The present invention relates to measuring and regulating concentration of a fibre suspension, particularly within t pulp and paper industry, although the invention may be use within other areas, such as measuring the dry substance content in sewage sludge, peat suspensions, mineral wool suspensions etc.

Three main groups of transducers may be recognised for measuring the concentration of fibre suspensions, namely shear force transducers, optical transducers and trans¬ ducers working with pressure drop changes or flow speed changes depending on pulp concentration. There are furthe groups of subordinate importance. Transducers working according to the shear force principle may be divided into two main groups, namely rotating and static transducers.

The transducer of the present invention may be suitably placed in the group "static transducers", although there are certain principle differences compared with other devices in the group. Colloquially, these transducers are called static or stationary transducers, shark fins, blade ^■ transducers and fins. The static transducers compete to a minor extent with the rotating type, since the latter usually have better measuring accuracy, higher sensitivity, less flow sensitivity shorter unactivated times etc. User install static transducers primarily in positions where pe formance requirements are not too high.

The static transducers known up to now include a pivotably suspended blade placed in the pulp flow. The suspension most usually comprises some form of mounting and a seal against the medium. Due to the flow speed in the conduit there is a shear force moment on the blade, this moment

being converted to a standardized output signal, which may be pneumatic or electric. The moment converters work eithe according to the weighing balance principle or the force balancing principle. It is thus the front edge of the blade exposed to the flow which cuts through the fibre flow to generate the shear force moment. Since the working prin¬ ciple requires that the medium is in movement, it is easy to understand that a certain, undesired flow sensitivity occurs. The manufacturers have attempted to solve this problem by resorting to more or less sophisticated imple¬ mentations of the blade. The results of these efforts vary but the general opinion is that the dependency on flow in static transducers is one of three dominating problem areas for the group as a whole. The consequence has thus been that the manufacturers stipulate relatively tight flow rang within which the transducers may be used, around 0.5-3 m/s and in some cases as tight as 0.3 - 1.5 m/s. To obtain a flow within these limits the conduit must be tapered diver¬ ging or converging in certain cases, which considerably increases the installation. The next problem area is also brought about by the measuring principle, since the blades are usually relatively voluminous and occupy 80-95 % of a 100 mm diameter conduit. The formation of plugs or the presence of larger solid bodies moving with the fibre flow in the conduit quite often damages the blade and its sus¬ pension. The front edge of the blade cannot, of cource, be protected by a barrier placed too close it, since the mea¬ suring result would then be compromised. A smaller barrier protecting the suspension shaft of the blade is usually integrated in the structure, however. Effective damping systems cause some makes to be less sensitive to vibrations than others.

It was mentioned in the introduction that rotating trans¬ ducers give greater sensitivity to concentration changes. This is a generally accepted fact-, and the chief reason for there being little competitionbetween the groups. The

higher., relative difference in speed between sensor and fi network in rotating transducers gives a higher moment lev in spite of the minor size of the sensor, and in turn this results in higher sensitivity and better signal/noise rel tionship. This becomes particularly noticeable for low co centrations. While static transducers may be used down to 1.5 - 1.75 % concentration, the rotating type often measur down in the concentration range of 0.8 - 1 %. To do justi to a comparison between rotating and static transducers, i should also be mentioned that the price of a static trans¬ ducer is 3-4 times lower than that of the rotating type.

The object of the present invention is to provide a new me and a new device for measurement of the kind mentioned in introduction, where the limitations and disadvantages of c ventional static transducers have been eliminated. This o ject is achieved by the method an"d ^~ device in' accordance wit the invention having been given the characterizing feature disclosed in the following claims.

By means of the present invention there is provided a meth and device for measuring and regulating the concentration of a fibre suspension where, in comparison with other al¬ ready known stationary transducers, the following advan¬ tages should be mentioned: the blade only blanks off a mi part of the conduit diameter and is entirely protected by barrier arranged in front of it, flow dependency is less because of the fully covering barrier, the relative moveme of the blade itself with respect to the fibre network re¬ sults in that the flow speed range can be increased down¬ wards, in certain concentration ranges the flow range will be 0.15 - 5 m/s (for a blade length of 0.15 m) , the active blade gives greater sensitivity for concentration changes within the concentration range of 1.5 - 3 % and that vibra tion sensitivity is low, since the blade is only active fo a very short time, 0.04 - 2.0 % of the total time, and is locked by magnetic force in its rest position, and finally that the number of moving parts is less than in other trans ducers.

The invention will now be described in detail with refe¬ rence to the accompanying drawing, on which

Fig. 1 illustrates, partly in section, a device in accor¬ dance with the present invention installed in a conduit, Fig. 2 is a schematic side view of the optical reading for illustrated in Fig. 1,

Fig. 3 is a side view of a blade,

Fig. 4 is a view from below of the blade illustrated in

Fig. 3, Fig. 5 is a section along the line A-A through the blade illustrated in Fig. 3,

Fig. 6 is a side view of an alternative embodiment of the inventive blade,

Fig. 7 is a view from below of the blade illustrated in Fig. 6,

Fig.8 is a section along the line B-B through the blade illustrated in Fig. 6,

Fig.9 is a side view of an alternative embodiment of the inventive blade, Fig. 10 is a section along the line C-C through the blade illustrated in Fig. 9,

Fig. 11 is a side view of an alternative embodiment of the inventive blade, and

Fig. 12 is a view from below of the blade illustrated in Fig. 11.

A transducer device in accordance with the present inventi is illustrated partially sectioned in Fig. 1, where it is shown with its blade pivotally mounted in a conduit, throu which flows the medium that is measured. The blade 1 is smaller than is general in static transducers and only occupies 30 - 40 % of the conduit diameter, where the dia¬ meter of the conduit is 100 mm. The whole of the front en 16 of the blade 1 is protected by a streamlined barrier 2, which is situated in front of the blade 1, seen in the flo direction. A system of forces in the form of an electro¬ magnetic system 3 is arranged to activate the blade 1 via a suspension shaft β, so that the blade 1 can pivot about a

bearing 4. The free or trailing edge 8 of the blade 1 mo a distance A-B during a given time. The time for the bla 1 to execute this angular stroke o* is a parabolic functio of the concentration of the medium. The inventive trans- ducer device works, as will be seen, according to the she force principle, but the blade 1 has a movement of its ow and is not dependent on the flow in the conduit for the occurrence of shear forces. After termination of the ang lar stroke A-B, the current to the electro-magnetic syste 3 is pole-reversed, and the trailing edge 8 of the blade 1 moves through the_distance B-A, i, e, it returns to its initial position.

The blade 1 moves at a rate ranging from one stroke per second (1.0 Hz) to one stroke every five seconds (0.2 Hz).

An electronic amplifier unit, not illustrated on the draw¬ ing, measures the time,for the stroke A-B with the aid of an optical reading fork 5 including an IR-light-emitting diode 9 (LED) and a detector 10, the gap 11 of which is traversed by a beam interrupter 7 attached to the suspen- sion shaft 6. The amplifier evaluation unit sums five timeperiods, for example, and gives a mean value thereof to form a representative time for the five executed stroke For each new stroke the first time of the five is subtrac¬ ted and the new time is added, which gives a continuous mean value formation of the time for one stroke. The system 3' is situated at the free end 12 of the suspension shaft 6, and comprises a solenoid magnet 14 fixed to the device housing 13 for co-action with a plunger coil 15 arranged at the free end 12 of the suspension shaft 6, suc as to provide the pivoting or swinging movement of the bla 1 about the bearing 4. The optical fork 5 is also fixed t the casing 13 in the region of the free end 12 of the sus¬ pension shaft 6. The bearing 4 carrying the blade 1 is situated on the suspension shaft 6, close to the forward end 16 of the blade. Between the bearing 4 and the free end 12 of the shaft 6 there are mechanical - stroke limitin

means 17, 18 for arresting the swinging movement of the suspension shaft 6.

The mentioned mean time value is presented by the amplifier unit as a standard output signal, e,g, 0-20 mA or 4-20 πiA.

In reality, the time measurement is made during a portion of the stroke, the beam interrupter 7 interrupting the lig beam between the legs of the optical fork 5. The beam inte rupter 7 has a width corresponding to. 50 - 80 % of the tota stroke. The reason for this is. to prevent a false signal, should there be bounce when the magnetic system reaches its end positions, if the beam interrupter 7 corresponds to or lies too close to, the total stroke. The system 3 is about 100 mm away from the bearing 4, and has a total stroke of about 4 mm. Consequently, the distance A-B moved through by the trailing edge 8 of the blade 1 is about 8 mm if the distance to the bearing 4 is about 200 mm. The measuremen time will, of course, vary heavily with the configuration of the blade 1, type of medium that is measured, beam inter rupter etc. Typical measurement times are 2-20 m/s.

As will be seen from Fig. 3-12, the blade 1 can be imple— ^{" ■} mented in several different ways to suit different types of medium which is to be measured. The basic type of blade fo long-fibre suspensions with a high network strength is a single blade with longitudinal grooves 19 on both sides, see Fig. 3-5. The grooves 19 increase the total area of the blade and reduce its weight. Furthermore, the number of edges and sides which are to cut through the fibre net¬ work increase with the number of grooves. This gives a com paratively high shear force level.

Short-fibre suspensions with low network strength sometimes require other configurations, e,g, a double blade with long tudinal grooves 19 on the outsides of the blades 1, see. Fig 6-8. For extremely low network strengths, both the single blade and the double blade may be provided with concave bottom surfaces 20 in the measuring direction, see Fig. -10

This provides a certain dewatering of the suspension in front of the blade 1 in the measuring direction and con- ^■ tributes to increasing the shear force level. Common to all the embodiments of the blade 1 is that its shape con- forms well to the shape of the barrier 2, so that together they form a profile which gives the least possible distur¬ bance to the flow picture, see Fig. 11-12. This contri¬ butes to small dependence on the flow. It has been stated hereinbefore that the transducer according to the present invention is not dependent on the flow of the medium to be measured for shear forces to occur. However, there is a least flow which may be accepted. This minimum flow is 0.15 m/s if the blade 1 is activated once per second for a blade length of 0.15 m, 0.4 m/s if the blade 1 is activa- ted twice per second and so. ^, on. Since more rapid activa¬ tion that once per second is not necessary, 0.15 m/s is a suitable minimum flow rate. It is thus guaranteed that the blade always cuts through a representative sample of the medium to be measured. If the blade 1 is activated more rapidly simultaneously as the flow rate is low, the result could be that fibre flocks in the suspension are thrust away and replaced by pure liquid, which lowers the output signal.