WO2025032128A1 - Anemometer device suitable for calibrating a gas sampler and a method of calibrating a gas sampler using such anemometer device - Google Patents

Abstract

The present application relates to an anemometer device suitable for calibrating a gas sampler, particularly at low flow rates, and a method of calibrating a gas sampler using such anemometer device.

Description

Anemometer Device Suitable for Calibrating a Gas Sampler and a Method of Calibrating a Gas Sampler using such Anemometer Device

Technical Field

The present application relates to an anemometer device suitable for calibrating a gas sampler, particularly at low flow rates, and a method of calibrating a gas sampler using such anemometer device.

Background

Gas sampling, particularly air sampling, is frequently performed in a range of cleanroom and manufacturing environments requiring low levels of particles, such as cleanroom environments for electronics manufacturing and aseptic environments for manufacturing pharmaceutical and biological products, such as sterile medicinal products.

Such gas sampling is generally performed using particle monitoring systems, such as microbial or active gas samplers and particle counters. Such gas samplers and particle counters are particularly advantageous as they allow for sampling a defined volume of gas and determining the risk of contamination within such cleanroom and manufacturing environments.

An example of a microbial gas or air sampler and method for sampling, detecting and/or characterizing particles, for example, via collection, growth and analysis of viable biological particles such as microorganisms is disclosed in EP 0 964 240 Al. This device includes an integrated sampler and impact surface, such as the receiving surface of a growth media, for collecting biological particles. The collected particles are then typically incubated to grow living particles and are in the following analyzed by different techniques including naked eyes inspection, microscopy, fluorescence or autofluorescence, ATP detection and others.

In a particle counter a laser beam is directed into a flow of the gas to be monitored. Particles crossing the laser beam will create signals which are subsequently detected by a photomultiplier. The output of the photomultiplier has several amplifiers with different gain stages that allow discrimination of particle number and particle sizes based on the evaluation of the signals, more specifically of the amplitudes of the signals. Because the volume of gas passed through the gas sampler is the product of gas flow rate and time, it is essential for reproducible results that the gas flow rate of the gas sampler is either continuously measured or - preferably as easier to realize - set to a precise predetermined value necessitating a simple but accurate calibration method and device.

A commercially available calibration device for a gas or air sampler is the MAS-100® Regulus, available from Merck KGaA, Darmstadt, Germany and MBV AG, Stafa, Switzerland, an anemometer using a fan to correlate the velocity of gas to the respective flow rate, which allows calibrating for a gas flow rate of 50 l/min to 200 l/min.

Other anemometers may, for example, make use of the pressure drop created within a laminar flow element, the Coriolis effect, the Venturi effect, and/or temperature effects, to name a few. For example, the TSI 5000 Series Digital Flow Meters, available from TSI Incorporated, Shoreview, Minnesota, USA, is an all-in-one gas flow meter integrating flow, absolute pressure, and temperature sensors.

US 2001/0029777 Al discloses a self-normalizing flow sensor to normalize a flow rate of a fluid in a main flow channel.

US 2014/0208755 Al discloses a method and system for measuring a mass flow rate in a portion of the flow path in an inlet duct of a gas turbine engine.

US 2015/0192445 Al relates to microprocessor-based thermal dispersion mass flow meters that use temperature sensing elements in its flow sensor probe(s).

US 2020/0049541 Al discloses a sensor apparatus and a method for measuring the flow rate of a shielding gas in a welding apparatus.

However, currently available anemometers still exhibit significant disadvantages for the calibration of gas samplers due to excessive pressure drop, inertia so as not reliably allow determining low flow rates, drifting of the base line, and/or an increase in relative uncertainty with decreasing flow rates.

Furthermore, so as to allow accurate and reproducible determination of rather low flow rates, such as in the range of about 30 l/min or less, it is necessary to assure consistent laminar gas flow at least in the measuring zone, i.e. avoiding any (or at least reducing to as little as possible) perturbation of the laminar gas flow.

Hence, there is still a need in industry for a reliable and/or easy-to-use anemometer suitable for calibration of a gas sampler over a wide range of flow rates, particularly low flow rates, such as flow rates down to 15 l/min or even lower.

Preferably such anemometer is also portable, and/or simple to assemble, and/or digital.

Summary

The present inventors have now surprisingly found that the above objects may be attained either individually or in any combination by the anemometer device and calibration method of the present application.

The present application therefore provides for an anemometer device comprising

(i) a duct comprising a maximum inner width R, a first open end, and a second open end, for conducting gas from the first open end ("entry") to the second open end ("exit");

(ii) a flow laminator covering the first open end of the duct; and

(iii) a thermal anemometer positioned inside the duct at a distance D from the flow laminator.

The present application also provides for a method of measuring an actual gas flow rate of a gas sampler, said actual gas flow rate being at most 28.3 l/min (and preferably at least 1 l/min), the method comprising the steps of providing a gas sampler, providing such anemometer device, attaching the second open end of the anemometer device to the gas intake of the air sampler, and determining the actual gas flow rate.

Said anemometer device is particularly suitable as a flow calibration device, i.e. for use in the calibration of gas samplers. Thus, the present application also provides for a method for calibrating a gas sampler, the method comprising the steps of

(a) providing a gas sampler to be calibrated;

(b) providing such anemometer device as defined above;

(c) attaching the second open end of the anemometer device to the gas intake of the gas sampler; and

(d) calibrating the gas sampler. Brief description of the drawings

Figure 1 shows the cross-section of an exemplary schematic representation of the present anemometer device (1), wherein the flow laminator (4) is set onto the duct (2).

Figure 2 shows the cross-section of an exemplary schematic representation of the present anemometer device (1), wherein the flow laminator (4) is set into the duct (2).

Figure 3 shows top views of an exemplary schematic representation of a flow laminator (4, 4') comprising hexagonal channels (7).

Figure 4 shows the pressure drop at various flow rates for an anemometer device as defined herein and a commercially available fan-type anemometer.

Detailed description

For the purposes of the present application, the term "regular polygon" is used to denote a polygon wherein all angles are equal and all distances are equal, with a regular n-sided polygon having rotational symmetry of order n.

In general terms, the present application relates to an anemometer device comprising a duct, a flow laminator, and a gas or air velocity sensor.

The shape of the duct is not particularly limited as long as it is suitable for conducting gas from the first open end (the "entry") to the second open end (the "exit"). It is preferred that the duct is straight, i.e. does not have any bends or curves. Generally stated, the duct may have a cross-section (with the cross-section of the duct being essentially orthogonal to the longitudinal axis of the duct) that is circular, or oval, or polygonal, i.e. in the shape of a circle, an oval, or a polygon. Preferably, the duct has a circular cross-section.

The polygon may be a n-polygon having n corners, wherein n is a natural number, with n being at least 3. Though not particularly limited, for practical reasons it is preferred that n is at most 100, for example 80, or 70, or 60, or 50, or 40, or 30, or 20. It is noted that with n ever increasing (n -> °°) the n-polygon approaches a circle and can eventually be considered a circle. Examples of a suitable n-polygon may have a cross-section selected from the group consisting of triangular, rectangular, square, pentagonal, hexagonal, heptagonal, octagonal, and any regular polygonal having at least 9 corners. Preferably the n-polygon is a regular n-polygon.

The size of the duct as generally defined by its length and its maximum inner width (or diameter) is not particularly limited. When defining the size of the duct, it needs to be kept in mind that the length and the maximum inner width of the duct have an influence on the pressure drop and thus on the overall performance of the present anemometer device. Generally an increase in length of the duct as well as a decrease in its diameter will result in an increased pressure loss. For given dimensions of the duct, the resulting pressure loss can easily be calculated using well known equations, such as the Darcy- Weisbach equation.

Generally, the maximum inner width of the duct is rather determined by the size of the gas inlet of the gas sampler. It is, however, preferred that the maximum inner width R, i.e. the longest straight line that can be placed in a cross-section of the duct (throughout this application also referred to as "diameter R") is at least 10 mm, preferably at least 20 mm, more preferably at least 30 mm, and most preferably at least 40 mm. Maximum inner width R or diameter R may, for example, be at most 100 mm, preferably at most 90 mm, more preferably at most 80 mm, still even more preferably at most 70 mm, and most preferably at most 60 mm.

In a preferred example, for allowing to calibrate gas or air samplers for particularly low flow rates, the duct has a diameter R of at least 45 mm and of at most 55 mm.

Equally, the length of the duct is not particularly limited. In any case, it is rather defined by the distance D as defined herein.

The flow laminator generally aids in reducing turbulences in the flow of gas into and through the duct, with the objective of allowing for reproducible and reliable determination of gas flow rates. Thus, the flow laminator may also be described as a means for generating an essentially laminar gas flow. The flow laminator comprises channels and walls, with the walls separating the channels from each other. Preferably, the flow laminator is a plate, or more preferably a disc, comprising channels and walls, with the walls separating the channels from each other.

Preferably the flow laminator has a thickness (or height) H of at least 5 % (for example, 5 % or 6 % or 7 % or 8 % or 9 % or 10 %), more preferably of at least 7 %, even more preferably of at least 8 %, still even more preferably of at least 9 %, and most preferably of at least 10 % of the distance D as defined herein.

Preferably, the flow laminator has a thickness (or height) of at most 20 %, more preferably of at most 18 %, even more preferably of at most 16 %, still even more preferably of at most 14 %, and most preferably of at most 12 % of the distance D as defined herein.

In a preferred embodiment, said flow laminator has a thickness H of at least 5 mm, preferably of at least 6 mm, more preferably of at least 7 mm, even more preferably of at least 8 mm, and most preferably of at least 9 mm, and though the upper limit of the thickness H of the flow laminator is not particularly limited, for reasons of handling, the flow laminator preferably has a thickness H of at most 50 mm or 40 mm, more preferably of at most 30 mm, even more preferably of at most 25 mm, still even more preferably of at most 20 mm, and most preferably of at most 15 mm.

The other two dimensions of the flow laminator i.e. in x- and z-direction if thickness H is considered to be in y-direction (see Figure 1), are not particularly limited as long as they are sufficient for the flow laminator to cover the first open end of the duct.

The flow laminator has a porosity of at least 60 %, preferably of at least 65 %, more preferably of at least 70 %, even more preferably of at least 75 %, and most preferably of at least 80 %. In general, higher porosity is preferred so as to keep any potential turbulences in the gas flow to a minimum, thereby also allowing for a shorter distance D.

Throughout this application the term "porosity" is used to denote the percentage of the (open) area of the channels allowing for continuous gas flow across the flow laminator in respect to the area of the first open end of the duct.

The flow laminator, for example in form of a plate, may either be inserted into the duct or be set onto the duct. If inserted or set into the duct, the flow laminator's dimension corresponds to the duct, e.g. the flow laminator has a diameter R. Alternatively, if set onto the duct, the flow laminator's dimension will be such that the flow laminator extends beyond the duct, for example by 2 % or 3 % or 4 % or 5 % or 6 % or 7 % or 8 % or 9 % or 10 % or even more of the duct's diameter R.

The channels may, for example, be introduced into the flow laminator, for example in form of a plate, by any suitable method, for example by drilling. It has, however, been found that the flow laminator is preferably produced either (i) by placing tubes defining the channels adjacent to each other or (ii) by directly producing the flow laminator by an extrusion method, injection molding method, or by a 3D printing method. All of these methods are well known to the skilled person and need not be explained in detail.

The shape or cross-section of the channels is not particularly limited and is preferably such as defined above in respect to the duct. For reasons of clarity it is noted that the crosssection of the channels and the duct may be selected to be the same or be different.

The dimensions of the channels may without difficulty be adapted to the specific conditions, particularly the flow rate, under which the present anemometer device is to be used.

For example, the maximum internal (open) length of the channels may be at least 1.0 mm, preferably at least 2.0 mm, more preferably at least 3.0 mm, even more preferably at least 4.0 mm, and most preferably at least 5.0 mm.

For example, the maximum internal (open) length of the channels may be at most 10.0 mm, preferably at most 9.0 mm, more preferably at most 8.0 mm, and most preferably at most 7.0 mm.

The gas velocity sensor is positioned inside the duct at a distance D from the flow laminator, wherein distance D is taken from the inner surface of the flow laminator, i.e. the surface facing the inside of the duct. Though not particularly limited, it is preferred that distance D is not too long so as to avoid excessive pressure drop.

Preferably, distance D is at least 100 %, more preferably at least 120 %, even more preferably at least 140 %, still even more preferably at least 160 %, and most preferably at least 180 % of the maximum inner width R of the duct.

Preferably, distance D is at most 250 %, more preferably at most 230 %, even more preferably at most 210 %, and most preferably at most 190 % of the maximum inner width R of the duct.

In a preferred embodiment distance D is at least 40 mm, more preferably at least 50 mm, even more preferably at least 60 mm, still even more preferably at least 70 mm, and most preferably at least 80 mm; and preferably distance D is at most 150 mm, more preferably at most 140 mm or 130 mm, even more preferably at most 120 mm, still even more preferably at most 110 mm, and most preferably at most 100 mm.

The present anemometer device preferably further comprises an adapter, located at the second open end ("exit") of the duct, allowing it to be gas-tightly (but removably) connected to a gas sampler. Preferably the adapter further comprises a seal configured to achieve gas-tight connection between the anemometer device and the gas sampler. Such gas-tight connection may be achieved by the adapter comprising a seal (for example, in form of an O-ring placed in a grove of the adapter) that is pressed against a corresponding surface of the gas sampler.

Not having the anemometer device fixedly integrated with the gas sampler offers a number of advantages for this specific application, notably that the anemometer device will in operation not interfere with the gas sampling process, potentially causing a loss of reproducibility, and further that a single anemometer device may be used for calibrating more than one gas samplers.

For the present anemometer it has been found that thermal anemometers are the best suited. Suitable types of thermal anemometers may be so-called "hot-wire anemometers" or alternatively "thermal flow meters".

In a hot-wire anemometer, a fine wire, for example a tungsten wire, is heated to a temperature above ambient temperature, i.e. the temperature of the gas flowing through the anemometer. The fine wire is then cooled by the gas flow, i.e. heat is transferred from the wire to the gas. Because most metals' electrical resistance is dependent upon the temperature of the metal, the velocity of the gas flowing past the wire can then be estimated from the change in electrical resistance of the wire. This principle may be implemented, for example, as constant current anemometer, constant voltage anemometer, or constant temperature anemometer, with electrical resistance being determined using Ohm's law.

In a thermal flow meter, the temperatures on the upwind and downwind side of one or element or hot wire, or alternatively the temperatures of one or more elements or hot wires placed apart from each other in the direction of the gas flow are determined. The gas flowing past the element(s) will transfer heat downwind thus increasing the temperature. The change in downwind temperature can then be correlated to the gas flow rate. Such thermal anemometers generally further comprise a transducer for driving and/or controlling the probe and for converting its signal to be accepted by a general purpose input output (GIRO) board for data acquisition and digitalizing the so-obtained data. On basis of a conversion algorithm this data is then converted by a processor into the gas flow rate. Such processor may either be part of the thermal anemometer or be separate from the thermal anemometer.

Thermal anemometers are available from a number of commercial suppliers. One example of a commercially available thermal anemometer suitable for use in the present anemometer device is the Air Velocity Transducer Model 8465 from TSI Incorporated, Shoreview, Minnesota, USA.

Due to the presence of the flow laminator the present anemometer device as described herein will show a non-linear drift. Thus, in order to be suitable as a calibration device the present anemometer device must itself be calibrated, which may be done using well known methods. It may, for example, be calibrated using a test bench equipped with sonic nozzles, wherein each head is able to generate a given gas flow (for example, any one or more selected from the group consisting of 15 l/min, 30 l/min, 60 l/min, 90 l/min, 120 l/min, and 150 l/min). In a first step the mean value of the raw measurements of the present anemometer device placed on a sonic nozzle of the test bench is computed during an acquisition phase with the following acquisition parameters: Filtering period (on the transducer): 5 s; acquisition frequency: 2 Hz; and acquisition period: 30 s. These discrete values are then used to determine the coefficients of a regression model, that will be used to adjust the measures of the system. For such regression model a power model may preferably be used.

Thus, the anemometer device (1) itself may be calibrated using a test bench with sonic nozzles comprising the steps of

(1) computing the mean value of the raw measurements obtained during an acquisition phase;

(2) subsequently, based on the values obtained in step (1), determine the coefficients of a regression model, preferably a power model.

The present anemometer is now to be described in an exemplary and non-limiting way in respect to the drawings. Figure 1 shows an anemometer device (1) as described herein comprising a duct (2), a flow laminator (4, 4') with an inner surface (3a), a thermal anemometer (5), and an adapter (6) for connecting the anemometer device (1) to a gas sampler, wherein the flow laminator (4, 4') is set onto duct (2).

Figure 2 also shows an anemometer device (1) as described herein comprising a duct (2), a flow laminator (4, 4') with an inner surface (3a), a thermal anemometer (5), and an adapter (6) for connecting the anemometer device (1) to a gas sampler, wherein, however, in contrast to the anemometer device (1) of Figure 1 the flow laminator (4, 4') is inserted into duct (2).

Figure 3 shows a schematic exemplary view of a flow laminator (4) in form of a disc (4') comprising hexagonal channels (7) with walls (8) separating the channels from each other. The example shown is particularly suitable for being inserted into the duct but may, preferably with a wider rim, the rim in this case then preferably not comprising any channels to minimize additional turbulences, also be set onto the duct.

The present anemometer device is well suited to the determination or measurement of low gas flow rates, such gas flow rate being, for example, at most 28.3 l/min and preferably at least 1 l/min, as typically used with gas or air samplers, with high accuracy. For obtaining consistent results it is preferred that the gas flow in the anemometer device is as laminar as possible. The present anemometer device is therefore preferably designed to have as few obstacles, bends or curves, and gas off-takes in the duct as possible. In a particularly preferred embodiment only the thermal anemometer is to intrude into the gas flow inside the duct.

For determining a gas flow rate of a gas sampler, an anemometer device as described herein is provided and its second open end (the "exit") attached to the gas intake of the gas sampler. Subsequently the gas flow rate may be determined.

The present anemometer device is particularly favorably used in the calibration of gas or air samplers. Thus, the present application also generally relates to a method for calibrating a gas or air sampler, the method comprising the steps of

(a) providing a gas sampler to be calibrated;

(b) providing an anemometer device as described herein; (c) attaching the second open end of the present anemometer device to the gas intake of the gas sampler; and

(d) calibrating the gas sampler.

The actual calibration in step (d) of the present method may either be performed by setting the gas flow rate of the gas sampler to a value determined by the gas sampler or to a value determined by the present anemometer device, and then calibrating the device.

Thus, for the present method for calibrating a gas sampler step (d) may comprise the steps of

(d'-l) setting the gas flow rate of the gas sampler to one or more desired values as determined by the gas sampler;

(d'-2) determining the actual gas flow rate of the gas sampler using the present anemometer device; and

(d'-3) calibrating the gas sampler.

Alternatively, for the present method for calibrating a gas sampler step (d) may comprise the steps of

(d"-l) increasing the gas flow rate of the gas sampler to one or more pre-selected actual gas flows as determined by the anemometer device; and

(d"-2) calibrating the gas sampler.

The step of calibration typically includes a step of comparing the measured values obtained by the device under test, i.e. the gas sampler, with the measured values obtained by the present anemometer. The comparison can essentially result in either finding that there is no significant error between the two devices; or there is a significant error and an adjustment is made to bring the error or deviation of the device under test to an acceptable level.

The present anemometer surprisingly allows accurate and reliable calibration of gas samplers even at low gas flow rates. Thus, the actual gas flow rate is preferably at most 28.3 l/min, more preferably at most 25 l/min, even more preferably at most 20 l/min, and most preferably at most 15 l/min.

Preferably the actual gas flow rate is preferably at least 1 l/min, more preferably at least 5 l/min, and most preferably at least 10 l/min. In a specific embodiment, an anemometer device specifically suited for such low flow rates as defined immediately above, has a maximum inner width R of the duct of at least 45 mm and of at most 55 mm, a thickness or height H of the flow laminator of at least 8 mm and of at most 12 mm; a distance D of at least 80 mm and of at most 100 mm; and a porosity of the flow laminator of at least 80 %. The channels preferably have a maximum internal (open) length of at least 5 mm and of at most 7 mm, and/or may, for example, be hexagonal.

Examples

The following examples are intended to illustrate the present anemometer in a nonlimiting exemplary way.

Example 1

The commercially available anemometer using a fan-type anemometer was compared to a commercially available mass flow sensor. A total of 10 flow rate measurements was done for each of the two devices with the results shown in the following Table 1.

Table 1

The significantly lower values for the commercially available mass flow sensor are attributable to a much higher pressure loss. Thus the data clearly indicates that such an anemometer is not suited for application in combination with a gas sampler at low gas flow rates.

Example 2

An anemometer device in accordance with the present application (with R ~ 50 mm; H ~ 10 mm; D ~ 90 mm; Porosity ~ 85 %) was tested at different gas flow rates on a test bench equipped with sonic nozzles. The results are shown in the following Table 2, wherein the "pre-set gas flow rate" indicates the gas flow rate of the respective sonic nozzle, the "measured gas flow rate" indicated the gas flow rate measured using the present anemometer device, and the "error" indicates the error between the pre-set gas flow rate and the measure gas flow rate in percent.

Table 2

The data clearly shows that the present anemometer device works with extremely high accuracy and thus can be used for calibrating gas samplers.

Example 3

An anemometer device in accordance with the present application (with R ~ 50 mm; H ~ 10 mm; D ~ 90 mm; Porosity ~ 85 %) and a commercially available fan-type anemometer were compared in respect to pressure drop on a test bench equipped with sonic nozzles. The results are indicated in Figure 4. Thus, generally stated the present anemometer as defined in the present application has proven to be very accurate for the measurement of gas flows, also at rates going as low as 15 l/min. This also renders it useful for the calibration of gas samplers, particularly of air samplers, that are used in controlling the cleanliness of clean rooms. At the same time the present anemometer is very simple in design and easy to use. It is also very robust in design, allowing it to be portable as well as be suitable for use by a field service technician. This permits calibration of gas samplers on the spot rather than having to send the gas sampler to a specifically equipped laboratory, which would either render it necessary to cope for some time without a gas sampler or require the purchase of at least one further gas sampler so as to be able to continue monitoring while the other gas sampler is away for calibration.

It is also noted that the actual flow rate determined with the present anemometer device will - in combination with respective gas temperature and pressure sensors, which may either be directly integrated into the present anemometer device or provided separate from the present anemometer device - allow for converting the flow rate from liter per minute ("l/min") to standard liter per minute ("slpm").

Furthermore, and this is quite important with regards to the ever increasing requirements of traceability the present anemometer also allows easy digital capture of the data, which can then be directly introduced, for example, into a laboratory management system (LIMS).

Claims

Claims

1. Anemometer device (1) comprising

(i) a duct (2) comprising a maximum inner width R, a first open end (3a), and a second open end (3b), for conducting gas from the first open end ("entry") to the second open end ("exit");

(ii) flow laminator (4) covering the first open end (3a) of the duct (2); and

(iii) a thermal anemometer (5) positioned inside the duct (2) at a distance D from the flow laminator (4).

2. Anemometer device (1) according to claim 1, wherein the duct (2) has a crosssection selected from the group consisting of circular, oval, and polygonal, preferably selected from the group consisting of circular, oval, and regular n- polygonal with n being at least 3, preferably wherein the duct (2) has a circular crosssection.

3. Anemometer device (1) according to claim 1 or claim 2, wherein the duct (2) has a maximum inner width R of at most 60 mm, preferably of at most 55 mm, and most preferably of at most 50 mm.

4. Anemometer device (1) according to any one of the preceding claims, wherein the duct (2) has a maximum inner width R of at least 35 mm, preferably of at least 40 mm, and most preferably of at least 45 mm.

5. Anemometer device (1) according to any one of the preceding claims, wherein the flow laminator (4,4') has a thickness (or height) H of at least 5 %, preferably of at least 6 %, more preferably of at least 7 %, even more preferably of at least 8 %, still even more preferably of at least 9 %, and most preferably of at least 10 % of the distance D.

6. Anemometer device (1) according to any one of the preceding claims, wherein the flow laminator (4,4') has a thickness (or height) of at most 20 %, preferably of at most 18 %, more preferably of at most 16 %, even more preferably of at most 14 %, and most preferably of at most 12 % of the distance D.

7. Anemometer device (1) according to any one of the preceding claims, wherein the flow laminator (4,4') has a porosity of at least 70 %, preferably of at least 75 %, and most preferably of at least 80 %, relative to the total surface of the flow laminator (4,4').

8. Anemometer device (1) according to any one of the preceding claims, wherein the distance D is at least 100 %, preferably at least 120 %, more preferably at least 140 %, even more preferably at least 160 %, and most preferably at least 180 % of the maximum inner width R.

9. Anemometer device (1) according to any one of the preceding claims, wherein the distance D is at most 250 %, preferably at most 230 %, more preferably at most 210 %, and most preferably at most 190 % of the maximum inner width R.

10. Anemometer device (1) according to any one of the preceding claims, wherein the anemometer device (1) comprises an adapter (6), located at the second open end (3b) of the duct, allowing it to be gas-tightly connected to a gas sampler, preferably the adapter (6) further comprising a seal configured to achieve gas-tight connection between the anemometer device (1) and a gas sampler.

11. Method of measuring an actual gas flow rate of a gas sampler, said actual gas flow rate being at most 28.3 l/min, the method comprising the steps of providing a gas sampler, providing an anemometer device (1) as claimed in any one of claims 1 to 10, attaching the second open end (3b) of the anemometer device (1) to the gas intake of the air sampler, and determining the actual gas flow rate.

12. Method for calibrating a gas sampler, the method comprising the steps of

(a) providing a gas sampler to be calibrated;

(b) providing an anemometer device (1) as claimed in any one of claims 1 to 10;

(c) attaching the second open end (3b) of the anemometer device (1) to the gas intake of the gas sampler; and

(d) calibrating the gas sampler.

13. Method for calibrating a gas sampler according to claim 12, wherein step (d) comprises the steps of

(d'-l) setting the gas flow rate of the gas sampler to a desired value;

(d'-2) determining the actual gas flow rate using the anemometer device; and

(d'-3) calibrating the gas sampler, or step (d) comprises the steps of

(d"-l) increasing the gas flow rate of the gas sampler to a pre-selected actual gas flow rate as determined by the anemometer device; and

(d"-2) calibrating the gas sampler.

14. Method for calibrating a gas sampler according to claim 12 or claim 13, wherein the actual flow rate is at most 28.3 l/min, more preferably at most 25 l/min, even more preferably at most 20 l/min, and most preferably at most 15 l/min.

15. Method for calibrating a gas sampler according to any one of claims 12 to 14, wherein the actual flow rate is at least 1 l/min, and preferably at least 5 l/min.

16. Method for calibrating a gas sampler according to any one of claims 12 to 15, wherein the anemometer device (1) itself is calibrated using a test bench with sonic nozzles comprising the steps of

(1) computing the mean value of the raw measurements obtained during an acquisition phase;

(2) subsequently, based on the values obtained in step (1), determine the coefficients of a regression model, preferably a power model.