GB2131169A - Sensor arrangement - Google Patents

Abstract

A sensor arrangement, e.g. for water or sewage treatment plants, comprises a sensor 12 mounted on a support element 10 which is movably mounted (for example by pivoting at 14) and arranged so that the sensor is disposed substantially below the free surface of the liquid and so that the support element moves relative to the liquid in response to flow and/or agitation of the liquid. This prevents accumulation on the sensor and support element of solid matter from the liquid, which might shield the sensor from the liquid it is intended to monitor. <IMAGE>

Description

SPECIFICATION Sensor arrangement This invention relates to a sensor arrangement particularly but not exclusively for the water industry (for example in water treatment or sewage treatment plants).

In for example water treatment or sewage treatment plants, sensors are immersed in the liquid at various points of its flow path or paths for the purpose of detecting or measuring various parameters of the fluid, for example dissolved oxygen content, ph, temperature etc. Generally such sensors are mounted rigidly, but they are extremely susceptible to solid matter attaching itself to the sensor or to the support mounting the sensor: the result of this is that within a short period of time so much matter has accumulated around the sensor that it is effectively shielded from the fluid which it is intended to monitor.Attempts have been made to alleviate this problem, for example by providing an apertured guard or shield member around the sensoe but in such case the apertures become clogged, or by providing a system of water jets or mechanical device for periodically cleaning the sensor but these arrangements are complex and often unsatisfactory.

We have now devised a sensor arrangement for the water industry (but doubtless having utility in other flowing fluid situations also), which in use stays clean for prolonged periods of time or indefinitely without requiring maintenance or any active self-cleaning systems or mechanism of its own.

In accordance with this invention, there is provided a sensor arrangement for a liquid, comprising a sensor mounted on a support element, the support element being movably mounted and arranged so that the sensor is disposed substantially below the free surface of the liquid and so that the support element moves relative to the liquid in response to flow and/or agitation of the liquid thus preventing accumulation on the sensor and support element of solid matter from the liquid.

The support element may comprise an elongate element extending downwardly from a point above the free surface of the liquid at which point the support element is pivoted to fixed structure, and having the sensor mounted at its lower end. The pivotal axis is generally transverse to the direction of flow and the support element is so counter balanced that even relatively light matter impinging on the sensor (or on the immersed length of the support element) causes pivoting of the support element to allow this piece of matter past. The support element is smooth-surfaced and streamline in profile, for example a circular-section plastics tube or a flat plastics tube arranged with its major opposite faces parallel to the direction of any flow.

Embodiments of this invention will now be described, by way of examples only, with reference to the accompanying drawings, in which: Figure 1 is a schematic side elevational view of one sensor arrangement in accordance with this invention; and Figure 2 is a schematic side elevational view of another sensor arrangement in accordance with this invention.

Referring to Figure 1 of the drawings, there is shown a sensor arrangement comprising an elongate support element in the form of a straight length of circular-section plastics (e.g. PVC) tube 10, having a sensor 12 mounted in one of its ends in a manner providing a smooth, hemispherical or otherwise rounded closure to that end of the tube 10. This tube 10 is pivoted ata point 14 substantially above the free surface of the liquid 16 (which is assumed to be flowing in the direction of arrow A), so that the tube 10 extends downwardly with the sensor 12 at its lower end and substantially below (say at least 1 foot below) the free surface of the liquid. The tube 10 may be arranged to hang (normally) vertically downwards, but preferably as shown there is a stop to prevent it returning to less than a certain angle B inclined to the vertical.This stop in the example shown comprises a fixed bar 18, mounted transverse to the direction of flow, to which bar a support 20 for the pivot pin 14 is mounted. The tube 10 is counter balanced so that, in operation, a piece of solid matter flowing with the liquid and impinging upon the sensor (or upon the immersed length ofthetube) will cause the tube 10 to pivot away from its rest position in which it is shown, to permit that piece of matter to continue its flow.

Movement of the support tube 10 relative to the liquid, in response to flow and/or agitation (e.g.

wave motion) of the liquid, is effective in keeping the sensor and supporttube clean, particularly by the consequent rubbing action of the pieces of solid matter upon the sensor and its support tube.

It will be particularly noted that the surface of the sensor arrangement at least below the liquid surface is smooth and streamlined to discourage matter attaching to it. The tube and sensor may be of such a material (or their surfaces so treated) as to have non-adherent properties (for example the surfaces may be coated with Teflon (R.T.M.) or the like), and/orto have anti-bacteriacidal properties.

Referring to Figure 2 of the drawings, the sensor arrangement shown therein differs from that shown in Figure 1 in that the tube 10 progressively curves towards its lower end, the sensor being mounted along the curved section in such a fashion that it remains parallel to the flow stream in normal operation. This has the effect of reducing impact velocity on the sensor of any solids in suspension whilst remaining optimum conditions for process reaction with the sensor.

Both of the arrangements shown preferably has a single-board computer or other electronic signal processing circuit mounted within the tube 10 and arranged to receive a weak analogue signal provided by the sensor and derive a digital output signal for transmission to a central station, for example. This in particular enables the sensor and its circuit to be calibrated in the factory rather than in the field, as opposed to known devices which produce analogue signals for transmission and which accordingly require calibration in the field.

For the purpose of guarding against possible faults in an instrument, three sensors may be installed at each site (either mounted at the end of the same tube 10 or in separate sensor arrangements) with their outputs connected to a logic circuit. This logic circuit is arranged so that the sensor outputs are regarded valid if all three are equal (within limits), or if two are equal (within limits) and the third is different than the outputs of those are regarded as valid, or if all three outputs are different then none is regarded as valid.

An alternative system of sensor fault-detecting may be employed where an installation includes (a large) three or more (number of) sensors, quite possibly monitoring a number of different parameters. Thus, it is known to provide a simulated model of a flow system or treatment plant, for use in determining the effect of certain eventualities (such as leaks at specific points etc). In an interactive system we now envisage, if one sensor output should provide an exceptional measurement, indicative of an unusual situation, then the outputs of at least selected other sensors are automatically evaluated to determine whether they also indicate the same unusual situation: if they do not, then the system concludes that the one sensor is faulty.

The arrangements described above are applicable to currently available sensors and to future generation sensors also.

For non-flowing or relatively slow-flowing situations or where additional movement of the sensor and its mounting tube is necessary relative to the liquid, the upper portion of the tube may be mounted to moving structure which is responsive to wave motion of the liquid itself. Thus the tube may be mounted to a system of gimbals or other arrangement in turn supported on a plurality of floats (e.g. 4floats) or even on a single float.

Reference has been made to use of the sensor arrangement in water treatment plants or in sewage plants, and in the latter case the sewage may include a significant amount of solid matter or sludge in suspension whilst still remaining fundamentally liquid. The sensor arrangement may however be used in other liquids.

Claims (12)

1. A sensor arrangement for a liquid, comprising a sensor mounted on a support element, the support element being movably mounted and arranged so that the sensor is disposed substantially below the free surface of the liquid and so that the support element moves relative to the liquid in response to flow and/or agitation of the liquid thus preventing accumulation on the sensor and support element of solid matter from the liquid.

2. A sensor arrangement as claimed in claim 1, in which the support element comprises an elongate element extending downwardly into the liquid and pivoted about a horizontal axis intermediate its ends.

3. A sensor arrangement as claimed in claim 2, in which said support element is counterbalanced so as to pivot in response to minimal flow and/or agitation of the liquid.

4. a sensor arrangement as claimed in claim 2 or 3, further comprising a stop to prevent the support element pivoting to less than a predetermined angle inclined to vertical.

5. A sensor arrangement as claimed in any one of claims 2 to 4, in which said support element comprises a length of tube.

6. A sensor arrangement as claimed in any one of claims 2 to 5, in which said support element is formed with a generally smoothly convex lower end.

7. A sensor arrangement as claimed in any one of claims 2 to 6, in which said support element progressively curves towards its lower end so as to become directed generally horizontally.

8. A sensor arrangement as claimed in claim 7, in which the sensor is mounted along the curved section of the support element and spaced from its said end.

9. A sensor arrangement as claimed in any preceding claim, in which the support element is mounted to a moving structure which is responsive to wave motion of the liquid.

10. A sensor arrangement as claimed in any preceding claim, in which said support element incorporates a signal processing circuit for receiving an analogue signal from the sensor and deriving a digital output signal for onward transmission.

11. A sensor arrangement as claimed in any previous claim, in which at least three said sensors are provided together with a logic circuit arranged to approve a sensor output as valid if it is equal (within limits) to either or both the other sensor outputs.

12. A sensor arrangement substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.