FIELD OF THE INVENTION

The invention relates to an aerosol diluter and to a method for diluting aerosol.

BACKGROUND OF THE INVENTION

Aerosol diluters are needed in conjunction with aerosol measurement applications. The purpose of dilution is to dilute the particle amount in the aerosol flow to a level where the measurement equipment does not clog, and the measurement signal does not saturate. Aerosols to be measured can exceed the concentration range of the measurement device or repeatable concentrations are needed for calibration of instrumentation. In calibration applications the generation of multiple different concentrations repeatably is challenging with existing techniques, the different concentration required can be conveniently produced by controlled dilution of the primary aerosol.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a new aerosol diluter and method for diluting aerosol. The invention is characterized by what is stated in the independent claims. Some embodiments of the invention are disclosed in the dependent claims.

In the presented solution the aerosol diluter comprises an input for input flow, a dividing chamber, a filter flow channel, a sample flow channel, a mixing chamber, and an output for diluted flow. The filter flow channel starts from the dividing chamber, has a filter, and ends to the mixing chamber. The sample flow channel starts from the dividing chamber and ends to the mixing chamber. The input flow is divided such that a portion of the input flow is guided to the filter flow channel as a filter flow and a remaining portion of the input flow is guided to the sample flow channel as a sample flow. The filter filters particles from the filter flow and thereby forms a filtered flow of the filter flow. The filtered flow and the sample flow are combined in the mixing chamber such that a diluted flow is formed. The diluted flow is guided to the output. The sample flow channel is configured such that the ratio of the width of the sample flow channel to the length of the sample flow channel is at least 1. If the sample flow channel is round in cross-section the width of the sample flow channel is the diameter of the sample flow channel. If the shape of the cross-section of the sample flow channel deviates from a round shape the width of the sample flow channel is the largest cross-sectional dimension of the sample flow channel. If the sample flow channel is square in cross-section, for example, the width of the sample flow channel is the diagonal of the square. If the sample flow channel is formed of a plurality of orifices next to each other the width of the sample flow channel is the sum of the widths of the orifices. According to an embodiment if the sample flow channel is formed of a plurality of orifices next to each other the ratio of the width of the smallest orifice to the length of the sample flow channel is at least 1. The length of the sample flow channel may be less than 10 mm, for example. Preferably the length of the sample flow channel is less than 5 mm and more preferably the length of the sample flow channel is 1 mm or less. The structure of the diluter is simple and reliable. No external air source is needed in connection with the diluter. There is also no need for an external power source and the diluter does not comprise continuously moving parts. Diffusion losses in the sample flow channel are very low. Because of low diffusion losses the diluter does not substantially affect on the measuring result. Pressure loss in the diluter is quite low and the contamination of the sample flow channel is also low.

According to an embodiment the sample flow channel and the mixing chamber are dimensioned such that a dwell time of the sample flow from the dividing chamber to that part in the mixing chamber where the sample flow and the filtered flow are essentially mixed is less than 0.5 seconds. A short dwell time provides small diffusion losses. Small diffusion losses are important especially when the flow has small particles or a high concentration of particles, for example. The higher the dilution ratio is the smaller is the sample flow and, in such case, the small dwell time reduces the diffusion losses remarkably. Thus, a high dilution ratio is easily achieved if the dwell time is rather short. Preferably the dwell time is less than 0.1 seconds. According to an embodiment the dwell time is less than 0.05 seconds. In this connection the sample flow and the filtered flow are essentially mixed when the dilution ratio of the diluted flow is at least 50 % of a target dilution ratio.

A flow rate of the diluter may be 5 to 20 litres/min, for example, and in such a case the volume of the mixing chamber may be 0,01 to 0,1 litres, for example. A small mixing chamber makes it possible to achieve a high dilution ratio.

According to an embodiment turbulent mixing is provided in the mixing chamber for the sample flow and the filtered flow. According to an embodiment the sample flow is guided to the mixing chamber in an axial direction of the mixing chamber and the filtered flow is guided to the mixing chamber substantially perpendicularly to the axial direction of the mixing chamber. In this connection substantially perpendicularly means that when entering the mixing chamber the direction of the filtered flow may be such that the angle between the direction of the filtered flow is from 60 to 120 degrees, or preferably from 75 to 105 degrees, to the axial direction of the mixing chamber. Such solutions provide an efficient mixing for the sample flow and the filtered flow. Thereby the short dwell time of the sample flow from the dividing chamber to that part in the mixing chamber where the sample flow and the filtered flow are essentially mixed is easily achieved.

According to an embodiment the filter flow channel comprises one or more orifices through which the filter flow flows and the sample flow channel comprises one or more orifices through which the sample flow flows. The orifices naturally restrict the flow to some extent. However, such a solution provides for a simple and reliable way of forming the structure. A pressure difference over an orifice causes a known flow through the orifice which guarantees a stable dilution ratio as long as the pressure loss in other elements does not distort the flows. A stable function with low pressure loss is achieved.

According to an embodiment the sizes of the orifices in the filter flow channel are substantially the same as the sizes of the orifices in the sample flow channel. This results in the feature that contamination on the orifices is the same. Thereby the flow ratio between the sample flow and the filter flow remains the same. Thus, the dilution ratio remains unchanged even though the orifices contaminate during the use of the diluter.

According to an embodiment the number of the orifices in the filter flow channel together with the number of the orifices in the sample flow channel determine the dilution ratio of the diluter. This is a simple solution for determining and implementing the desired dilution ratio. The dilution ratio remains stable even though the flow changes.

According to an embodiment the diluter comprises a splitter plate comprising the orifices of the filter flow channel and the orifices of the sample flow channel. Providing the orifices on one splitter plate is a simple and reliable way of providing the solution.

According to an embodiment the dividing chamber is on one side of the splitter plate and the mixing chamber is on the other side of the splitting plate whereby the thickness of the splitting plate defines the length of the sample flow channel. This enables a reliable and simple realization of the structure. The thickness of the splitter plate may be rather small whereby it is easy to implement a short sample flow channel. According to an embodiment the splitter plate is exchangeable. Thereby it is easy to change the dilution ratio simply by changing the splitter plate to another splitter plate having different amount and/or distribution of orifices.

According to an embodiment the diluter comprises in connection with the splitter plate a mask plate, whereby the splitter plate and mask plate are movable with respect to each other such that in a certain mutual position of the mask plate and the splitter plate the mask plate allows a flow through orifices in the splitter plate and in another mutual position the mask plate prevents flow through at least some orifices in the splitter plate. By moving the splitter plate and the mask plate with respect to each other it is simple and easy to amend the dilution ratio.

According to an embodiment the mask plate comprises openings which when positioned in alignment with the orifices allow flow through the orifices and if none of the openings is not positioned in alignment with a certain orifice flow through that orifice is prevented. This an easy and simple way of implementing allowing and preventing flow through an orifice.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of some embodiments with reference to the attached drawings, in which

Figure 1 is shows schematically a diluter;

Figure 2 shows schematically a part of another diluter in an oblique top view;

Figure 3 shows schematically the part of Figure 2 in an oblique top view partly cut out;

Figure 4 shows schematically a side view of the part of Figure 2;

Figure 5 shows schematically a cross sectional side view of the part of Figure 2 along line A-A in Figure 4;

Figures 6a, 6b, 6c, and 6d show schematically a splitter plate and a mask plate in different mutual positions; and

Figure 7 shows schematically a diagram of diffusion losses of two different diluters.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows an embodiment of a diluter 1. The diluter 1 comprises two tubes 2a, 2b. The tubes 2a, 2b are positioned crosswise such that a wall of the first tube 2a contacts a wall of the second tube 2b. The material of the tubes 2a, 2b may be metal such as copper or steel, for example. An inner diameter of the tubes 2a, 2b may be 4 to 10 mm, for example.

The first tube 2a comprises an input 3 for the input flow Qin. The input flow Qin may be an aerosol flow having particles in the size of 1 to 300 nm, for example.

From the input 3 the input flow Qin flows to a dividing chamber 4. The dividing chamber 4 is in the first tube 2a. In the dividing chamber 4 the input flow Qin is divided to a filter flow QF and to a sample flow Qs. Thus, a portion of the input flow Qin is guided to a filter flow channel 5 as a filter flow QF and a remaining portion of the input flow Qin is guided to a sample flow channel as a sample flow Qs. The input flow Qin, the filter flow QF, and the sample flow Qs each have the same particle concentration.

The filter flow channel 5 starts from the dividing chamber 4, has a filter 6, and ends to a mixing chamber 7. The mixing chamber 7 is in the second tube 2b. The filter 6 may be a low-pressure loss HEPA filter, for example. The filter 6 filters particles from the filter flow QF and thereby forms a filtered flow of the filter flow QF. Thus, the filtered flow comprises less particles than the filter flow QF, which means that the particle concentration of the filtered flow is lower than the particle concentration of the filter flow QF.

The filter flow channel 5 may be formed of a tube or a hose, for example. The material of the filter flow channel 5 may be metal or plastic, for example. The inner diameter of the filter flow channel tube or hose may be 4 to 15 mm, for example. The length of the filter flow channel tube or hose may be 10 to 100 cm, for example.

The sample flow channel starts from the dividing chamber 4 and ends to the mixing chamber 7. The reference marking Qs for the sample flow also denotes the sample flow channel. The first tube 2a and the second tube 2b may be connected to each other by welding, for example. The sample flow channel may be formed by making orifices or holes to the walls of the tubes 2a, 2b. Thereby the thicknesses of the walls of the tubes 2a, 2b define the length of the sample flow channel. On the other hand, one of the tubes may comprise a small orifice and the other a substantially larger hole. In such case the larger hole is a part of the dividing chamber (if the larger hole is in the first tube 2a) or a part of the mixing chamber (if the larger hole is in the second tube 2b) and the wall of the tube having the small orifice defines the length of the sample channel.

According to an embodiment the tubes 2a, 2b have a wall thickness of 1 mm. The first tube 2a comprises an orifice having a diameter of 1 mm. The second tube 2b has a hole having a diameter of 3 mm in connection with the orifice of the first tube. In such case the length of the sample channel is 1 mm. The ratio of the width of the sample flow channel to the length of the sample flow channel is thus 1.

In the presented solution the ratio of the width of the sample flow channel to the length of the sample flow channel is at least 1. Preferably said ratio is at least 3.

The filtered flow and the sample flow Qs are combined in the mixing chamber 7 such that a diluted flow Qout is formed. The second tube 2b comprises an output 8 for the diluted flow Qout. The diluted flow Qout is guided to the output 8.

The particle concentration of the diluted flow Qout is smaller than the particle concentration of the input flow Qin. The input flow Qin is divided into the filter flow QF and the sample flow Qs such that Qin=Qp+Qs. A dilution ratio DR of the diluter 1 is DR=Qin/QF=(QF+Qs)/Qs-

The orifice or orifices forming the sample flow channel restrict the sample flow. The filter flow channel may also be provided with one or more orifices through which the filter flow flows for restricting the filter flow. The sizes and numbers of the orifices define the dilution ratio. The orifices cause a pressure loss on the flow that flows through the orifice. Because the pressure loss of the filter 6 is small and the inner dimeter of the tube or hose of the filter flow channel is so large, they do not substantially cause any pressure loss in the filter flow channel 5 or their impact on the total pressure loss is less than 10 %.

The sizes of the orifices in the filter flow channel 5 may be substantially the same as the sizes of the orifices in the sample flow channel. In such case only the number of the orifices in the filter flow channel 5 together with the number of the orifices in the sample flow channel determine the dilution ratio of the diluter 1. Each orifice in the filter flow channel 5 may have an equal size. Also, each orifice in the sample flow channel may have an equal size. According to an embodiment the orifices in the filter flow channel have different sizes. According to an embodiment the orifices in the sample flow channel have different sizes.

Figures 2, 3, 4, and 5 relate to an embodiment that corresponds to the embodiment shown in Figure 1 but Figures 2, 3, 4, and 5 show a part of the diluter that replaces the crossing tubes 2a, 2b of the embodiment in Figure 1. Figures 2, 3, 4, and 5 show only the beginning and the end of the filter flow channel but does not show the tubes or hoses and the filter. The tubes or hoses and the filter of the filter flow channel in connection with the embodiment shown in Figures 2, 3, 4, and 5 may correspond to the tubes or hoses and the filter of the filter flow channel in connection with the embodiment shown in Figure 1.

The part shown in Figures 2, 3, 4, and 5 comprises a body 9 and a top 10. The top 10 is removably attached to the body 9. The removable attachment may be implemented by using screws, for example, or any other suitable attachment means.

The top 10 is provided with the input 3 of the diluter. The output 8 of the diluter is arranged in the body 9.

A splitter plate 11 is provided between the top 10 and the body 9. The top 10 may comprise in its middle part supports or props 12. The props 12 ensure that the splitter plate 11 remains firmly against the body on its middle part, also. There may be seals, such as O-rings, between the top 10 and the splitter plate 11 and between the body 9 and the splitter plate 11 such that the splitter plate 11 is tightly against the top 10 and against the body 9.

In its middle part the splitter plate 11 comprises orifices 13 of the sample flow channel. The orifices 13 in the splitter plate 11 form the sample flow channel.

Near its outer circumference the splitter plate 11 comprises orifices 14 of the filter flow channel. The filter flow channel starts from the orifices 14 of the splitter plate 11. The body 9 comprises an output 5a of the filter flow channel and an input 5b of the filter flow channel. From the orifices 14 the filter flow flows to the output 5a of the filter flow channel. Arrows A demonstrate the route of the filter flow QF.

The tube or hose of the filter flow channel leading to the filter is connected to the output 5a of the filter flow channel. The tube or hose of the filter flow channel leading the flow from the filter is connected to the input 5b of the filter flow channel.

The dividing chamber 4 is provided in connection with the top 10 on one side of the splitter plate 11. The mixing chamber 7 is provided in connection with the body 9 on the other side of the splitter plate 11. Thus, the thickness of the splitter plate 11 defines the length of the sample flow channel. The diameters of the orifices 13, 14 may be from 0,5 to 3 mm, for example. The thickness of the splitter plate 11 may be from 0,2 to 5 mm, for example. Preferably the thickness of the splitter plate 11 is 1 mm or less. The orifices may have an aspect ratio (diameter to axial length) of at least 1, preferably 5 or more. Each orifice 14 in the filter flow channel 5 may have an equal size. Also, each orifice 13 in the sample flow channel may have an equal size. According to an embodiment the orifices 14 in the filter flow channel have different sizes. According to an embodiment the orifices 13 in the sample flow channel have different sizes.

In the presented solution the ratio of the width of the sample flow channel to the length of the sample flow channel is at least 1. Preferably said ratio is at least 3.

The mixing chamber 7 starts from the splitter plate 11 and ends at the output 8. The mixing chamber 7 has an axial direction which is denoted with line B in Figure 5.

A length of the mixing chamber 7 in its axial direction B may be 1 to 5 cm, for example. A volume of the mixing chamber 7 may be 0,01 to 0,1 litres, for example. A flow rate of the diluter may be 5 to 20 litres/min, for example.

In the embodiment shown in Figures 2, 3, 4, and 5 the mixing chamber 7 comprises a pre-mixing chamber 7a. The pre-mixing chamber 7a is the part of the mixing chamber 7 where the sample flow first comes from the sample flow channel. The pre-mixing chamber 7a is the cylindrical part of the mixing chamber 7 and it starts from the splitter plate 11 and ends to the point where the cross section of the mixing chamber 7 widens.

The sample flow and the filtered flow start to mix already at the splitter plate 11. However, at first the sample flow and the filtered flow are not completely mixed but they are essentially mixed at the end of the pre-mixing chamber 7a. In the embodiment of the Figures 2, 3, 4, and 5 the dwell time of the sample flow from the dividing chamber to that part in the mixing chamber where the sample flow and the filtered flow are essentially mixed is less than 0.5 seconds.

It is clear to a skilled person that the structure of the diluter that achieves the feature that the dwell time of the sample flow from the dividing chamber to that part or position in the mixing chamber where the sample flow and the filtered flow are essentially mixed is less than 0.5 seconds may vary from the solutions disclosed herein. However, a skilled person may easily define the part or position in the mixing chamber where the sample flow and the filtered flow are essentially mixed by simulating the diluter and the flows therein or by making a prototype and providing it with a sensor or sensors that measure the dilution ratio in the mixing chamber, for example. A target dilution ratio is naturally first determined for the diluted flow. The sample flow and the filtered flow are essentially mixed when the dilution ratio of the diluted flow is at least 50 % of the target dilution ratio. When said part or position is defined the dwell time is easily calculated. In the calculation the flow rate is also taken into consideration.

Turbulent mixing is provided in the mixing chamber 7 for the sample flow and the filtered flow. The sample flow is guided to the mixing chamber 7 in the axial direction B of the mixing chamber 7. The filtered flow is guided to the mixing chamber 7 from the input 5b of the filter flow channel substantially perpendicularly to the axial direction B of the mixing chamber 7 as shown with arrow C in Figure 5.

In the embodiment shown in Figure 3 the splitter plate 11 comprises seven orifices 13 of the sample flow channel and eight orifices 14 of the filter flow channel. The orifices 14 in the filter flow channel and the orifices 13 in the sample flow channel are each equal in size. In such case the dilution ratio is 15/7. The splitter plate 11 may be exchangeable. Thus, the top 10 may be detached from the body 9 and the splitter plate 11 may be removed. Thereafter another splitter plate 11 having different number of orifices 13, 14 may be positioned in the diluter. Thereby the dilution ratio of the diluter may be easily amended. The new splitter plate 11 may comprise four orifices 14 of the filter flow channel and one orifice 13 of the sample flow channel, for example. In such case the new dilution ratio of the diluter is 5/1.

The dilution ratio may also be amended by amending the distribution of the orifices 13, 14 in the splitter plate 11. For example, if the splitter plate 11 comprises five orifices 13 of the sample flow channel and ten orifices 14 of the filter flow channel the dilution ratio is 15/5. Thus, even though the number of orifices 13, 14 in the splitter plate 11 is equal to the number of orifices 13, 14 in the splitter plate 11 in the embodiment shown in Figure 3 the dilution ratios are different because the distribution of the of the orifices 13, 14 in the splitter plate 11 is different.

The diluter 1 may comprise in connection with the splitter plate 11 a mask plate. The splitter plate 11 and the mask plate may be movable with respect to each other such that in a certain mutual position of the mask plate and the splitter plate 11 the mask plate allows a flowthrough orifices 13, 14 in the splitter plate 11 and in another mutual position the mask plate prevents flow through at least some orifices 13, 14 in the splitter plate 11. By moving the splitter plate 11 and the mask plate with respect to each other it is simple and easy to amend the dilution ratio, for example.

The mask plate may comprise openings which when positioned in alignment with the orifices in the splitter plate 11 allow flow through the orifices and if none of the openings is not positioned in alignment with a certain orifice flow through that orifice in the splitter plate 11 is prevented. The openings in the mask plate may be substantially larger than the orifices in the splitter plate 11. Thereby the mask plate does restrict the flow and therefore does not affect on the length of the restricting flow route through the splitter plate and the mask plate.

Referring to Figures 6a to 6d the co-operation of the mask plate and the splitter plate is demonstrated in a simplified manner. Figures 6a to 6d show a top view of the mask plate and splitter plate positioned one above the other. The splitter plate may be fixed in its place and the mask plate is positioned on top of the splitter plate and may be rotatable with respect to the splitter plate. In this example the splitter plate comprises one orifice 13 that forms the sample flow channel. The splitter plate also comprises orifices 14a to 14d of the filter flow channel. The mask plate comprises a central opening 15 and six outer openings 16a to 16f.

In the position shown in Figure 6a the central opening 15 is in alignment with the orifice 13 thereby allowing flow through the sample flow channel. The outer openings 16a to 16d are in alignment with the orifices 14a to 14d. Thereby flow through four openings 14a to 14d of the filter flow channel is allowed. The dilution ratio is then 5/1.

In Figure 6b the mask plate is rotated 45° with respect to the position shown in Figure 6a. In the position shown in Figure 6b the central opening 15 is still in alignment with the orifice 13 thereby allowing flow through the sample flow channel. The outer openings 16a to 16d are not in alignment with any of the orifices. However, outer openings 16e and 16f are in alignment with orifices 14a and 14c. Thereby flow through two orifices of the filter flow channel in the splitter plate is allowed. The dilution ratio is then 3/1.

In Figure 6c the mask plate is rotated 67,5° with respect to the position shown in Figure 6a. In the position shown in Figure 6c the central opening 15 is still in alignment with the orifice 13 thereby allowing flow through the sample flow channel. None of the outer openings 16a to 16f is in alignment with any of the orifices. Thereby no flow through the filter flow channel is allowed. Thus, the particle concentration of the flow in the output 8 of the diluter is equal to the particle concentration of the flow in the input 3 of the diluter.

In Figure 6d the mask plate is rotated 90° with respect to the position shown in Figure 6a. In the position shown in Figure 6d the central opening 15 is not in alignment with the orifice 13. Thereby no flow is allowed through the sample flow channel. The outer openings 16a, 16b, 16c, and 16d are in alignment with the orifices 14b, 14c, 14d, and 14a. Thereby only flow through the filter flow channel is allowed. Thus, the particle concentration of the flow in the output 8 of the diluter is close to zero.

The positions illustrated in Figures 6c and 6d may be used for calibrating purposes and/or for checking dilution ratio and leaks, for example.

Referring to Figure 7, one beneficial feature of the presented solution is demonstrated. Given a typical sample flow rate of a particle concentration measurement instrument of 1 1pm, we can calculate the diffusion losses of particles based on well-known and published diffusion losses of aerosol particles. The calculation is done according to well-known formulas given for example in [Hinds, William C. Aerosol technology: properties, behavior, and measurement of airborne particles. John Wiley & Sons, 1999.]. It is important to note that diffusion losses of particles in a transfer line are dependent only on the length of the path and flow rate if the shape of the transfer line and the flow regime stays the same. Thus, increasing or decreasing the diameter of the tube or orifice does not affect the losses, but the as the dilution ratio increases, the flow rate of the sample flow decreases and thus diffusion losses increase accordingly.

A comparison of typical sample flow lengths between state of the art and the diluter presented in this description, we can give 0.3mm and 80mm flow path lengths. Diffusion losses for particle size ranging from 1 to 300nm for dilution ratio of 1:20 is given in Figure 7. For example, at 23nm, about 10% of the particles are lost in the conventional diluter, more than ten times the losses compared to short path length diluter. At 4nm, more than half of the particles are lost in a conventional diluter system.

It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.