Suspended sediment meter

Abstract

The application provides a device for measuring at least one parameter value of a suspended sediment of a fluid. The device includes a backscattering transducer module, a storage unit, and a calculation unit. The backscattering transducer module comprises a source module and a receiver module. The source module transmits at least three acoustic signals with different fixed characterizing measurement frequencies while the receiver module measures at least three echo level values of echo signals, which correspond with the at least three acoustic signals. The storage unit stores a data set of pre-determined echo level values with a data set of pre-determined suspended sediment parameter values. The calculation unit derives at least one suspended sediment parameter value from the data sets and the at least three echo level values.

Claims

1. A device for measuring at least one parameter value of a suspended sediment of a fluid, the device comprising a backscattering transducer module that comprises a source module for transmitting at least three acoustic signals with different fixed characterizing measurement frequencies and a receiver module for measuring at least three echo level values of echo signals, which correspond with backscattered acoustic signals of the at least three acoustic signals with said different fixed characterizing measurement frequencies, a storage unit that stores a data set of pre-determined echo level values with a data set of pre-determined suspended sediment parameter values, a calculation unit for deriving at least one suspended sediment parameter value according to the data set of pre-determined echo level values with the data set of pre-determined suspended sediment parameter values and according to the at least three echo level values, wherein the data set of pre-determined suspended sediment parameter values comprises at least one concentration suspended sediment parameter value.

2. The device according to claim 1, wherein the data set of pre-determined suspend sediment parameter values comprises at least one median parameter value of a particle function of the suspended sediment.

3. The device according to claim 1, wherein the data set of pre-determined suspended sediment parameter values comprises at toast one slope parameter value of the particle function of the suspended sediment.

4. The device according to claim 1, wherein the calculation unit comprises a module for detecting a presence of air bubbles in the signal path of the acoustic signals by deriving at least one parameter value of the echo signal.

5. The device according to claim 4, wherein the parameter value of the echo signal comprises a phase parameter value of the echo signal.

6. The device according to claim 4, wherein the parameter value of the echo signal comprises an amplitude parameter value of the spectrum of the echo signal.

7. A method of determining at least one parameter of a suspended sediment of a fluid, the method comprising providing a data set of pre-determined echo level values with a data set of pre-determined suspended sediment parameter values, directing at least three acoustic signals with different fixed characterizing measurement frequencies at the suspended sediment of the fluid, measuring at least three echo levels of echo signals, which correspond with backscattered acoustic signals of the at least three acoustic signals with said different fixed characterizing measurement frequencies, deriving at least one parameter value of the suspended sediment according to the data set of pre-determined echo level values with the data set of pre-determined suspended sediment parameter values and according to the measurements of the at least three echo levels, wherein the data set of pre-determined suspended sediment parameter values comprises a pre-determined concentration suspended sediment parameter value.

8. The method according to claim 7, wherein the data set of pre-determined suspended sediment parameter values comprises a pre-determined median parameter value of a particle function of the suspended sediment.

9. The method according to claim 7, wherein the data set of pre-determined suspended sediment, parameter values comprises a pre-determined slope parameter value of the particle function of the suspended sediment.

10. The method according to claim 7 further comprising detecting a presence of air bubbles in the signal path of the acoustic signals.

11. The method according to claim 10, wherein the detecting of the presence of air bubbles comprises deriving at least one phase parameter value of the echo signal.

12. The method according to claim 10, wherein the detecting of the presence of air bubbles comprises deriving at least one amplitude value of the spectrum of the echo signal.

13. The method according to claim 7, wherein the deriving of the at least one suspended sediment parameter takes in account effects of attenuation due to the fluid.

14. The method according to claim 7, wherein the deriving of the at least one suspended sediment parameter takes in account effects of attenuation due to particle sizes of the suspended sediment.

15. The method according to claim 7, wherein the deriving of the at least one suspended sediment parameter takes in account effects of concentration of the suspended sediment.

Description

(1) FIG. 1 illustrates a suspended sediment meter,

(2) FIG. 2 illustrates a transducer head of the suspended sediment meter of FIG. 1, the transducer head includes a plurality of transducers,

(3) FIG. 3 illustrates an operating field of the transducer head of FIG. 2,

(4) FIG. 4 illustrates a receiving sensitivity graph of one of the transducers of the transducer head of FIG. 2,

(5) FIG. 5 illustrates a transmitting sensitivity graph of one of the transducers of the transducer head of FIG. 2,

(6) FIG. 6 illustrates a flow chart of a method for measuring suspended sediment concentration (SSC),

(7) FIG. 7 illustrates a flow chart of a method of using the sediment meter of FIG. 1 to determine suspended sediment parameters,

(8) FIG. 8 illustrates a table of stored values of the sediment meter of FIG. 1 of the method of FIG. 7,

(9) FIG. 9 illustrates ranges of measurement values of the method of FIG. 7,

(10) FIG. 10 illustrates measurement points of backscattered signals of suspended sediment of a method for detecting air bubbles,

(11) FIG. 11 illustrates a view of phases of the backscattered signal of FIG. 10,

(12) FIG. 12 illustrates another view of phases of the backscattered signal of FIG. 10,

(13) FIG. 13 illustrates a signal spectrum of a backscattered signal of the suspended sediment meter of FIG. 1 of another method of detecting presence of air bubbles,

(14) FIG. 14 illustrates a further signal spectrum of a backscattered signal BS of the suspended sediment meter of FIG. 1 for detecting presence of air bubbles, and

(15) FIG. 15 illustrates another signal spectrum of a backscattered signal BS of the suspended sediment meter of FIG. 1 for detecting presence of air bubbles.

(16) In the following description, details are provided to describe embodiments of the application. It shall be apparent to one skilled in the art, however, that the embodiments may be practiced without such details.

(17) Some parts of the embodiments, which are shown in the Figs., have similar parts. The similar parts have the same names or similar part numbers with a prime symbol or with an alphabetic symbol. The description of such similar parts also applies by reference to other similar parts, where appropriate, thereby reducing repetition of text without limiting the disclosure.

(18) FIG. 1 shows a suspended sediment meter 10. The sediment meter 10 comprises a processor 15, a memory unit 17, and a transducer head 20. The processor 15 is connected to the memory unit 17 and to the transducer head 20. The sediment meter 10 also includes an air bubble detector 18.

(19) As seen in FIG. 2, the transducer head 20 includes a transducer 41, a transducer 42, a transducer 43, and a transducer 44. Each of the transducers 41, 42, 43, and 44 has an acoustic source and an acoustic receiver.

(20) In use, the suspended sediment meter 10 is used for determining one or more parameters or characteristics of a suspended sediment of a fluid. An example of the parameter is a concentration of the suspended sediment. The suspended sediment is also called suspended material. The fluid here refers to a liquid, such as water.

(21) For operational efficiency, the transducers 41, 42, 43, and 44 are often in physical contact with the fluid. A user directs the transducers 41, 42, 43, and 44 towards an area or a volume of the fluid that contains the suspended sediment, wherein the fluid area is of interest to the user.

(22) The acoustic sources of the transducers 41, 42, 43, and 44 are intended for transmitting short pulses of source acoustic signals with various different frequencies to the fluid area, which is of interest to the user.

(23) Pulse widths and frequencies of the transmitted pulses of the source acoustic signals are selected according to expected particle sizes of the suspended sediment, which are expected to be present in the fluid area. The source acoustic pulses then interact with the said suspended sediment, wherein the suspended sediment scatters the source acoustic pulses and reflects the source acoustic pulses back to the acoustic receivers of the transducers 41, 42, 43, and 44. In other words, the suspended sediment backscatters the source acoustic pulses to the said acoustic receivers. The reflected acoustic pulses are also called echo signals, raw signals, or backscattered signals.

(24) In one example, the transducer 41 produces an acoustic signal with an operating frequency of 0.5 MHz (megahertz) while the transducer 42 produces an acoustic signal with an operating frequency of 1 MHz. The transducer 43 produces an acoustic signal with an operating frequency of 2 MHz and the transducer 44 produces an acoustic signal with an operating frequency of 4 MHz.

(25) In another example, the source acoustic pulses have a transmission range of about several meters, a pulse width of about 10 microseconds, and a frequency ranging from about 0.250 MHz (megahertz) to about 5 MHz for measuring particle sizes with diameter ranging from about 2 mm (millimeter) to about 20 μm (micrometer).

(26) The air bubble detector 18 acts to detect presence of air bubbles in the fluid, which can affect readings of suspended sediment.

(27) FIG. 3 shows an operating area or field of acoustic signals of a sonar transducer 11 that corresponds to the transducers 41, 42, 43, and 44 of the transducer head 20 of the suspended sediment meter 10.

(28) The operating area can be roughly divided into two regions, namely a near field 50 and a far field 54. The near field 50 is placed between the transducer 11 and the far field 54. The far field 54 includes a measurement bin 55.

(29) The measurement bin 55 refers to a volume that contains a portion of a suspended sediment of a fluid, which is of interest to the user. The length of the measurement bin, which is measured radially along a line that originates from the transducer 11 to the measurement bin, defines a number of cycles of a frequency of the acoustic signals that is required to extend throughout the measurement bin to ensure that the measurement bin contains the acoustic signals for backscattering the acoustic signals.

(30) In one implementation, the suspended sediment is located at least 30 cm (centimeter) from the surface of the transducer 11. The length of the measurement bin 55 is 3 cm. In this example, for an acoustic signal with a frequency of 1 MHz, the number of pulses for extending throughout the measurement bin is about 20 cycles whereas for an acoustic signal with a frequency of 4 MHz, the number of pulses for extending throughout the measurement bin is about 80 cycles.

(31) The transducer 11 acts as a source of acoustic signals that are directed at the said sediment portion. The shape and the dimensions of the transducer 11 have an impact on spatial resolution of the suspended sediment and also an impact on characteristics of backscattered signals, which are associated with the acoustic signals.

(32) The acoustic signals in the near field 50 are usually more complex to analysis than the acoustic signals in the far field 54. In the far field 54, the acoustic signals of the acoustic source can be treated and be considered as originating from a point source. If the transducer 11 has a circular shape, the interacting volume of its acoustic signal in the far field 54 may be approximated as having a cone shape with a tip of the cone shape being located at the transducer 11. On the other hand, in the near field 50, the acoustic signals of the acoustic source can be considered as be emitted in a narrow column to the near field 50.

(33) In a general sense, the sonar transducer 11 can also produce specific, non-conical beam patterns. The measurement bin 55 can also be in located in the near field 50, instead of being located in the far field 54.

(34) Referring to the acoustic receivers of the transducers 41, 42, 43, and 44, they measure intensities of echo signals, which are associated with the source acoustic signals of the acoustic sources of the transducers 41, 42, 43, and 44. The intensity data comprises magnitude data and phase shift data. The magnitude data is also called strength data.

(35) The acoustic receiver measures backscattered signals BaS of the source acoustic signals in millivolts (mV). The millivolt value is then converted to dB (decibel) values by applying specific transducer sensitivity SE for the relevant frequency using an equation that is shown below.
EL[db]=20 log.sub.10(BaS)+SE  (2),
wherein EL=(received) echo level, BaS=measured backscattered signals SE=transducer sensitivity

(36) The value of the transducer sensitivity SE is shown in a receiving sensitivity graph 51, which is illustrated in FIG. 4. The transducer sensitivity SE is also called transducer responsivity. The transducer responsivity is defined as gain of output signal with reference to input signal.

(37) The echo level EL is also known as an intensity of the backscattered signals.

(38) In a similar manner, the decibel values of the source level SL are calculated using a transmitting sensitivity graph 52, which is illustrated in FIG. 5, for a signal that is produced by a transducer with a 1 MHz frequency.

(39) The measured intensity of the echo signals can be described using a mathematical equation, called a “Sound Navigation And Ranging” (SONAR) equation, which is shown below.
EL=SL−TL+BS  (1),
wherein EL=(received) echo level, SL=source level, TL=transmission loss, and BS=backscatter strength.

(40) Taking into account speed of sound in the fluid, sound propagation characteristics, and scattering strength of the suspended sediment, a relationship between the source level SL signals and characteristics of the suspended sediment can be developed. This relationship is dependent on the frequencies of the source level SL signals.

(41) An echo level EL of a backscattered signal from a uniform field of suspended sediment particles in a fluid with constant concentration is considered to vary inversely with a distance between an acoustic source of a source level SL signal and particles of the suspended sediment and to vary according to correlation factors for attenuation due to the fluid and to the suspended sediment particles.

(42) The echo level EL of the backscattered signal can thus be correlated with a concentration and particle size of the suspended sediment and with time delay between transmitting of the acoustic signal and receiving of the backscattered signal of acoustic signal.

(43) A mathematical relationship model for the above described relationship is shown below.

(44) $\begin{array}{cc}\mathrm{EL}=\mathrm{SL}+C-20\log \left(\eta R\right)-2{\alpha }_{w}R-2\left[.Math.\left(C_1\frac{{\chi }_i{\mathrm{SSC}}_i}{a_i{\rho }_s}\right)\right]R+10\log \left(\psi \frac{c\tau }{2}\right)+10\log .Math.\left(C_2{a}_i^2{f}_{\#,i}^2{N}_i\right)& \left(3\right)\end{array}$
wherein EL=(received) echo level, SL=source level, C=coefficient that is dependent on the transducer, R=distance between an acoustic source and a measurement bin, the bin being defined as a range between two distances R1 and R2, wherein the selection of the two distances R1 and R2 is done carefully to ensure that the measurement bin contains the suspended sediment of interest for measuring the desired information, for example particle mixture, η=f(R,a.sub.t)=near field correction, α.sub.w=f(f.sub.R,S,T, etc)=absorption due to the fluid, wherein f.sub.R=frequency, S=salinity, and T=temperature, ψ=solid opening angle of the transducer, SSC.sub.i=Suspended Sediment Concentration of i-th fraction, c=f(f.sub.R,S,T, etc)=sound speed, τ=duration of the transmitted signals, N.sub.i=number of particle per unit volume, a.sub.i=particle radius, χ.sub.i=normalized total scattering cross-section, f.sub.#,i=form function, ρ.sub.S=density of sediments, C.sub.1=function of sediment concentration, and C.sub.2=function of sediment concentration.

(45) The equation (3) takes into consideration transmission loss or attenuation of signal due to spreading, to fluid and to the suspended sediment.

(46) In particular, the term 20 log(ηR) represents a component of the transmission loss TL that is due to spreading. The term 2α.sub.wR represents another component of the transmission loss TL that is due to the fluid. The term

(47) $2\left[.Math.\left(C_1\frac{{\chi }_i{\mathrm{SSC}}_i}{a_i{\rho }_s}\right)\right]R$
represents a further component of the transmission loss TL that is due to the suspended sediments.

(48) The terms

(49) $10\log \left(\psi \frac{c\tau }{2}\right)+10\log .Math.\left(C_2{a}_i^2{f}_{\#}^2,{}_{i}N_i\right)$
represent the backscatter strength BS of the source level SL signals.

(50) The coefficients or parameters C.sub.1 and C.sub.2 are also defined as functions of the sediment concentration, instead of constant values of the sediment concentration, for improving accuracy of determining the echo levels EL over a wider range of the suspended sediment concentration (SSC) values.

(51) Referring to the memory unit 17, it serves to store a table of the determined echo levels EL with corresponding sediment parameters.

(52) The processor 15 acts to determine parameters of the suspended sediment according to the determined echo levels EL.

(53) Referring to the suspended sediment, the suspended sediment can be described with three sediment parameters, namely the suspended sediment concentration parameter together with the median parameter and with the slope parameter of the particle function of the suspended sediment. This is explained below.

(54) Different methods for determining the suspended sediment parameters are possible.

(55) FIG. 6 shows a flow chart 112 of a method of metering for measuring suspended sediment concentration (SSC) using multi-frequency acoustic backscattering (ABS).

(56) The flow chart 112 includes a step of transmitting three different signals of acoustic pulse to the suspended sediment of the fluid of interest, wherein the three signals have three corresponding frequencies. After this, backscattered signals, which are associated with the three different signals, are measured.

(57) FIG. 7 shows a flow chart 115 of a method of using the sediment meter 10 to determine the suspended sediment parameters is described below.

(58) The method includes a preparation step and an operating step.

(59) The preparation step comprises calculation of a plurality echo level EL values for pre-determined source level SL signals using the equation (3) for various pre-determined values of the suspended sediment parameters, in a step 117.

(60) These values are then stored in the memory unit 15 of the sediment meter 10, in a step 119.

(61) FIG. 8 shows a table 120 of stored data set of sediment parameters.

(62) The table 120 comprises several columns of data. The data includes a column of data of ranges of SSC values of a suspended sediment, a corresponding column of data of ranges of median values of particle function of the sediment, a corresponding column of data of ranges of slope values of the particle function of the sediment, a corresponding column of data of ranges of echo levels for a source signal with a frequency of 0.5 Mhz, a corresponding column of data of ranges of echo levels for a source signal with a frequency of 1.2 Mhz, a corresponding column of data of ranges of echo levels for a source signal with a frequency of 2.5 Mhz, and a corresponding column of data of ranges of echo levels for a source signal with a frequency of 5.0 Mhz.

(63) Referring to the operating step, it comprises sending three source level SL signals with three different corresponding frequencies of the sediment meter 10 to the suspended sediment, in step 121.

(64) The sediment meter 10 later measures three echo level EL values that are associated with the three source level SL signals, in a step 123.

(65) FIG. 9 shows a table 124 of ranges of the measured echo level EL values.

(66) 4

(67) Using the stored values in the memory unit 15, three parameters of the suspended sediment are then estimated, in a step 125.

(68) The three sediment parameters represent three unknown factors. The three source level SL signals with three different corresponding frequencies are then sufficient to obtain the values of the three sediment parameters.

(69) For measured echo levels with readings between the range of 146.7 to 147.2 (dB) for a source signal with a frequency of 0.5 Mhz, the Suspended Sediment Concentration (SSC) parameter of the suspended sediment is then estimated as between 1.0 and 1.1 (kg/m3) with a median size parameter of particle function of the sediment estimated as between 53 and 55 μm (micrometer) and a slope parameter of the particle function of the sediment estimated as between 0.0161 and 0.0162, in a step 125.

(70) The user often transmits more than three different signals with corresponding different frequencies, which can be four or five different frequencies, to ensure that the desired information is obtained from these signals. One of the frequencies of the acoustic signals may not generate backscattering signals when the said frequency does not generate backscattering signals for a particular particle size of the sediment. When this occurs, measurements of the backscattering signals will not contain any desired information.

(71) The above method for determining suspended sediment parameters can also include steps for detecting air bubbles in the suspended sediment. The presence of air bubbles can adversely change the measured or determined sediment parameter values.

(72) FIGS. 10 and 11 illustrate the evaluation of phases of backscattered signals of a suspended sediment of a method of detecting air bubbles, in a measurement bin of the fluid.

(73) The method includes a step of taking discrete measurements 130 of the backscattered signals, as shown in FIG. 10.

(74) The processor 15 then provides a Fourier transformation, such as Fast Fourier Transform (FFT), of the discrete measurements 130 in order to evaluate its phase spectrum. The phase values of the measurement frequencies in the phase spectrum have typical pre-determined range of values for fluids without air bubbles. Deviations from these phase values indicate the presence of air bubbles.

(75) FIG. 11 shows different phases of the backscattered signals of the suspended sediment, namely phases 140 of suspended sediment with no air bubbles and shifted phases 135 of suspended sediment with air bubbles.

(76) FIG. 12 shows another view of phases of the backscattered signal of the suspended sediment. FIG. 12 shows a solid line 145 of typical phases of backscattered signals from only suspended sediments, a dotted line 147 of shifted phases of backscattered signals from only air bubbles, and a dotted connected line 150 of shifted phases of backscattered signals from both suspended sediments and air bubbles.

(77) When the presence of air bubbles is detected, the user is warned or alerted about this.

(78) This allows the user to take appropriate actions when the air bubbles are detected. The air bubbles can adversely affect the determined suspended sediment parameter values.

(79) FIGS. 13 to 15 show different signal amplitude spectrums derived by calculating the Fourier Transform of the backscattered signals of a suspended sediment of another method of detecting the presence of air bubbles.

(80) The amplitude spectrums are then analyzed to identify occurrence of significant amplitude values at higher harmonic frequencies in the amplitude spectrum, wherein the said occurrence indicates presence of air bubbles in the fluid. The significant amplitude values are bigger than a pre-determined threshold value, which is stored in the memory unit 15 of the sediment meter 10.

(81) FIG. 13 shows a signal amplitude spectrum 160 of the backscattered signal of the suspended sediment. The signal is backscattered from only suspended sediments of a fluid. The backscattered signal is associated with an acoustic source signal AS with an operating frequency of 120 kHz. The amplitude spectrum 160 shows one main frequency of 120 kHz with essentially no higher harmonic frequencies.

(82) FIG. 14 shows a signal amplitude spectrum 162 of the backscattered signal. The signal is backscattered from only air bubbles of a fluid. The backscattered signal is associated with an acoustic signal AS with an operational frequency of 120 k kHz. The amplitude spectrum 162 shows a main frequency of 120 kHz and higher harmonic frequencies that includes a higher harmonic frequency of 240 kHz and a higher harmonic frequency of 360 kHz.

(83) FIG. 15 shows a signal amplitude frequency spectrum 165 of a backscattered signal. The signal is backscattered from suspended sediments and from air bubbles of a fluid. The backscattered signal is associated with an acoustic signal AS with an operating frequency of 120 kHz. The amplitude spectrum 165 shows a main frequency of 120 kHz and higher harmonic frequencies that includes a higher harmonic frequency of 240 kHz and a higher harmonic frequency of 360 kHz.

(84) This detection of air bubbles is then used to warn the user that the measured sediment concentration can be incorrect due to the presence of air bubbles.

(85) This step allows the user to take appropriate steps regarding the air bubbles. The air bubbles can adversely affect the determined suspended sediment parameter values.

(86) In a general sense, the step of detecting the air bubbles can be done in parallel or at the same time as the step of determining the concentration of the suspended sediment. The step of detecting the air bubbles can be also done before or after the step of determining the concentration of the suspended sediment.

(87) The embodiments can also be described with the following lists of features or elements being organized into items. The respective combinations of features, which are disclosed in the item list, are regarded as independent subject matter, respectively, that can also be combined with other features of the application. 1. A device for measuring at least one parameter value of a suspended sediment of a fluid, the device comprising a backscattering transducer module that comprises a source module for transmitting at least three acoustic signals with different fixed characterising measurement frequencies and a receiver module for measuring at least three echo level values of echo signals, which correspond with the at least three acoustic signals, a storage unit that stores a data set of pre-determined echo level values with a data set of pre-determined suspended sediment parameter values, a calculation unit for deriving at least one suspended sediment parameter value according to the data set of pre-determined echo level values with the data set of pre-determined suspended sediment parameter values and according to the at least three echo level values 2. The device according to item 1, wherein the data set of pre-determined suspended sediment parameter values comprises at least one concentration suspended sediment parameter value. 3. The device according to item 1 or 2, wherein the data set of pre-determined suspended sediment parameter values comprises at least one median parameter value of a particle function of the suspended sediment. 4. The device according to one of the above-mentioned items, wherein the data set of pre-determined suspended sediment parameter values comprises at least one slope parameter value of the particle function of the suspended sediment. 5. The device according to one of the above-mentioned items, wherein the calculation unit comprises a module for detecting a presence of air bubbles in the signal path of the acoustic signals by deriving at least one parameter value of the echo signal. 6. The device according to item 5, wherein the parameter value of the echo signal comprises a phase parameter value of the echo signal. 7. The device according to item 5, wherein the parameter value of the echo signal comprises an amplitude parameter value of the spectrum of the echo signal. 8. A method of determining at least one parameter of a suspended sediment of a fluid, the method comprising providing a data set of pre-determined echo level values with a data set of pre-determined suspended sediment parameter values, directing at least three acoustic signals with different fixed characterising measurement frequencies at the suspended sediment of the fluid, measuring at least three echo levels of echo signals, which correspond with the at least three acoustic signals, deriving at least one parameter value of the suspended sediment according to the data set of pre-determined echo level values with the data set of pre-determined suspended sediment parameter values and according to the measurements of the at least three echo levels. 9. The method according to item 8, wherein the data set of pre-determined suspended sediment parameter values comprises a pre-determined concentration suspended sediment parameter value. 10. The method according to item 8 or 9, wherein the data set of pre-determined suspended sediment parameter values comprises a pre-determined median parameter value of a particle function of the suspended sediment. 11. The method according to one of items 8 to 10, wherein the data set of pre-determined suspended sediment parameter values comprises a pre-determined slope parameter value of the particle function of the suspended sediment. 12. The method according to one of items 8 to 11 further comprising detecting a presence of air bubbles in the signal path of the acoustic signals. 13. The method according to item 12, wherein the detecting of the presence of air bubbles comprises deriving at least one phase parameter value of the echo signal. 14. The method according to item 12, wherein the detecting of the presence of air bubbles comprises deriving at least one amplitude value of the spectrum of the echo signal. 15. The method according to one of items 8 to 14, wherein the deriving of the at least one suspended sediment parameter takes in account effects of attenuation due to the fluid. 16. The method according to one of one of items 8 to 15, wherein the deriving of the at least one suspended sediment parameter takes in account effects of attenuation due to particle sizes of the suspended sediment. 17. The method according to one of the items 8 to 16, wherein the deriving of the at least one suspended sediment parameter takes in account effects of concentration of the suspended sediment.

(88) Although the above description contains much specificity, this should not be construed as limiting the scope of the embodiments but merely providing illustration of the foreseeable embodiments. The above stated advantages of the embodiments should not be construed especially as limiting the scope of the embodiments but merely to explain possible achievements if the described embodiments are put into practice. Thus, the scope of the embodiments should be determined by the claims and their equivalents, rather than by the examples given.

REFERENCE NUMBERS

(89) 10 suspended sediment meter 11 sonar transducer 15 processor 17 memory unit 18 air bubble detector 20 transducer head 41 transducer 42 transducer 43 transducer 44 transducer 50 near field 51 receiving sensitivity graph 52 transmitting sensitivity graph 54 far field 55 measurement bin 112 flow chart 115 flow chart 117 step 119 step 120 table 121 step 123 step 124 table 125 step 130 discrete measurement 135 phase 140 phase 145 line 147 line 150 line 160 frequency spectrum 162 frequency spectrum 165 frequency spectrum