GB2288238A - Monitoring device for liquid levels - Google Patents

Abstract

Liquid level monitoring means is provided intended to be of particular use in controlling the level (11) of water in a steam raising boiler (10). A set of probes (12 - 15) is provided, usually four in number, to detect when water supply is to be initiated or interrupted, and to detect low water level at which boiler shut off should take place. The resistances across pairs of the probes are measured and compared to monitor the water levels; the probe 15 is a reference probe and continuously immersed. <IMAGE>

Description

Monitoring Device for Liquid Levels This invention relates to a monitoring device for monitoring liquid levels, in particular the level of electrically conducting liquid.

Electrodes or probes are frequently used for detecting and monitoring the level of liquid in vessels especially in environments in which mechanical devices cannot be used.

Typically such probes are arranged to detect when a high level of liquid is reached, when a lower intermediate level of liquid is reached requiring further liquid to be introduced into a vessel, and when an unacceptably low level of liquid is present in the vessel.

Such monitoring arrangements are used in steam raising boilers in which the environment is harsh and the water level to be detected is subject to turbulence and, in some cases, to the presence of foam. Hitherto, in order to overcome the problems of turbulence and foam, there has been provided a secondary vessel connected to the boiler in which the probes are located, the water level in the secondary vessel being the same as in the primary boiler. Such a solution to the problem is effective but is expensive particularly in the case where the boiler is a pressure vessel.

Steam raising boilers provide a potentially hazardous situation and need to be carefully monitored to ensure satisfactory operation without inadvertent shut down and without the boiler continuing to operate when water levels are unacceptably low.

An object of the invention is to provide an improved monitoring device for liquid levels which overcomes disadvantages of existing devices.

According to the invention a monitoring device for liquid levels comprises at least three electrodes or probes, a vessel for containing electrically conducting liquid, the probes being arranged to detect and monitor the level of liquid in said vessel, and circuit means connecting the probes, the circuit means being arranged to measure the resistance between pairs of said probes and to make comparisons between the resistances measured whereby to monitor the resistances and give an indication of the liquid level in said vessel.

Preferably at least two of said three probes are located to be immersible in said liquid with one of said two probes extending into said liquid to a lower level in the liquid than the other probe to indicate lower and higher levels of liquid in said vessel.

Conveniently the third probe of said three probes is a reference probe providing a reference to monitor the resistance between the reference probe and each of said two probes and the third probe is either permanently immersed in the liquid in use or is electrically connected to the vessel which is electrically conductive.

In another arrangement there is provided a fourth probe, in which case one of said probes is arranged to monitor a high level of liquid in said vessel, another probe is arranged to monitor an intermediate level of liquid in said vessel, whereby the higher level probe and the intermediate level probe are arranged to control the introduction of liquid into the vessel, a third probe is arranged to monitor a low level of liquid in the vessel, and the fourth probe is a reference probe to monitor the resistance between the reference probe and the other three probes.

Accordingly when there are three probes resistances across three pairs of probes are measured and compared, and when there are four probes the resistances across six pairs of probes may be measured and compared. Comparison is by a microprocessor having memory and comparison means and takes account of different resistances which may arise during operation of, say, a water boiler in which the resistance of the water decreases significantly as the water temperature rises, or according to other variable parameters. Thus the comparison of the resistances can detect when a probe is covered in foam even though the resistance of the water is significantly lower when hot than when cold because the microprocessor is constantly monitoring and comparing the resistances of the pairs of probes.If the probe is covered in foam it will have a higher resistance, but less than if it were completely uncovered, and a sufficiently higher resistance than a probe covered in water that, by the comparison of the resistance of a probe covered in water and the resistance of the probe covered in foam, a signal will be generated by the microprocessor which will actuate the relevant relay, for example a pump on relay or a shut down relay.

The probes are arranged to monitor a condition of the liquid in the vessel which is outside desired predetermined conditions and in these circumstances the circuit is arranged to actuate cut off means for interrupting the operation of power means to the vessel, for example heating of the liquid in the vessel.

According to a further aspect of the invention there is provided a monitoring device for a water boiler which comprises a boiler for containing water, at least three electrodes or probes for monitoring and controlling the level of water in the boiler, at least two of the probes being located to be immersible in the body of water in the boiler according to the level of said water, the probes being connected by a circuit means which is arranged to measure the resistance between pairs of said probes and to make comparisons between the resistances measured, whereby to monitor the resistances and give an indication of the water level in the boiler and to cut off power to the boiler or to energise means for introducing water into the boiler, depending on the liquid level detected in the boiler.

Preferably there is provided at least three water level probes and a reference probe, the three water level probes being located at three different levels in the boiler, a pair of said water level probes located at different higher levels of water in the boiler, and arranged to control the flow of water into the boiler, a third water level probe being located at a lower level in the boiler than said pair of water level probes and being arranged to control operation of the boiler in the event that the water level in the boiler reaches said lower level.

The reference probe may be immersed in the liquid or water, or is electrically connected to an electrically conductive shell of the vessel.

Conveniently the circuit means selects pairs of probes for measurement of resistance between the probes, in sequence, and compares the resistance measured for each pair with the other pairs of probes, whereby if resistances measured are within or outside a predetermined range of resistance a signal is generated dependent on a comparison of the resistances.

The circuit means may be arranged to reverse the polarity of the electrical supply to said pairs of probes to enable a resistance measurement to be taken in each direction between the pairs of probes.

According to a further aspect of the invention in a method of monitoring liquid levels of electrically conductive liquid there are provided at least three probes or electrodes contactable with said liquid, and circuit means including a microprocessor, whereby the resistance between pairs of probes is measured, the resistance measurements are stored in memory means of the microprocessor, and comparison means of the microprocessor compares the resistance measurements with one another and the measurements are monitored to detect the level of liquid in a container of the liquid, whereby to generate signals according to said level and, according to the level, the signals are arranged to actuate alarm means and/or pump means for introducing liquid into the container.

Further features of the invention will appear from the following description of an embodiment of the invention given by way of example only and with reference to the drawing which is a schematic drawing of a vessel containing liquid, and a monitoring circuit.

Referring to the drawing there is shown an arrangement specifically adapted for steam raising boilers. A boiler shell 10 contains water at a level 11 (shown at a normal operating level) and located within the boiler are four probes or electrodes 12, 13, 14 and 15.

Probe 12 is a high level probe, in this case shown uncovered and above the water level 11, which probe 12 is provided to actuate a pump-off signal for a water pump (not shown).

Extending into the shell to a lower level than the probe 12 is the probe 13, shown immersed in the water, and intended to energise a switch or relay 20 to initiate a pump-on signal for the pump whereby water is pumped into the boiler when the level 11 of the water falls below the probe 13. Probe 14 extends to a level below probes 12 and 13 and is shown immersed in the water. The probe 14 is intended to generate a signal when the water level in the shell 10 goes below the level of the probe 14 indicating a dangerous situation of the boiler when the boiler should be shut down, the signal initiating a relay such as shown at 20 to shut down the burner for the boiler.

Probe 15 extends below the level of all the probes 12, 13 and 14 and constitutes a reference probe which is always immersed in the water in use of the boiler. Instead of the probe 15 extending into the water the probe 15 can be replaced by an electrical connection with the boiler shell to electrically connect the shell to the power supply, the shell being electrically conducting, so that the boiler shell acts as the reference probe.

In a steam raising boiler the boiling water in the boiler can develop turbulence, and foam can also be generated on the surface of the water. The function of the probes and associated circuitry is to enable the probes to extend into the main body of the boiler shell 10 and, even under these conditions, still provide effective monitoring of water level.

The probes 12-15 are connected to an electrical circuit shown at the left-hand side of the drawing. In the circuit there are provided analogue switches IC2 and IC3 which are shown as mechanical switches for clarity. A microprocessor IC1 provides a binary address to switches IC2 through connections AO, Al and A2 and such address closes the one of the switches 1-8 which is addressed. Switches IC3 are addressed in a similar but independent manner through connections BO, B1 and B2.

The probes 12-15 are connected to the circuit, connection A being to the upper probe 12, connection B being to the intermediate probe 13, connection C being to the low water probe 14, and connection D being to the reference probe 15.

Each of the switches IC2 and IC3 contain eight switches 1-8 which are selectively connected into the circuits for each pair of probes when switches 1-8 are actuated.

The arrangement of the circuitry enables the microprocessor to sequentially select a measurement of resistance across pairs of probes, in the illustrated arrangement across the pairs of probes 12 and 13, 12 and 14 and 12 and 15, across the pairs of probes 13 and 14 and 13 and 15, and across the pair of probes 14 and 15, i.e. six measurements. In addition the measurement of resistance across the pairs of probes can be effected by a reversing of the polarity of the current flow through the probes. The resistances measured are stored in the memory of the microprocessor IC1 which also includes comparison means for comparing the-resistances measured.

The objective of the measurements is to establish whether the relevant probe is immersed in water or is not, and the circuitry establishes which state applies. The resistance across pairs of probes can vary due to the temperature of the liquid and, even when no liquid is present a resistance reading can be obtained, for example when probes are short circuited or if foam is present in the boiler shell 10.

The microprocessor IC1 selects the switches for operation in the analogue switches IC2 and IC3 in accordance with a predetermined sequence. Thus, for example when switch 1 of IC2 and switch 2 of IC3 are both closed current will flow through resistance R to the pump-off probe 12 and return via the pump-on probe 13 and by measuring the voltage drop across the probes 12 and 13 the microprocessor is able to calculate the resistance between the probes. This measurement can now be repeated with switch 2 of IC2 and switch 1 of IC3 closed which gives a measurement with the current flowing in the reverse direction between the probes 12 and 13.This procedure has the advantage that by reversing the current electrolytic corrosion of the probes is prevented and the measurements of resistance are averaged which reduces any error due to electrolytic emf which can arise if the probes are of dissimilar metals.

Such measurements are repeated for every permutation of pairs of probes, i.e. six measurements in the illustrated embodiment, and the microprocessor IC1 compares the resistances measured. A check can be made for any inconsistency with the measurements. For example with probes 13 and 14 covered if the reference probe 15 to low water probe 14 resistance is greater than the resistance between reference probe 15 and the pump-on probe 13 then the measurements must be faulty and the circuit actuates a fault indication signal.

This ensures that the burner for the boiler is turned off, an alarm turned on and the water pump is turned on.

In practice the resistance of the water measured by the probes falls as the water in the shell heats up. The microprocessor IC1 in effect keeps a record of water resistivity which provides a comparison with the recorded resistance to determine if the relevant probe is immersed in water or otherwise. Thus the system is able to distinguish between the presence of water and the presence of foam which gives a higher resistance. In addition it can distinguish between the presence of water or a short circuit across the probes giving a lower resistance.

It will also be evident that if switch 1 of IC2 and switch 1 of IC3 are operated together and separately then by reading the changes in analogue input the microprocessor ICI is able to verify that the switches are working and to measure the internal resistance of the switch. This is repeated for all the switches at regular intervals and any fault in the switches which is found is treated as serious such that the relays to operate the shell burner, the alarm and the pump are actuated to ensure that the burner is turned off, the alarm is turned on and the pump is turned on.

It will be seen that the circuit is arranged to measure the resistance between pairs of probes and to compare each of the resistances measured with the others.

It will be appreciated that in some monitoring arrangements it is only required to know if the liquid level is above or below two predetermined levels in which case three probes can be used instead of four. In this case two of the probes are located one at a high level and the other at a low level, and the third probe is a reference probe.

As a further alternative more than four probes may be provided for, for example, monitoring more than three levels of liquid in a vessel.

It will be appreciated that with the arrangement described it is not the specific resistance derived from the resistance measurement for each pair of probes which is relevant but that resistance in comparison with the resistance measurements of the other pairs of probes. The need for this comparison is evident given an understanding of the possible changes of resistance in the system. Thus, for example, the resistance across a pair of probes, both immersed in water, at a cold temperature of the water may be of the same order as the resistance across a pair of probes one of which is immersed in hot water and the other of which is located above the water level in foam. Hence a measurement of actual resistance would not give an indication of the presence of foam unless it were possible to incorporate temperature compensation according to the temperature of water.

In the present arrangement due to the resistance comparison used there is automatic compensation for temperature changes and other variable factors such as water purity.

In practice the arrangement is called upon to distinguish between situations such as (a) both probes in a pair immersed in water, (b) one probe immersed in water and the other in air or steam, (c) one probe immersed in water and the other in foam, (d) the probes in a pair short circuited. In each case a comparison of the differences between the resistances measured monitors the relevant situation because the differences in resistances are of sufficient order of magnitude as to be distinguished from one another.

Preferably the measurements of resistances are taken in sequence and are repeated at short intervals; averages of the measurements may be calculated by the microprocessor and the generation of alarm or other signals may not take place until after a predetermined time has elapsed, such as 10-30 seconds.

This ensures that a checking procedure can be followed before such signals are generated, obviating signal generation due, for example, to turbulence in the water.

The arrangement described accommodates changing resistances and other changeable parameters, for example reducing resistance as water is heated, increasing resistance when the water is pure, while still being able to distinguish between water and foam.

Claims (12)

Claims

1. Monitoring device for liquid levels which comprises at least three electrodes or probes, a vessel for containing electrically conducting liquid, the probes being arranged to detect and monitor the level of liquid in said vessel, and circuit means connecting the probes, the circuit means being arranged to measure the resistance between pairs of said probes and to make comparisons between the resistances measured whereby to monitor the resistances and give an indication of the liquid level in said vessel.

2. A device according to claim 1 wherein at least two of said probes are located to be immersible in said liquid with one of said two probes extending into said liquid to a lower level in the liquid than the other of said two probes to indicate lower and higher levels of liquid in said vessel.

3. A device according to claim 2 wherein a third probe is a reference probe providing a reference to monitor the resistance between the reference probes and each of the other probes and the reference probe is either permanently immersed in the liquid in use or is electrically connected to the vessel which is electrically conductive.

4. A device according to any one of the preceding claims comprising at least four probes, one first probe being arranged to monitor a high level of liquid in said vessel, another second probe being arranged to monitor an intermediate level of liquid in said vessel whereby the first and second probes are arranged to control the introduction of liquid into the vessel, a third probe arranged to monitor a low level of liquid in the vessel, and a fourth probe which is a reference probe to monitor the resistance between the reference probe and the other three probes.

5. A device according to any one of the preceding claims wherein the circuit means includes microprocessor means having memory and comparison means whereby measurement of resistances are regularly taken, stored in the memory and compared with one another and the circuit means is connected to cut off means whereby upon receiving a signal from the circuit means the cut off means is actuated.

6. A monitoring device for a water boiler comprising a boiler vessel for water, at least three electrodes or probes for monitoring and controlling the level of water in the boiler, at least two of the probes being located to be immersible in the body of water in the boiler at different levels in said vessel, the probes being connected by electrical circuit means arranged to measure the resistance between pairs of said probes and to make comparisons between the resistances measured whereby to monitor the resistances and give an indication of the water level in the boiler and to cut off power to the boiler and/or to energise means for introducing water into the boiler depending on the liquid level detected in the boiler.

7. A device according to claim 6 wherein there is provided at least three water level probes and a reference probe, the three water level probes being located at three different levels in the boiler, two of said water level probes being located at different higher levels in the boiler and arranged to control the flow of water into the boiler, a third water level probe being located at a lower level in the boiler than said two probes and arranged to control operation of the boiler should the level of water in the boiler reach said lower level.

8. A device according to claim 7 wherein the reference probe is immersed in water or is electrically connected to an electrically conductive shell of the boiler.

9. A device according to claim 6, 7 or 8 wherein the circuit means is arranged to select pairs of probes for measurement of resistances between said pairs of probes in sequence, and compare the resistance measured for each pair with the resistances for the other pairs whereby to determine whether the resistances are within or outside a predetermined range of resistance and to generate a signal dependent on said comparison of resistances.

10. A device according to any one of claims 6-9 wherein the circuit means is arranged to reverse the polarity of the electrical supply to said pairs of probes to enable a resistance measurement to be taken in each direction between the pairs of probes.

11. A method of monitoring the liquid level of electrically conductive liquid wherein the resistances between pairs of probes in a set of at least three probes or electrodes are measured, the resistance measurements are stored in memory means of a microprocessor and the resistance measurements are compared with one another, at least two of the probes being locatable to contact the liquid, the measurements being monitored to detect the level of the liquid in a container of the liquid whereby to generate signals according to said level which signals are arranged to actuate pump means for introducing liquid to raise the level and/or alarm means according to the level of liquid.

12. Liquid level monitoring means substantially as described with reference to the drawings.