GB2227365A - Level detector - Google Patents

Abstract

A level detector comprises a hollow tubular body 3 of synthetic polymeric material, an electrically conductive liquid 9 (eg Hg) to which that material is lyophobic, an insulating seal 4, 5 at each end of the tubular body, two electrodes 6 extending through one of the seals to present a low surface area to the interior of tubular body 3, and an electrical circuit to indicate whether the electrodes are in electrical contact with each other within the body. Materials of components 3, 4, 5, 6 are exemplified as are some dimensions. The tubular body and seals may be a one piece or two piece molding. Electrodes 6 may have a protective oxide coating (eg titanium dioxide), and tubular body 3 may be transparent. <IMAGE>

Description

Level Detector The present invention relates to devices suitable for use in detecting whether a piece of equipment is set in an accurately level position and in detecting small angular departures from such a position.

Various devices are available, under the general name of "tilt switches", which are designed to respond to angular zDvement. Such devices include switches in which an electrically conductive liquid, especially mercury, is free to flow, in response to a change in the angle of orientation of the device, between a position in which the liquid forms an electrical bridge between two spaced electrodes and an alternative position in which no such bridge is formed.

Devices of the foregoing types have, in general, only been used in applications wherein a relatively low degree of sensitivity is required, for example in ooarsely setting a piece of equipment in a roughly level orientation. Fbr this purpctse, a significant angular tolerance is acceptable. Thus in terms of the differential switching angle, which is the angular difference between the positions in which the switch is just electrically closed and just open respectively, this angle is typically in the range fran 5 to 15 degrees of arc.

However, there are nwtrous potential applications of devices of the tilt switch general type for which available devices are wholly inadequate. For example, electronic surveying instruments, many security devices and military ranging/siting equipment currently rely on complex electronic systems for levelling. Devices of the tilt switch type would have to be much ncre sensitive, that is have a greatly reduced differential switching angle, before they could be adopted for such purposes.

It is an object of the present invention to provide a level detector which is significantly more sensitive than at least inany of the liquid tilt switch devices heretofore available and wherein some at least of the disadvantages of prior such switches are reduced or overoome.

The level detector according to the present invention coRprises a hollow tubular body made of a synthetic polymeric material, a quantity of an electrically conductive liquid to which said polymeric material is lyophobic contained within said tubular body, an electrically insulating seal disposed at each end of said tubular body to retain said liquid therein, two electrodes extending through one of said insulating seals so as each to present to the interior of the tubular body a surface not greatly exceeding the cross-sectional area of the electrode, and an electrical circuit to indicate whether or not said surfaces of the electrodes within the tubular body are in mutual electrical contact, the quantity of the conductive liquid being sufficient to form an electrical bridge between said electrode surfaces in some only of the possible orientations of the tubular body.

The level detector of the invention requires, among other features, an electrically conductive liquid and a synthetic polymeric material which is lyophobic to that liquid.

In this way, the liquid forms itself into a discrete bubble upon the interior surface of the tubular body. bvenrtnt of the bubble in response to a change of inclination of the tubular body brings the bubble into or out of a position in which it is in oontact with the exposed surfaces of both electrodes, thereby forming or not forming an electrical bridge between the electrodes.

Any practicable stable oodbination of conductive liquid and lyophobic material may be used but it is particularly preferred that the liquid be mercury and that the polymeric material therefore be lyophobic to mercury. Numerous plastics materials are suitable for this purpose, including polystyrene, polyethylene (especially high density and intermediate-density polymers), polypropylene, polyvinyl chloride, polycarbonates and poly(tetrafluDroethylene), that is PrFE. Among these materials, polyethylene and polycarbonates are preferred.

The material selected may be transparent, or sufficiently so to enable the position of the liquid bubble in the tubular body to be visually detectable, but this is not an essential characteristic as that position is detected by the electrodes. Indeed the tubular body may if desired be surrounded by a protective sheath of another material, for example a nrttal.

The tubular body preferably is rotationally symmetriral and most preferably is generally cylindrical. Thus the body may readily be formed by extrusion, the open ends then being sealed by plugs of the same or a different material. However one desirable alternative is to form the tubular body and the end seals as a one-piece or twO-oiece moulding into which the electrodes are incorporated.

The end-seals, whether or not they are formed integral with the tubular body, preferably present a generally flat surface to the interior of the tube. Fbr example, the endseals may be solid generally cylindrical plugs of a suitable dimension to form a close sealing fit within the ends of the tubular body. Such plugs may, for example, be formed of an epoxy resin or of another polymeric material, e.g. polyethylene.

Through one of the endseals, two electrodes extend.

Such electrodes may be formed of one of the noble Itletals, especially platinum, or of any material of which electrodes for this general purpose are conventionally made. An attraction of platinum is that it is a strong, durable, conductive material which does not form an amalgam with mercury. The electrodes may, if desired, be provided with a protective oxide coating, for example of titanium dioxide.

The electrodes should preferably have no sharp exposed edges and be of minimum surface roughness, in order to avoid significant breaking of the physical farm of the liquid burble.

It is an important feature of the present invention that the electrodes shculd present only a small surface to the liquid bubble. This has the effect of limiting the current rating of the device but that is acceptable. An important advantage of the small extent of the exposed electrode surface is that the sensitivity of the device is thereby greatly improved. Thus the electrodes may sixthly present the exposed cross-section of their ends, set flush with the seal surface, to the liquid; alternatively the electrodes may extend by a very small amount into the interior of the tubular body. As a further alternative, the ends of the electrodes may be shaped to form small buttons or domes, of slightly greater diameter than the electrodes themselves.

The electrodes are disposed in an electrical circuit which is oampleted when, and only when, the liquid bubble is in oontact with the exposed ends of both electrodes. The circuit may be of the type conventionally employed in mercury tilt switches of known type. When the circuit is closed, the resulting signal may simply indicate the fact that the equipment associated with the device is level, or the signal may, for example by actuating one or more electrical or mechanical relays, modify the orientation of the equipment until the latter is accurately set level.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, wherein Fig. 1 is a longitudinal cross sectional view of one embodiment of the level detector acoording to the invention; and Fig. 2 illustrates, in cross-sectional view and to a larger scale, an alternative forum of electrodes.

The illustrated level detector comprises a cylindrical body 3 of polyethylene, which in this specific embodiment is approximately 1.5 cm long and approximately 0.5 cm in diameter, closed at its ends by epoxy resin seals 4, 5. Two electrodes 6 of platinum or oxide-coated transition metal extend from the outside of the body, through the seal 5, and terminate flush with the flat inner surface 5a of the seal. The outer ends of the electrodes 6 are connected via spot welds 7 to copper leads 8, which in turn connect the electrodes into an electrical detector circuit (not shown).

A quantity of mercury is enclosed within the detector body 3 and, because the polyethylene is lyophobic (that is, essentially non-wetting) with respect to mercury, the mercury forms itself into a discrete burble 9. The position of the burble within the cylindrical body 3 is determed by gravity and therefore depends solely upan the orientation of that body.

Thus in a position in which the body is tilted very slightly towards the seal 5, the bubble 9 lies against the seal and in contact with both of the electrodes 6, as shewn in Fig. 1.

In this position, the mercury therefore forms an electrical bridge between the electrodes and the detector circuit is thereby closed. A very slight tilting of the body 3 towards the seal 4 causes the mercury burble 9 to rEDve away from the seal 5 and out of contact with the two electrodes, thus breaking the detector circuit.

Very good sensitivity may be achieved by the illustrated detector, especially as a result of the lyophobic property of the polyethylene and the very small contact area of the electrodes. In experiments, a differential switching angle at least as low as 0.2 degrees of are has been readily achieved.

In the form of the electrodes illustrated in Fig. 2, which represents one alternative to that of the electrodes 6, electrodes 10 project by a very small amount beyond the surface 5a of the seal 5 and terminate in small domes 10a, which are of slightly greater diameter than the electrodes proper. In this way, the surface areas of the electrodes are slightly increased relative to the cross-sectional areas of the electrodes themselves.

The very high sensitivity achievable by the level detector according to the invention makes it suitable for use in applications which until now could only be satisfied by complex electronic systems, for example in security and military equipment such as discussed above. By using the signals generated by one or mare detectors to actuate electrical or mechanical relays operating servoactuators, the detector may control the setting of a self-levelling device, for example as a feature of surveying equipment.

Claims (11)

1. A level detector, comprising a hollow tubular body made of a synthetic polymeric material, a quantity of an electrically conductive liquid to which said polymeric material is lyophobic contair within said tubular body, an electrically insulating seal disposed at each end of said tubular body to retain said liquid therein, two electrodes extending through one of said insulating seals so as each to present to the interior of the tubular body a surface not greatly exceeding the cross-sectional area of the electrode, and an electrical circuit to indicate whether or not said surfaces of the electrodes within the tubular body are in irutual electrical contact, the quantity of the conductive liquid being sufficient to form an electrical bridge between said electrode surfaces in some only of the possible orientations of the tubular body.

2. A level detector as claimed in claim 1, wherein the electrically conductive liquid is mercury.

3. A level detector as claimed in claim 2, wherein the synthetic polymeric material is polystyrene, polyethylene, polypropylene, polyvinyl chloride, a polycarbonate or poly (tetraflroethylene).

4. A level detector as claimed in any of the preceding claims, wherein the hollow tubular body is generally cylindrical.

5. A level detector as claimed in any of the preceding claims, wherein the hollow tubular body and the end seals are formed as a one-piece or two-piece moulding.

6. A level detector as claimed in any of claims 1 to 4, wherein the end seals are solid generally cylindrical plugs.

7. A level detector as claimed in claim 6, wherein the plugs are of an epoxy resin or of another polymeric material.

8. A level detector as claimed in any of the preceding claims, wherein the electrodes terminate flush with the inner surface of said seal.

9. A level detector as claimed in any of claims 1 to 7, wherein the ends of the electrodes exposed to the inside of the tubular body form small buttons or dates, of slightly greater diameter than the electrodes themselves.

10. A level detector as claimed in any of the preceding claims, wherein the electrodes are electrically connected such that a signal therefrom may actuate one or itore electrical or mechanical relays, whereby to nudity the orientation of equipment associated with the detector.

11. A level detector substantially as hereibefore described with reference to, and as illustrated in,Fig. 1 or Fig. 2 of the accompanying drawings.