GB1602151A

GB1602151A - Fluid monitoring

Info

Publication number: GB1602151A
Application number: GB3105777A
Authority: GB
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1978-05-26
Filing date: 1978-05-26
Publication date: 1981-11-11

Description

(54) FLUID MONITORING (71) We, WATER RESEARCH CENTRE, a British Company, Limited by Guarantee, of 45 Station Road, Henley-on-Thames, Oxfordshire RG9 1 BOW, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is concerned with the mom.- toring of the properties of fluids. In this specification, the term 'fluids' embraces both liquids and gases except where the context provides otherwise.

It is known to monitor the properties of fluids, such as their temperature, pH value, conductivity and (in the case of liquids) their turbidity and dissolved gas, e.g. oxygen, content by contacting the fluids with appropriate sensors. These sensors may operate in various ways, e.g. electrically or optically. The efficiency of such a monitoring process varies depending on, for example, the nature of the fluid under test, the nature of the sensor, the duration of exposure of the sensor and the frequency of cleaning and/or re-calibration of the sensor.

In some of the more sophisticated monitors, automatic cleaning of sensors is carried out on a periodic basis using, for example, mechanical means or ultrasound to dislodge matter which may have adhered to the sensor surface or membrane, or by regular chemical cleaning by substitution or injection techniques. This may be followed by a recalibration sequence which may also be set to occur automatically. The passage of a sample of calibration fluid which possesses characteristics corresponding to the low end of the range of the sensor may precede the presentation of a sample whose characteristics correspond to the high end of the range.

In this way the sensitivity and range of the sensor may be checked periodically, and in some cases, the transducer circuitry automatically reset to the preferred values.

In some applications, where reliability is of paramount importance, two or more sensors may be used to measure the test fluid and an average value taken as the correct reading.

However, if the two or more sensors are of the same type, they may be expected to exhibit the same decay in performance, if exposed to the fluid under test in an identical manner.

Thus the readings from each of the sensors may not be any more reliable than if only one sensor were used.

We have now devised an improved method and apparatus for monitoring fluids. According to the invention, there is provided a method of monitoring a property of a fluid which comprises passing the said fluid under test in contact with a first sensor arranged to provide a signal dependent on a property of the fluid in contact therewith; simultaneously passing, independently of said test fluid, a calibration fluid having a known value of said property in contact with a second sensor arranged to provide a signal dependent on the said property of the calibration fluid in contact therewith; intermittently changing over the flow paths of the two fluids whereby (at the first change) the calibration fluid is passed in contact with the first sensor and the test fluid is passed in contact with the second sensor, the signals from the sensors being monitored indepen dently of each other to provide, respectively, information about the test fluid and information as to the accuracy of the sensor in contact with the calibration fluid.

The invention also includes apparatus for monitoring a property of a test fluid, which apparatus comprises a first sensor and s second sensor for sensing that property of a fluid, means for passing the test fluid in contact with the first sensor, means for passing independently of said test fluid a calibration fluid in contact with the second sensor, means for intermittently changing over the flow paths of the two fluids to bring the calibration fluid in contact with the first sensor and the test fluid in contact with the second sensor, and means for independently monitoring the signals from each sensor to provide, respectively (in use) information about the property of the test fluid and information about the accuracy of the sensor in contact with the calibration fluid.

In the method of the invention, the flow paths of the fluids are intermittently changed.

Thus, the sensor which has been in contact with the fluid under test is than subjected to calibration fluid. In some cases, it will be necessary first to contact the sensor with wash fluid to clean it, and then to contact it with calibration fluid for calibration purposes. In other cases, a single fluid can serve both as a wash fluid and as a calibration fluid. Thus, for example, in the case of dissolved oxygen sensors, clean aerated water containing a small quantity of biocide would serve both as a wash and as a calibration fluid. In some cases, it may not even be necessary to include any special wash agent in the calibration fluid since the latter on its own may serve satisfactorily to clean the sensor.

The signal from that sensor which is in contact with the fluid under test is monitored to provide information about the test fluid. The signal from that sensor in contact with calibration fluid is also monitored as a check on that sensor. In this way, the performance of the sensor in contact with the calibration fluid can be assessed to ensure that it is in good condition to provide accurate results when the flow paths are changed and it is brought into contact with the test fluid. In the event of either sensor giving an unusually low or high signal, the flow paths can be changed to test the sensor in question. The signals from both sensors may be displayed to show the characteristics of the fluid under test and the performance of the second sensor receiving calibration fluid.

On a periodic or demand basis, the fluids supplied to the two sensors are interchanged (and the exhausts from each sensor are preferably similarly changed over to avoid interaction of the sample fluid and the calibration fluid). Thus it will be possible to continue to measure the sample fluid with the freshly calibrated sensor, while the previously contaminated sensor may be cleaned and recalibrated.

During the changeover period, there may be some interaction ofthe two fluids and it is preferred that the mixed fluids be exhausted to waste for a short period until both fluids have again reached steady-state conditions, when the calibration fluid may be re-directed to the source of this fluid.

An advantage of the method and apparatus of the invention is that substantially no date of the conditions of the sample fluid are lost, whereas using one or more sensors in the sample fluid and calibrating these together (as in prior known procedures), data are lost during the calibration period.

The programming of the changeover periods may be selected to suit prevailing conditions.

In some instances it may only be necessary to change over the sensors for a short period and return them to the original condition soon after. In this case, it may be desirable to reverse the roles of the sensors after a longer period, which may typically be from several hours to several days.

In other instances the condition of the sample fluid may necessitate the regular change over of sensors and a longer recovery period for the sensor previously subjected to the sample fluid, to allow for the removal of contaminants and their influence on sensor performance. In this case, each sensor may be exposed to the sample fluid for approximately equal times which may vary typically from several minutes to several hours.

It can be advantageous to provide an alarm detection system such that when the sensor receiving sample fluid exhibits an unusual (low or high) value, a change over would be initiated thus immediately providing confirmation or denial of the potential alarm state. This is particularly useful in the control of critical processes and protection of influent and effluent conditions to treatment processes.

When the unusual value is detected in the output of either the sensor monitoring the test fluid or the one monitoring the calibration fluid, electronic equipment such as a digital computer may be used to determine whether this condition is due to a failure of one or other of the sensors, or whether a real alarm situation in the test or calibration fluid has occurred.

An unusual condition may be defined in several different ways, e.g. if the output exceeds predetermined high or low values or if the new data are inconsistent with an extrapolated value computed from previously recorded data.

The change-over of sample and calibration fluids between the two sensors which is initiated on detection of an unusual condition is used to obtain two more sets of data which are used in conjunction with the previous data to distinguish between sensor failure and real alarm conditions. For example, if TA, TB, CA, CB are values obtained for sensor A on the test solution, sensor B on the test solution, sensor A on the calibration solution and sensor B on the calibration solution, respectively, then if TA and CA both have nominally equal low values, sensor A is assumed to have failed. The other possible conditions can be detected by comparing the four numbers, within tolerances, as pairs.

In order that the invention may be more fully understood, reference is made to the drawings accompanying the provisional specification, in which: Figure 1 is a diagrammatic longitudinal cross-sectional view of a spool valve (in one operating position), which may be used for interchanging the flow paths of the test fluid and wash or calibration fluid; Figure 2 is similar to Figure 1 but illustrates the valve in a second operating position; Figure 3 is a diagram of one arrangement of apparatus for carrying out the invention for monitoring dissolved oxygen in water; and Figure 4 is a diagram of another arrangement of apparatus for carrying out the invention, for simultaneously monitoring four parameters of river water.

Referring to Figures 1 and 2 of the drawings, there is shown a spool valve having a body 1 and spool member 2 slideable in a bore 3 in a body 1. The body and spool are provided with channels shown whereby fluid A can be passed either to exit 1 (Figure 1) or exit 2 (Figure 2).

Likewise, fluid B can be passed to exit 2 (Figure 1) or exit 1 (Figure 2).

In use, fluid B is the test fluid and fluid A is the wash or calibration fluid, and exits 1 and 2 each communicate with a sensor. The flow paths of the two fluids are interchanged by sliding the spool 2 from one operating position to the other.

The spool valve illustrated is merely one of many forms of valve which can be used for changing the fluid flow paths.

Referring to Figure 3, there is illustrated diagrammatically an arrangement for monitoring according to the invention the dissolved oxygen content of water. The water to be tested passes through a strainer 1 and is pumped (by pump A) to a first spool valve (6).

The water passes through the valve 6 to flow cell 2 where it contacts a dissolved oxygen sensor 4 therein. The water passes through cell 2 to a second spool valve 7 and thence to water.

A supply tank 10 is arranged to supply wash or calibration fluid (e.g. clean aerated water containing a small quantity of biocide) to spool valve 6 via pump B.

From spool valve 6, the water passes through flow cell 3 containing a dissolved oxygen sensor 5, and thence to spool valve 7. The water is then drained to waste or returned to tank 10.

The sensor 4 and 5 are connected to monitors (not shown) which in turn are connected to a chart recorder. Hydraulic control of the spool valves and other valves is achieved by a control unit incorporating a timer.

In operation, the test water and wash or standard water pass through cells 2 and 3, respectively, as described. The signals from the sensors 4 and 5 are recorded and displayed, thus giving information concerning the dissolved oxygen content of the test water. After a given time, the valve control alters the spool valves 6 and 7 to direct test water through cell 3 and then on to waste, and wash water through cell 2 and then on to waste. After a period sufficient to flush the system, wash or calibration fluid may be passed to tank 10 by alteration of valve 12. The test fluid now contacts sensor 5, and sensor 4 is exposed to the wash fluid and so cleaned ready for contact again with test fluid.

Whilst Figure 3 has been described with reference to dissolved oxygen sensors, other sensors may be used. No detailed description of the sensors or of the electrical and electronic control and monitoring equipment is given since this will be well understood by those skilled in the art.

Referring to Figure 4, there is shown an arrangement which is very similar to Figure 3 except that four pairs of flow cells are arranged in parallel, each pair (1A, 1B; 2A, 2B; 3A, 3B and 4A, 4B) having different sensors therein so as simultaneously to monitor four different parameters of the test liquid. Each pair of cells has spool valves upstream and downstream thereof. Operation is essentially the same as in the arrangement of Figure 3.

Among the parameters of liquids, such as water, which can be monitored according to the invention are dissolved oxygen, temperature, pH value, conductivity, redox potential, ammonia, nitrate ion, chloride ion, tubidity and colour. Among the parameters of gas, such as air, which can be monitored according to the invention are oxygen, CO2, SO, CO, NO, NO2 and hydrocarbons such as methane.

Whilst the invention is applicable to monitoring gases, it is especially useful for monitoring water and aqueous materials such as sludges. The method and apparatus can be operated on a continuous basis with automatic control, and if desired an alarm system can be included to draw attention to abnormal function (arising from abnormal changes in the test liquid or machine malfunction).

WHAT WE CLAIM IS: 1. A method of monitoring a property of a fluid which comprises passing the said fluid under test in contact with a first sensor arranged to provide a signal dependent on a property of the fluid in contact therewith; simultaneously passing, independently of said test fluid, a calibration fluid having a known value of said property in contact with a second sensor arranged to provide a signal dependent on the said property of the calibration fluid in contact therewith; intermittently changing over the flow paths of the two fluids whereby (at the first change) the calibration fluid is passed in contact with the first sensor and the test fluid is passed in contact with the second sensor; the signals from the sensors being monitored independently of each other to provide, respectively, information about the test fluid and information as to the accuracy of the sensor in contact with the calibration fluid.

2. A method according to claim 1, whereipon, at, or immediately before, changing the flow paths of the fluids, the sensor which has been in contact with the test fluid is connected with a wash fluid prior to being brought into contact with the calibration fluid.

3. A method according to claim 1 or 2 wherein the flow paths of the fluids are changed at regular intervals controlled automatically by a control apparatus.

4. A method according to any of claims 1 to 3 wherein, when an unusually high or low signal is provided by either sensor, the flow paths of the fluids are changed.

5. A method according to any preceding claim wherein a liquid is monitored for its

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. simultaneously monitoring four parameters of river water. Referring to Figures 1 and 2 of the drawings, there is shown a spool valve having a body 1 and spool member 2 slideable in a bore 3 in a body 1. The body and spool are provided with channels shown whereby fluid A can be passed either to exit 1 (Figure 1) or exit 2 (Figure 2). Likewise, fluid B can be passed to exit 2 (Figure 1) or exit 1 (Figure 2). In use, fluid B is the test fluid and fluid A is the wash or calibration fluid, and exits 1 and 2 each communicate with a sensor. The flow paths of the two fluids are interchanged by sliding the spool 2 from one operating position to the other. The spool valve illustrated is merely one of many forms of valve which can be used for changing the fluid flow paths. Referring to Figure 3, there is illustrated diagrammatically an arrangement for monitoring according to the invention the dissolved oxygen content of water. The water to be tested passes through a strainer 1 and is pumped (by pump A) to a first spool valve (6). The water passes through the valve 6 to flow cell 2 where it contacts a dissolved oxygen sensor 4 therein. The water passes through cell 2 to a second spool valve 7 and thence to water. A supply tank 10 is arranged to supply wash or calibration fluid (e.g. clean aerated water containing a small quantity of biocide) to spool valve 6 via pump B. From spool valve 6, the water passes through flow cell 3 containing a dissolved oxygen sensor 5, and thence to spool valve 7. The water is then drained to waste or returned to tank 10. The sensor 4 and 5 are connected to monitors (not shown) which in turn are connected to a chart recorder. Hydraulic control of the spool valves and other valves is achieved by a control unit incorporating a timer. In operation, the test water and wash or standard water pass through cells 2 and 3, respectively, as described. The signals from the sensors 4 and 5 are recorded and displayed, thus giving information concerning the dissolved oxygen content of the test water. After a given time, the valve control alters the spool valves 6 and 7 to direct test water through cell 3 and then on to waste, and wash water through cell 2 and then on to waste. After a period sufficient to flush the system, wash or calibration fluid may be passed to tank 10 by alteration of valve 12. The test fluid now contacts sensor 5, and sensor 4 is exposed to the wash fluid and so cleaned ready for contact again with test fluid. Whilst Figure 3 has been described with reference to dissolved oxygen sensors, other sensors may be used. No detailed description of the sensors or of the electrical and electronic control and monitoring equipment is given since this will be well understood by those skilled in the art. Referring to Figure 4, there is shown an arrangement which is very similar to Figure 3 except that four pairs of flow cells are arranged in parallel, each pair (1A, 1B; 2A, 2B; 3A, 3B and 4A, 4B) having different sensors therein so as simultaneously to monitor four different parameters of the test liquid. Each pair of cells has spool valves upstream and downstream thereof. Operation is essentially the same as in the arrangement of Figure 3. Among the parameters of liquids, such as water, which can be monitored according to the invention are dissolved oxygen, temperature, pH value, conductivity, redox potential, ammonia, nitrate ion, chloride ion, tubidity and colour. Among the parameters of gas, such as air, which can be monitored according to the invention are oxygen, CO2, SO, CO, NO, NO2 and hydrocarbons such as methane. Whilst the invention is applicable to monitoring gases, it is especially useful for monitoring water and aqueous materials such as sludges. The method and apparatus can be operated on a continuous basis with automatic control, and if desired an alarm system can be included to draw attention to abnormal function (arising from abnormal changes in the test liquid or machine malfunction). WHAT WE CLAIM IS:

1. A method of monitoring a property of a fluid which comprises passing the said fluid under test in contact with a first sensor arranged to provide a signal dependent on a property of the fluid in contact therewith; simultaneously passing, independently of said test fluid, a calibration fluid having a known value of said property in contact with a second sensor arranged to provide a signal dependent on the said property of the calibration fluid in contact therewith; intermittently changing over the flow paths of the two fluids whereby (at the first change) the calibration fluid is passed in contact with the first sensor and the test fluid is passed in contact with the second sensor; the signals from the sensors being monitored independently of each other to provide, respectively, information about the test fluid and information as to the accuracy of the sensor in contact with the calibration fluid.

dissolved oxygen content, temperature, pH value, conductivity, redox potential, or ammonia, nitrate ion or chloride ion content, or its turbidity or colour.

6. A method according to any of claims 1 to 4 wherein a gas is monitored for its oxygen, carbon dioxide, sulphur dioxide, carbon monoxide, nitrous oxide, nitric oxide or hydrocarbon content.

7. A method according to claim 1 substantially as herein described with reference to Figures 1 to 3 or Figure 4 of the drawings.

8. Apparatus for monitoring a property of a test fluid, which apparatus comprises a first sensor and a second sensor for sensing that property of a fluid, means for passing the test fluid in contact with the first sensor, means for passing independently of said test fluid a calibration fluid in contact with the second sensor, means for intermittently changing over the flow paths of the two fluids to bring the calibration fluid in contact with the first sensor and the test fluid in contact with the second sensor, and means for independently monitoring the signals from each sensor to provide respectively (in use) information about the property of the test fluid and information about the accuracy of the sensor in contact with the calibration fluid.

9. Apparatus according to claim 8 which also includes control means for automatically changing over the flow paths of the fluids at regular intervals, or upon demand.

10. Apparatus according to claim 8 and 9 which also includes control means for automatically changing over the flow paths of the fluids if an unusually low or high signal is received from a sensor.

11. Apparatus according to claim 8,9 or 10 which also includes means for continuously recording the signals from one or more of the sensors.

12. Apparatus according to claim 8 substantially as herein described with reference to Figures 1 to 3 or Figure 4 of the drawings accompanying the Provisional Specification.