The invention relates to level meters.

Such meters are already known in which a vial is substantially filled with liquid and arranged so that an entrapped air bubble centralises when the vial is level. Such levels, often called "spirit-levels" require the user to compare the position of the bubble with graduates or other marks requiring skill and experience. The user must have a good line-of-sight, reducing the satisfactory use of the meter in some situations, for example, where only very limited access or poor lighting. The vial, often made of glass, must also be at least fairly exposed in its housing so as to be readily visible and is thus liable to damage. Common spirit levels cannot normally be adjusted or calibrated without disassembly and re-fitting or re-cementing the vial in its housing.

According to the invention there is provided a level meter including a housing, a sealed elongate vial mounted in the housing substantially filled with liquid to entrap an air bubble which centralises within the vial when the vial is level and moves to the left or to the right in the vial if the vial is tilted up at the left or up at the right respectively, means for directing light towards the centre of the vial transverse to the longitudinal axis of the vial, a light intensity detector t each end of the vial, and means for monitoring the light received at each

detector reflected at a lower surface of the bubble for providing an output whenever there is a balance in the intensities received at the detectors to indicate that the vial is level.

The monitoring means may be arranged to provide respective output signals indicative that the vial is tilted to the left or to the right.

The meter may include an audible signal generator and the monitoring means arranged to initiate the generator whenever the vial is level.

The vial may be pivotably mounted in the casing and means included for holding and moving the vial about the pivot to adjust and hold the vial relative to the housing for calibrating the meter.

The vial may be held in the housing by a relatively rotatable holder, and the holder moved or set at different positions corresponding for different angular orientations of the vial with respect to the housing.

Means may be provided for adjusting the sensitivity of the meter.

A level meter according to the invention will now be

described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a top outside view of the meter;

Figure 2 is an elevation of the meter;

Figure 3 to δ are diagramatic sectional views of optical components of the meter;

Figure 6 is an elevation of the operative components of the meter;

Figure 7 is a sectioned side view of Figure 6; and

Figure 8 is a supply circuit for a light emitting diode in the meter.

Referring to the drawing, in Figures 1 and 2 mounted in a casing 10, which houses the components of the meter, is an ON-OFF switch 11 and a buzzer 12. Three lamps 13, 14 and 15 provide indications of whether the meter, that is the base of the casing as seen in Figure 2, is level. This will be described in more detail below. A circular support panel 16 is rotationally mounted in the casing 10 and has an arrow formed or marked on its face which in practice is positioned next to graduations 17 mounted or marked on the face of the casing 10.

SUBSTITUTE SHEET

A slotted end 18 of an eccentric shaft 19 (see Figures 6 and 7) enables the meter to be adjusted by using a small screwdriver, as described below. Further adjustment is provided by a manual movable slide 19A as described below.

The optical components of the meter are shown in Figures 3 to 5. A sealed vial 20 which is curved at least slightly downwards is filled with a liquid 21 (usually alcohol) to entrap an air bubble 22 which, when the vial 20 is symmetrically disposed about a vertical plane (that is, when the vial is normally described as "level"), is positioned centrally as shown in Figure 3. The lower surface of the bubble is curved as shown and as is inherent in such an arrangement. A light emitting diode (LED) 23 is mounted centrally and below the vial 20 and two light intensity detectors 24 are mounted at each end of the vial 20.

In the configuration shown in Figure 1, light from the LED 23 is reflected in use at the lower surface of the bubble 22 such that the intensity of light received by detectors 22 is equal, or in balance (see discussion later). Thus by monitoring the detectors 22, a signal is generated to indicate that the meter is level by illuminating the lamp 14 and/or energising the buzzer 12.

If the meter is tilted to the left (Figure 4) or to the right (Figure 5), the light intensity received by the

detectors 22 becomes unequal. In Figure 4, significantly more light is reflected at the lower surface of the bubble towards the left, for example. In that case a signal is generated to turn on the lamp 13. When the meter is tilted to the right the lamp 15 is turned on.

Referring to Figures 6 and 7, the vial 20 is held between jaws 30 and 31 integrally formed on a base 32. The base is supported in the support panel 16 by a pivot 33 at its lower end and cup-shaped aperture 34 at its upper end. The eccentric shaft 19 fits into and engages the sides of the aperture 34. If the shaft 19 is rotated the top of the base 32 moves slightly to the left and right, as seen in Figure 6, so that the vial 20 consequently tilts to the left and to the right, respectively, with respect to the support panel 16. Thus, by using a screw-driver in the slotted end 18, the meter can be effectively initially calibrated, and from time to time adjusted if necessary by the user, by slight movement of the relative position of the vial 20 with respect to the support panel 16 and thus the casing 10. A printed circuit board 35 is supported by the base 32 on which electrical components of a monitoring and control circuit are mounted.

Referring to Figure 2, a further adjustment can be made or used. The panel 16 can be off-set, by rotating it in the casing 10, by a chosen angle, say 10 or 15°. This is done by setting the arrow to a 10° or 15° mark of the graduations

at 17. In this fashion the level meter can be used to determine when a surface is at IOC; or 15C to the horizontal or level. Alternatively, when the meter is used and either the lamp 13 or 15 is illuminated, the panel 16 can be moved until the lamp 14 is turned on. As soon as the lamp 14 turns on, the position of the arrow relative to the graduations 17 indicates how many degrees and in which direction the meter is then from level.

It will be noted that the calibration facility provided by the eccentric shaft 19 and the manner of supporting the vial 20 in the meter can readily be used and effective to compensate for variations in the 'optical' components. In this manner for example relatively cheap light detectors 24 can be used. If their light responses are not matched, as they would be if high quality detectors are used, the eccentric shaft can be used to compensate for lack of close matching. In other words, a true level position for the meter can be set up when the vial 20 is not truly level so that somewhat more light then falls on one of the detectors 24 than the other to compensate for their somewhat different relative light responses. ever-the-less for the purposes of monitoring whether the meter, that is effectively the casing 10, is level, a "balance" of light intensities can be used to provide an accurate indication even where the optical components are not necessarily particularly accurate or well-matched in their responses, and/or the optical paths or other optical components are

not wholly symmetrical.

Although the specification refers to light source and light detectors, it will be appreciated that light in the non-visible frequency ranges may be used. It will also be appreciated that the optical components can be, and are in the described arrangement, totally enclosed in the casing

10. This ensures their relative safety against damage.

Further, the described meter although making use of a well-known form of "spirit-level" (the vial filled with alcohol) does not advantageously rely on visual assessment by the user of the position of the bubble or require the user to have a clear line-of-sight of the vial.

In Figure 8, a supply circuit for the LED 23 includes a variable resistor 40 connected in series with the LED 23. A slider contact 41 of the resistor 40 is -moved by the slider 19A (Figure 2).

In normal use, the described meter is accurate to a level of approximately 0.1mm per metre but for many practical applications such a sensitivity is too accurate. In fact, few "level" surfaces are level to this accuracy and it may be impractical or not useful to try and provide surfaces as level as this when using the meter. By adjusting the power supply to the LED 23 to increase the intensity of light output of the LED 23, the effective sensitivity of the meter can be suitably reduced according to these

adjustments to, say, 5mm per meter. The slider 19A is provided with a marker which slides next to adjacent markings on the case indicating the sensitivity of the meter. The markings show meter sensitivity in increments from 5mm to 0.1mm. For convenience these markings are preferably evenly spaced in which case the resistor 40 is suitable formed to alter, as the slider contact 41 is moved, the power through the LED 23 according to a logarithm function or similar. In this way the light output intensity varies generally linearly as the slider contact is moved.

In an alternative arrangement, the LED 23 is provided with power supply arranged to adjust the light intensity in one or more steps. In that case, markings on the casing simply indicate a level of sensitivity, for example "high" , "medium" and "low" sensitivity.

In a further arrangement, the adjustment of the effective light intensity can be made using a variable density filter which is mechanically coupled to the slider 19A and moved across the light path between the LED 23 and the bubble 22 to reduce either in steps or gradually, as required, the intensity of the light falling on the undersurface of the bubble.

Normally, the electrical circuit is arranged to respond very accurately to when the balance of light intensities

detector circuit monitoring the detectors 22 is arranged with greater latitude of comparison, the sensitivity of the meter is also be affected, that is reduced. Thus, a comparator circuit with wider tolerances reduces the sensitivity and it is possible to provide a comparator circuit in which the tolerence can be adjusted in a manner generally equivalent to altering the light output intensity of the LED 23.