound signals having a second polarity opposite the first one are emitted.

15. A device according to claim 14, characterized in that the control means are arranged to provide control signals, so that the groups comprise first and second groups alternating with each other, in the first groups three sound signals being emitted having the 5 first polarity and three sound signals having the second polarity and in the second groups one sound signal being emitted having the first polarity and one sound signal having the second polarity.

16. A device according to claim 12, characterized in that the measuring means comprise detection means to perform, when receiving the sound signals a detection of a 0 predetermined position in the sound signals, and inversion means to perform, before the detection, for sound signals having a predetermined polarity, an inversion in respect of polarity or phase, whereby the detection means always perform a determination of the predetermined position in sound signals having the same polarity opposite the predetermined polarity.

17. A device according to claim 12, characterized in that the measuring means comprise memory means for storing a value of a set time period and that they furthermore

5 comprise timing means and detection means, the detection means connected to the timing means and to the sound emitters/receivers to perform, when receiving the sound signals a detection of a predetermined position in the sound signals, so that the detection, determined by a signal from the timing means, for each emitted sound signal is performed after the set time period counted from the time of emitting the sound signal. o 18. A device according to claim 17, characterized in that the memory means are arranged to store, for the set time period for the first emitted sound signal a value of a preset time period and that the detection means are arranged not to perform the detection of a predetermined decision in the corresponding received sound signal after this preset time period until the received sound signal has become lower than a positive threshold level or larger than s a negative threshold level.

19. A device according to claim 17, characterized in that the memory means are arranged to store, for the set time period, a value determined from measurement of run time for previously emitted sound signals.

20. A device according to claim 12, characterized in that the sound emitters/ receivers o are arranged to emit/receive sound signals in a direction forming an angle to the flow velocity of the fluid, the angle being an acute angle, in particular an angle between 5 and 60°.

21. A device according to claim 12, characterized by a pulse emission unit connected to the sound emitters/receivers for activating, in the emitting of sound signals from the sound emitters/receivers, them to emit sound signals comprising pulses having a frequency 5 substantially corresponding to the eigenfrequency of the sound emitters/receivers.

22. A device according to claim 12, characterized in that the pulse emission unit is arranged to activate, in the emitting of sound signals from the sound emitters/receivers, them to emit for each sound signal a small number of pulses having a frequency substantially corresponding to the eigenfrequency of the sound emitters/receivers, in particular two or three 0 pulses and preferably two pulses.