GB2297843A

GB2297843A - Conductivity sensing apparatus

Info

Publication number: GB2297843A
Application number: GB9502751A
Authority: GB
Inventors: Andrew Ceri Davis
Original assignee: Spirax Sarco Ltd
Current assignee: Spirax Sarco Ltd
Priority date: 1995-02-13
Filing date: 1995-02-13
Publication date: 1996-08-14
Anticipated expiration: 2015-02-13
Also published as: GB2297843B; GB9502751D0

Abstract

The system comprises two excitation electrodes 21, 22, with electrode 21 having a larger liquid contacting area, and a single sensing electrode 23. A potential 11 is applied across the excitation electrodes, and conductivity is determined from the current 12 flowing between the excitation electrodes and the voltage 13 between the sensing electrode and the larger excitation electrode 21. The applied potential is preferably AC and the liquid contacting area of the larger excitation electrode 21 is preferably between ten and one million times the area of the smaller excitation electrode 22. The sensing electrode 23 and the smaller excitation electrode 22 may be mounted on a probe, with the wall of the vessel holding the liquid constituting the larger excitation electrode 21.

Description

CONDUCTIVITY SENSING APPARATUS This invention relates to a conductivity sensor for measuring the conductivity of a liquid.

Conductivity sensors for measuring the conductivity of a liquid have wide application. For example, a conductivity sensor is commonly used to monitor the condition of water in a boiler of a steam generating plant. In such a plant the concentration of impurities in the water is of critical importance.

Such impurities take the form of dissolved gases or solids and suspended solids. The measurement of the conductivity of the water provides one means of ascertaining the concentration of total dissolved solids (TDS). If the TDS concentration is allowed to rise excessively, the process efficiency can be reduced, and serious damage can result to steam and condensate systems in the plant as a result of corrosion and deposition on heat transfer surfaces.

It is known to monitor the level of TDS in the boiler and, when the concentration is unacceptably high, to discharge (or "blowdown") a proportion of the contaminated water and to introduce fresh water into the system. However, blowdown loses energy and water treatment chemicals from the system and therefore instances of blowdown should be minimised as far as possible. This results in a need for accurate monitoring of the level of impurities in the water in the boiler. This can be achieved by monitoring the conductivity of the water and controlling blowdown in dependence on the measured conductivity.

One simple known conductivity sensor consists of two electrodes immersed in the liquid. A voltage is applied to the electrodes and the resulting electrical current flow through the liquid is measured. The conductivity of the liquid can be easily obtained from the voltage and current measurements.

However the measurement of conductivity by this method is inaccurate because polarisation of the electrodes and fouling of the electrodes by scale and suspended solids increases the resistance of the circuit and therefore results in a lower reading for the current flowing through the circuit. As a result the conductivity reading is lower than the true reading of the liquid.

A four electrode probe as shown in Figure 1 has been developed to overcome the disadvantages of the conductivity sensor, mentioned above. Figure 2 represents the electrical circuit set up in operation of the probe of Figure 1. The probe body 1 is made of an electrically insulating material and four conductive rings 2, 3, 4 and 5 are incorporated into the body 1.

The probe is immersed in the liquid and a voltage is applied by a voltage source 11 between electrodes 2 and 3, referred to as "excitation electrodes". The electrical current flowing in the liquid between electrodes 2 and 3 is measured by means of an ammeter 12 connected in the voltage supply circuit. The voltage established between the electrodes 4 and 5 (referred to as "sense electrodes") by virtue of the current flowing through the liquid is measured by a voltmeter 13 connected across the sense electrodes 4 and 5.

In Figure 2 resistances 14, 18 arise at the excitation electrode/liquid interfaces and vary as a result of fouling or polarisation of the electrodes.

Resistance 15 represents the resistance of the liquid between the excitation electrode 2 and the sense electrode 4, resistance 16 represents the resistance of the liquid between the two sense electrodes 4, 5, and resistance 17 represents the resistance of the liquid between the sense electrode 5 and the excitation electrode 3.

The conductivity of the water can be calculated from the readings of current and voltage. Since the voltage reading taken by the voltmeter 13 is taken directly across the liquid, and the current~flowing in the liquid is known from the output of the ammeter, the conductivity of the liquid can be accurately determined, and fouling or polarisation of the electrodes has no effect on the accuracy of the conductivity reading. Thus a four electrode system enables conductivity to be measured accurately and is especially useful where the conductivity of the liquid is high leading to polarisation errors, or where the liquid contains contaminants which foul the electrodes.

However, four electrode high pressure probes are large and mechanically complex, and are therefore difficult to install into pressurised systems. In addition, in many situations the liquid is held in a conductive container which is earthed. In this situation it is necessary to isolate the sensing system from the container in order to prevent undesirable currents circulating.

According to the present invention there is provided conductivity sensing apparatus for measuring the conductivity of a liquid, comprising first and second excitation electrodes and a sense electrode for contact with the liquid, the second excitation electrode having a larger liquid contacting surface than the first excitation electrode, the apparatus further comprising operating circuitry for applying a potential difference between the excitation electrodes and for determining the conductivity of the liquid from the current flowing between the excitation electrodes and from the voltage developed at the sense electrode.

In use of such a conductivity sensing apparatus, the potential difference applied between the excitation electrodes generates a current in the liquid. Since the second excitation electrode has a larger liquid contacting surface than the first excitation electrode, the voltage drop at the liquid/electrode interface at the second excitation electrode will be relatively small, and so the voltage difference between the sense electrode and the second excitation electrode can be used to provide a reliable conductivity measurement for the liquid.

Preferably the second excitation electrode is sufficiently large that the liquid/electrode interface resistance is negligible in comparison with the resistance of the liquid between the sense electrode and the second excitation electrode.

This feature enables the voltage measurement to be made more easily since it allows the voltage developed across a portion of the liquid measured between the two sense electrodes in the four electrode prior art to be approximated by measuring the voltage developed by virtue of the resistance between the sense electrode and the second excitation electrode of the present invention and the relatively insignificant liquid/ electrode interface resistance of that excitation electrode.

The second excitation electrode may be sufficiently large that polarisation and/or fouling of that excitation electrode during use does not significantly increase its liquid/electrode interface resistance.

This feature prevents the polarisation or fouling of the electrode adversely affecting the voltage measurement.

The liquid contacting surfaces of the excitation electrodes may differ in area from each other by a factor of at least ten, preferably by a factor of at least fifty. In a preferred embodiment, they may differ in area by a factor of at least one million.

The second excitation electrode may be constituted by the vessel holding the liquid.

The sense electrode and the smaller excitation electrode may be incorporated into a single probe, or alternatively may be provided separately.

The present invention is suitable for monitoring the condition of water in a boiler, as previously described. However, the invention is equally suitable for use in any application where the conductivity of a liquid must be measured.

According to a second aspect of the present invention there is provided a method of measuring the conductivity of a liquid comprising the steps of: applying a voltage to the liquid by means of two excitation electrodes, one of the excitation electrodes having a larger liquid contacting surface than the other, determining the voltage developed at a sensing point within the liquid; measuring the electrical current flowing through the liquid; and determining the conductivity of the liquid from the current and voltage measurements.

For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a known conductivity probe using four electrodes; Figure 2 is a diagram showing an electrical circuit equivalent to the conductivity probe shown in Figure 1 in use; Figure 3 shows conductivity sensing apparatus in accordance with the present invention; Figure 4 is a diagram of an electric circuit equivalent to the conductivity sensing apparatus of Figure 3; Figures 5a-5c show different arrangements of a conductivity probe suitable for use in the apparatus of Figure 3.

The apparatus of Figure 3 is used for measuring the conductivity of a liquid in a tank 21.

The tank may be a boiler for steam generation in which case the liquid is water, containing various impurities in the form of dissolved gasses, dissolved solids and suspended solids.

The apparatus comprises a first excitation electrode 22 for supplying an electrical current to the liquid in the tank 21. The wall of the tank 21 is conductive, and acts as a second excitation electrode with a liquid contacting surface significantly larger in area (more than 106 times larger, and possibly around 109 times larger) than that of the electrode 22. An a.c. voltage generator 11 and an ammeter 12 are connected in series with the excitation electrodes 21 and 22. The ammeter 12 measures the current supplied by the voltage generator to the water via the excitation electrodes 21, 22.

A sense electrode 23 is also provided close to the first excitation electrode 22 and a high input impedance voltmeter 13 is connected between the sense electrode and the wall of the tank 21.

During operation of the apparatus, the voltage generator 11 applies a voltage between the first excitation electrode 22 and the wall of the tank 21, in order to generate an electrical current through the liquid. The ammeter 12 measures the electrical current flowing through the liquid. The voltage generated between the sense electrode 23 and the wall of the tank 21 is measured and the conductivity of the liquid is calculated from the measurements of voltage and current in accordance with the formula: o I V In a preferred method of determining the conductivity, the voltage applied by voltage source 11 is varied such that the voltage of the sense electrode 23 is constant at a predetermined value. The current flowing through the liquid is then proportional to the conductivity, in accordance with the above equation.

As polarisation or fouling occurs, the voltage applied between the excitation electrodes 21, 22 will need to increase to maintain the constant voltage at the sense electrode 23. At a predetermined maximum value of the voltage between the excitation electrodes, an alarm can be activated to indicate excessive polarisation or fouling, necessitating replacement or cleaning of the probe or probes.

The same reference numerals are used in Figure 4 to indicate the same or similar elements as described above in connection with Figure 2. Resistance 14 represents the resistance attributable to the interface between the first excitation electrode 22 and the liquid, including any resistance attributable to polarisation or the build up of scale on the electrode 22; resistance 15 represents the resistance associated with the liquid between the first excitation electrode 22 and the sense electrode 23; resistance 16 represents the resistance of the liquid between the sense electrode 23 and the second excitation electrode (tank wall) 21, and resistance 18 represents the resistance attributable to the interface between the second electrode 21 and the liquid, including any resistance associated with polarisation or the build up of scale on the second excitation electrode 21.

The liquid contact surface of the tank wall 21 is sufficiently large compared with that of the first excitation electrode 22 that the resistance 18, and consequently the voltage drop across the liquid/electrode interface at the second excitation electrode 21, are negligible in relation to the resistance 14 and the voltage drop at the liquid/electrode interface at the first excitation electrode 22. The voltage measured by voltmeter 13 will thus be a highly accurate measurement of the voltage across the liquid itself. In addition, the effect of polarisation and fouling of the second excitation electrode 21 is reduced.

Although not shown in Figure 3, assuming the voltage at the sense electrode 23 is kept constant, the output of the ammeter 12 may be applied to control circuitry for controlling blowdown from the tank 21, when it constitutes a boiler. Thus, when the conductivity of the liquid reaches a predetermined level indicating a limit value of total dissolved solids, a blowdown operation is automatically initiated, to discharge a quantity of liquid from the boiler and to replace it by a fresh supply.

In the embodiment of Figure 3 the second electrode is constituted by the metal tank used to hold the liquid. However, any suitable large conductive area may be used instead. In particular, it is possible to provide a single probe in which all three electrodes are incorporated.

Figure 5a shows a probe having two electrodes 24, 25 mounted on a body 26 of insulating material. The electrode 24 is circular, and surrounded by the electrode 25 which is annular. The electrodes 24, 25 are exposed to the liquid only at the tip of the probe.

Either of the electrodes 24, 25 could serve as the first excitation electrode 22 and sense electrode 23 of Figure 3.

Another embodiment of a probe is shown in Figure Sb. In this embodiment the electrodes 24', 25' are in the form of two spaced conductive rings mounted along the length of the body 26'.-- It is possible to provide the sense electrode 23 and the first excitation electrode 22 on separate probes, or on two separate limbs extending from a common support. Figure Sc illustrates an embodiment of such a probe consisting of two rods 24", 25" which are electrically insulated by sleeves 27" from the liquid except at their tips.

Apparatus as described above is able to compensate for the polarisation or fouling of the excitation electrodes leading to a more accurate measurement of the conductivity. Furthermore the arrangement of the present invention can utilise a conductive vessel holding the liquid as one of the excitation electrodes enabling the simplification of the electrical circuit necessary to measure the conductivity of the liquid.

Claims

1. Conductivity sensing apparatus for measuring the conductivity of a liquid, comprising first and second excitation electrodes and a sense electrode for contact with the liquid, the second excitation electrode having a larger liquid contacting surface than the first excitation electrode, the apparatus further comprising operating circuitry for applying a potential difference between the excitation electrodes and for determining the conductivity of the liquid from the current flowing between the excitation electrodes and from the voltage developed at the sense electrode.

2. Conductivity sensing apparatus as claimed in claim 1 wherein the operating circuitry includes an alternating voltage source for applying an alternating voltage to the liquid by means of the excitation electrodes.

3. Conductivity sensing means as claimed in claim 1 or 2 wherein the liquid contacting surface area of the second excitation electrode is at least ten times as large as that of the first excitation electrode.

4. Conductivity sensing apparatus as claimed in claim 3 wherein the liquid contacting surface area of the second excitation electrode is at least fifty times as large as that of the first excitation electrode.

5. Conductivity sensing apparatus as claimed in claim 4 wherein the liquid contacting surface area of the second excitation electrode is at least one million times as large as that of the first excitation electrode.

6. Conductivity sensing apparatus as claimed in any preceding claim wherein the liquid contacting surface area of the second excitation electrode is sufficiently large that the liquid/electrode interface resistance of the second excitation electrode is negligible in comparison with the resistance of the liquid between the sense electrode and the second excitation electrode.

7. Conductivity sensing apparatus as claimed in any preceding claim wherein the liquid contacting surface area of the second excitation electrode is sufficiently large that polarisation and/or fouling of the second excitation electrode during use does not significantly increase the liquid/electrode interface resistance of the second excitation electrode.

8. Conductivity sensing apparatus as claimed in any preceding claim wherein the second excitation electrode is constituted by the wall of a vessel holding the liquid.

9. Conductivity sensing apparatus as claimed in any preceding claim wherein the sense electrode and the first excitation electrode are each formed as a conducting rod electrically insulated from the liquid except at the tip.

10. Conductivity sensing apparatus as claimed in any preceding claim wherein the sense electrode and the first excitation electrode are incorporated into a single probe.

11. Conductivity sensing apparatus as claimed in claim 10 wherein the sense electrode and the first excitation electrode comprise two conductive rings which are supported on a common support and are electrically insulated from each other.

12. Conductivity sensing apparatus as claimed in claim 10 wherein the sense electrode and the first excitation electrode are provided on an end face of a common support, one of the sense electrode and the first excitation electrode being annular and surrounding the other.

13. A conductivity sensing apparatus as claimed in any preceding claim wherein the operating circuit comprises means to vary the potential difference applied between the excitation electrodes in such a way that the voltage developed at the sense electrode is constant.

14. Conductivity sensing apparatus substantially as herein described with reference to any one of Figures 3, 4, 5 and 6 of the accompanying drawings.

15. A boiler having conductivity sensing apparatus as claimed in one of claims 1-14.

16. A method of measuring the conductivity of a liquid comprising the steps of: applying a voltage to the liquid by means of two excitation electrodes, one of the excitation electrodes having a larger liquid contacting surface than the other, determining the voltage developed at a sensing point within the liquid; measuring the electrical current flowing through the liquid; and determining the conductivity of the liquid from the current and voltage measurements.

17. A method of measuring the conductivity of a liquid, as claimed in claim 16, wherein the method also includes the additional step of adjusting the voltage applied to the liquid so as to keep constant the voltage developed at the sensing point within the liquid.