WO1990013792A1

WO1990013792A1 - Procedure and device for the detection of direction or angle

Info

Publication number: WO1990013792A1
Application number: PCT/FI1989/000083
Authority: WO
Inventors: Viljo Lukkarinen
Original assignee: Metrum Oy
Priority date: 1987-11-09
Filing date: 1989-05-05
Publication date: 1990-11-15
Also published as: FI874952A0; FI80788C; FI80788B; FI874952A

Abstract

The invention concerns a procedure and a device for the detection of direction or angle, whereby the position and direction of movement of the level indicating means (3) of an easily tilted bubble-spirit glass (1) or equivalent are monitored by means of a light source and light detector system consisting of optoelectric components, whereby beams of light reflected from the level indicating means (3) are detected and, when the electric signals generated by different light beams are mutually balanced, the user is informed about this by means of signalling devices. At least one light source (4a, 4b) is directed at the boundary surface between the liquid and the level indicating means in the libellum (1) in such manner that, when the level indicating means (3) is in the middle position, essentially equal portions of the light beams emitted by each source and reflected by the level indicating means fall on the surface of one or more detectors (5) placed symmetrically relative to the light sources at esssentially right angles thereto.

Description

PROCEDURE AND DEVICE FOR THE DETECTION OF DIRECTION OR ANGLE

The present invention relates to a procedure and a device for the detection of direction or angle, whereby the posi¬ tion and direction of movement of the level indicating means of an easily tilted bubble-spirit glass or equivalent are monitored by means of a light source & light detector system consisting of optoelectric components.

In prior art, there are devices based on the observation of a gas bubble eclosed in a bubble-spirit glass, as exempli¬ fied by US patent no. 3,863,067, West-German patent publica¬ tion no. 2,636,706 and Finnish patent application no. 841864 filed by the present applicant. In all ^hese examples, the boundary surface of the bubble is observed by the aid of one light source and one detector by using either transillu ina- tion or direct reflection. On the other hand, there are devices based on the observation of the motions of a gas bubble in a bubble-spirit glass by means of two light source & detector pairs, as exemplified by US patent no. 4,182,046, British patent no. 1,528,445 and Finnish patent application no. 870737 filed by the present applicant.

Almost all of the previously known devices involve numerous drawbacks. One of these is that the gas bubble in the bubble-spirit glass undergoes considerable changes of size and shape with changes of temperature. This causes serious difficulties regarding calibration and interpretation, especially in countries like Finland where the temperature variations are large. In devices using two pairs of light source & detector pairs, variations of temperature are a serious problem especially because of the drift of the characteristic values of the phototransistors. In theory, this could be remedied by using adapted pairs (there are no photoelectronic components available on the market) or by compensation, but for practical applications such solutions are too difficult, expensive and unreliable. Furthermore, it has been observed that with previously known devices the result obtained by repeating the same measure¬ ment varies irregularly and approaches the correct value only after some time. This is due to the presence of a thin film of liquid remaining at least for some time on the sur¬ face of the liquid tube above the bubble. The thickness and evenness of this liquid film varies irregularly, because there is a continuous process of drying and flow within the film, which has an effect both on the light penetrating the film and on the light reflected from it. Thus, the measure¬ ment results are always inaccurate if the monitoring light of the position sensor passes through the bubble tube in the direction from top to bottom or vice versa.

The object of the present invention is to eliminate the drawbacks referred to and to achieve a procedure for the detection of direction and a device applying the procedure that are capable of accurate detection of an angle and the direction of its change regardless of temperature or the formation of a liquid film as mentioned above. To achieve this, the procedure of the invention, whereby beams of light reflected from the level indicating means of a spirit glass are detected and, when the electric signals generated by different light beams are mutually balanced, the user is informed about this by means of a signalling device, is characterized in that at least one light source is directed at the boundary surface between the liquid and the level indicating means of the spirit glass in such manner that, when the level indicating means is in its middle position, essentially equal portions of the light emitted by each source and reflected by the level indicating means fall on the surface of one or more detectors placed symmetrically relative to the light sources at essentially right angles thereto.

Depending on the embodiment, the invention provides the following advantages over previously known techniques: - high accuracy, since the beam of light to be measured never passes through the liquid film above the bubble

- reliable detection of direction, large tilt permitted

- operation of the device unaffected by any normal phenomena occurring in the liquid tube due to variations of tempera¬ ture and affecting the bubble or the liquid.

A preferred embodiment of the procedure of the invention is characterized in that the light beams produced by at least two light sources are directed at the boundary surface between the liquid and the level indicating means of the spirit glass, and that rays of light reflected from the level indicating means are gathered on the surface of a single detector. This embodiment allows all of the aforemen- tioned advantages to be achieved by a construction of maximal simplicity.

Another preferred embodiment of the procedure of the invention is characterized in that the light beam produced by a single light source is directed at the boundary surface between the liquid and the level indicating means of the spirit glass, and that reflected rays of light are gathered on the surface of at least two detectors. As this embodiment uses several detectors, it is mainly suited for applications where the temperature remains essentially constant.

The features of the other preferred embodiments of the procedure of the invention are as presented in the claims to follow.

The device of the invention, consisting of one or more light sources, e.g. light-emitting diodes, and one or more light detectors, e.g. phototransistors, mounted around an easily tilted bubble-spirit glass known in itself, and their control electronics together with signalling devices for the verification of a given angle or direction, is characterized in that the device comprises at least one light source directed at the boundary surface between the liquid and the level indicating means of the spirit glass and one or more detectors placed symmetrically relative to the light sources and at essentially right angles thereto in such manner that the beams of light emitted by the light sources and reflected by the level indicating means when in the middle position are essentially equal in strength when detected.

A preferred embodiment of the device of the invention is characterized in that at least two light sources, such as LEDs, are directed at the boundary surface between the liquid and the level indicating means of the spirit glass, and that the device has a single detectoi* for detecting the light reflected from the level indicating means.

Another preferred embodiment of the procedure of the inven¬ tion is characterized in that a single light source, such as a LED, is directed at the boundary surface between the liquid and the level indicating means of the spirit glass, and that the device has at least two detectors for detecting the light reflected from the level indicating means.

The features of the other preferred embodiments of the device of the invention are as presented in the claims to follow.

In the following, the invention is described in greater detail by the aid of examples, reference being made to the drawing attached, wherein:

Fig. 1 presents an example of the arrangement of a spirit glass and a light source & detector combination as provided by the invention, seen from one side of the spirit glass.

Fig. 2 presents the arrangement in fig. 1 as seen from one end of the spirit glass. Fig. 3 presents the arrangement in fig. 1 in top view.

Fig. 4 presents another example of the arrangement of the invention.

Fig. 5 presents an example of an arrangement of the invention using more than two light sources.

Fig. 6 presents the arrangement in fig. 5 in lateral view.

Fig. 7 presents the electronics and signalling devices of the device of the invention.

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Fig. 8 presents the signal forms appearing in the circuit of fig. 7 in different situations.

Fig. 9 shows how the tolerance range of the device of the invention is determined.

The following description concentrates on embodiments using two or more light sources and one detector, the latter being placed below the level indicating means of the spirit glass. It is obvious that, in embodiments using two light sources, the light sources and the detector can be interchanged without leaving the domain of the invention or altering the operation of the device. On the other hand, if the light sources are replaced with detectors and the detector with a light source, the result is a reversed application which, due to variations in the temperature sensitivity of the detectors, is best suited for constant temperature applica¬ tions. On this precondition, such a solution achieves all the above-mentioned advantages without deserting the basic idea of the invention.

Fig. 1 illustrates the arrangement of the components in relation to the spirit glass as provided by the invention, the spirit glass itself being identified by reference number 1, the liquid space and the liquid inside the spirit glass by number 2 and the level indicating means or gas bubble in the liquid by number 3. Mounted closely against one side of the spirit glass 1 are two light sources 4a and 4b, herein¬ after referred to as light-emitting diodes (LEDs). The LEDs, arranged parallel to each other and essentially symmetrical¬ ly relative to the equilibrium axis A of the spirit glass, are directed at the level indicating means 3 of the spirit glass 1 from one side in such manner that, in the position of equilibrium, a substantial part of each light beam falls tangentially on the level indicating means and is reflected downward from it onto a detector below. The detector, which in this case consists of a phototransistor 5, is located under the level indicating means 3 on the equilibrium axis A of the spirit glass. Here it is assumed that, in the horizontal position of the spirit glass, the centre of the level indicating means 3 is coincident with the equilibrium axis A. If for some reason this is not the case, the centre of the level indicating means is naturally the determining factor, and the LEDs and detectors have to be positioned accordingly. The manner of attachment of the LEDs and phototransistor is not described here, but there are numerous ways of securing them, e.g. by using screwed joints or glue.

Fig. 2 illustrates the passage of the light beams emitted from the LEDs to the phototransistor 5, and so does fig. 3, which shows both of the diodes 4a and 4b and the correspond¬ ing arrows B and C indicating the passage of the light. The LEDs are so located relative to the bubble 3 that most of the light is reflected from the curved surface of the bubble 3 instead of penetrating it and passing to the other side of the spirit glass. Obviously not all of the light emitted by the diodes 4a,4b reaches the phototransistor 5 but some of it is scattered in other directions as well. There is no need to focus all of the reflected light on one spot, be¬ cause sufficent accuracy is achieved by having the LEDs and the phototransistor 5 so arranged that they constitute a symmetrical configuration relative to the bubble 3 when the latter is in the position of equilibrium in the middle of the bubble tube. Therefore, the exact vertical position of the LEDs and the exact position of the phototransistor rela¬ tive to the axis A in fig. 2 are not of critical importance since these components remain symmetrically arranged. Thus, the most decisive factor regarding accuracy is the quality and tuning of the electronics controlling the phototransis¬ tor 5 (see fig. 7). To enable the electronics to distinguish between light beams emitted by different diodes, it is generally necessary to render the beams into pulse trains multiplexed in such manner that at any given instant the phototransistor actually detects light emanating from one diode only.

As is obvious from fig. 2, the direction of the light beam in this embodiment may just as well be reversed. In this case, the LEDs are located under the spirit glass while the phototransistor is placed on either side of it.

Fig. 4 shows another embodiment of the invention, in which the LEDs 6a and 6b are placed at the ends of the spirit glass 1 while the phototransistor 5 is in the same place as before in figs. 1-3. In other respects, too, the arrangement in fig. 4 corresponds to the embodiment described in connection with figs. 1-3.

Figs. 5 and 6 illustrate an embodiment in which the prin¬ ciple of the invention is applied in a multidirectional sen¬ sor using more than two light sources. In this example, a level indicating means or bubble 9 is formed in the conven¬ tional manner in a cylindrical liquid space 8 inside a cy¬ lindrical multidirectional spirit glass 7. The spirit glass 7 is surrounded by eight identical and symmetrically placed LEDs 10a...10h, among which each pair of opposite diodes functions in the way explained above e.g. in connection with Fig.4. By having these pairs (e.g. 10c and lOg in Fig. 6) alternately switched on together with the phototransistor 6 in a sufficiently fast tempo, a sensor like this will provide unambiguous information regarding the direction of tilt e.g. for use in an alarming or measuring device.

Fig. 7 shows an example of a circuit which can be used to implement the invention, and fig. 8 shows the wave forms of the principal signals appearing in the circuit in fig. 7 in different situations. Fig. 8 is divided into three zones I, II and III, representing different positions of the level indicating means or bubble of the spirit glass.

In the following, the action of the circuit illustrated in fig. 7 is described with reference to figs. 7 and 8. The circuit includes an oscillator 12 producing two rectangular waves of opposite phase, which are fed into resistors R1-R4 at points D and E (see also fig. 8). The oscillator frequen¬ cy is of the order of 1 kHz, but nothing prevents it from being increased e.g. to 100 kHz if necessary. When the logic state at point D is one, the logic state at point E is zero and transistor T1 conducts, thereby turning on LED 13a, which is connected to a trimmer R5 by means of which the intensity of the light emitted by the diode can be adjusted. In this situation, no current flows through the other LED 13b, which is therefore turned off. In a corresponding manner, during the next pulse when E=1 and D=0, LED 13b is turned on via transistor T2 and LED 13a is turned off. Thus, the two LEDs emit light to the phototransistor 14 by turns. When the light emitted by either one of the LEDs 13a,13b falls on the surface of the phototransistor 14, it generates a current in the transistor, causing transistor T3 to conduct for a short time, during which the RC circuits C1,R8 and C2,R9 are charged. Transistors T4 and T5 take care that RC circuit C1 ,R8 is charged by the current generated by LED 13a and RC circuit C2,R9 by the current generated by LED 13b. The operational amplifier 15 is fed by the voltages set up at points F and G, which vary in step with the charging and discharging of the RC circuits. If the bubble in the spirit glass is in the position of equilibrium, it reflects equal amounts of light from different (in this case two) direc¬ tions to phototransistor 14, so that the RC circuits are equally charged. In zone I in fig. 8, this can be seen from the equal mean voltages at points G and F. At the points of intersection of the two curves, the operational amplifier, which compares the voltages at its inputs, changes the state of its output, producing a rectangular voltage wave H as shown in fig. 8. This signal is inverted by NAND-gate 16, which produces a signal at point I that iέ opposite in phase to the signal at point H. These two signals are rectified and fed into another pair of RC circuits C3,R10 and C4,R11. The signals generated by these are represented by the curve forms at points J and K in fig. 8. In the zone I repre¬ senting the state of equilibrium, the voltages at both point J and point K remain at logic one, so that the output of AND-gate 17 is also one, which in turn causes transistor T7 to conduct, thereby turning on a LED L1 which indicates that the position of equilibrium has been reached.

If the level indicating means of the spirit glass is out of equilibrium, i.e. the situation is as illustrated by zone II (tilt left) or zone III (tilt right) in fig. 8, then the voltages at points F and G are continuously unequal, so that the output of the operational amplifier comparing these voltages is also continuously either logic zero or logic one, depending on the case. As is obvious to a person skilled in the art, this has the result that the output of AND-gate 17 always remains zero while either transistor T6 or transistor T8, depending on the case, receives a control signal that turns it on, thereby also turning on the relevant LED 1_2 or L3 indicating the direction in which the position of the spirit glass needs to be corrected. Operational amplifier 18 takes care, by means of transistor T9 acting as a current switch, that no direction data is given when the signals obtained from the phototransistor 14 are very weak, which is the case when the angle of inclina¬ tion of the spirit glass is very large, e.g. 10°. Otherwise, conflicting situations might arise, for instance if LEDs 13a and 13b should produce signals of about equal magnitude. If the device represented by fig. 7 is used in an embodiment constructed as illustrated by fig. 5, then it has to be provided with a simple additional logic which enables the different pairs of LEDs to be monitored by turns in the manner explained above. Designing such a circuit is an obvious matter to a person skilled in the fert.

The embodiment based on the use of one light source and several light detectors employs a control technique corre¬ sponding to that described above, yet with the difference that (provided that the complexity of the electronics is kept at the same low level as in the case of fig. 7) the light source is continuously illumined while the detectors are so controlled that they receive reflected beams of light by turns. Altering the circuit in fig. 7 so as to accomplish this should be no problem to a person skilled in the art.

The graph in fig. 9 represents the phototransistor output voltage levels as a function of the position of the level indicating means, i.e. angle α of tilt of the spirit glass. Level N represents the detection limit, in this case ±10°, and curves O and P represent the output voltage obtained from a phototransistor connected as provided by the inven¬ tion for different angles α of the spirit glass. It can be seen that the peak intensity values of the beams reflected from the bubble to the photodetector do not coincide with the horizontal position of the spirit glass. The detection of the horizontal position is based exclusively on the de¬ tection of the intersection of the two curves, and the pre¬ cision typical of the invention is achieved by virtue of the fact that the curves O and P remain symmetrical and uniform in shape in all situations, which again is due to the fact that they are produced by one and the same solid-state com¬ ponent. The tolerance range R, mainly determined by adjust¬ ing the resistance values of resistors R8 and R9, is typi¬ cally 0.005°, which corresponds to the accuracy achieved by the best of the previously known devices, which are consid¬ erably more complex and expensive. Adjustment of the preci¬ sion range can be implemented e.g. by connecting a transis¬ tor in parallel with each of the RC circuits C1,R8 and C2,R9 and adjusting their gain. Fig. 9 also shows the ranges of voltages within which the indicator LEDs L1-L3 are illumin¬ ed, making it easy to understand the grafch with respect to fig. 7. Final adjustment of the shape of the curves 0 and P can be effected using variable resistors R5 and R6. For this reason the LEDs need not be precisely symmetrically located relative to the axis A in fig. 1, because possible differ¬ ences in the strength of the detected reflection signal can be easily compensated by adjusting the intensity of emission of the LEDs by varying the setting of said resistors. In the same way, it is possible to compensate permanent differences in the intensity of the detected light that are due to fac¬ tors other than the object under measurement.

It is obvious to a person skilled in the art that different embodiments of the invention are not restricted to the ex¬ amples described above, but that they may instead be varied within the scope of the following claims. Thus, the indica¬ tor LEDs can be replaced by any other type of device pro¬ ducing signals perceivable to the human senses, e.g. sound signals. Also, it is possible to use a combination of different signal devices.

Claims

1. Procedure for the detection of direction or angle, whereby the position and direction of movement of the level indicating means (3;9) of an easily tilted bubble-spirit glass (1;7) or equivalent are monitored by means of a light source & light detector system consisting of optoelectric components, whereby beams of light reflected from the level indicating means (3;9) are detected and, when the electric signals generated by different light beams are mutually balanced, the user is informed about this by means of a signalling device, c h a r a c t e r i z e d in that at least one light source (4a,4b;6a,6b;10a-10h) is directed at the boundary surface between the liquid and the level indi- eating means in the spirit glass (1;7) in such manner that, when the level indicating means (3;9) is in the middle position, essentially equal portions of the light emitted by each source and reflected by the level indicating means fall on the surface of one or more detectors (5;11 ) placed symmetrically relative to the light sources at essentially right angles thereto.

2. Procedure according to claim 1, c h a r a c t e r ¬ i z e d in that the light beams produced by at least two light sources (4a,4b;6a,6b;10a-10h) are directed at the boundary surface between the liquid and the level indi¬ cating means (3;9) in the bubble-spirit glass, and that rays of light reflected from the level indicating means are gathered on the surface of a single detector (5;11).

3. Procedure according to claim 2, c h a r a c t e r ¬ i z e d in that the two light beams produced by the light sources (4a, b) are directed at the level indicating means (3) of the bubble-spirit glass (1 ) from one side, said beams being arranged parallel to each other and essentially symmetrically relative to the equilibrium axis (A) of the level indicating means of the spirit glass in such manner that, in the position of equilibrium, an essentially equal portion of each beam falls on the level indicating means (3) and is reflected downward from it onto the detector (5), placed under the spirit glass.

4. Procedure according to claim 2, c h a r a c t e r ¬ i z e d in that the two light beams produced by the light sources are directed at the level indicating means (3) of the bubble-spirit glass (1) from below, said beams being arranged parallel to each other and essentially symmetri¬ cally relative to the equilibrium axis (A) of the level in¬ dicating means of the spirit glass in such manner that, in the position of equilibrium, an essentially equal portion of each beam falls on the level indicating* means (3) and is reflected sideways from it onto the detector (5), placed at one side of the spirit glass.

5. Procedure according to claim 2, c h a r a c t e r ¬ i z e d in that the two light beams produced by the light sources (6a,6b) are directed at the boundary surfaces be¬ tween the level indicating means (3) and the liquid in the bubble-spirit glass (1) from opposite directions from the ends of the spirit glass in such manner that, in the position of equilibrium, an essentially equal portion of each beam falls on the level indicating means (3) and is reflected downward from it onto the detector (5), placed under the spirit glass.

6. Procedure according to claim 2, c h a r a c t e r ¬ i z e d in that the light beams produced by more than two light sources (10a-10h) are directed at the level indi¬ cating means (9) of a circular symmetrical bubble-spirit glass (7) sideways symmetrically in pairs of opposite beams in such manner that, in the position of equilibrium, an essentially equal portion of each beam in each pair of beams falls on the level indicating means (9) and is reflected downward from it onto the detector (11), placed under the spirit glass.

7. Procedure according to claim 1, c h a r a c t e r ¬ i z e d in that the light beam produced by a single light source is directed at the boundary surface between the level indicating means (3;9) and the liquid in the spirit glass (1 ;7), and that reflected rays of light are gathered on the surface of at least two detectors.

8. Procedure according to claim 7, c h a r a c t e r ¬ i z e d in that the light beam produced by a single light source is directed at the level indicating means (3) of the bubble-spirit glass (1) from one side in such manner that, in the position of equilibrium, a substantial portion of the beam falls on the level indicating means (3) and is reflected downward from it onto the two detectors, placed under the spirit glass essentially symmetrically relative to its equilibrium axis (A).

9. Procedure according to claim 7, c h a r a c t e r ¬ i z e d in that the light beam produced by a single light source is directed at the level indicating means (3) of the spirit glass (1) from below in such manner that, in the position of equilibrium, a substantial portion of the beam falls on the level indicating means (3) and is reflected sideways from it onto the two detectors, placed essentially symmetrically relative to its equilibrium axis (A).

10. Procedure according to claim 7, c h a r a c t e r ¬ i z e d in that two light detectors are directed at the level indicating means (3) of the bubble-spirit glass (1 ) from the ends of the spirit glass in opposite directions in such manner that, in the position of equilibrium, a sub¬ stantial portion of the light beam produced by the light source, placed under the level indicating means, falls on the level indicating means (3) and is reflected sideways onto the detectors.

11. Procedure according to claim 7, c h a r a c t e r ¬ i z e d in that more than two light detectors are directed at the level indicating means (3) of a circular symmetrical bubble-spirit glass (1) sideways in pairs of opposite beams in such manner that, in the position of equilibrium, a substantial portion of the light beam produced by the light source, placed under the level indicating means, falls on the level indicating means (3) and is reflected sideways from it onto each detector.

12. Device for implementing the procedure of claim 1 , consisting of one or more light sources, e.g. light- emitting diodes (4a,4b;6a,6b;10a-10h), fend one or more light detectors, e.g. phototransistors (5;11), mounted around an easily tilted bubble-spirit glass (1;7) known in itself, and their control electronics together with a signalling device (L1 ) for the verification of a given angle or direction, c h a r a c t e r i z e d in that the device comprises at least one light source (4a,4b;6a,6b; 10a-10h) directed at the boundary surface between the liquid and the level indicating means (3;9) of the spirit glass (1 ;7) and one or more detectors (5;11 ) placed symmet¬ rically relative to the light sources and at essentially right angles thereto in such manner that the beams of light emitted by the light sources (4a,4b;6a,6b;10a-10h) and reflected by the level indicating means (3;9) when in the middle position are essentially equal in strength when detected.

13. Device according to claim 12, c h a r a c t e r ¬ i z e d in that at least two light sources, such as LEDs (4a,4b;6a,6b;10a-10h) , are directed at the boundary surface between the liquid and the level indicating means (3;9) in the bubble-spirit glass (1;7), and that the device has a single detector (5;11 ) for detecting the light reflected from the level indicating means.

14. Device according to claim 13, c h a r a c t e r ¬ i z e d in that two light sources, such as LEDs (4a,4b), are directed at the boundary surface between the liquid and the level indicating means (3) of the bubble-spirit glass (1 ) from the sides in parallel directions and essentially symmetrically relative to the equilibrium axis (A) of the spirit glass in such manner that, in the position of equilibrium, the optical axis of each LED (4a,4b) meets the level indicating means (3), and that the detector (5) is placed under the spirit glass (1).

15. Device according to claim 13, c h a r a c t e r ¬ i z e d in that two light sources, such as LEDs (4a,4b), are directed at the boundary surface between the liquid and the level indicating means (3) of the bubble-spirit glass (1 ) from below in parallel directions and essentially symmetrically relative to the equilibrium axis (A) of the spirit glass in such manner that, in the position of equilibrium, the optical axis of each LED (4a,4b) meets the level indicating means (3), and that the detector (5) is placed at one side of the spirit glass (1 ) .

16. Device according to claim 13, c h a r a c t e r ¬ i z e d in that two light sources, such as LEDs (6a,6b), are directed at the boundary surface between the liquid and the level indicating means (3) of the bubble-spirit glass (1) from the ends of the spirit glass in such manner that, in the position of equilibrium, the optical axis of each LED (6a,6b) meets the level indicating means (3), and that the detector (5) is placed under the level indicating means.

17. Device according to claim 13, c h a r a c t e r ¬ i z e d in that more than two light sources, such as LEDs (10a-10h), are directed at the boundary surface between the liquid and the level indicating means (9) of a circular symmetrical bubble-spirit glass (7) sideways symmetrically forming pairs of opposite beams in such manner that, in the position of equilibrium, the optical axis of each pair or LEDs (10a,10e;10b,10f... ) meets the level indicating means (9), and that the detector (11) is placed under the level indicating means.

18. Device according to claim 12, c h a r a c t e r ¬ i z e d in that a single light source, such as a light emitting diode, is directed at the boundary surface between the liquid and the level indicating means (3;9) of the bubble-spirit glass (1 ;7), and that the device has at least two detectors for detecting light beams reflected from the level indicating means. *

19. Device according to claim 18, c h a r a c t e r ¬ i z e d in that a single light source, such as a LED, is directed at the boundary surface between the liquid and the level indicating means (3) of the bubble-spirit glass (1) from one side of the spirit glass, and that the device has two detectors placed below the spirit glass essentially symmetrically relative to the equilibrium axis (A) of the spirit glass (1 ) .

20. Device according to claim 18, c h a r a c t e r ¬ i z e d in that a single light source, such as a LED, is directed at the boundary surface between the liquid and the level indicating means (3) of the bubble-spirit glass (1 ) from below, and that the device has two detectors placed at one side of the spirit glass essentially symmetrically re¬ lative to the equilibrium axis (A) of the spirit glass (1).

21. Device according to claim 18, c h a r a c t e r ¬ i z e d in that two light detectors are directed at the boundary surface between the liquid and the level indi¬ cating means (3) of the bubble-spirit glass (1) from opposite directions from the ends of the spirit glass 1 , and that a light source such as a LED is provided below it.

22. Device according to claim 18, c h a r a c t e r ¬ i z e d in that more than two light detectors are directed at the boundary surface between the liquid and the level indicating means (9) of a circular symmetrical bubble- spirit glass (7) sideways symmetrically in pairs from opposite directions in such manner that, in the position of equilibrium, the optical axes of said pairs or detectors meet the level indicating means (9) symmetrically, and that the light source is placed under the level indicating means.