The invention refers to a process according to the generic part of claim 1, a filtration device and an extinction measuring device to complete the process.
It is well-known that different measuring methods are available for the analysis of turbid suspensions of different kinds e.g. waste waters or digestion solutions. One measuring method frequently used involves the measurement of the extinction of a suspension at certain wave lengths which provides useful information about the composition of the suspension and in particular about particularly interesting substances with a characteristic absorption spectrum.
The absorption of light by chemical substances is, as is generally known, ascertained by measuring extinction E in wave length ranges typical for the substance in question. Extinction E is determined according to Lambert's Law from the log of the quotient from the intensity of the light entering the sample to the intensity of the light emerging from the sample and is proportional to the concentration and the layer thickness of the sample. Lambert's Law is not valid for some substances and substance mixtures and an additional calibration is required, however, the mensuration technique remains largely uninfluenced by the additional calibration.
Starting with the established manual methods to measure extinction, which harbour the disadvantage of requiring a great deal of time and labour and moreover supply only discrete values, from which one can only determine tendencies in the system via extrapolations, a great deal of effort has gone into simplifying the procedure of analysis by automating the individual steps Particularly, the development of H process, in which the extinction of a suspension can be measured in one step continuously, represents a technical problem .
Regular flow properties in the sample, which are necessary to ensure an exact measurement e.g. to obtain reproducible sample volumina, which are known with respect to clear liquids, are not present in the suspension due to the existence of particles of different sizes which are moreover swollen in part. Coarse particles can lead to blockages, circuit interruptions and consequently damage in the mensuration apparatus. For the measurement itself, the sample must be freed of all particles.
In many cases, pressure and temperature fluctuations occur not only as a result of the consistency of the suspension but also due to the treatment to which the suspension is

subjected during its respective processing e.g. in a digestion tank. These can spoil the measurement. Gas bubbles, which also occur in the sample, prevent the determination of reproducible values.
Until now, these problems have prevented the development of a reliable one-step continuous process for the measurement of suspensions.
The difficulties named above occur in particular with respect to the measurement of digestion solutions, particularly cooking acid during the chemical digestion of wood.
It is an established fact that chemical digestion must be performed to produce pulp from wood in order to dissolve out the lignine and hemicellulose contained in the material. Cellulose is left in the residual material from the digestion which serves as a raw material for various industrial-scale products such as man-made fibres.
Different processes for the chemical digestion of wood arc used today at industrial level. The main basic processes to be used involve sulphite or sulphate digestion. Whilst the sulphate process is characterized by the use of soda lye NaOH and sodium sulfide Na2S and is also designated as an alkaline process due to the mainly alkaline milieu, the sulphite process is characterized in the main by the presence of sulphur dioxide S02- In an aqueous solution sulphur dioxide is balanced with sulphurous acid H2SO3. The digestion solution, which also comprises metal ions from the sulphites employed, is, therefore, often named "cooking acid" or "digestive acid" for this reason.
To perform chemical digestion, the raw mntcrinl is usually treated with the cooking acid for several hours at raised temperature. The digestion itself is often described as "cooking" and the digestion tank as a "cooker" or "digester". Following a certain period of time, the so-called stopping process is introduced; this means that the temperature and excess pressure in the digester are slowly reduced. It has been shown that the absorption behaviour of the digestion solution, which has changed as a result of a change in the composition of the digestive acid, supplies important information to evaluate the stopping process. Extinction is hereby preferably measured at a wave length of 430nm.
According to Lambert's Law a look at the absorption values of the cooking acid directly allows some conclusion about the reduction in concentration of the cooking acid in the digester and, therefore, about the condition of the system as a whole. It is, therefore, absolutely necessary to control the absorption values of the cooking acid in order to avoid possible serious quality impairments in the final product as a result of faulty process control when

stopping. As a result of the measured values obtained it is, for example, possible to influence the speed of stopping and thereby obtain the desired quality of the end product.
Until now, the measurement of the absorption of cooking acid was generally done by hand and in a discontinuous manner. A small sample of cooking acid is taken from the digester at a bleeder connection at regular intervals of approx. 20 minutes and is analysed in the laboratory.
This method has various disadvantages: the long periods of time between the individual measurements allow very little insighrjnto the time progress of the stopping process so that it is very possible that considerable changes occur in the system between two measurements which can lead to quality impairments if they arc not immediately removed. Moreover, longer-term tendencies, from which conclusive statements can be made about the length of the stopping process and the control thereof cannot be concluded from the discrete values, since an extrapolation is afflicted with a high rate of inaccuracy.
Moreover, the method demands lots of time and personnel and allows for the mistakes common with respect to analyses conducted by hand such as, for example, dilution errors.
In the attempt to provide a continuous measuring process which avoids these disadvantages, however, the following technical problems, as already described above, occur:
It must be possible to exclude disturbances with respect to the flow of the sample from the digestion tank to the measuring device. This is difficult to accomplish since a considerable share of solid substance in the sample hinders the flow, and the pressure in the digester during the stopping process fluctuates considerably. In the initial stage of the stopping process, pressures of about 7 bar are mostly measured whilst pressures of no more than about 3 bar arc present in the final phase. Moreover, considerable gas formation takes place in the sample.
Since dilution of the sample, as foreseen in the manual process, is very complicated, the measurement of extinction is also difficult due to the large measurement range. In addition, in many cases the known state of the art extinction measuring devices are only suitable to a very limited extent and are very work-intensive with respect to the continuous measurement of turbid suspensions and of digestion solutions in particular.
The object of the present invention, whilst overcoming the above-named disadvantages and problems, is to provide a process for the measurement of the extinction of turbid suspensions which makes it possible in a simple and user-friendly manner to observe the progress of the absorption values of the interesting substances in the suspension in continuous fashion.

Moreover, the object of the invention is also to provide a filtration device and an extinction measuring device with which the process according to this invention can be performed in a simple fashion and without the need for lots of devices.
The aim of the process according to this invention is to make it possible to continuously measure the progress of the absorption of digestion solutions, and in particular of cooking acid in the chemical digestion of wood.
This task is resolved by a process according to this invention characterized in that the following steps are performed continuously in the given sequence:
• taking of samples from the suspension tank
• separation off of larger and smaller particles
• degassing of the sample
measurement of the sample in an extinction measuring device
For the first time it is possible with the process according to this invention to measure, fully automatically, the absorption behaviour of turbid suspensions in one step. It is not necessary to perform conversion work on the devices during the measurement since a regular flow of the sample is guaranteed due to the individual steps of the process even when circumstances in the system change.
Once the sample has been extracted from the suspension tank e.g. a tank or a digestion container, the particles to be found in the suspension are separated off and the sample is freed of gas bubbles forming which guarantees interference-free circulation and the exact measurement of the sample by the downstream extinction device.
A sample measuring procedure according to the invented process, therefore, only takes a few seconds, depending on the embodiment, up to a maximum of one minute and is thus considerably shorter than the manual analysis. In addition, a continuous measuring curve can be drawn up as a result of the measuring values which occur one after the other in rapid succession, from which tendencies and erratic changes in the system are immediately recognisable.
The task is resolved in an advantageous manner according to the invention by the fact that the separation of the particles is performed by a two-step filtration whereby in a first filtration device coarser particles with a diameter, preferably larger than 1 mm, are separated and in a

second filtration device finer particles with a diameter preferably larger than 50 µm. The separation of the particles is thereby possible with a low level of apparatus in particular,
Since particles of different sizes are present in the suspension and a one-step process is, therefore, relatively work-intensive, it has proved to be particularly advantageous for the process according to this invention to conduct a filtration in two steps. In a particularly advantageous manner coarser particles with a diameter of over 1 mm are separated in a first filtration device and finer particles with a diameter of over 50 µm in a second downstream filtration device. In this way it is possible to efficiently filter the suspension using two relatively simple apparatuses.
In an advantageous manner, the process according to this invention is further characterized in that the sample is alternately led into at least two degassing vessels in parallel connection after filtration whereby at least one part of the sample is led into respectively one degassing vessel and degassed and, at the same time, another part of the sample which is already degassed is led to the extinction measuring device from resp. one other degassing vessel.
In this embodiment of the invention it is possible to completely free the sample of gas bubbles without thereby having to interrupt the transport of the sample to the measuring device and the measurement itself. Whilst one of the degassing receptacles in parallel connection is filled with the sample from the filtration device and the sample is degassed, the sample flows from another degassing receptacle, in which another part of the sample has already been degassed, to the measuring device. It is ensured that the sample loads reach the measuring device in the right sequence by means of the corresponding established control of the supply and discharge functions.
One further preferred embodiment of the process according to the invention comprises the relative measurement of extinction in the extinction measuring device with at least two different layer thicknesses. Due to this measuring technique it can be ensured that any factors which might cause an interference, with respect to the sample, the measuring device itself and the large measuring range of the undiluted sample, will not impair the measuring result.
Common methods to measure extinction are characterized in that an alignment is carried out by the effect of two beams of a different wave length, a measuring beam and a comparative beam. It is possible to ascertain an absolute extinction value only on the basis of the comparison calculated between the blank and measured values obtained. Devices with one and two-beam techniques and one and two cuvette systems are well known whereby all of these

systems have in common the fact that complicated arrangements such as choppers and special mirror/lense systems are necessary to change from the measuring to the comparative beam.
The relative method differs from these conventional processes in that it is not the incidental beam of light but rather the layer thickness of the sample to be measured which is varied. Since according to Lambert's Law (or rather in empirical formula for systems derived from the former in which Lambert's Law does not apply absolutely) the thickness of the layer is always taken as one variable quantity, one arrives with the measurement of one sample with one and the same measuring beam but two different layer thicknesses, at one system of equations from which the extinction of the sample can be calculated in a simple manner.
This method has the outstanding advantage for the process according to this invention that only one beam of light of a defined wave length is required and, therefore, the optical arrangement in the measuring device can be organised in a very simple and economical manner,
It has proved to be particularly advantageous in the process according to the invention when the layer of thickness is varied by adjusting the layer thickness of the sample volume. Normally the layer thickness is varied by using two sample vessels of different thicknesses (e.g. two cuvettes) which demands the use of lots of apparatus since these cells have to be constantly exchanged. It has been shown that it is possible to obtain different layer thicknesses without using several sample vessels by providing a sample volume in which the layer thickness is directly adjustable. This leads to a considerable reduction in the effort required for the relative measurement.
One further preferred embodiment of the process according to this invention is characterized in that the sample is led through at least one pressure vessel before the particles are separated off.
In this way it is simple to compensate the possibly large drop in pressure in the suspension tank and other possible fluctuations in pressure so as to guarantee a regular sample flow. The level reached in the pressure vessel is dependent on the pressure in the suspension tank. The pressure vessel must be of the size required so that a sufficient amount of the sample can be pressed in, with respect to the minimum pressure to be expected, in order to sufficiently fill the measuring system. On the other hand, the amount of sample present in the surplus can be diverted via an overflow pipe at higher pressures i.e. in the area of the degassing receptacle.
The filling and emptying of the surge tank takes place in a matter of seconds which permits a fast measuring procedure.

The process according to this invention is ideally suited for the measurement of the extinction of digestion solutions and in particular of cooking acid in the chemical digestion of wood. Due to the process steps according to this invention, the particles located in the digestion suspension can be easily separated off and large fluctuations in pressure in the digester can be compensated (from 7 bar at the beginning of the stopping procedure up to 3 bar at the end of the stopping procedure). A continuous measurement carried out in this way provides excellent results and much more information compared to the manual process. The stopping procedure can be controlled in such a flexible way by the process according to this invention that it is possible to react to minor changes in the system and, therefore, no quality impairments occur in the end product.
A filtration device is particularly well suited to perform the process according to this invention, made of a filtration vessel with a device for sample inlet, a device for the discharge of the filtered sample which is connected to a filter medium, a separate device for the discharge of unfiltcred sample and a device for the rinsing of the filtration vessel characterized in that the device for the sample inlet is arranged in the form of a nozzle and that the filter medium is inclined at an angle of 33° to 47°, preferably by 39° to 41°, towards the axis of the filtration device.
Due to this new kind of construction, the filtration device is self-cleaning which means that it is not for example necessary to take additional cleaning steps in the filtration device during a measuring process which covers the entire stopping process.
As a result of this design, particles with a diameter larger than the average hole diameter of the filter medium are separated from the liquid which can penetrate unhindered through the filter medium. The particles located on the filter medium are subsequently carried away by the sample beam which follows and rinsed off due to the slanted arrangement of the filter medium. Additional cleaning or rinsing of the filter medium is, therefore, not necessary.
There are different possibilities how to develop a filtration device in self-cleaning fashion whereby according to the present invention a self-cleaning is obtained in a particularly advantageous fashion in that the sample in the filtration device is sprayed via a nozzle onto a filter medium which is inclined at an angle of 33° to 47°, and preferably from 39° to 41°, towards the axis of the filtration device.
One preferred embodiment of the filtration device according to this invention is characterized in that the nozzle has a diameter of 2 to 4 mm, and preferably 3 mm.

Due to the envisaged nozzle diameter, the danger of particle blockage can be practically completely avoided. Surprisingly it is possible, despite the relatively large nozzle opening, to spray suspensions, which contain smaller particles, onto the filter medium in a way that self-cleaning can take place in a simple and reliable manner.
In a further advantageous manner, the filtration device according to this invention can be designed so that the filter medium, onto which the sample is sprayed, is designed in the form of a wire sieve. Other well-known filter media are also suitable for the process.
In a particularly preferred embodiment the wire sieve has a mesh size of 40 to 70 u.m, and preferably 45 to 55 urn. In this way it is possible to separate off particles with a diameter of up to 50 µm.
A filtration device according to the construction as defined in this invention is naturally well suited to numerous applications and in particular for use in the process according to this invention. This device allows the self-cleaning of the filter in a particularly simple manner without the need for complicated devices.
The object of the present invention is also resolved by the use of an extinction measuring device, comprising a light source, an Optical system, a sample volume and a detector system and means to adjust the layer thickness of the sample volume for the relative measurement of the extinction of turbid suspensions with at least two different layer thicknesses.
Conventional extinction measurement devices, e.g. a photometer, are, as is well known, principally built on the basis of the following scheme: light source - optical system - sample volume - detector system. The light source transmits light in a particular wave length range (in rare cases the source of light transmits monochromatized light, i.e. light with only one specific wave length). The beam of light is concentrated on the one hand by an optical system made for example of lenses, monochromators etc. and on the other hand, the specific wave length necessary to measure extinction is set. The beam of light, which is monochromatized in this manner, penetrates the sample in the sample volume, which for example normally is a cuvette in the usual devices, whereby one part of the light is absorbed and the sample is transferred to a higher energy level depending upon the wave length and chemical substance of the sample. The intensity of the beam of light emerging from the sample is detected using known detectors e.g. photodiodes or photoelectric multipliers and compared with the intensity of the light beamed in.

Different types of devices have been suggested for the continuous measurement of extinctions. Some of these are described for example in Ullmans Encyclopedia of Technical Chemistry, 4th edition, volume 5, page 889 f. These are one and two-cuvette systems and one and two-beam systems. As already described above, complicated optical systems are required for these types of devices. Often several sample vessels (cuvettes) are required through which the beams of light penetrate in alternating mode.
For this reason it has been shown to be advantageous to perform the measurement with two different layer thicknesses instead of comparing two beams of light whereby here it is likewise disadvantageous if one works with two sample vessels with different layer thicknesses since these would have to be constantly exchanged.
From AT-B 303.425 a device is already known for the optical analysis of liquid samples in an analysis chamber which is located between two plates with parallel front surfaces and is provided with pliable walls equipped with two measuring zones lying opposite each other and the two plates. A sliding device is foreseen in order to press together one part of the analysis chamber at the time of the measurement.
DE-OS 23 61 752 describes a device for the analysis of liquids, in particular turbidimeters and similar devices, whereby within a flow pipe at least one of the windows of the lighting equipment is movable using a setting device.
From these two documents it is known to adjust the layer thickness of the sample volume located either between two plates or in a flow pipe. The setting of the layer thickness in AT-B 303,425, however, merely pursues the goal of adjusting a certain layer thickness of the sample with a high level of precision. There is absolutely no indication in this document that the extinction of one and the same sample can be measured as a result of adjusting the layer thickness with two different layer thicknesses.
The adjustment of layer thickness in the sample volume in DE-OS 23 61 752 merely serves the purpose of adjusting the length of each path of light due to the different dullness of different samples in such a way that a reproducible measurement can take place. In different samples, therefore, different layer thicknesses can be set. However, the measurement itself is performed according to DE-OS 23 61 752 whilst keeping the length of the path of light constant and the ascertainment of the extinction of one and the same sample by measuring with two different layer thicknesses cannot be deduced from this document either.

If however, extinction measuring devices, in which the layer thickness of the sample volume can be adjusted, are used to perform the relative measurement of the extinction of turbid suspensions with at least two different layer thicknesses, then one obtains excellent measuring results without the need for complicated optical systems. This is quite surprising since this new use is not at all revealed by publications of the state . Rather one skilled in the art is encouraged to measure the extinction of the sample with two different wave lengths.
In an advantageous manner the extinction measurement device is used in a manner that the adjustment of the layer thickness is performed by the shifting of a movable plate, which forms one limitation with respect to the sample volume, against a rigid plate arranged in parallel fashion to the movable plate which forms the other limitation of the sample volume. The layer thicknesses which can be set in this way for example equal approx. 0.4 or rather 0.7 mm when measuring the extinction of the cooking acid. These layer thicknesses can, if necessary, be adjusted by moving the two plane-parallel plates towards one another or away from one another until for example a mechanical stop is reached.
In a further advantageous form, the extinction measuring device is used in a manner that the movable plate is shifted by the introduction of air applied under pressure.
Surprisingly it has been shown that despite the simple construction of the measuring device according to this invention, the measurements produce excellent reproducible values which completely agree with the values of a conventional complicated extinction measuring device.
It is clear that using this particular embodiment of the invention, the plane-parallel plates can be set one against the other in a particularly easy manner.
Advantageous embodiments of the invention are described in greater detail further on in drawings whereby possible designs of the invention should not of course be limited to the content of these drawings
Figure 1 shows a diagram of a preferred embodiment of the process according to the invention
for the measurement of the extinction of cooking acid in the chemical digestion of wood in a
quiescent condition.
Figure 2 is the cross-section of an embodiment of a filtration device according to this
invention.
Figure 3 is a cross-section through an embodiment of an extinction measuring device used
according to this invention.

Figure 4 shows a diagram in which measured values from the traditional manual process arc compared with measured values obtained using the process according to this invention for a stopping process in the chemical digestion of wood.
From figure 1 it can be seen that the sample which originates from the digester (not shown in drawing) first of all flows via a valve 1 into a pressure tank 2. With pressure tank 2 pressure fluctuations from the digester are compensated. Pressure tank 2 is formed so that its volume is sufficient to absorb enough of the sample for the measurement even when the pressure is low. When pressure tank 2 has absorbed enough of the sample, valve 1 closes. Valve 3 opens and as a result of the pressure which has built up in the pressure tank, the digestion suspension flows into a pre-filter 4 in which larger particles are separated off. Prefilter 4 can be of a well known construction design and is of a size at which it is not necessary to clean the filter for the length of time of the stopping process (60 to 120 minutes).
The precleaned sample flows from prefilter 4 into main filter 5 in which smaller particles are separated off This filter is self-cleaning. The filtrate flows further and is led via one of the valves 6, 6' into one of thc degassing vessels 7, 7'. The degassing vessels 7, 7' arc equipped with an overflow pipe 71, 71' to deviate excess sample amounts at the beginning of the stopping process. Whilst the sample is degassed in a degassing vessel 7 with closed valves 6 and 8, an already degassed sample load flows from the second degassing vessel 7' with open valve 8' and closed valve 6' to extinction measuring device 9. As soon as the degassing vessel T has emptied its content, valve 8' closes and valve 6' opens in order to absorb the next sample load which has in the meantime been lead from the pressure-equalizing vessel via the two filters.
Extinction measuring device 9 is first of all rinsed of the sample load and finally the extinction is measured and evaluated with two different layer thicknesses.
One preferred filtration device according to figure 2 is made of a casing 303 basically of cylindrical form with a cover 300. The casing 303 contains a discharge device 304 for washed off particles. Cover 300 contains an, inlet device 310 for the suspension to be filtered which is preferably designed in the form of a nozzle. An inlet 311 is also located in cover 300 for rinsing water to keep the device damp following a complete stopping procedure. Furthermore, the casing 303 contains a collection device 305 for the filtered sample. Collection device 305 is basically of cylindrical design and has an opening on the side 306 to guarantee pressure compensation and a discharge 309 for the filtered sample on the lower limitation . The entire collection device 305 is inclined to the axis of the filtration device by an angle of 312 of approx. 40° and is sealed on the side with casing 303. A wire sieve 308 is located on the upper

limitation of the collodion device 305. The wire sieve is made of steel and has a mesh size of 35 urn.
In the following the mode of operation of this embodiment of the filtration device is described according to figure 2: the sample to be filtered enters nozzle 310 and is sprayed by the nozzle onto the wire sieve 308. The particles thereby remain on the upper side of the wire sieve 308 whilst the liquid penetrates the wire sieve 308 and enters the collection device 305 and from here it leaves the filtration device via discharge 309. The particles which remain on wire sieve 308 are carried away by the sample which follows and rinsed off from the wire sieve due to the slanted arrangement of the wire sieve to the discharge device 304.
The filtration device, therefore, cleans itself during its operation. Following a complete stopping procedure the device can be rinsed with water via inlet 311. Rinsing with water prevents the apparatus from running dry and therefore, from it being impaired in its function.
Figure 3 shows simplified a preferred embodiment of an extinction measuring device 9 used in the invention to perform the process whereby the extinction measuring device 9 basically comprises a light source 901 with a lamp, a lense system 902 to direct the light beam, a detector 903 eg a photodiode and a sample volume 904. The sample volume 904 is in the form of a gap and is formed by two plane-parallel arranged plates 905 and 905'. These plates are, in essence, formed like a cylindrical segment and in the area in which the ray of light is to penetrate the sample the plates are made of a translucent material e.g. quartz glass. One of the two plates 905' is movable and sealed to the side e.g. via a membrane 906 made of Teflon. The movable plate 905' can be shifted sideways via a ring-shaped shift element 907 to which the plate is attached. Figure 3 shows the movable plate 905' in the foremost position i.e. so that the sample volume displays the lowest layer thickness. A sideways protruding part 908 is attached to shift element 907 which gears into a recess 909 which is wider by a certain value than the protruding part 908. The recess 909 is connected to two air pipes 910 and 910' through which air is forced under pressure in alternating fashion. A sample inlet 911 and a discharge 912 are connected to the sample volume 904.
To measure the absorption of a sample the sample flows through inlet 911 into the gap-like sample volume 904 and is penetrated by the beam of light source 901 in the area of the two plates 905, 905', which are made of translucent material. Detector 903 measures the intensity of the beam of light emerging from the sample. The sample flows further through the gap-like sample volume and emerges at discharge 912 from the extinction measuring device 9 whilst new sample flows continuously into inlet 911.

Once the extinction of the sample has been completed in this way with a certain layer thickness (figure 3 shows the measurement with respect to the smaller of the two adjustable layer thicknesses), the layer thickness of the sample volume is now adjusted in the following manner: In order to get from a lower to a higher layer thickness, air is pressed into the recess 909 via the air pipe 910 under pressure whereby the sideways protruding part 908, the shift clement 907 connected to this, and the plate 905' attached to the shift element 907 are moved backwards to the stop of the protruding part 908 on the left limitation of recess 909. In this way a defined wider gap is formed between the two plates 905 and 905' through which the sample flows. Now a measurement is performed with this enlarged layer thickness. Finally, the movable plate 905' can be returned to the front position via the opposite means of procedure (pressing in of air via air pipe 910) whereby the lower layer thickness is set.
It is quite clear that in this manner samples can be measured continuously with two different layer thicknesses whereby the extinction measuring device according to the invention produces excellent measuring results for samples of different types and origins.
Example:
Following sulphite wood digestion, the digester stopping process was introduced in the usual conditions. At intervals of approx. 20 minutes samples were taken by hand and the extinction was measured in an extinction measuring device at approx. 430 nm. At the same time, a continuous measurement was conducted according to the process in this invention using a filtration device according to the invention and the method of relative measurement.
In the form of a diagram, figure 4 displays the results of the two parallel points whereby the continuous line displays the measured values for the continuous measurement and the points marked with "x" show the measured values for the discrete measurements. It is quite clear that the progression of the curve corresponds in excellent manner with the discrete measured values. Moreover, it can be seen that tendencies can be easily concluded from the curve for the continuous measurement which, with respect to individual measured points, is only possible with the risks involved in an extrapolation.

We claim:
1) Process for the measurement of the extinction of turbid suspensions characterized in that the
following steps are performed continuously in the sequence which follows:
taking of samples from the suspension tank separation off of larger and smaller particles • degassing of the sample
measurement of the sample in an extinction measuring device
2) Process according to claim 1 characterized in that the separation of the particles takes place in a two-step filtration whereby coarser particles with a diameter preferably larger than 1 mm are separated off in a first filtration device and finer particles, with a diameter preferably larger than 50 u.m, are separated off in a second filtration device.
3) Process according to one of the claims 1 or 2 characterized in that the sample is alternately led into at least two degassing vessels in parallel connection after filtration whereby at least one part of the sample is led into respectively one degassing vessel and degassed and, at the same time, another part of the sample which is already degassed is led to the extinction measuring device from resp. one other degassing vessel.
4) Process according to at least one of claims 1 to 3 characterized in that a relative measurement of the extinction is performed in the extinction measuring device with at least two different layer thicknesses.
5) Process according to claim 4 characterized in that the layer thickness is varied by adjusting the layer thickness of the sample volume.
6) Process according to at least one of claims 1 to 5 characterized by the fact that the sample is led through at least one pressure vessel before separation of the particles.
7) Process according to at least one of claims 1 to 6 characterized in that the extinction of a digestive solution, particularly cooking acid during the chemical digestion of wood, is measured.
8) Filtration device, in particular for the performance of the process according to one or several of claims 1 to 7 comprising a filtration vessel (303) with a device for the inlet of the

sample (310), a device for the discharge of filtered sample (309) which is connected to a filter medium (308), a separate device for the discharge of unfiltered sample (304) and a device for rinsing the filtration vessel (311) characterized in that the device for the inlet of the sample is in the form of a nozzle and that the filter medium is inclined towards the axis of the filtration device by an angle of 33° to 47°, and preferably by 39° to 41°.
9) Filtration device according to claim 8 characterized in that the nozzle (3 10) has a diameter
of 2 to 4 mm, and preferably 3 mm.
10) Filtration device according to at least one of the claims 8 or 9 characterized in that the filter medium (308) is in the form of a wire sieve.
11) Filtration device according to claim 10 characterized in that the wire sieve has a mesh size of 40 to 70 u.m, and preferably 45 to 55 u.m.
12) Use of an extinction measuring device comprising a light source (901), an optical system (902), a sample volume (904) and a detector system (903) and means (910, 910') to adjust the layer thickness of the sample volume (904) to perform a relative measurement of the extinction of turbid suspensions with at least two different layer thicknesses.
13) Use of an extinction measuring device according to claim 12 characterized in that the adjustment of the layer thickness is performed by shifting a movable plate (905') which forms one limitation of the sample volume (904) against a rigid plate (905) lying parallel to another movable panel (905') which forms the other limitation of the sample volume (904).
14) Use of an extinction measuring device according to claim 13 characterized in that the shifting of the movable plate (905') is performed by the pressing in of air under pressure.

15. Filtration device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.