GB2235768A - Flow sensor - Google Patents

Description

FLOW SENSOR This application is a divisional application of British Patent Application No. 8727499.

This invention relates to a flow sensor system and in particular to a system for sensing water flow in pipes, for example, pipes associated with fire sprinkler systems in buildings. However, the flow sensor of the present invention has other applications such as in oil refineries, gas plants and pumping systems.

Building fire sprinkler systems usually comprise a main pipe which feeds to sub-main pipes which in turn feed to individual sprinkler pipes. It is known to place flow sensor devices at least in each sub-main pipe and also in each individual sprinkler pipe.

A known flow sensor includes a paddle which, when moved by water flowing in an associated aprinkler pipe, will move a mechanical switch to complete an electrical circuit.

For reasons of safety, it is not considered satisfactory that that known sensor should close a switch in a two wire system consisting of a power supply wire and a return wire as any break in either of the wires or in an associated connection would render the closing of the switch ineffective.

Instead, a three wire system is used in which two wires are respectively power supply and return wires and the third wire is a signal carry wire through which current passes when the switch is closed. A load resistor is located adjacent the sensor and a small current is continually passed through the supply and return wires. The three wires run to a remote monitor panel where current sensor means is located and adapted to give an alarm in the event that current flow in the supply and return wires exceeds a first value (as may occur in a short circuit) or falls below a second value (as may occur in an open circuit). Other current sensor means is adapted to give a signal if current flows in the signal carry wire.

It is particularly to be noted that whilst the integrity of the supply and return wires is continually monitored, the integrity of the signal carry wire is not continually monitored. This last is considered by the present inventors to be an unsafe practice.

Further, since three wires must be run from each sensor to the control panel there is substantial cabling expense.

Further, the known sensor utilizing a mechanical switch is considered by the present inventors to be unsatisfactory.

It is one object of the present invention to produce a flow sensor for use in a two wire system.

However, it is also another object of this invention to produce a flow sensor having other features and which could be used in a conventional three wire system.

A flow sensor according to the present invention comprises light emission means, light detection means and two circuit elements forming current supply means, current return means and signal carrier means, the light emission means being connected across the two circuit elements, the sensor being so arranged that when a predetermined flow condition occurs, light emanating from the light emission means will be detected by the light detection means, and monitor means coupled to the current supply and return means for sensing changes in the resistance of the sensor, the arrangement being such that if the light emission means should fail and go open or short circuit or an output signal be produced, on detection of light, by the light detection means, the resistance of the flow sensor will change so as to be detectable at the monitor means.

Another problem of the known sensor discussed above is a tendency to give a false alarm signal due to fluctuations in mains pressure, water hammer or during testing.

Accordingly, it is preferred that circuit means be associated with the sensor system which is adapted to delay outputting of an output signal unless a condition initiating an output signal is maintained for not less than a predetermined period of time.

In this last instance it is preferred that output from the light detector means be directed to a timing circuit which will only produce an output for signaling purposes if the output from the light detector means is maintained for more than said period of time.

Preferably remote monitoring means are provided to detect changes in the resistance of the flow sensor system. As well as an ability to give a signal to a remote control panel it is also desirable that the system of this invention be adapted to give, at or adjacent the sensor, a visually or audibly perceivable signal of the fact that it is giving such a signal to the panel.

This last will be found to be useful when a serviceman or like is adjacent sensors and remote from the panel in quickly detecting which one of many sensors is outputting.

The invention will now be described further, by way of example, with reference to the accompanying drawing which is a circuit diagram of a sensor system according to the invention.

This circuit is preferred to be used with the flow sensor described in co-pending British Application No. 8727499, of which this Application is a divisional.

The circuit is a sensor circuit located within the body of a flow sensor and is to be connected to wires 61 and 62 of a two wire system in a building fire sprinkler alarm system. The wire 61 is a nominal positive, the wire 62 is a nominal negative, an end of line resistor (EOL) is provided and terminals a,b and c are provided for connection to terminals A, B and C of the sensor circuit.

The sensor circuit comprises an infra red light emitting diode LED1 which is the light emitter 25 and a protective diode D1 which will protect against reverse polarity. In the event of a reverse polarity connection, the diode D1 will short circuit, causing a detectable current flow in the wires 61 and 62. In addition, a voltage limiting zenner diode ZD1 and a voltage limiting resistor are provided to limit current flow. A jumper J1 is normally positioned to connect terminals D and F.

Under normal conditions, i.e. without flow of water in a pipe with which the sensor is associated, only LED1 will be energised and thus current flow will be small. It will thus be realised that current flow to check the integrity of the wires 61 and 62 through the EOL resistor will also drive LED1 and, further, if LED1 should fail by going open circuit or short circuit then current flow will change in a way which can be detected.

Under normal conditions, light from LED1 will incide black end 41 and not be reflected but if the slider 34 has been moved by the arm 32 in consequence of water flow light will be reflected from silvered end 42 onto a phototransistor PT which is the light detector 26.

When light strike PT it becomes conducting and via transistors T1 and T2 there is amplification of the current flow in PT. Resistor R1 provides negative feedback to reduce gain.

The output of T2 is connected as an input signal to, and power source for, IC1 which is a 3905 precision timer. Thus, prior to T2 output, which is dependent on PT conducting, the timer IC1 is not energised and is consuming no current.

R5 is a variable resistor and C1 is a capacitor which together form a RC timing circuit. By adjustment of R5 the timing is varied.

Provided that output from PT and hence T2 is maintained for more than the time period set by R5 and C1, IC1 will output at terminal 7 to transistor T3.

If, however, PT should cease outputting before that time period has elapsed, as might be the event in a potential false alarm condition, no output to T3 will occur and thus false alarms are unlikely.

Output from T3 passes via a current limiting resistor R3 to LED2 which is a light emitting diode.

This may be omitted but is usefully mounted outside or to be viewable from outside the sensor body 1 to give a visual indication that the particular sensor is outputting. This last is desirable as in a large building with many such sensors, the fact that a remote control panel is indicating that a particular sensor is outputting is of little help to a serviceman who is not adjacent that remote control panel but is adjacent to a number of sensors and is uncertain which of them is outputting.

Output from T3 also passes through current limiting resistor R4, terminals B and b and wire 62 to give a detectable current flow at the remote monitor panel.

Resistor R2 will allow the timer IC1 to reset after testing.

The above described flow sensor and its circuit are suitable for use in building fire sprinkler alarm systems and also in other applications of fluid flow monitoring.

With jumper J1 connecting terminals D and E the flow sensor will be able to act as a supervising monitor in which its own circuit is also supervised.

However, if jumper J1 is removed and in lieu a jumper J2 is used to connect terminals D and E then the circuit is no longer monitored but the sensor can be used to directly control a relay or other switching device for giving a signal or operating means such as might be required in an airconditioning plant and in this instance resistor R7 serves as a current limiting resistor for LED1.

In a modification, a coil spring mounted on the axis 31 is used in lieu of the spring 36.

Since modifications with the scope of the invention may be readily effected by persons skilled in the art, it is to be understood that the invention is not limited to the particular embodiment described, by way of example, hereinabove.

Claims (7)

CLAIMS:

1. A flow sensor comprising light emission means, light detection means and two circuit elements forming current supply means, current return means and signal carrier means, the light emission means being connected across the two circuit elements, the sensor being so arranged that when a predetermined flow condition occurs, light emanating from the light emission means will be detected by the light detection means, and monitor means coupled to the current supply and return means for sensing changes in the resistance of the sensor, the arrangement being such that if the light emission means should fail and go open or short circuit or an output signal be produced, on detection of light, by the light detection means, the resistance of the flow sensor will change so as to be detectable at the monitor means.

2. A flow sensor as claimed in claim 1, including delay means for preventing alteration of the flow sensor resistance by the light detector means unless a condition initiating the alteration is maintained for not less than a predetermined period of time.

3. A flow sensor according to claim 2, wherein the output from the light detector means is connected to a timing circuit which produces an output if the output from the light detector means is maintained for more than said period of time.

4. A flow sensor according to any of claims 1-3, wherein if the light emission means should go open or short circuit, the monitor means will indicate a fault or alarm condition respectively.

5. A flow sensor according to any preceding claim, wherein remote monitoring means are provided to detect changes in the resistance of the flow sensor.

6. A flow sensor as claimed in claim 5, having means, at or adjacent the sensor, to produce a perceivable signal indicative of the fact that it is providing a signal to said monitor means.

7. A flow sensor substantially as hereinbefore described with reference to the accompanying drawing.