EP1436573A1

EP1436573A1 - Surface switch for determining the surface level

Info

Publication number: EP1436573A1
Application number: EP02758500A
Authority: EP
Inventors: Matti Kari
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-09-12
Filing date: 2002-09-12
Publication date: 2004-07-14
Also published as: FI20011798A0; WO2003023333A1; FI20011798A; FI116421B

Abstract

The invention comprises a surface switch for determining the surface level of an explosive liquid stored in a container (15), for instance. Information about changes in the surface level is transmitted from the surface switch sensor (10) through an optic fibre cable (11) to the receiver (14) of a control unit (12). The surface level information correlates with the intensity of a returning light beam is controlled by means of a glass ball (6).

Description

SURFACE SWITCH FOR DETERMINING THE SURFACE LEVEL

The invention relates to a surface switch as defined in the preamble of claim 1.

When it is desirable to determine the liquid surface level in containers holding ex- plosive liquids, the sensors used for the determination should have a structure such that they do not cause any risk of explosion during use. The surface level of such explosive liquids has previously been determined using either mechanical or electric devices.

When mechanical devices are used, the transmission of surface level data and alarm data to the user involves a problem, frequently requiring the user to go to the container in order to check the surface level. In addition, the mobile parts of mechanical surface level sensors get into contact with the liquid, and are thus susceptible to malfunctions caused by fouling. Such a mechanical surface level sensor may consist for instance of a float fixed to a wire cable, and then information about the move- ment of the float together with information about the surface level is transmitted via the wire cable outside the container.

The surface level can also be determined by conducting compressed air into a pipe in the container, the pipe end being located under the liquid surface. As the liquid surface sinks, the pressure in the pipe decreases and an alarm of dropping surface level can be emitted. However, the implementation of such a system is expensive.

Electric surface level sensors are represented e.g. by sensors of reed-relay type and surface switches suspended from their conductor. In reed relay sensors, a float floating on the liquid surface has been attached about the shaft. As the liquid surface rises or sinks, the shaft transmits the movement of the float to the shaft joint loca- tion, where there is a switch that is disconnected and transmits an electric signal further. In a second sensor of reed relay type, the shaft remains in position and the float surrounding the shaft moves along with the liquid surface. As the liquid surface rises, a magnet provided in the float disconnects the read relay located within the shaft. In a surface switch suspended from its conductor, the float provided at the conductor end holds a switch, which is disconnected as the float position changes as a result of the liquid surface rising or sinking and transmits an electric signal forwards. When electric liquid surface level sensors are used, the sensors have nevertheless to be manufactured in conformity with Exi protection requirements in order to avoid any risk of explosion. This is not always possible, and then a separate Exi relay has to be used, the sensor being connected via the safe current circuit of this to the measurement system. This increases the cost of the sensor and entails more complicated circuitry required for connecting the sensor. What is more, electric sensors usually do not have any self-diagnostics. There is no warning of damage to the sensor or its cable until damage actually occurs.

There are also known sensors, which use an optical fibre cable for transmitting sur- face level information. However, these known sensors for determining a liquid surface level by means of an optical fibre cable are susceptible to fouling. They also have a design such that either no information about a broken cable is given or then continual surface level monitoring requires special provisions.

Conventional liquid surface level sensors are also large-sized. Usually the sensors can be inserted into the container only through a small hole, through which conventional surface level sensors do not pass. With sensors mounted through the container wall, again, provisions must be made for adequate sealing and pressure resistance between the sensor and the container wall, and consequently the sensors are more difficult to install.

Moreover, there is often a need for continuous monitoring of the liquid surface level in the container. Conventional electric surface level sensors do not usually allow continuous monitoring. They only indicate that specific limit values have been exceeded, such as the container being emptied or overfilled.

The purpose of this invention is to provide a new surface switch, which does not have the shortcomings mentioned above.

To achieve this, the surface switch of the invention is characterised by the features defined in claim 1.

Preferred embodiments of the invention are characterised by the features defined in the dependent claims.

The surface switch of the invention has smaller size than previously known surface switches, and thus it can be used in containers having smaller holes. Moreover, the surface switch of the invention does not comprise electric components, and it can consequently be safely used in explosive spaces. Its manufacturing costs are also lower than those of a conventional Exi-protected electric switch.

In addition, the surface switch of the invention does not have moving parts getting into contact with the liquid, and hence there will be no malfunctions caused by fouling. Should the switch or its wire cable fail, the malfunction is easily detected.

The invention is explained below by means of examples and with reference to the accompanying drawings, in which

figures la, lb and lc illustrate the surface switch of a preferred embodiment of the invention in cross-section, in horizontal and vertical positions,

figures 2a and 2b illustrate the surface switch of a second preferred embodiment of the invention in cross-section, in horizontal and vertical positions, and

figure 3 shows the use of a surface switch of a preferred embodiment of the invention for monitoring the surface level of a liquid.

The surface switch in a preferred embodiment of the invention as shown in figures la, lb and lc comprises a float 1, an air space 2, a plug 3, an optic fibre 4, a mounting support 5, a glass ball 6, a support base 7 and a mirror 8.

The surface switch in a second preferred embodiment of the invention as shown in figures 2a and 2b comprises a float 1, an air space 2, a plug 3, an optic fibre 4, a mounting support 5, a glass ball 6 and a support base 7.

The use of a surface switch in a preferred embodiment of the invention as shown in figure 3 for monitoring the surface level of a liquid comprises a sensor 10, an optic fibre cable 11, a control unit 12, a transmitter 13, a receiver 14 and a container 15.

When the surface switch of the invention is used in monitoring the surface level of a liquid in a container 15 to be monitored, the optic fibre cable 11 of the surface switch is inserted into the container 15 through a hole in the upper surface of the container 15. By adjusting the length of the optic fibre cable 11, one sets the sensor 10 suspended from the end of the optic fibre cable 11 to the desired height. When an alarm about overfilling of the container 15 is desired, the sensor 10 is lowered to a height such that the sensor turns upside-down once the liquid surface has risen to the alarm level. If, again, it is desirable to receive an alarm about the container 15 being empty, the sensor 10 is lowered so as to float on the liquid surface on a height such that the sensor swings into upright position when the liquid surface sinks to the alarm level.

The optic fibre cable 11 is inserted into the sensor 10 through a tight plug 3 provided at the end of the sensor 10. The optic fibre 4 is fixed to the mounting support 5 and the end of the optic fibre 4 is oriented towards a reflective surface, such as a mirror 8, or towards the other end of the optic fibre. A light beam is fed to the optic fibre cable 11 from the transmitter 13 of the control unit 12, the return of the light beam through the optic fibre cable 11 being detected in the receiver 14 of the control unit 12. The control unit 12 reports the surface level of the liquid to the user or, if necessary, gives an alarm at the upper or lower limit set for the surface level.

The surface switch of a preferred embodiment of the invention shown in figure 1 provides information about the surface level in the container 15. The surface switch is fitted by suspending from the optic fibre cable 15 of the surface switch, at the desired height at the upper edge of the container 15. Then the float 1 of the surface switch will not be in contact with the liquid and the sensor 10 is in vertical position as shown in figure la. Then the glass ball 6 within the float is located in the centre of the support base 7. The glass ball 6 acts as a lens, and there will be double light beam intensity reflected from the mirror 8 to the optic fibre 4.

As the surface level of the liquid in the container 15 rises and the liquid surface reaches the surface switch sensor 10, the surface switch starts floating on the liquid surface owing to the buoyancy provided by the float 1. As the surface level of the liquid rises, the position of the surface switch changes as shown in figure lc, and the glass ball 6 switches into a position between the mounting support 5 and the support base 7. There will then be a decrease in the intensity of the light beam reflected from the mirror 8. When the sensor 10 of the surface switch has turned completely upside-down as shown in figure lb, the glass ball switches into a position in the centre of the mounting support 5. As the glass ball 6 thus acts as a lens, the light beam reflected from the rnirror 8 will have high intensity.

The control unit 12 infers the prevailing position of the sensor 10 and the surface level of the liquid in the container 10 from the intensity of the light beam returning through the optic fibre cable 10. In the case of the embodiment shown in figure 1, high intensity corresponds to the upper limit of the surface level, medium intensity corresponds to the lower limit of the surface level and high intensity corresponds to midway between the alarm range surface levels.

The surface switch of a second preferred embodiment of the invention shown in figure 2 may provide an alarm about the surface level of the liquid in the container 15 having reached either the upper or the lower limit. If information about the upper limit having been reached is desired, the sensor 10 should be lowered to such a level in the container 15 that the sensor turns into horizontal position as the liquid surface level reaches the upper limit. Accordingly, when it is desirable to receive information about the lower limit having been reached, the sensor 10 should be lowered to a level such that the sensor swings into vertical position as the liquid surface level reaches the lower limit.

As the sensor 10 floats, the float 1 on the liquid surface in the container 15 is in horizontal position as shown in figure 2b. Then the glass ball 6 is located between the mounting support 5 and the support base 7. Then there will be a decrease in the intensity of the light beam returning to the optic fibre through the glass ball 6 acting as a lens.

As the liquid surface level drops, the sensor 10 turns into upright position as shown in figure 2a, and the glass balls 6 switches into a position in the centre of the support base 7. The light beam returning to the optic fibre 4 will then pass through the central point of the glass ball 6 and the returning light beam will have high inten- sity.

In the case of figure 2, the liquid surface level is inferred in a manner corresponding to the case of figure 1. In the embodiment of figure 2, high intensity corresponds to the lower limit of the surface level, and high intensity corresponds to the upper limit of the surface level.

It is obvious to those skilled in the art that the invention is not confined to the description above, but may vary within the scope of the following claims. For instance, surface switches of the invention can be used in the measurement of the sur- face level of granular explosive material, such as grain stored in a grain silo. Besides alarms about upper and lower limits, all of the sensors mentioned above can be used in controlling solenoid valves or pumps.

Claims

1. A surface switch for determining for instance the surface level of an explosive liquid stored in a container (15), comprising a float (1), an air space (2), a plug (3), an optic fibre (4), a mounting support (5), a ball (6, 9) and a support base (7), characterised in that

- surface level information is transmitted from a sensor (10) to the receiver (14) of a control unit (12) through an optic fibre cable (11),

- the surface level information correlates with the intensity of a returning light beam, and

- the intensity of the returning light beam is controlled by means of a glass ball (6, 9).

2. A surface switch as defined in claim 1, characterised in that the light beam is re- fleeted from a mirror (8).

3. A surface switch as defined in any of the preceding claims, characterised in that a break of the light beam returning to the control unit (β) generates an alarm to the user.