The invention relates to a device for determining the (mean) velocity in a flowing fluid, of the type including an ultrasound transmitter and a receiver coupled therewith for receiving ultrasounds reflected from the fluid, which are amplified and demodulated so as to produce a low-frequenc Doppier-signal, that is subsequently analised by a spectrumanalyser, con¬ sisting of a number (n) of bandpass filters, peakdetectors being provided to determine the energy values of the frequencycomponents obtained by said bandpassfilters, and a calculation unit being used to calculate that frequency, at which the low-frequency Doppler-signal appears to have a maximum, the latter being a measure for the (mean) flowing velocity.

A device of this type is known from the DE-PS 1,791,191 in which the application for determining the flow rate of blood in blood vessels is decribed. The velocity values obtained by this well-known device device appear to deviate rather substantially from the values obtained by means of an other weli-known type of meter, the electromagnetic flowmeter, which is generally considered as an accurate one.

The measuring device, operating according to the Doppler principl however, has the practical advantage over other types of flowmeters, that it may also be used in circumstances in which other types of measuring devices would not be useful or less appropriate. For as a matter of fact this meter can be easily put in place, is not subject to wastage and is relatively inexpensive.

It is therefore an object of this invention to improve the device of the type above referred to and more particularly to improve its easu- ring accuracy.

Tests run in flowing fluids of different composition, density and volume rate, have led to the insight that the interference of the Doppler-spectrum caused by background noise, is taking place in a rather fixed distribution over the individuai frequency bands. It has been found that the successively measured frequency components may be approximately corrected to the actual Doppler-spectrum with the aid of a simple correcti curve, which is substantially determined by the intensities, measured by the bandpass filters at the lower and upper limits of the spectrum res¬ pectively. Starting from this insight, the device according to the invention is characterized in that the calculation unit includes components, which a

designed to cause the energy values of the individual frequency components of the frequency spectrum to decrease according to a monotonously decreasin correction curve.

According to a further feature of the invention, the calculation unit is adapted to operate according to a correction curve, which connects a location representing a value between 65% and 85% of the energy value measured by the lowest bandpassfilter with a location representing a value between 35% and 45% of the energy value measured by the highest bandpassfil ter. The invention will hereinafter further explained, by way of exampl with reference to the accompanying drawings wherein:

Fig. 1 is a block diagram showing the device according to the - invention, fig. 2 shows a Doppler-energy spectrum of a medium and fig. 3 shows the calculation unit of the spectrum analyser block of fig. 1.

The device for determining the flow rate of a fluid flowing throug a pipe, as diagrammatically shown in fig.l, comprises a transmitter 3 place on the pipewall 2, which ^' transmits a beam of sound waves 4. The beam of sound waves 5 reflected from the flowing fluid is received by a receiver 6 also placed on the pipewall 2. The Doppler-effect causes a frequency shift between the transmitted and received beams of sound waves. This frequency shift is expressed in the energy spectrum of the output signal of receiver Upon amplification in block 7 and demodulation in block 8 the low-frequency Doppler-signal is obtained from this signal. An example of the energy spectrum of a low-frequency Doppler-signal is indicated by the line 15 in f

Fig. 2 also shows two further curves 16 and 17. The curve 16 represents the noise in the total energy spectrum 15 and curve 17 represent the corrected energy spectrum. The Doppler-signal 9 is applied to a spectrum analyser in block 10, which comprises a series of n bandpassfilters, which cover the total spectrum and the bandwidths of which are preferably equal. The output of each of the n bandpassfilters are connected to a peak detector, which deter mines the energy intensity of the respective frequencyband (see the measuring points in fig. 2) . The output signals of the n peak detectors are supplied to a calculation unit, making part of block 10. So far the measuring device corresponds to the prior art.

Unlike the prior art, where the mean frequency as a measure of the

flow rate is immediately determined from the n peak detector signals by th following equation:

mean m m

in which I is the intensity measured by bandpassfilter m and f is the 5 filter frequency, the calculation unit of the device according to the invention is arranged in such a way, that first n modified or corrected intensity values are determined from the individual peak detector signals. These modified or corrected values are represented by the curve indicated by the broken line 17 in tig. 2, which is obtained by subtracting the lϋ intensity values of curve 16 from the respective intensity values o± curve 15. An example of the calculation unit in the device according to the invention is shown in fig. 3.

In fig. 3 the n output signals of the n peak detectors are supplied to a multiplexer 18. The multiplexer 18 successively connects its

15 n inputs with its output. The n intensity signals I. - I , which successiv ly apjpear at the output ot the multiplexer 18, are converted into a digita value by means of an analog-digital converter 19. The n digital intensity values are stored in n operating registers 20-1 to 20-n. The monotonously decreasing curve 16 in fig. 2 may, as proven by a large number of test

Z0 measurements, be simply approximated by a straight line. The latter is determined by the intensity values at the limits of the energy spectrum 15 i.e. the intensities determined by the first and n-th peak detectors. The noise contribution to the total energy intensity is between 65% and 85% at the lowest frequency (f.) and between 35% and 40% at the highest fre-

25 quency. The intensity values I ₁ and I from the registers 20-1 and 20-n are multiplied by a factor a, (0,65 - 0,85) and a (0,35 - 0,45)

I n respectively in the multiplier devices 21 and 22 respectively The outputs of the multiplier devices 21 and 22 are connected with a device 23, which performs the following equation:

The intensity values of the noise contribution curve 16 in fig. 2 may be determined, by approximation, from the output signals of the multiplier devices 21 and 22 and device 23 according to the following equation:

in which I is the noise intensity at frequency f and m has a value between

1 and n.

The noise contribution intensities (correction curve) I ' to I ' are stored in a correction table 24. The n values from the correction table 24 are supplied to the inverting input of an adding device 25. The intensity values I. to I from the n registers 20-1 to 20-n are supplied to the non-inverting input of the adding-device 25. The noise contribution intensities I, ¹ to I ' are subtracted from the respective total intensities i n

I, to I by means of device 25, so that a table 26 of corrected intensi- 1 n ties II," to In" is obtained. The corrected intensities II" to In" constitute curve 17 in fig ^β . 2. Thereafter the mean freq-iuency. f _mean " is determined from this table by means of device 27, including adders, multipliers and dividers, in a well-known manner, e.g. the manner described in the aforement German Patent Specification, according to the following equation:

Finally, this mean frequency f " is converted, in block 11 (fig. 1), into a value indicative of the flow rate by means of a conversion factor, determined by the transmitting frequency and the sensor geometry.

The functions and processes described hereinabove with reference to the diagram of fig. 3 can be easily accomplished in a simple processor specifically designed for this purpose.