GB2034040A - Improvements in or Relating to Liquid Level Alarms - Google Patents

Abstract

A liquid level alarm comprises two electrodes 43, means for monitoring the resistance between them and giving an alarm when the resistance crosses a threshold value and a float switch 44 which disables the alarm when liquid is below a predetermined value. The alarm is used in a drainage system including oil interceptors from each of which oil must be removed when it reaches a preset level to prevent contamination. The float switch is arranged to short circuit the electrodes when liquid is absent thus simulating the presence of liquid. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Liquid Alarms This invention relates to a liquid alarm, and in particular to an electronic alarm adapted to give an alarm dependent upon liquid level in a tank.

The alarm of this invention is especially suitable for use with a waste-water oil interceptor, though it may be used in a variety of other circumstances, when an alarm is to be given dependent upon liquid levels.

With increasing emphasis on pollution control, it is now a common practice to install an oil or grease interceptor in a waste or surface water drain in situations where considerable quantities of oil products are likely to be found in waste water. For instance, the surface water drains from garage forecourts normally have a three-stage interceptor installed therein to trap oil products before the surface water is discharged to a main drain or other water course, and commercial kitchens often have a single-stage interceptor installed in the sink waste drains.

A known design of interceptor comprises at least one tank having a high-level waste-water inlet and a relatively low-level water outlet arranged so that the normal liquid level in the tank is significantly higher than the outlet. This may be achieved for instance by designing the outlet as a dip-pipe opening into the tank well below the normal liquid level, defined by the position of the outlet from the dip-pipe. In use, oil and oil-based products tend to float on water within the tank and thus are separated from the water. To minimise the possibility of flush-through of collected products, in times of high flow rates, three such tanks often are arranged in series.

An interceptor as described above has to have the oil removed therefrom at varying intervals, depending upon the conditions of use, for otherwise the water level would gradually fall as more and more oil collects, until oil passes out of the low-level outlet. Up to now, it has been necessary frequently to inspect the interceptor, and then to take the appropriate emptying action if required, but unfortunately such mainten-ance is often neglected until poliution actually occurs.

Emptying may be performed either by pumping the contents of the tank into a tanker vehicle for subsequent disposal, or by skimming under gravity into a separate storage tank for subsequent collection. The latter is much preferred, because in theory only oil collects in the storage tank and collection by tanker need be arranged much less frequently. Unfortunately, should skimming not be required yet performed, water as well as oil will collect in the storage tank.

For the above reasons, there would be considerable advantages in a system which operated automatically to indicate when emptying or skimming of an interceptor is required. Electronic liquid level detectors are known which monitor resistance between probes and though with such a detector it would be possible to detect when more than a predetermined amount of oil was present because of the differing conductivities of oil and water-these known detectors have been found to be unsuitable for use with an interceptor. This is because the detector must be arranged to monitor resistance at the level of the maximum allowable depth of oil below the normal liquid surface level in the interceptor, and thus normally to detect the presence of water. When too much oil is present, the resistance between the probes rises and an alarm must be given.If now the interceptor is emptied or skimmed, the probes will either be left with air or oil therebetween and an alarm will still be given until the water level rises again within the interceptor-but in dry conditions this may not be for some days or even weeks.

To overcome the above problem, either the interceptor could be artificially filled with water (which is most wasteful and in any event complicates the maintenance) or an alarm cancel switch could be incorporated in the alarm. The latter case is most unsatisfactory because once operated, the cancel switch will inhibit operation of the alarm until reset, and an operator would have to remember to reset the switch after the interceptor has refilled, possibly some weeks later.

It is an aim of this invention to provide a liquid alarm especially suitable for use with an oil interceptor and which overcomes the various problems discussed above.

According to this invention, there is provided an electronic liquid level alarm for use in conjunction with a tank for containing liquids, which alarm comprises an electronic circuit adapted to monitor the resistance between two points and to give an alarm when the resistance crosses a threshold value, and a probe assembly for insertion into the tank which probe assembly includes an electrode for contacting contained liquid and co-operate with a second electrode so that the resistance may be monitored between the electrodes, and a float switch responsive to liquid level in the tank and connected to disable the alarm when liquid in the tank is below a predetermined level.

It will be appreciated that with the alarm of this invention, an alarm may be given only if the resistance between the probes is to one side of a threshold level (that is, either above or below the threshold level, depending upon the circuit configuration) and if the liquid level in the tank is sufficiently high to operate the float switch. For an oil interceptor, the monitored resistance will normally be low, but will be high in the event of oil or air between the electrodes-but an alarm will only be given when oil is present. The alarm automatically is disabled after emptying or skimming by the float switch, but also is automatically reset to enable the alarm as the liquid level rises again in the tank.

The float switch could be connected back to the electronic monitoring circuit to disable the alarm, for instance by inhibiting an output from the circuit which drives the alarm. It is however preferred for the float switch to be connected appropriately to at least one of the electrodes to inhibit operation of the alarm by preventing monitoring of the change in resistance across the electrodes. In the preferred arrangement intended for use with an oil interceptor, the float switch should be of the normally-open type, but which closes when the liquid surface in the tank falls below a pre-set level, the switch being connected directly between the two electrodes to effect a short circuit therebetween when closed.It will be appreciated that in this way, the float switch may simulate the presence of water between the electrodes when the level in the tank is low and thus after emptying or skimming until the tank refills.

The probe assembly advantageously mounts both electrodes and the float switch. However, in the case of a metal tank, the tank itself may form one of the electrodes and the resistance would then be monitored between the tank wall and the other electrode mounted on the probe assembly.

Conveniently, the probe assembly is in the form of an elongate member having the two electrodes and the float switch mounted at one end thereof, and arranged so that the member may be clamped to the tank to extend generally vertically with the electrodes at the lower end thereof.

The electronic circuit preferably is adapted to monitor the resistance using an alternating current signal, to minimise polarisation effects at the electrodes. It has been found that an alternating current signal in the range of 500 Hz to 4 kHz produces optimum results.

The alarm given by the electronic circuit may be a visual alarm, an audible alarm, or both. If an audible alarm is provided as well as a visual alarm, it is preferred for there to be a cancel switch for the audible alarm only, but in this case the circuitry should be arranged so that if cancelled, when the monitored resistance returns back to a non-alarm value, the audible alarm sounds again until the cancel switch is reset.

For the case of a multi-stage interceptor, the electronic circuit may be arranged to operate a single alarm if the resistance monitored in any one of the tanks crosses the threshold, there being a secondary indicator to show from which tank the alarm originates. Moreover, for an interceptor having an oil-storage bay or tank into which oil may be skimmed, a float switch can be provided for that bay or tank and the electronic circuit arranged to be responsive to operation of the float switch when that bay or tank is full, thereby to give an alarm warning that the oil storage bay or tank should be emptied.

By way of example only, one specific embodiment of a liquid alarm arranged in accordance with this invention will now be described, reference being made to the accompanying drawings, in which Figure 1 is a block diagram of the liquid alarm of this invention; Figure 2 is a circuit diagram of part of the alarm of Figure 1; Figure 3 is a view of a probe assembly for use in the alarm of Figure 1; and Figure 4 is a cut-away view of part of an oil interceptor fitted with a probe assembly as shown in Figure 5.

Referring to Figure 1, there is shown a block diagram of liquid alarm intended for use with an oil interceptor having three interceptor (or settlement) tanks arranged in series and an oil storage bay into which oil collected in the interceptor tanks can be skimmed. The alarm has three probes 10, 11 and 12 each of which has a pair of electrodes (not shown) between which the electrical resistance is monitored, and a fourth probe 1 3 fitted with a float switch. The probes 10 to 12 are intended for installation in the interceptor tanks at such positions that the electrodes lie substantially at the level to which the associated tank is skimmed, and the probe 1 3 is intended for installation in the oil storage bay so that the float switch is operated when the oil reaches the maximum permitted level in the bay.

Each probe 10 to 12 also includes a float switch 14 to 16, arranged so as to close after a skimming operation, but to be open when the interceptor is operating normally.

Probes 10 to 1 2 each has a detector circuit 1 7 to 1 9 associated therewith, to monitor the resistance across the electrodes of the associated probe. If the detected resistance rises above a pre-set threshold, then an indication of this is given on an associated channel indicator 20 to 22, and a signal passed to an alarm driver 23. The state of the float switch of probe 13 is monitored by a further detector circuit 24, and an indication given by an indicator 25 as well as a signal passed to the alarm driver 23 when the float switch closes.

Whenever the alarm driver 23 receives a signal from any one of the detectors 17 to 19 or 24, a main alarm lamp 26 is illuminated, and an audible alarm 27 (such as a buzzer) is energised through a cancel logic network 28. This network is arranged so that if operated to cancel the audible alarm 27, when next the alarm driver receives no signals from any of the detectors 1 7 to 1 9 or 24, the audible alarm is energised once again until the network 28 is re-set, to ensure that the alarm will sound again next time an alarm proper is to be given.

The alarm driver 23 also drives a relay 29, which may be used for example to control the operation of a pump 30, and to operate a remote alarm 31 in the form of a lamp or buzzer.

When in use in a three-stage oil interceptor with an oil storage bay, the above described alarm arrangement operates to give both a visual alarm and an audible alarm whenever any one of the three interceptor tanks contains the maximum permissible depth of oil for reliable performance, or when the oil storage bay contains the maximum permissible amount of oil to allow skimming thereinto from the interceptor tanks.

The tank or bay from which the alarm originates is also indicated, on indicators 20 to 22 or 25. The audible alarm may be cancelled by means of network 28, but not the visual alarm, if for instance maintenance cannot be effected for some while.

Normally, water is present between the electrodes of the probes 10 to 12, and thus the associated detector senses a low resistance. If the amount of oil intercepted in any one tank reaches the maximum permissible depth, the water is displaced by oil floating on the water, and the resistance sensed rises over a pre-set threshold.

This triggers the associated detector to give an alarm, both on the channel indicator and on the main alarm lamp. If the tank is then either skimmed or pumped out to remove the intercepted oil, the electrodes either will be immersed in a thin layer of oil or will be in air; in either case a relatively high resistance exists therebetween. However, the float switch associated with the probe of the tank in question will close as the liquid level in that tank falls on maintenance, and thus will provide a short circuit between the electrodes: the detector thus senses a low res;stance and no alarm is given. When water next flushes through the interceptor, the tank refills to its normal level, thus re-setting the float switch (i.e. the switch opens) but the electrodes will be immersed in water until next the amount of oil present reaches the maximum permissible amount.

Figure 2 shows a circuit diagram of the alarm of Figure 1. This circuit is based on the National Semiconductor LM 1830 integrated circuit, which provides, from an internal oscillator, an alternating waveform to a pair of electrodes and monitors the resistance therebetween. One channel 35 of the alarm is shown in detail; the two other channels 36 and 37 shown by longchain lines are essentially simiiar to that shown in detail. The power supply 38 is shown in block form and should provide a smoothed direct current supply at about 15 v.

Referring specifically to channel :35, capacitor C1 sets the frequency of the internal oscillator of the LM 1830, and capacitor C2 is a filter capacitance to ensure the output from the LM 1830 is of step-form. Capacitor C3 supplies the signal from the internal oscillator to the probes, and the circuit is arranged to trigger when the impedance across the probe leads (at the integrated circuit) is substantially 1 3 k. By using external resistors, this impedance can however be set to any required value within a wide range.

The indicator for each channel (including the oil bay channel) comprises a light-emitting diode LED, in series with a current-limiting resistor R1.

When the resistance detected by the integrated circuit rises over the threshold, the output of the LM 1 830 is in effect earthed, thereby causing the associated light-emitting diode to light. Also, transistor T1 is caused to conduct, the base of this transistor being connected to the output of each channel by means of a discrete diode ORgate, made up of diodes D1 to D4. Conduction of transistor T1 illuminates the visual alarm lamp 26, energises the coil of the relay 29 and energises a solid-state buzzer 27 through a double-pole change-over switch 39.If the switch is operated to its other position from that shown, the buzzer 27 is muted when transistorT1 conducts, but when that transistor no longer conducts, the buzzer will be energised through resistor R2 and through the relay coil and lamp 26, to sound until the switch is re-set to the position shown.

Figure 3 shows a probe assembly for connection to one of the channels 35, 36 or 37 of the circuit of Figure 2. This probe assembly comprises a connector box 40, from which depends a plastics tube 41. At the lower end of the tube 41 is a housing 42 from which project a pair of stainless steel electrodes 43, and which supports a float switch 44. The connections to the float switch and electrodes are potted within an epoxy potting compound, to protect the wires and connections when in use. The wires are taken back to the box 40, for connection to wiring external to the interceptor, back to the alarm circuit itself.

Figure 4 shows diagrammatically a part of an interceptor fitted with a probe assembly for an alarm of this invention. The interceptor has three interceptor tanks 50, 51 and 52 arranged in series with dip-pipes 53 interconnecting the tanks. An oil storage bay 54 is disposed to one side of the tanks 50 to 52 and oil may be drained into the bay 54 from any one of the tanks through a skimming arrangement 55. Each probe assembly 56 is set so that the electrodes thereof are substantially at the level of the skimming arrangement, whereby an indication is given of the need to skim oil into the bay 54 when sufficient has accumulated in a tank. The oil bay 54 has a probe 57 which has a float switch only, whereby an indication is given when so much oil has been skimmed into the bay that the bay itself needs emptying. The float switch should thus be located at the maximum permissible level of oil in the bay.

Claims (14)

Claims

1. An electronic liquid level alarm for use in conjunction with a tank for containing liquids, which alarm comprises an electronic circuit adapted to monitor the resistance between two points and to give an alarm when the resistance therebetween crosses a threshold value, and a probe assembly for insertion into the tank which probe assembly includes an electrode for contacting contained liquid and co-operable with a second electrode, electric conductors connecting the electrodes to the electronic circuit so that the resistance may be monitored by the circuit between the electrodes, and a float switch responsive to liquid level in the tank and connected to disable the alarm when liquid in the tank is below a predetermined level.

2. An electronic liquid level alarm according to claim 1, wherein the float switch is appropriately connected to at least one of the electrodes to inhibit operation of the alarm by preventing monitoring of the change in resistance between the electrodes.

3. An electronic liquid level alarm according to claim 2, wherein the float switch is of the normally-open type, but which closes when the liquid surface in the tank falls below a pre-set level, the switch being connected directly between the two electrodes to effect a short circuit therebetween when closed.

4. An electronic liquid level alarm according to any of the preceding claims, wherein the probe assembly mounts both electrodes and the float switch.

5. An electronic liquid level alarm according to claim 4, wherein the probe assembly is in the form of an elongate member having the two electrodes and the float switch mounted at one end thereof, and arranged so that the member may be clamped to the tank to extend generally vertically with the electrodes at the lower end thereof.

6. An electronic liquid level alarm according to any of the preceding claims, wherein the electronic circuit is adapted to monitor the resistance using an alternating current signal.

7. An electronic liquid level alarm according to claim 6, wherein the alternating current signal has a frequency in the range of 500 Hz to 4 kHz.

8. An electronic liquid level alarm according to any of the preceding claims, wherein the alarm given by the electronic circuit is at least one of a visual alarm and an audible alarm.

9. An electronic liquid level alarm according to any of claims 1 to 7 wherein the alarm given by the electronic circuit is both a visual alarm and an audible alarm, there being a cancel switch for the audible alarm only.

10. An electronic liquid level alarm as claimed in claim 9, wherein the electronic circuit is arranged so that when the audible alarm is cancelled and the monitored resistance subsequently returns to a non-alarm value, the audible alarm sounds again until the cancel switch is reset.

11. An electronic liquid level alarm as claimed in any of the preceding claims, wherein a plurality of probes are connected to the electronic circuit and the electronic circuit is arranged to operate a singie alarm if the resistance monitored between any one pair of electrodes crosses the threshold, there being a secondary indicator to show from which probe the alarm originates.

12. An electronic liquid level alarm substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.

1 3. An electronic liquid level alarm as claimed in any of claims 1 to 12, in combination with an oil interceptor, the or each probe of the alarm being arranged to sense the liquid level in one or more tanks respectively of the interceptor.

14. The combination as claimed in claim 13 and in which the interceptor has an oil-storage bay or tank into which oil may be skimmed, wherein a float switch is provided for that bay or tank and the electronic circuit is arranged to be responsive to operation of the float switch when that bay or tank is full, thereby to give an alarm warning that the oil storage bay or tank should be emptied.