RU2790001C2 - Method and device for measurement of gas jet flow rate - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to a method for measurement of gas jet (14) flow rate (vG), including stages, at which: (a) measurement with time resolution of infrared radiation parameter (E) in infrared radiation of gas jet (14) is performed at first measurement point (P1) beyond gas jet (14), thereby obtaining first curve (Eg1,1(t)) of the infrared radiation parameter, (b) measurement with time resolution of infrared radiation parameter (E) is performed at second measurement point (P2) beyond gas jet (14), thereby obtaining second curve (Eg1,2(t)) of the infrared radiation parameter, (c) time (τ1) of passage from first curve (Eg1,1(t)) of the infrared radiation parameter and second curve (Eg1,2(t)) of the infrared radiation parameter is calculated, in particular, by means of cross-correlation, and (d) flow rate (vG) is calculated based on passage time (τ1). In this case, (e) gas jet (14) is a jet of a gas mixture, which contains first gas (g1) and at least second gas (g2). At the same time, (f) first gas (g1) has excitation wavelength (λg1) of the first gas, characterized by that (g) temperature (T) of gas jet (14) is at least 200°C, while (h) first infrared radiation parameter (Eg1) is measured by a photoelectric method with wavelength (λg1) of at least 780 nm, (i) frequency (f) of measurements is at least 1 kHz, and (j) infrared radiation parameter (E) is intensity of infrared radiation emitted with the first gas.

EFFECT: possibility of measurement of a gas flow rate with higher accuracy as there is no need for IR light source.

10 cl, 3 dwg

Description

The invention relates to a method for measuring the flow rate of a gas jet. According to a second aspect, the invention relates to a gas jet flow velocity measurement device with (a) a first infrared sensor for time-resolved measurement of a first infrared parameter in the infrared radiation of the gas jet to obtain a first infrared parameter curve, (b) a second infrared sensor for measuring the second infrared parameter in the infrared radiation of the gas jet to obtain the second infrared parameter curve and (c) an estimator that is configured to automatically calculate the transit time between the first infrared parameter curve and the second infrared parameter curve, in particular by cross-correlation, and flow rate calculations based on transit time.

The flow rate of gases needs to be measured in many cases. This measuring task is especially difficult if the gases are very hot and/or aggressive. In the case of high temperatures, for example, more than 1000°C, it is necessary to use materials that are resistant to high temperatures, which is expensive. Aggressive gases lead to increased wear. For example, if the gas jet carries solid particles such as ash, carbon, slag, or cement particles, this can result in significant abrasion of the measuring device in use. If the gas contains oxidizing components, for example, this can also lead to chemical wear. Despite potentially unfavorable environmental conditions, a high degree of measurement accuracy is desirable as this leads to improved controllability of the technical installation in which the flow rate is measured.

It is a known practice to measure temperature fluctuations in a gas jet at points located at a distance from each other, and to determine the time shift of two temperature curves by means of cross-correlation. The flow velocity of the gas jet can be determined based on the time offset and the distance between two measuring points.

The disadvantage of this method of measuring the flow rate is that it is difficult to achieve high degrees of accuracy.

DE69921009T2 describes an optical flow meter, specifically for natural gas pipelines, in which scattered light on particles is measured. If the particle concentration is too low, more particles are added.

DE3827913A1 describes a method and apparatus for determining the flow rate, which includes the measurement of scattered light on particles. The respective measuring points are located at a distance from each other. The speed is obtained using the calculation of the correlation of the measurement results.

US9157778B2 details a gas flow measurement method in which the absorption of radiation is measured at two measurement points at a distance from each other. The flow velocity is calculated by determining the transit time of the perturbation. This perturbation can be caused, for example, by the introduction of a gas.

The invention is directed to improving the measurement of the gas jet flow velocity.

The invention solves the problem by a method comprising the steps of (a) performing a time-resolved measurement of an infrared parameter in the infrared radiation of the gas jet at a first measurement point outside the gas jet, thereby obtaining a first infrared parameter curve, (b) performing a time-resolved measurement of an infrared parameter at a second measurement point outside the pipe, thereby obtaining a second infrared parameter curve, (c) performing a transit time calculation from the first infrared parameter curve and the second infrared parameter curve, specifically by cross-correlation, and (d) performing a flow rate calculation based on transit time, wherein (e) an infrared parameter is measured photoelectrically at a wavelength of at least 780 nm, especially 1.5 µm.

According to a second aspect, the invention solves the problem by a device according to the preamble of the claims, wherein the infrared sensors are photoelectric infrared sensors and have a measurement range whose lower limit is at least 0.78 µm, and a measurement frequency of at least 1 kHz.

The advantage of the invention is that the flow rate can be measured with a higher degree of accuracy. The reason for this is that absolute photoelectric measurement of the infrared radiation parameter is possible; in the case of a pyrometric measurement, for example, this is generally only possible if the emission factor is constant, which often cannot be guaranteed.

It is particularly preferred if the infrared radiation parameter is measured at a wavelength of at least 0.78 µm, in particular at least 1.5 µm. In this case, the influence of black body radiation is used effectively. Black body radiation can arise, for example, from the walls of a pipe through which a gas jet passes, or from particles in a gas jet. Gases with excitation wavelengths above 1.5 µm absorb and re-emit blackbody background radiation in this wavelength range so that fluctuations in gas concentrations are particularly noticeable. A time-constant background, for example, is irrelevant when calculated using cross-correlation.

Preferably, the infrared radiation parameter is measured at a wavelength of at most 6 µm, in particular at most 5.3 µm. It has been shown that a particularly high degree of accuracy in flow rate measurements can be achieved in this way.

The invention is based on the knowledge that local deviations or inhomogeneities in the infrared parameter are similar as long as these deviations move at the same speed as the gas jet itself. These fluctuations can have several causes. First, it may be a matter of thermal fluctuations, which means that the temperature of the gas jet is spatially non-uniform at a given point in time. If this inhomogeneity moves with the flow rate of the gas jet, the flow rate can be determined based on temperature fluctuations.

If the gas is a mixture of different gases, i.e. if the gas is a mixture of gases according to the preferred embodiment, fluctuations in gas concentration may occur. It has been proven that the spatial distribution of gas concentrations is locally more stable than the temperature distribution. The reason for this may be that three mechanisms are known to balance differences in temperature, namely mixing, thermal conduction and thermal radiation. Conversely, deviations in concentration can only be balanced by diffusion. The local distribution of concentration differences is therefore more stable over time. As a result, the first infrared parameter curve and the second infrared parameter curve are more similar to each other so that the calculation of the transit time can be achieved with a lower degree of measurement uncertainty.

Within the scope of the present description, the term "infrared parameter" should be understood to mean a value or vector that denotes the intensity of the electromagnetic infrared radiation caused by the infrared radiation of the gas jet in the measurement range. If the density, temperature, and composition of the gas jet change, then the parameter of infrared radiation also changes.

The jet of gas preferably pokes in the pipe and the infrared parameter is measured from a measurement point outside the pipe. Alternatively, it is also possible for the gas jet to spread freely, for example flowing out of the outlet and exiting into the environment or a larger cavity.

The measurement frequency is preferably at least 1.5 kHz, particularly preferably at least 16 kHz. In general, the higher the frequency of measurements, the lower the degree of measurement uncertainty used to determine transit time. However, until now there have been limitations to increasing the frequency of measurements, since the prior art uses only pyrometric measurements, not photoelectric.

Preferably, the radiation parameter is measured in analog form but then converted to digital form, the bit depth being preferably 16 bits.

According to a preferred embodiment, the gas jet is a jet of a gas mixture that contains a first gas and at least a second gas, the first gas having an excitation wavelength of the first gas, and the infrared radiation parameter is the emission intensity of the infrared radiation sensor at a wavelength excitation of the first gas. The first gas can be, for example, water vapor, nitric oxide, methane, carbon dioxide, carbon monoxide, sulfur oxide or sulfur trioxide, NOx, H ₂ S, HF, NH ₃ and all infrared active molecules. The second gas is a gas different from the first gas and is also, for example, steam, nitrogen oxide, methane, carbon dioxide, carbon monoxide, sulfur oxide or sulfur trioxide.

The feature that the infrared parameter is the intensity of the radiation at the excitation wavelength of the first gas is to be understood particularly to mean that a change in the concentration of the first gas leads, under conditions otherwise remaining the same, to a change in the infrared parameter . Preferably, radiation components that lie outside a predetermined measurement range, which contains the excitation wavelength of the first gas, are filtered out. The width of this measurement interval is preferably less than 0.5 μm, preferably less than 0.4 μm.

Preferably, the second gas has an excitation wavelength of the second gas, and the method includes the steps of (a) performing time-resolved detection of a second infrared parameter in the form of an emission intensity at the excitation wavelength of the second gas at the first measurement point, thereby obtaining a first radiation intensity curve, (b) performing time-resolved detection of a second infrared parameter at a second measurement point, thereby obtaining a second radiation intensity curve, (c) performing a calculation of a second transit time between radiation intensity curves, in particular by cross-correlation, and (d) performing a flow rate calculation based on the first transit time and the second transit time. In other words, transit times are measured using two different fluctuations in concentration. This has the advantage that the degree of measurement uncertainty can be reduced.

Infrared radiation from the gas jet that does not lie within a predetermined measurement range, such as ±0.3 µm from the excitation wavelength of the first gas, or within a predetermined range of ±0.3 µm from the excitation wavelength of the second gas, is preferably filtered out. It is particularly advantageous if infrared radiation is filtered out which does not lie within predetermined intervals of ±0.2 μm from the respective excitation wavelength. This has the advantage that the degree of measurement uncertainty can be further reduced as there is less overlap with other fluctuations in the radiation components that can lead to an averaging effect.

The temperature of the gas jet is preferably at least 200°C, particularly preferably at least 1000°C. The advantages of the invention are especially evident at high temperatures.

An indium arsenide-artemonide detector is preferably used to measure the infrared parameter. Alternatively or additionally, a cadmium mercury telluride detector may be used.

With the device according to the invention, the measurement range of infrared sensors preferably lies between 1 and 6 µm, especially between 1.5 and 6 µm.

Preferably, the evaluation unit is configured to automatically carry out the method according to the invention. This is to be understood as meaning that the estimator automatically executes the method without human intervention.

Preferably, the device has a pipe for conducting a gas jet, in which the first infrared sensor and the second infrared sensor are located with the possibility of detecting infrared radiation from the outside of the pipe. In particular, infrared radiation sensors are located on the outside of the pipe. If the temperature of the gas jet during operation of the device is more than 200°C, the infrared sensors are preferably located at such a distance from the gas jet that the temperature at this point is not more than 100°C, preferably not more than 80°C. The location of the infrared sensors at a distance from the gas jet has the additional advantage that chemical and/or abrasive wear can become negligible.

Preferably, the device according to the invention has (a) a first measuring line that extends transversely to the gas jet pipe and is configured to conduct a first infrared radiation beam from the gas jet to the first infrared radiation sensor, (b) a second measuring line that extends transversely to the gas jet pipe and is configured with the possibility of passing the second beam of infrared radiation from the gas flow to the second sensor of infrared radiation, and the measuring lines are located in such a way that the beams of infrared radiation form an angular deviation ϕ of no more than 45°, in particular no more than 20°, preferably no more than 10°. Thus, the turbulence patterns at the first measurement point and the second measurement point are particularly similar to each other, thereby ensuring that a low degree of uncertainty in flow velocity measurements can be achieved.

Preferably, the infrared sensors are insensitive below a wavelength of 1.5 µm, preferably below 780 nm. This is to be understood as meaning that the spectral sensitivity below this wavelength is not more than one third, in particular not more than one tenth, of the maximum spectral sensitivity. Spectral sensitivity is indicated in amps/watts.

Preferably, the infrared sensors are also insensitive above 15 µm, preferably above 5.5 µm. The excitation wavelengths of the vibrations commonly encountered gases such as carbon dioxide, carbon monoxide and water lie in the wavelength range between 1.5 and 6 microns. At the same time, as noted above, the black body background radiation is sufficiently intense to obtain a good signal-to-noise ratio.

The infrared radiation sensors are preferably arranged such that the maximum diameter of the infrared radiation beam is at most 200 millimeters. The smaller the diameter of the infrared beam, the less deviations are averaged and the more the signal fluctuates. Preferably, the minimum infrared beam diameter is at least 1 millimeter. If the infrared beam diameter becomes too small, the signal-to-noise ratio deteriorates.

Preferably, (a) the first infrared sensor is positioned such that the first infrared radiation beam travels in a first straight line, (b) the second infrared radiation sensor is positioned such that the second infrared radiation beam travels in a second straight line, and a minimum distance line between both continues in straight lines in the direction of flow. The distance between the two straight lines is the measuring distance. The measuring distance is preferably at least 50-1000 millimeters, especially not more than 600. It is also preferable if the measuring distance is not more than 600 millimeters.

It is particularly preferred if the two straight lines run parallel in the technical sense, which means that perfect parallelism in the mathematical sense is preferred but usually cannot be achieved. For this reason, deviations of, for example, ±5° are permissible.

The measuring distance between two straight lines preferably corresponds to the quotient of the flow rate and 1000 hertz and/or not more than the quotient of the flow rate and 100 hertz. At this distance, the degree of measurement uncertainty in determining the flow velocity is already very low due to the degree of uncertainty with transit time. In addition, the degree of uncertainty due to a change in the nature of the inhomogeneity is not yet large enough to have too strong a negative effect on the degree of measurement uncertainty.

The device preferably does not protrude into the pipe. This is to be understood as meaning that no part of the apparatus protrudes more than one tenth into the cross section of the pipe. Prior art systems often have lances that generate turbulence in the gas stream. The disadvantage of this is that it causes a decrease in the flow rate and therefore a decrease in the efficiency of the controlled installation. In other words, the infrared parameters are preferably measured on an undisturbed or not actively disturbed gas jet.

In the following, the invention will be explained in more detail with the help of the attached figures. They are showing:

fig. 1 shows an apparatus according to the invention for carrying out the method according to the invention according to a first embodiment;

fig. 2 shows an apparatus according to the invention for carrying out the method according to the invention according to a second embodiment;

fig. 3 shows an apparatus according to the invention for carrying out the method according to the invention according to a third embodiment.

Fig. 1 shows a furnace 10 in which a gas jet 14, in this case in the form of an exhaust gas jet, is formed by combustion or other exothermic processes or external fuel heat supply via a burner 12. The temperature T of the gas jet 14 is above T=1400°C, for example. As in the present case, the furnace 10 may be a device for heating a metal bath or a glass bath 16. The furnace may also, for example, be part of a power plant or a cement plant. A furnace, power plant or cement plant with a measurement device according to the invention is also the object of the present invention. The jet 14 of gas passes through the pipe 18.

Fig. 1 also shows a measurement device 20 for measuring the flow velocity vG of the gas jet 14 . The flow rate vG is the average flow rate which, when multiplied by the cross-sectional area A of the pipe 18, gives the volumetric flow rate of the gas. In the present case, the pipe is round, so that the cross-sectional area results in A = πD ² /4.

The measurement device 20 includes a first infrared sensor 22.1 and a second infrared sensor 22.2. The first infrared sensor 22 is arranged to detect the first infrared beam 24.1 which propagates along the measuring line 25.1.

If the schematically depicted molecule 26.1 located in the first infrared beam 24.1 emits an infrared photon 28 that travels in the first infrared beam 24.1 towards the first infrared sensor 22.1, it reaches the sensor element 30.1 in the form of an InAsSb photodetector, which subsequently generates voltage. Thus, the photovoltage U ₁ generated by the sensor element 30.1 depends on the intensity of the radiation incident on the sensor element 30.1. Element 30.1 of the sensor is located at a distance from the pipe 18.

The measuring line 25.1 does not protrude into the pipe 18, thereby largely preventing the formation of additional turbulence.

The sensor element 30.1 has a measurement range M = [λ _min , λ _max ] with a lower critical wavelength λ _min and an upper critical wavelength λ _max . In the present case, λ _min = 0.78 µm and λ _max = 5.3 µm.

The infrared sensor 22.1 measures the curve E _g1,1 (t) of the infrared parameter as a function of time t with a measurement frequency f _mess of at least 1 kHz, in the present case f _mess = 16 kHz. Preferably, the measurement frequency f _mess is at most 1 MHz. The analog raw data is converted into digital values using the analog-to-digital converter of the radiation sensor 22.1. The sample bit depth is 8 to 24, preferably 16 bits.

The second infrared sensor 22.2 is configured to measure the radiation from the infrared beam 24.2 that propagates along the second measuring line 25.2. The infrared radiation of the second infrared beam 24.2 comes from, for example, the second molecule 26.2. The first beam 24.1 of infrared radiation passes along the first straight line G1; the second beam 24.2 of infrared radiation passes along the second straight line G2. Two straight lines G1, G2 are at a measuring distance d from each other. As shown in the present case, they preferably run parallel to each other.

The measuring distance d is preferably at most 500 mm, for example 350 ± 50 mm.

Photovoltage U ₁ , U ₂ generated by the respective elements 30.1, 30.2 sensors are sent to the block 32 evaluation. The photovoltage U ₁ is a measure of the radiation intensity E ₁ measured by the sensor element 30.1 and constitutes an infrared radiation parameter. The intensity E ₂ of the radiation is measured by element 30.2 of the second sensor and also depends on time.

кросскорреляции достигает е` максимума, где

- символ оператора для кросскорреляции.The estimator 32 calculates the transit time as the time for which the function

cross-correlation reaches e` maximum, where

- operator symbol for cross-correlation.

If the local concentration c of the first gas g1, such as methane, water, carbon dioxide, carbon monoxide, sulfur trioxide, sulfur dioxide or nitrogen oxide, fluctuates in the exhaust gas jet 14, this results in a change in the emission intensity E _g1,1 when the corresponding the oscillation passes through the area of the first infrared beam 24.1. The spatial inhomogeneities of the concentration remain substantially constant over the measurement distance d, thereby leading to similar curves of the respective radiation intensities E _g1.1 (t) and E _g1.2 (t) at the first sensor element 30.1 and the second sensor element 30.2.

Blackbody radiation from wall 34 in tube 18 does not interfere with this measurement. If, for example, H ₂ O is selected as the first gas, it has a first gas excitation wave length λ _g1 of 3.2 μm. In this case, it is preferable if the infrared sensors 22.1, 22.2 have a measurement interval M = [λ _g1 -0.3 µm, λ _g1 +0.3 µm].

If, as provided by the preferred embodiment, a second gas g2 is selected whose excitation wavelength λ _g2 of the second gas does not lie within the measurement interval M for the first gas g1, the degree of measurement accuracy can often be increased. For example, carbon dioxide can be used as the second gas, whose excitation wavelength of the second gas is λ _g2 = 4.27 µm.

Fig. 2 schematically shows a jet engine 36 on which the measuring device 20 is positioned so that the gas jet 14 is measured, which in this case leaves the jet engine 36 through the outlet 38.

Fig. 3 schematically shows a part of an electric arc furnace 40 with a melting chamber 42 in which steel scrap is melted by means of an electric arc between the electrodes 43.1, 43.2, 43.3, thereby forming a metal pool 16. On the right side of the image, an enlargement of the area circled by a dotted line is shown. The exhaust gases generated during melting form a jet 14 of gas and are discharged through a pipe 18. The pipe 18 has an annular gap 44 through which air 46 can also enter the pipe 18. In order to measure the gas jet 14, the measuring device 20 is pipe gap 18.

List of reference positions

10 - oven

12 - burner

14 - gas jet

16 - metal bath

18 - pipe

20 - measuring device

22 - infrared sensor

24 - beam of infrared radiation

25 - measuring line

26 - molecule

28 - infrared photon

30 - sensor element

32 - evaluation block

34 - wall

36 - jet engine

38 - outlet

40 - electric arc furnace

42 - melting chamber

43 - electrode

44 - annular gap

46 - air

λ _min - lower critical wavelength

λ _max - upper critical wavelength

λ _g1 - excitation wavelength of the first gas

λ _g2 - excitation wavelength of the second gas

τ - transit time

A - cross-sectional area

c - concentration

D - diameter

d - measuring distance

E - radiation intensity

E(t) - infrared parameter curve

f _mess - measurement frequency

f _g1 - excitation wavelength of the first gas

f _g2 - excitation wavelength of the second gas

M - measurement interval, measurement range

vG - flow velocity

T- temperature

t - time

U ₁ - photovoltage

Claims (38)

1. A method for measuring the velocity (vG) of the gas jet (14) flow, which includes the steps of: (a) performing a time-resolved measurement of the infrared radiation parameter (E) in the infrared radiation of the gas jet (14) at the first measurement point (P1) outside the gas jet (14), thereby obtaining the first curve (E _g1,1 (t) ) parameter of infrared radiation, (b) performing a time-resolved measurement of the infrared parameter (E) at the second measurement point (P2) outside the gas jet (14), thereby obtaining a second curve (E _g1,2 (t)) of the infrared parameter, (c) calculating the transit time (τ ₁ ) from the first infrared parameter curve (E _g1,1 (t)) and the second infrared parameter curve (E _g1,2 (t)) specifically by cross-correlation, and (d) performing a flow velocity (vG) calculation based on transit time (τ ₁ ), where (e) the gas jet (14) is a gas mixture jet that contains a first gas (g1) and at least a second gas (g2), wherein (f) the first gas (g1) has a first gas excitation wave length (λ _g1 ) characterized in that (g) the temperature (T) of the gas jet (14) is at least 200°C, (h) wherein the first parameter of infrared radiation (E _g1 ) is measured by a photoelectric method at a wavelength (λ _g1 ) of at least 780 nm, (i) the frequency (f) of the measurements is at least 1 kHz, and (j) the infrared parameter (E) is the intensity of the infrared radiation emitted by the first gas. 2. The method according to claim 1, characterized in that (i) the second gas (g2) has an excitation wave length (λ _g2 ) of the second gas, and (ii) the method comprises the following steps, where: (a) performing time-resolved detection of the second parameter (E _g2 ) of infrared radiation in the form of radiation intensity over the excitation wave length (λ _g2 ) of the second gas at the first measurement point (P1), thereby obtaining the first curve (E _g2.1 (t )) radiation intensity, (b) performing time-resolved detection of the second infrared parameter (E _g2 ) at the second measurement point (P2), thereby obtaining a second radiation intensity curve (E _g2.2 (t)) (c) performing the calculation of the second transit time (τ ₂ ) between the intensities (E _g2.1 (t)) (E _g2.2 (t)) of the radiation, especially by cross-correlation, and (d) performing a calculation of the flow velocity (vG) based on the first transit time (τ ₁ ) and the second transit time (τ ₂ ). 3. Method according to claim 2, characterized in that the infrared radiation parameter (E) is measured by means of an InAsSb photoelectric sensor. 4. The method according to any one of the above paragraphs, characterized in that the infrared radiation parameter (E _g1 ) is measured at a wavelength (λ _g1 ) of a wavelength of not more than 15 μm. 5. Method according to any one of the above-mentioned claims, characterized in that the temperature (T) of the gas jet (14) is at least 1000°C. 6. A device for measuring the flow rate of a jet (14) of a gas containing a gas mixture containing a first gas (g1) and at least a second gas (n2) and particles, and passing through a pipe with: (a) a first infrared sensor (22.1) for measuring with time resolution the first infrared parameter (E _g1 ) of the infrared radiation of the gas jet (14) to obtain the first curve (E _g1,1 (t)) of the infrared radiation parameter, (b) a second infrared sensor for measuring with time resolution the infrared parameter (E _g1 ) of the infrared radiation of the gas jet (14) to obtain a second curve (E _g1,2 (t)) of the infrared radiation parameter and (c) an estimator (32) that is configured to automatically - calculating the transit time (τ ₁ ) between the first curve (E _g1,1 (t)) of the infrared parameter and the second curve (E _g1,2 (t)) of the infrared parameter, in particular by cross-correlation, and - calculation of the speed (vG) of the flow based on the time (τ ₁ ) of passage, characterized in that (d) infrared sensors (22) are photoelectric sensors of infrared radiation and have a measurement range M whose lower critical wavelength (λ _min ) is at least 1.5 nm and whose upper critical wavelength (λ _max ) is not more than 15 nm, and have a measurement frequency (f _mess ) of at least 1 kHz, and configured to measure the flow rate based on the intensity of infrared radiation caused by infrared radiation emitted by the first gas. 7. Device according to claim 6, characterized in that the upper critical wavelength (λ _max ) is at most 15 nm. 8. Device according to claim 6 or 7, characterized in that the IR sensors (22) are InAsSb photoelectric sensors. 9. The device according to any one of paragraphs 6-8, characterized in that the evaluation unit (32) is configured to automatically execute the method according to any one of paragraphs. 1-5. 10. The device according to any one of paragraphs. 6-9, differing in that it has (a) a pipe (18) for conducting a jet (14) of gas, wherein the first infrared sensor (22.1) and the second infrared sensor (22.2) are arranged to detect infrared radiation from the pipe (18), or (b) an outlet or through hole (38), wherein the first infrared sensor (22.1) and the second infrared sensor (22.2) are arranged to detect the infrared radiation of the gas jet (14) flowing out of the outlet (38).

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