AT527574A4 - Structure and method for determining the speed of sound in a gas - Google Patents

Abstract

A setup (1) for determining the speed of sound in a gas comprises a sensor unit (3) for generating an acoustic output signal and for detecting an acoustic measurement signal. Furthermore, the setup (1) comprises a calibration element (6) with a permeable length (L) extending between an inlet (7) and an outlet (8), which is formed at least in sections by a pipe (9). The inlet (7) is acoustically conductively connected to the sensor unit (3).

Description

for detecting an acoustic measurement signal.

Furthermore, the invention relates to a method for determining the

Speed of sound in a gas.

To monitor the performance of oil pumps in oil wells, it is common practice to determine the fluid level in the well at regular intervals. Based on the position of the fluid level, it is possible to calculate whether and how high the fluid column is above the lower end of the pipe. This information helps prevent the oil pump from running dry. Other parameters can also be derived from monitoring the fluid level in the well over a certain period of time, such as the reservoir

Inflow capacity and the skin factor.

To measure the fluid level in the borehole, a pressure pulse (i.e., an acoustic signal) is typically introduced into the borehole, and the time it takes for a reflection of the acoustic signal (an acoustic echo) to return to the starting point is measured. Since the production string does not completely fill the borehole, a gas-filled circular gap or annulus is formed around the production string, across which the acoustic

Signal can propagate into the depths and back.

If the speed of sound in the gas filling the gap is known, the height of the liquid level can be determined from the measured time difference between the emission and reception of the reflection of the acoustic signal as follows:

become:

where D is the distance to the liquid level measured from the source of the acoustic signal, v is the speed of sound in the gas and t,g is the measured time difference. (The distance traveled by the signal is multiplied by % since the acoustic signal travels both towards the liquid level and from

fluid level must travel back.)

The gas contained in the gap between the production string and the borehole can have a different composition depending on the location of the borehole and the type of reservoir being developed (e.g., oil reservoir), so the speed of sound in the gas (or the speed of sound of the gas) is not known in advance, but varies between boreholes and must be determined on-site. This is usually done

by means of “pipe socket counting”.

In "pipe socket counting," a pressure pulse (i.e., an acoustic signal) is introduced into the annular gap between the production string and the borehole. The pressure pulse travels downwards toward the oil reservoir, passing through the joints (the socket-spigot connections) between the tubular sections of the production string. At these joints, the production string exhibits a widening of the outer diameter due to the expanded socket of one section, into which the spigot of the subsequent section is inserted. This widens part of the pressure pulse and thus redirects it toward the source of the pressure pulse. A measuring unit (e.g., a piezo sensor, a loudspeaker membrane, etc.) records acoustic signals emerging from the borehole, and therefore also the reflections that occur at the individual widenings of the production string. The determined time difference between the

Sending the pressure pulse and detecting the reflections

gas surrounding the production string.

The disadvantage of “joint counting” is that this method is very error-prone, since deformed joints between the sections of the production string, but also material in the gap and the walls of the borehole, can produce misleading, disturbing or unclassifiable reflections, which often leads to an inaccurate

Determination of the speed of sound.

From CH 653 441 A5, a device for determining the speed of sound propagation in a changing medium is known. The device comprises a measuring tube with an end opening, wherein a loudspeaker is placed at a distance from the end opening and a sound sensor is arranged on the casing of the measuring tube. A disadvantage of such a device is that, due to the design of the measuring tube, it can only be used in a gas-tight container or room if the device is to be used to measure the speed of sound propagation in an explosion-proof manner in an ignitable or explosive gas. Furthermore, the device according to CH 653 441 A5 is complicated, imprecise and

susceptible to failure.

The invention is based on the object of providing a structure and a method for determining the speed of sound in a gas

which addresses the problems raised in such a way

can be determined.

This object is achieved according to the invention with a structure having the features of claim 1 and with a method,

which has the features of claim 10.

Preferred and advantageous embodiments of the invention are

Subject matter of the subclaims.

According to the invention, the structure comprises a calibration element with a predefined, permeable length extending between an inlet and an outlet, which is formed at least in sections by a pipe, and that the inlet is acoustically conductively connected to the

sensor unit is connected.

The inventive setup can be used, for example, to determine the speed of sound in a gas located in an annular gap or annular space around the production string in an oil well. However, the setup can also be used in other technical fields, in principle wherever the speed of sound of a gas needs to be determined, for example in gas wells or in

geothermal power plants (for depth measurement).

Within the scope of the invention, the “predefined, permeable length” of the calibration element is defined as an elongated internal volume or a space that is essentially closed to the outside and has a predefined longitudinal extent and, in particular, is only enclosed at its longitudinal ends via the inlet and the

Output of the calibration element is accessible.

gas-permeable length.

In the context of the invention, "acoustically conductive" means that the calibration element is connected to the sensor unit in such a way that an acoustic signal from the sensor unit can be introduced through the inlet of the calibration element into the gas contained in the calibration element, and that an acoustic signal from the gas contained in the calibration element can be conducted back to the sensor unit. The sensor unit does not have to be arranged directly at the inlet of the calibration element, but can also be indirectly connected to the inlet in an acoustically conductive manner, e.g., via at least one chamber and/or at least one line.

stand.

Within the scope of the invention, a pipeline is understood to mean in particular a substantially rigid pipe, but also a flexible

pipe or hose.

The invention also relates to a method for determining the speed of sound in a gas. For the method, a structure is used which comprises a sensor unit for generating an acoustic output signal and for detecting an acoustic measurement signal and furthermore a calibration element, wherein the calibration element has a permeable length extending between an inlet and an outlet, which is formed at least in sections by a pipe, and wherein the inlet is acoustically conductively connected to the sensor unit

is connected.

following steps:

First, in step a. the calibration element is flooded (=filled) with the gas over its entire flowable length, i.e. from the inlet via the pipe to the

Exit.

Then, in step b, a time-limited acoustic output signal is generated with the sensor unit and transmitted via the

Inlet into the gas-filled calibration element.

The sensor unit is capable of detecting, in step c, an acoustic measurement signal emerging from the input directly after generating and transmitting the acoustic output signal (or even during this process). For this purpose, the sensor unit can be switched from a transmit mode to a receive mode or can be configured to generate acoustic signals in parallel and simultaneously receive acoustic signals. The detected acoustic measurement signal is, in particular,

recorded or at least temporarily stored.

In a step d., a control unit connected to the (inventive) structure determines a signal change in the acoustic measurement signal that is generated or occurs due to a reflection of the acoustic output signal at the output of the calibration element. This signal change is an echo of the acoustic output signal that is generated in the area of the output of the calibration element.

is thrown back.

In practice, this means that the sensor unit immediately after sending the output signal sends the acoustic measurement signal

which initially shows an inconspicuous (i.e. consistently

7732

Control unit is/are registered as a signal change.

In a step e., the control unit uses calculation logic to determine a time difference between the introduction of the acoustic output signal into the calibration element and the detection of the signal change, i.e., the time interval until the signal change occurs. The introduction of the acoustic signal occurs (almost) simultaneously with the generation of the acoustic signal (i.e., the acoustic signal is preferably generated directly in front of the calibration box).

generated).

Subsequently, in a step f, the speed of sound in the gas is calculated using the calculation logic based on the determined time difference and the known flowable length of the calibration element. For this purpose, the

The following calculation formula is used:

v= (2+*L)/T

where v is the speed of sound in the gas, L is the known traversable length of the calibration element (which is traveled from the acoustic signal to the output and back again) and T is the determined time difference between the initiation (in particular generation) of the acoustic

output signal and detecting the signal change.

partly consists of methane).

To flood the flowable length of the calibration element, the structure can be connected to the borehole or another device carrying the gas, whereby the gas either flows into the calibration element independently (due to an existing overpressure or gas flow) and floods it or by means of a

device is pumped into the calibration element.

The sensor unit preferably comprises a pressure box with an integrated loudspeaker that can both generate and receive acoustic signals (i.e., generate acoustic signals from electronic signals and vice versa). Alternatively, the sensor unit can also comprise a device for generating acoustic signals and another, separate device for receiving acoustic signals.

have.

It is particularly preferred if the pipeline is wound in the calibration element in several superimposed and/or adjacent turns. This means, in particular, that the pipeline is fixed in the wound form. Due to the wound form of the pipeline, it takes up only a small amount of space, while at the same time the flowable length of the calibration element is sufficiently long to enable a precise determination of the speed of sound. The structure according to the invention can therefore be used as a whole

be quite compact and small.

components are required, resulting in lower costs.

Within the scope of the invention, it can be provided that a closure valve (i.e., a proximal closure valve) is arranged at the proximal pipe end of the pipeline. Such a closure valve can prevent continuous flow through the calibration element and therefore a continuous gas escape from the assembly. Preferably, the assembly is first connected to a gas-carrying device or facility (e.g., the borehole) and only shortly before the determination of the sound velocity in the gas is the

Shut-off valve at the proximal end of the tube opened.

In one possible embodiment, the pipeline has a distal pipe end in the region of the outlet. The outlet can be formed directly by the distal pipe end. Alternatively, a particularly tubular ventilation element connected to the distal pipe end with one end can form the outlet with its other, unconnected end. The ventilation element serves, in particular, to direct the flow of the fluid from the calibration element when flowing through the calibration element.

the gas exiting the outlet from a device using the structure

person away or release the gas from the calibration box

to lead out.

Within the scope of the invention, it can be provided that a closure valve (i.e., a distal closure valve) is arranged at the distal end of the pipeline. With such a configuration, an acoustic signal passing through the flowable length of the calibration element can be reflected by the closed closure valve. The closure valve at the

distal end of the tube can also be a pressure relief valve.

In versions with a proximal and a distal closure valve, these can be closed when the calibration box is removed from the drill string to prevent the gas contained in the permeable length from escaping or to allow it to be released or flushed out in a secure environment or outdoors.

can be.

Particularly preferably, at least part of the pipeline is arranged in a housing. In embodiments in which the inlet is formed on the connecting element and/or the outlet on the ventilation element, the entire pipeline (optionally with the closure valve arranged on the proximal pipe end and/or the distal pipe end) can be arranged within the housing. Such a housing protects the calibration element and enables easy transport thereof. In such embodiments, it is particularly preferred if the pipeline - as already described - is wound up in windings and secured in the housing against

unwinding is fixed.

Possible, although not preferred, are embodiments in which a section of the pipeline in the area of the inlet and/or a section of the pipeline in the area of the outlet from the

In such embodiments,

a connecting element and/or a ventilation element is omitted

become.

If the pipeline has at least one closure valve as described above, the housing can in particular be openable and closable (e.g. via a cover) in order to access the

to have valve(s) arranged in the housing.

Within the scope of the invention, the pipeline preferably has a length of at least 0.5 m, preferably at least 2 m, in particular at least 5 m. Likewise, the pipeline preferably has a length of at most 100 m, in particular at most 70 m. The pipeline is particularly preferably approximately 50 m long. Such a long pipeline enables the determination of the speed of sound in the gas with sufficiently high precision, without the structure according to the invention becoming unwieldy or cumbersome.

becomes too big/heavy.

Particularly preferably, the pipeline has an inner diameter of a maximum of 20 mm, preferably a maximum of 15 mm, in particular a maximum of 10 mm. Such an inner diameter is sufficient for the most precise determination of the speed of sound and, at the same time, enables a space- and weight-saving design of the structure. Within the scope of the invention, embodiments with a pipeline with a larger inner diameter are also conceivable, for example, a maximum of 30 mm or a maximum of 40 mm; however, such embodiments are

quite large or heavy.

The structure according to the invention for determining the speed of sound in a gas is particularly suitable or intended to be used in the method according to the invention or to implement the method according to the invention

It is understood that features that are only

Structure are described, also in an adequate manner in the process

can be realized and vice versa.

In the method according to the invention, the reflection causing a signal change can be the acoustic output signal encountering and being reflected back from a closed outlet of the calibration element. This is the case, for example, if the pipeline of the calibration element is equipped with a closure valve at its distal end, which is closed after the calibration element has been completely flooded and before the

acoustic output signal is closed.

However, the reflection causing a signal change can also be a pressure wave formed due to a reflection at the open end of the tube. In this variant, the gas can be continuously released, i.e., even during the determination of the

speed of sound flowing through the calibration element.

In the method according to the invention, the acoustic output signal preferably has a predefined duration, beginning at a first frequency and ending at a second frequency. The second frequency can be different from the first frequency, in particular higher (changing frequency band), or equal to the first frequency (constant frequency band). Passing through a frequency spectrum when generating the acoustic output signal produces a particularly well-measurable reflection or a particularly clearly detectable signal change, since this ensures that the output signal reaches the output at the (previously unknown) frequency at which the strongest reflection (in particular due to reflection at the closed output or

by exiting the open outlet).

It is preferred if, in the method according to the invention, the predefined duration of the acoustic output signal is a maximum of 250 milliseconds, preferably a maximum of 200 milliseconds, in particular a maximum of 150 milliseconds. The duration can also be less than 150 milliseconds, e.g., between 50 and 100 milliseconds (but also less than 50 milliseconds). In particular, the predefined duration is shorter than the time required for the acoustic signal to travel along the permeable length to the outlet and back again, and is therefore dependent on the length of the permeable length or the length of the pipeline. This is intended to ensure that the generation of the acoustic output signal ends before the reflection of the beginning of the output signal reaches the input of the

calibration element is reached.

The first frequency is preferably at least 1 Hertz (Hz), preferably at least 3 Hertz, in particular approximately 5 Hertz, and the second frequency is preferably at most 200 Hertz, preferably at most 100 Hertz, in particular approximately 50 Hertz. An acoustic output signal having such a frequency spectrum is highly likely to generate a particularly strong reflection, which leads to a significant signal change.

leads.

If the frequency or frequency band of the acoustic output signal is constant, the output signal has a frequency between 1 and 100 Hertz, preferably between 5 and 50 Hertz, in particular between 10 and 30 Hertz

(for example, approx. 20 Hertz).

In particular, the acoustic output signal in the sensor unit is generated from an electronic output signal (an audio signal) and the recorded acoustic measurement signal in

the sensor unit into an electronic measuring signal.

The electronic output signal can be generated, for example, by an analogue output controlled by the control unit (and in particular amplified by an amplifier arranged in front of the sensor unit) and the electronic measurement signal can be recorded at an analogue input monitored by the control unit

become.

In particular, the analogue output and input can be connected to the sensor unit via a switching relay, wherein the switching relay can be switched back and forth between a connection of the analogue output to the sensor unit and a connection of the analogue input to the sensor unit by a digital output signal also generated by the control unit. An amplifier for amplifying the electronic

output signal (i.e. the analog audio signal).

The calculation logic is particularly in a computing unit of the

Control unit implemented.

Furthermore, the control unit can be connected to a temperature sensor for measuring the temperature of the gas for which the speed of sound is to be determined.

have such a temperature sensor.

Further details, features and advantages of the invention will become apparent from the following description with reference to the attached drawings, in which preferred

Embodiments are shown. It shows:

Fig. 1 a highly simplified conceptual view of a

Embodiment of the structure according to the invention in

Connection to a control unit,

Fig. 2 is a highly simplified plan view of a calibration element of the inventive structure, and

Fig. 3 is a flow chart of a method according to the invention.

Fig. 1 shows the structure 1 according to the invention in a highly simplified conceptual view, wherein the structure 1 according to the invention is connected to a control unit 2. Both the structure 1 according to the invention and the control unit 2 are indicated in Fig. 1 by dashed lines as

Functional group marked.

The structure 1 according to the invention has a sensor unit 3, which in the illustrated embodiment consists of a pressure box 4 with an integrated loudspeaker 5. The loudspeaker 5 can

both generate and receive acoustic signals.

A calibration element 6 is connected to the sensor unit 3 and has an inlet 7 and an outlet 8. Between the inlet 7 and the outlet 8, the calibration element 6 has a permeable length L, which, in the operating state of the structure 1 according to the invention, is filled with a gas

is flooded.

The floodable length L is at least partially

a pipeline 9.

In Fig. 1, the flowable length L is formed entirely by the pipe 9, with the inlet 7 of the calibration element 6 being connected to a proximal pipe end 11 connected to the sensor unit 3 and the outlet 8 of the calibration element 6 being connected to a freely projecting distal

Pipe end 12 is formed (or is formed by it).

The input 7 of the calibration element 6 is acoustically conductive

connected to the sensor unit 3, i.e. that an acoustic

Signal from the sensor unit 3 can be introduced into the permeable length L of the calibration element 3. The acoustic signal can thus propagate or move along the permeable length L through the gas absorbed in the calibration element 6. Likewise, an acoustic signal propagating from the output 8 to the input 7 through the gas absorbed in the permeable length L from the calibration element

6 can be fed back into the sensor unit 3.

The control unit 2 connected to the structure 1 according to the invention and required for the method according to the invention has, in the embodiment shown, an analog output 13 and an analog input 14, each for generating and receiving

an electrical signal.

The analog output 13 is connected to a relay switch 16 via an amplifier 15 to amplify the analog audio signal (output signal). The analog input 14 is also connected to the relay switch 16, as is a digital output 17 of the control unit 2. Via a digital signal generated by the digital output 17, the relay switch 16 can be switched between a first conductive connection of the analog output 13 with the sensor unit 3 and a second conductive connection of the analog input 14 with the

Sensor unit 3 can be switched back and forth.

In the illustrated embodiment, the control unit 2 further comprises a temperature sensor 18 for detecting the temperature of the gas contained in the flow-through length L, so that the temperature at which the gas exhibits the speed of sound determined by the structure or method can be determined and recorded. The measurement can

but can also be carried out without a temperature sensor 18.

The control unit 2 further comprises a calculation logic, implemented in particular in a computing unit 19, for calculating

the speed of sound of the gas or in the gas.

Fig. 3 shows a flow chart of the sequence of a method according to the invention with the method steps a. to

f., which take place one after the other in chronological order.

To determine the speed of sound in the gas, the gas is first introduced into the calibration element 6 (via its inlet 7) in a step a. until the calibration element 6 is flooded with the gas over its entire permeable length L

is.

Subsequently, an analogue electrical output signal is generated by the control unit 2, amplified by the amplifier 15 and transmitted via the relay switch 16 (which provides a conductive connection between the analogue output 13 and the

Sensor unit 3) is introduced to the sensor unit 3.

In the sensor unit 3, in a step b, an acoustic output signal is generated from the electrical output signal, which has a predefined duration and which, for example, increases over this duration from a first frequency to a second frequency. The acoustic output signal is introduced into the calibration element 6 during its predefined duration and propagates through the flow-through length L towards the output 8 of the

Calibration element 6 continues.

Immediately after the acoustic output signal has been generated (i.e. after the predefined time period has elapsed), the relay switch 16 (which may have one or more relays) is activated by a digital signal from the digital output 17

switched so that a conductive connection is established between the

analog input 14 and the sensor unit 3 is established. From this point on (or even during the generation of the acoustic output signal), the sensor unit 3 detects in a step c. a signal coming from the input 7 of the

Acoustic measuring signal emerging from calibration element 6.

A reflection occurs at the open outlet 8 of the calibration element 6 - which in the embodiment shown in Fig. 1 is the distal tube end 12 or is formed at the distal tube end 12. A pressure wave formed by this reflection at the open tube end and traveling in the opposite direction, i.e., from the outlet 8 to the inlet 7 of the calibration element 6, which is a reflection or echo of the acoustic output signal

can be seen, propagates towards entrance 7.

The reflection of the acoustic output signal generates a signal change (i.e., an amplitude increase) in the acoustic measurement signal detected by the sensor unit 3 and also in the electronic measurement signal forwarded to the control unit 2. The signal change generated by the reflection of the acoustic output signal is

d. determined with control unit 2.

The calculation logic determines in a step e. a time difference between the initiation (in particular generation) of the acoustic output signal and the occurrence of the signal change and calculates in a step £f. based on this time difference and the known distance between the input 7 and the output 8 of the calibration element 6, i.e. the traversable length L, the speed of sound in

Gas.

Fig. 2 shows a preferred embodiment of the structure 1 used in the invention.

Calibration element 6.

In this embodiment, the calibration element 6 is arranged in a housing 21, which can be opened and closed in particular by means of a cover or slide 6 (not shown).

is.

The pipe 9 is wound in several turns lying above and/or next to each other and is fixed with fastenings 22 in the

Housing 21 fixed.

At the proximal pipe end 11, i.e. that end of the pipeline 9 which, when the calibration element 6 is connected to the sensor unit 3 (seen in the flow direction), is arranged closer to the sensor unit 3 and in the area of the inlet 7 of the calibration element 6, a proximal

Shut-off valve 23 is arranged.

A tubular connecting element 24 passed through an opening in the housing 21 connects the proximal closure valve 23 to the sensor unit 3, so that an end of the connecting element 24 connected to the sensor unit 3

forms the input 7 of the calibration element 6.

A pressure sensor is also provided on the distal pipe end 12 of the pipe 9, i.e. on that end of the pipe 9 which, when the calibration element 6 is connected to the sensor unit 3 (seen in the direction of flow), is arranged further away from the sensor unit 3 and in the area of the outlet 8 of the calibration element 6.

distal closure valve 25 is arranged.

A tubular ventilation element 26 is connected to the distal closure valve 25, which is led out through a further opening in the housing and whose end not connected to the distal closure valve 25 forms the outlet 8 of the

Calibration element 6 forms.

The flowable length L of the calibration element 6 in the embodiment according to Fig. 2 is thus determined by a length of the connecting element 25, a length of the pipe 9 and a

Length of the ventilation element 26.

List of reference symbols:

1 Structure

2 control unit

3 Sensor unit

4 print box

5 speakers

6 Calibration element 7 Input

8 Exit

9 Pipeline

10 .--

11 proximal tube end

12 distal tube end

13 analog output

14 analog input

15 amplifiers

16 relay switches

17 digital output

18 Temperature sensor

19 Computing unit

20 .--

21 housings

22 Fastening

23 proximal closure valve 24 connecting element

25 distal closure valve

26 Ventilation element

L Floodable length

Claims (1)

Patent claims: Structure (1) for determining the speed of sound in a gas, wherein the structure (1) has a sensor unit (3) for generating an acoustic output signal and for detecting an acoustic measurement signal, characterized in that the structure (1) has a calibration element (6) with a predefined flowable length (L) running between an inlet (7) and an outlet (8), which is formed at least in sections by a pipe (9), and that the inlet (7) is acoustically conductively connected to the sensor unit (3) is connected. Structure according to claim 1, characterized in that the pipeline (9) in the calibration element (6) is arranged in several superimposed and/or adjacent windings. Structure according to claim 1 or 2, characterized in that the pipeline (9) has a proximal pipe end (11) in the region of the inlet (7), and that the proximal pipe end (11) forms the inlet (7), or that a particularly tubular connecting element (24) is connected to one end of the proximal pipe end (11), the other end of which forms the entrance (7). Structure according to claim 3, characterized in that at the proximal pipe end (11) of the pipeline (9) a proximal Shut-off valve (23) is arranged. Structure according to one of claims 1 to 4, characterized in that the pipeline (9) has a distal pipe end (12) in the region of the outlet (8), and that the distal tube end (12) forms the outlet (8), or 22 that a particularly tubular ventilation element (26) is connected to the distal tube end (12) with one end, the other end of which forms the outlet (8) forms. Structure according to claim 5, characterized in that at the distal pipe end (12) of the pipeline (9) a distal Shut-off valve (25) is arranged. Structure according to one of claims 1 to 6, characterized in that the pipeline (9) is in a housing (21) is arranged. Structure according to one of claims 1 to 7, characterized in that the pipeline (9) has a length of at least 5 m and/or a length of at most 60 m has. Structure according to one of claims 1 to 8, characterized in that the pipe (9) has an internal diameter of maximum 20 mm, preferably maximum 15 mm, in particular a maximum of 10 mm. Method for determining the speed of sound in a gas, wherein a structure (1) is used for the method, which structure has a sensor unit (3) for generating an acoustic output signal and for detecting an acoustic measurement signal and furthermore a calibration element (6), wherein the calibration element (6) has a predefined permeable length (L) running between an inlet (7) and an outlet (8), which is formed at least in sections by a pipe (9), and wherein the inlet (7) is acoustically conductively connected to the sensor unit (3), characterized by the sequential steps: 12. 23 a. Flowing the calibration element (6) over its entire flowable length (L) with the gas, b. generating a time-limited acoustic output signal with the sensor unit (3) and introducing the acoustic signal via the input (7) into the gas-filled calibration element (6), c. detecting an acoustic measurement signal emerging from the input (7) immediately after the acoustic output signal has been emitted, d. Determining a signal change in the acoustic measurement signal generated by a reflection of the acoustic output signal at the output (8) of the calibration element (6) with a control unit (2), e. Determining a time difference between the introduction of the acoustic output signal into the calibration element (6) and the occurrence of the signal change with a calculation logic of the control unit (2), and f. Calculate the speed of sound in the gas using the calculation logic based on the determined time difference and the known predefined flowable length (L) of the calibration element (6). Method according to claim 10, characterized in that for carrying out the method a structure (1) according to one of claims 1 to 9. Method according to claim 10 or 11, characterized in that the reflection is the signal impinging on a closed outlet (8) of the calibration element (6) and reflected acoustic output signal or that the reflection is a pressure wave that is due to a reflection is formed at the open output (8). 14. 15. 24 Method according to one of claims 10 to 12, characterized characterized in that the acoustic output signal is a has a predefined duration, first frequency begins and at a, first frequency different, second where it is preferably from the Frequency ends. Method according to claim 13, characterized in that the predefined time duration is a maximum of 250 milliseconds, preferably a maximum of 200 milliseconds, in particular a maximum of 150 milliseconds, amounts. Method according to claim 13 or 14, characterized in that the first frequency is at least 1 Hertz, preferably at least 3 Hertz, especially 5 Hertz, and the second Frequency maximum 200 Hertz, preferably maximum 100 Hertz, in particular 50 Hertz.