Measuring devices of this kind are used to measure the diffusion flow of a gaseous substance through the surface of a material. The gaseous substance to be measured is particularly water vapour, but it can quite as well be ammonia, or any other gas for which a suitable content measuring sensor exists.

The measurement of water vapour is required particularly when measuring moisture in structures, in which it is possible to measure the rate of the evaporation and/or absorption of moisture in a desired structure or building material. On the basis of the measured data and of calculations made using them, several useful parameters can be obtained.

The prior art concerning diffusion measurement is disclosed in publications such as WO 95/14223 and WO 00/03208. Patent publication GB 1 554 441, on the other hand, discloses one known solution for the measurement of moisture. The latter publication does not, however, concern the measurement of diffusion.

Publication WO 97/46853, in turn, discloses a method and a micro-sensor for measuring transfer coefficients, such as a diffusion coefficient or a flow velocity. In the micro- sensor, there is a reservoir, in which there is an aperture and a detector. The point of the detector is located in the aperture, which can be equipped with a membrane or a plug.

The reservoir contains one or several gases or substances dissolved in a liquid, which it is intended to diffuse through the aperture into a medium outside the opening of the reservoir. The detector measures the partial pressure of the gas or the concentration of the substance dissolved in the liquid, in the aperture, which allows the diffusion coefficient or the flow velocity to be determined.

US patent publication 4,066,068 discloses a method and device for measuring the flow of moisture evaporating from a surface. The method and device are particularly intended

for measuring the moisture evaporating from the surface of the skin. The method is based on the use of a tubular case open at both ends. One end of the case is placed against the surface of the skin while the other end is left in unrestricted contact with the surrounding air. As the skin evaporates moisture, the humidity of the air in the vicinity of the surface of the air increases to become greater than the humidity of the surrounding air. This creates a humidity gradient in the air inside the case. The humidity gradient is measured inside the case by means of two moisture sensors located at two points spaced apart in the normal direction to the surface of the skin. The temperature gradient prevailing in the case is measured using two temperature sensors located correspondingly. The flow of moisture being evaporated from the surface of the skin can be calculated from the measured humidity and temperature gradients with the aid of diffusion equations.

The device disclosed in publication US 4,066,068 has the drawback that air currents in the surroundings may cause vortices within the case. In eddying or flowing air, the moisture flow is no longer determined only on the basis of the diffusion flow, and the use of the device may give an erroneous measurement result.

Another measurement device and measurement method based on diffusion flow is disclosed in Finnish patent application number 981295. In the solution disclosed in the application, a substantially vapour-tight measurement chamber is formed above the surface being measured, moisture being able to diffuse through the airspace contained in which chamber, either from the surface being measured to the other end of the chamber, or in the opposite direction. The moisture content of the air is determined at at least two points in the measurement chamber and the moisture flow through the measurement chamber is calculated from the results. Alternatively, the moisture flow can also be determined in such a way that the moisture content is measured at only one point on the surface of the material and can be assumed to be known at the other end of the measurement chamber.

Figure 1 shows the solution described in patent application FI 981295 in greater detail.

The measuring device of Figure 1 includes a cylindrical body A open at the first and

second ends, within which there is a measurement chamber G. The measuring device also includes a cover B arranged to close the second open end of the body A, a moisture binder C or moisture source C arranged between the body A and the cover B, a seal D arranged at the first end of the body A, and temperature and moisture sensors E, F located in the measurement chamber G near to the first and second ends of the body A.

The figure also shows a material H, the moisture evaporating from which is measured.

The diffusion of the moisture is measured by setting the measurement device against the surface I of the material H. Surface J is, in turn, the surface of the moisture binder C or moisture source C that binds or correspondingly releases moisture.

In the measurement method disclosed in application FI 981295, the measuring device is placed tightly against the surface I of the material H being measured, so that an essentially undisturbed diffusion path is created inside the measuring device. The moisture tends to equalize between the surfaces I, J and the air next to them, and a moisture gradient arises in the airspace of the chamber G, in the direction between the surfaces I, J. The moisture gradient in turn causes the diffusion of moisture in the airspace of the diffusion chamber, from the moister end to the drier end.

After this, the partial pressure of the water vapour at two points is determined from the airspace of the diffusion chamber. The first measurement point is located close to the surface I being measured and the second close to the moisture binding or releasing surface J. The partial pressure p w of the water vapour can be determined, for example, by measuring the relative humidity RH and the temperature T of the air, and calculating the partial pressurepw from these quantities. The partial pressurepW can then be calculated from the formula: RH T) *<BR> 100 pws U') La in whichpw is the partial pressure of the saturated water vapour. The partial pressure of the saturated water vapour, in turn, is obtained, for example, from tables or by calculation from the formula:

If the water vapour pressure p w, at the first measurement point differs from the water vapour pressure ? at the second measurement point, a diffusion flow W will arise between the vapour pressure levels represented by the measurement points. The diffusion flow W from the first end of the diffusion chamber G to the second end can be calculated using generally known flow equations, on the basis of the difference in the vapour pressures, the distance between the vapour pressure levels represented by the measurement points, the cross-sectional area of the diffusion path, and the permeability of the water vapour. If the moisture transfers from the material H being measured to the moisture binder C, the diffusion flow W is positive. If, on the other hand, are moisture source C is used, from which moisture is transferred to the material H being measured, the diffusion flow W obtained as a result will be negative.

If the surface I of the material H being measured can release or receive moisture, a diffusion flow W will begin to form immediately after the formation of the diffusion chamber G. At the very start, it will not be possible to measure the diffusion flow reliably, because the moisture gradients in airspace of the diffusion chamber G and between the surfaces I and J and the air near them will not yet have stabilized. The stabilization stage is typically followed by a stage of a large diffusion flow, when moisture is rapidly evaporated from the surface I of the material H into the air dried by the moisture binder C. Once the moisture of the surface I of the material H has diminished, the diffusion flow W also diminishes rapidly. If the diffusion flow W is measured within this time range, it is possible to determine the maximal evaporation capacity of the surface I being measured, under the prevailing moisture conditions.

When the measurement is continued, the surface parts of the material H being measured dry, and can no longer significantly release moisture from themselves. If, however, there is moisture in the interior parts of the material H being measured, the moisture will begin to move from the interior parts of the material H towards the surface I. At this stage, the diffusion flow approaches an equilibrium flow, examined as absolute values on an exponential curve. This basic rate of evaporation in turn depicts the properties of the material H. If the permeability of the material is known, then the equilibrium moisture content of the material can be decided on the basis of the basic rate of evaporation.

By measuring not only the diffusion flow, but also the equilibrium moisture content of the material at a specific depth d, the permeability of the material in question can be calculated on the basis of the said equilibrium moisture content, the measurement depth, and the reading of the sensor E, from the formula: [kg*rri 1*s-1*Pa 1], 3<BR> Pwm-Pws in which ~ jUm is the permeability of the material to water vapour, -W is the diffusion flow in kilograms per square metre per second, -p wm is the partial pressure of the water vapour inside the material at the depth d, measured by equilibrium moisture content measurement, and -pws is the partial pressure of the water vapour measured on the surface of the material by the sensor E of the diffusion chamber.

The achievement of equilibrium can be monitored by measuring the diffusion flow as a function of time. Equilibrium has been achieved, once the absolute change in the diffusion flow is no longer significant.

The material's equilibrium moisture content is determined by the amount of water contained by the material and by its ability to bind water. The water binding ability is material specific. For the same water content, the equilibrium moisture content of a material with a large water binding capacity is lower than that of a material with a small water binding capacity. The moisture physics of structures and the properties of building materials are described in greater detail in the book'Manual on Moisture Control in Buildings' (ISBN: 0-8031-2051-6).

When measuring the moisture absorption capacity of the material through the surface I, the procedure corresponds to that described above. The measurement differs in that the diffusion process is then in the opposite direction.

The maximal evaporation or moisture absorption ability of the surface I and the basic

evaporation rate or moisturizing rate of the equilibrium range can be determined by measuring these quantities is the manner described above. Alternatively, if the diffusion process can be depicted using an exponential equation, it is possible to measure a sufficient number of points as a function of time, in order to determined the exponential relation. After this, the desired quantities can be extrapolated from the exponential equation that has been formed.

The material H being measured does not, however, always show an exponential dependency. This can be due to, for example, the structure being measured itself, or the fact that there are large moisture gradients in the material, due to the moisture history prior to the moment of measurement. Deviations due to the structure being measured can be caused by, for example, moisture conductors close to the measurement area, such as cracks, or materials or joints between materials, which conduct moisture well. Even in that case, valuable information on the structures, their moisture conditions and/or their moisture behaviour can be obtained from measurements.

Application FI 981295 also discloses a method, in which the partial pressure of the water vapour is measured only from a single point in the diffusion chamber G. In such a case, the measurement point should be close to the surface I being measured. The second partial pressure required in the calculation of the diffusion flow is then calculated using the assumption that the relative humidity of the diffusion chamber G on the surface J of the moisture binder C is zero. Correspondingly, the relative humidity of the air on the surface J of the moisture source C can be assumed to be 100 %. In order to improve the accuracy of the measurement, it is possible to use experiential values, for example, in the ranges 1-3 % and 97-99 %, which deviate from the previously assumed values of 0 % and 100 %.

A drawback in the measuring device disclosed in application FI 981295 is that a relatively long measuring time is required. This is mainly due to the fact that the moisture profile of the diffusion chamber must be allowed to stabilize for several minutes before a reasonable measurement can be begun. The stabilization in turn takes longer the longer the diffusion path is between the material being measured and the moisture source/moisture binder. However, the diffusion path cannot be shortened to at

least less than five centimetres, as otherwise the accuracy of the measurement will suffer substantially. The reduction in the accuracy of the measurement is due, among other factors, to the fact that the moisture gradient at the sensor becomes too great.

A second drawback in measuring devices of the kind disclosed in application FI 981295 has been that the relatively large surface area of the interior of the measurement chamber has made the absorption and desorption of moisture in the wall of the measurement chamber itself a source of error.

The invention is intended to create a measurement device based on an entirely new type of measurement principle, which will permit a faster measurement that the solutions described above, but, however, in such a way that the accuracy of the measurement will not at least suffer essentially.

The invention is based on equipping the measuring device with means, with the aid of which it is possible to resist the diffusion of a gaseous substance in the diffusion path of the measurement device. This resistance to diffusion refers to the fact that means are used to affect the permeability of the diffusion path, in such a way that the permeability is reduced compared to the permeability of the normal measurement atmosphere, i. e. usually air. The permeability of the diffusion path is affected in the area that comes after the content measurement point, when seen looking away from the surface being measured. In an embodiment, in which the diffusion path contains two content measurement points, the permeability of the diffusion path is affected in the area between these measurement points.

The measurement point or points, on the other hand, are preferably located in the normal measurement atmosphere, and not in the area of reduced permeability. Thus, the gradient of the content of the substance being measured remains small in the area of the measurement point, which affects the measurement accuracy beneficially. If the measurement point is located in the area of reduced permeability, a larger content gradient will prevail at the measurement point and the measurement accuracy will diminish. In other words, the means arranged to resist the diffusion of the gaseous substance have an area of effect and at least one content sensor is arranged to measure

the content of the gaseous substance outside the said area of effect. The term area of effect then refers to the area, in which the permeability of the substance being measured has been reduced.

More specifically, the measuring device according to the invention is characterized by what is stated in the characterizing portion of Claim 1.

By means of the invention, the considerable advantage is gained that the diffusion path can be designed to be extremely short without essentially decreasing the measurement accuracy, so that the stabilization of the moisture profile is considerably accelerated.

This is because, with the aid of the invention, a content difference between the measurement point and the second assessment point that is sufficiently large to maintain the measurement accuracy can be created even with a short diffusion path. The second assessment point can be either a second measurement point or, for example, the end point of the diffusion path. The end point of the diffusion path, which is referred to here, can be a moisture binder, a moisture source, or the surrounding atmosphere, i. e. in any case, a point at which the content of the gaseous substance can be assessed with sufficient accuracy. In a preferred embodiment, two measurement points are used and the content difference is formed with the aid of a membrane, which has a known permeability, between the measurement points.

In addition, with the aid of the invention the gradient of the substance being measured is not greater at the measurement point than it is in known solutions, as most of the change in the content takes place in an area in which the permeability has been reduced.

The invention also has several preferred embodiments, which offer significant additional benefits.

In one preferred embodiment, the permeability of the diffusion path is affected by setting the diffusion path to run through a membrane with a known permeability to water vapour. By selecting the membrane so that the hygroscopicity of its surfaces is extremely low, the important additional advantage is achieved that the effect of the surface phenomena of the measurement chamber on the measurement is eliminated. The

effect of the surface phenomena of the measurement chamber is eliminated, because when applying the invention is the preferred manner, practically all that is measured is the content difference that is formed over a membrane with a known permeability. Thus, by means of this preferred arrangement, it is also possible to solve the second drawback of the known solutions referred to above.

In addition, with the aid of the invention, the further advantage is achieved that the measurement device can be built to be extremely small.

In the following, the invention is examined with the aid of examples and with reference to the accompanying drawings.

Figure 1 shows the solution disclosed in patent application FI 981295 in greater detail.

Figure 2 shows a cross-section of one measuring device according to the invention, placed on a surface to be measured.

Figure 3 shows a cross-section of a second measuring device according to the invention, placed on a surface to be measured.

Figure 4 shows a cross-section of a third measuring device according to the invention.

Figure 5 shows a cross-section of one operating assembly of a fourth measuring device according to the invention.

Figure 6 shows a cross-section of the measuring device of Figure 5, in which the components that are designed to be detachable are detached.

Definitions: The term absorption refers to the phenomenon, in which a material binds another substance, for example, a liquid or gaseous substance, from the environment to itself.

The term desorption refers to the phenomenon, in which a material releases another

substance, for example, a liquid or gaseous substance, into the environment.

The term diffusion refers to a thermodynamic phenomenon, in which molecules move from a higher concentration to a lower concentration.

The term hygroscopicity refers generally to the ability of a material to absorb water. In this publication, hygroscopicity is given a wider meaning, and is used to refer to the ability of a material to absorb the substance being measured at the time.

The term moisture source refers to an element, which is able to release water vapour, or any substance being measured at the time, at a significant rate.

The term moisture binder correspondingly refers to an element, which is able to absorb water vapour, or any substance being measured at the time, at a significant rate.

The measuring device of Figure 2 includes a body 1, in the lower part of which there is the first contact surface, through which the interior 7 of the measuring device is in contact with the upper surface 9 of the material 8 being measured. The contact surface is usually only an opening in the body 1 of the measuring device. In some embodiments, the contact surface can, however, comprise a filter that is highly permeable to the substance being measured, with the aid of which the interior of the measuring device can be protected from external mechanical and chemical stresses. In the body 1, there is also a second contact surface, which is located at the opposite end of the body 1. In the case of the figure, the second contact surface is formed by the upper surface of the measuring device. The purpose of the second contact surface is to connect the interior 7 of the measuring device with the surrounding airspace or accessories. These accessories include different kinds of moisture sources and moisture binders.

The task of the interior 7 of the body is to delimit an undisturbed diffusion path within the measuring device, along which the gaseous substance being measured can diffuse between the first and the second contact surface. The device of Figure 2 also includes a content sensor 5, for measuring the content of the gaseous substance at a content measuring point located in the diffusion path. When measuring a moisture flow, the content sensor 5 is a moisture sensor and the requisite temperature measurement

properties may be added to it, in order to achieve the desired accuracy of measurement.

Further, the device of Figure 2 includes a divider diaphragm 20, the permeability of water vapour in which is known and which creates a content difference of a significant amount of the substance being measured, between the interior 7 and the atmosphere surrounding the measuring device. The diffusion flow of the substance being measured can be calculated with the aid of the known permeability of the diaphragm 20 and the said content difference.

The device of Figure 3 includes not only the elements described in connection with the device of Figure 2, but also a seal 4 that runs around the first contact surface of the body 1, and a second content sensor 6, which corresponds to the first content sensor 5, but which is located on the opposite side of the diaphragm 20. Further, the device includes a moisture source or moisture binder 3 arranged against the second contact surface of the body 1, and a cover 2, which surrounds the moisture binder 3 and closes the second end of the body 1.

The use of the second content sensor 6 brings the advantage that an accurate content value is then obtained from the other side of the diaphragm too.

The device of Figure 4 includes not only the elements of the device of Figure 2, but also a moisture source or moisture binder 3 arranged against the second contact surface of the body 1. In addition, the device includes a second content sensor 6 manufactured in connection with the first content sensor 5, the measuring characteristics of which correspond to those of the first content sensor 5. The double sensor 56 thus formed is located in such a way that it penetrates the diaphragm 20 at a hole in the diaphragm made for this purpose. In the same way as the embodiment of Figure 3, the content sensors 5 are used to measure the content on both sides of the diaphragm 20, so that the content difference over the diaphragm 20 can be determined. In such an embodiment it is good to ensure that the connection between the diaphragm 20 and the double sensor 56 is tight, so that the substance being measured cannot leak from a gap between the diaphragm 20 and the double sensor 56. A possible leak would cause an unpredictable change in the permeability of the diaphragm 20 and thus influence the accuracy of the

measurement.

The measuring device shown in Figure 5, which is thus shown with the components detached in Figure 6, differs, on the other hand, from the device of Figure 4 mainly in that the body 1 is divided into two pieces, pieces 1 and 1'. In this case, the body pieces 1 and 1'and the diaphragm 20 can be formed in such a way that the diaphragm 20 can be supported between the body pieces 1 and 1'. An embodiment of this kind permits the diaphragm 20 to be changed easily between measurements, or for maintenance. In the measuring device, there is also a cover 2, which is designed similarly to be compatible with the moisture binder or moisture source 3 and with the upper part of the body piece 1. The joints between the body pieces 1 and 1'and cover 2 and body piece 1 can be implemented as, for instance, friction joints, which provide the necessary sturdiness, or, for example, with the aid of threads. The location of the joints and the actual shapes of the components can, of course, be designed in very many ways, and the examples of the. figures are in no way intended to limit the invention to certain shapes or manufacturing techniques of the mechanical components. The figures should also be understood as schematic drawings, which illustrate the implementation of the invention by means of clear examples, so that the proportions of the devices of the figures, for example, do not necessarily correspond to the proportions used in practical applications.

In the figure, support means 12 are also shown, on which the double sensor 56 is supported. At one end, the support means 12 are attached to the body piece 1. The necessary leads for connecting the double sensor 56 to external electronics can also be arranged inside or on the surface of the support means 12. The external electronics required for this is not, however, described in detail in this publication, as it is generally known to those versed in the art and does not relate to the scope of this invention.

With the aid of the invention, the measuring device can be made extremely small. For example, in a solution similar to that shown in Figure 5, the thickness of the diaphragm 20 can be well under 1 mm and the free airspaces beneath and above the diaphragm 20 can have a thickness of, for example 1-3 mm. With the aid of the invention, the measuring device can thus be made as low as is practicably possible. The width of the measuring device, on the other hand, is affected by the desired measuring surface area,

which can be, for example, from 10 to 150 cm2.

The essential part of the operation of the devices of the example is the diaphragm 20. In the following, preferred diaphragms for use in conjunction with the measuring device described above are examined in greater detail.

The central property of the diaphragm 20 was thus that its permeability to the substance being measured must be less than that of a corresponding layer of air. The magnitude of the diffusion resistance formed by the diaphragm 20 should be typically designed to be at least 10 times greater that the resistance of a layer of air of corresponding thickness.

Usually, the most practicable membranes are, however, those in which the said coefficient is between 100 and 1000. In that case, the diaphragm 20 can be made thin, so that the surface phenomena in the membrane itself remain small. This is particularly the case, if the diaphragm is manufactured from a solid substance with a very small hygroscopicity.

The diaphragm 20 can be manufactured form such a preferred material, which itself does not allow the substance being measured to permeate, by making a sufficient number of holes of a suitable size in a membrane of the material in question. The permeability of a membrane of a certain thickness can then be affected by altering the surface area of the holes in relation to the total surface area of the membrane. Possible parameters for the membrane are, for example, the following: -membrane thickness 100 um-1 mm, -diameter of the holes 10 urn-100 urn, and -ratio of the surface area of the holes to the surface area of the membrane 1/100- 1/1000.

Other parameters too can, of course, be used according to the required application and the membrane material and manufacturing technique used. Preferred manufacturing materials include polytetrafluoroethylene or polished metal, for example, steel. The holes, on the other hand, can be made, for example, by a mechanical method, such as impact, or with a laser.

It is also possible to manufacture the diaphragm 20 from a porous material, for example, from rigid cellular plastic or cellular rubber. If desired, holes can also be made in such a membrane, in order to increase permeability.

The diaphragm 20 can also be manufactured form a suitable fabric, for example, from polytetrafluoroethylene fabric. The diaphragm 20 can also be manufactured from thin non-hygroscopic cloth, for example, from cloth manufactured from suitable plastic. A polytetrafluoroethylene membrane containing holes can also be laminated into a cloth of this kind. The resistance of the diaphragm 20 can be set as desired by layering a sufficient number of the said cloth, fabric, or laminated layers on top of each other.

In the manufacture of the diaphragm 20, it is possible to exploit also a suitable composition containing gel or grease, which can be spread on top of or between, for example, membranes or layers of the kind referred to above.

In practical measurements, an important property of the diaphragm 20 is that the permeability behaves consistently as a function of time. Thus, the permeability of the diaphragm 20 should not change substantially during a measurement or between measurements. In this sense, the perforated polytetrafluoroethylene membrane or metal membrane described above are good diaphragms. Membranes of this kind have also other extremely advantageous characteristics. That is, they can be washed and are mechanically durable. Further, when applying a construction, in which the membrane can be easily detached and put back in place, mechanically durable membranes can be changed according to the measurement situation, for example, in order to change the measurement range. In that case, a measurement membrane with a greater permeability can be used in a situation, in which the diffusion flow being measured becomes large, whereas, in a situation with a small diffusion flow, a membrane with a smaller permeability can be placed in the measuring device. By means of such an arrangement, using suitable membranes, a precise measurement range, which is typically content differences of 20 % to 80 %, can be set for the content difference measured over the diaphragm 20 of the measuring device.

Once the diaphragms 20 have been manufactured, their permeability L must be

determined. This can be done in principle by calculation, but in practice the permeability is, however, determined by measurement separately for each diaphragm. The permeability L can be determined by measuring the diffusion flow W for the difference of the partial pressures pi and P2 over the diaphragm 20. The following equation is then obtained for the diffusion flow: = (P2-P7) . (4) This equation can be easily applied in conjunction with the formulae 1-3.

If extremely great measurement accuracy is required, the calculation can also take into account the change in content that takes place in the gaseous atmosphere on both sides of the diaphragm 20 before the measurement points. This may be required particularly in such devices, in which there is a great distance between the diaphragm 20 and the content sensor 5 or 6, or in which an exceptionally great permeability has been selected for the diaphragm 20. In that case, diffusion resistance that takes place outside the diaphragm 20 has significance. The diffusion resistance of the gaseous atmosphere in series with the diffusion resistance of the diaphragm 20 can be taken into account in the calculations by means of a simple calculation formula well known to one versed in the art. The series resistance can, if desired, be taken into account already when determining the permeability of the diaphragm 20 and the permeability value of the diaphragm can be expressed in such a way that it already includes the device-specific calibration coefficients.

One advantageous special feature made possible by an exchangeable diaphragm 20, is that a reference element, with the same permeability in the initial situation as the diaphragm element 20, can be manufactured for each diaphragm element 20. With the aid of such a reference element, it is possible to check the condition of the diaphragm element 20, by means of a simple measurement, in which the measuring device is placed in a large content and a small content in turn. These measurements are made through both the reference element and the diaphragm 20 and the values shown by the device are mutually compared. If, when using the diaphragm 20, the measurement values obtained differ substantially from the values obtained when using the reference element, the

diaphragm element should be replaced or cleaned. When using a measuring device, in which it is easy to change the diaphragm element 20, the above procedure makes it possible to check the condition of the measuring device before each measurement series and thus achieve repeatedly reliable measurement results.

In moisture measurements, the condition of the measuring device can be checked even more simply with the aid of a water vessel and a reference membrane. This is because the check can be made by placing the reference membrane on top of the water vessel and the measuring device in turn on top of the reference membrane. A moisture binder 3 is used in the measuring device. The relative humidity in the airspace between the water surface of the water vessel and the reference membrane is then nearly 100 % and the almost zero at the upper sensor 6 of the measuring device. At the lower sensor 5 of the measuring device, the relative humidity of the air settles at a value, which is determined according to the ratio between the permeabilities of the diaphragm and the reference membrane. If identical permeabilities have been selected for the reference membrane and the diaphragm 20, the moisture value given by the lower sensor 5 should be very close to 50 %, if the measuring device is in good condition. If the moisture value deviates from this value, it can be decided whether the permeability of the diaphragm 20 has decreased or increased from the magnitude and direction of the deviation. The diaphragm 20 can then be cleaned or changed as required, or the possible installation error in the diaphragm 20 can be corrected. If, on the other hand, the moisture value given by the upper sensor 6 differs substantially from zero, it can be decided that the absorption ability of the moisture binder 3 has diminished and that it must be changed or regenerated.

In the measurement itself, the diffusion flow of the substance being measured can be calculated simply from formula 4, with the aid of the permeability L determined for the diaphragm 20 and the partial pressurespl andp2 of the substance being measured. As stated above, the second partial pressure can also be determined by estimation. When seeking a good measurement accuracy, it is preferable, however, to use at least two measurement points. If desired, the measurement points can also be located in the measuring device, for example, at three or four measurement levels, and also in such a way that, in each measurement level, there are, for example, two, three, or four separate

sensors. In this case, the term measurement level refers to assumed levels of uniform content.

If desired, additional functions can also be added to the measuring devices described above, or the device can be connected to a computer. The method also does not restrict the material being measured. The method can be used to also measure, for example, ground. The building material 8 being measured can, in turn, be nearly anything at all, for example, concrete, timber, brick, tile, or gypsum, wood, or chip board. The point being measured can also be a joint between different materials. The material 8 can also be either a loose building material or can form part of a fixed structure. One particular application could be the measurement of a gas, for example, ammonia, released from a building material. In that case, the content of the gas being measured is typically very small in room air compared to the content prevailing in the building material, and a second measuring sensor 6 may not be needed at all.

The body 1 can also be shaped in such a way that an opening is made in the side of the body 1, through which, for example, a moisture binder 3, a moisture source, or a diaphragm 20 can be pushed inside the measuring chamber.

The task of the body 1 is, together with the cover 2, to delimit the measuring chamber 7 on top of the surface 9 being measured. In addition, the body supports some parts of the measuring device. There are practically no limits in principle to the shape of the measuring chamber 7 and thus of the body 1 itself. However, the edges at the first end of the body 1 should be, when measuring flat surfaces, as well and evenly as possible in one level, to that a good tightness is achieved between the body 1 and the surface 9 being measured. The cross-section of the measuring chamber 7 in the direction of the surface 9 being measured, on the other hand, can be, in principle, of any shape at all.

However, the preferred shapes in terms of manufacturing technique are a rectangle, a square, and particularly a circle. In other ways, it is preferable to shape the measuring chamber 7 in such a way that, in the measuring position, the measuring chamber 7 continues shaped as a beam in the direction normal to the surface 9 being measured and has a uniform cross-section. The preferable shapes for the measuring chamber 7 are thus, for example, a cube or a cylinder. In principle, the measuring chamber 7 can, however,

narrow, widen, or change its shape when moving from the first end of the measuring chamber 7 to the second end, but a regular shape will facilitate the modelling of the diffusion phenomenon and thus the measurement. The diffusion path, which can be defined as a path within the measuring chamber 7 from the surface 9 being measured at the first end of the body 1 to the second end of the body 1, is thus preferably of an even width and in the direction of the normal to the surface 9 being measured. The direction of the diffusion path can also be the opposite of than described above. The measuring chamber 7 thus operates as a diffusion chamber. In a preferably shaped measuring chamber 7 the surface being measured and the surface binding or releasing moisture are parallel.

The material of the body 1 is preferably of a material that does not bind the substance being measured and which is a good conductor of heat, so that the measuring chamber 7 itself will affect the measurement as little as possible. Suitable materials are, for example, aluminium, steel, or polytetrafluoroethylene. The body 1 can also comprise several different materials.

It is preferable to design the moisture binder 3, the moisture source, and the diaphragm 20 in such a way that the said elements essentially cover the entire cross-sectional surface of the diffusion path. When using a preferably shaped measuring chamber 7, such shapes can be used to avoid a moisture gradient arising within-the measuring chamber 7 parallel to the said elements. The substance binding moisture in the moisture binder 3 can be, for example, silica gel, calcium chloride, or some other similar substance, which is able to bind the substance being measured. It is important, that the moisture binder 3 is able to bind moisture at least at more or less the same rate that the surface 9 being measured releases it. The substance of the moisture source 13 releasing moisture can, in turn, be, for example, felt wetted with the substance to be measured. An important property is that the moisture source can, when necessary, release the substance being measured at at least the same rate as the surface 9 being measured can bind it at the second end of the diffusion path.

Further, the measuring device can be equipped with a dimmer-type mechanism, which is set on top of the diaphragm 20 and with the aid of which part of the surface of the

diaphragm, or a hole in it can be closed or correspondingly opened. Such an arrangement can also be used to regulate the permeability of the diaphragm 20. The measuring device can also be designed to be such that the permeability of the diaphragm 20 can be altered without interrupting the measurement. This is possible with the aid of, for example, a double-membrane construction. In such a constructions, there are two membranes on top of each other, in such a way that by rotating one membrane it is possible to close and correspondingly open a desired amount of the holes in the actual perforated diaphragm.

Thus, the permeability of the diaphragm totality 20 can be altered in a controllable manner, without interrupting the actual measurement.