WO2013072700A1 - Long lasting gas and liquid sensor - Google Patents

Long lasting gas and liquid sensor

Info

Publication number
WO2013072700A1
WO2013072700A1 PCT/GB2012/052849 GB2012052849W WO2013072700A1 WO 2013072700 A1 WO2013072700 A1 WO 2013072700A1 GB 2012052849 W GB2012052849 W GB 2012052849W WO 2013072700 A1 WO2013072700 A1 WO 2013072700A1
Authority
WO
Grant status
Application
Patent type
Prior art keywords
signal
incident
detected
radiation
monitor
Prior art date
Application number
PCT/GB2012/052849
Other languages
French (fr)
Inventor
David Walker
Mark Osborne
Original Assignee
Crowcon Detection Instruments Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date

Links

Classifications

    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/62Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/64Fluorescence; Phosphorescence
    • G01N21/6408Fluorescence; Phosphorescence with measurement of decay time, time resolved fluorescence
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/62Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/64Fluorescence; Phosphorescence
    • G01N21/6428Measuring fluorescence of fluorescent products of reactions or of fluorochrome labelled reactive substances, e.g. measuring quenching effects, using measuring "optrodes"
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/75Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated
    • G01N21/77Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator

Abstract

A safety monitor for determining the level of a target analyte in a fluid comprising optically sensitive material in the optical sensor (18) arranged to re-emit radiation incident on it, a radiation source (14) arranged to emit radiation incident on the optically sensitive material, a detector (20) arranged to detect radiation emitted by the optically sensitive material, a controller (12) comprising a processor (13) configured to control emission of radiation from the radiation source (14) using a pulse-modulated and amplitude-modulated incident signal, and to generate a detected signal based on the radiation detected by the detector (20), wherein the controller (12) is configured to vary the characteristics of the incident signal, and is further configured to determine a level of analyte using information derived from a comparison of the incident signal and detected signal.

Description

Long lasting gas and liquid sensor

Technical field

The present invention relates to a fluids (gases and liquids) sensor, and in particular to a safety monitor for determining the level of a target analyte in a fluid having an extended and predictable operational lifetime.

Background of the Invention

Gas level monitors comprising an optically sensitive material are known in the art to monitor target analytes, such as oxygen and carbon dioxide, in the gaseous phase and in solution. The optically sensitive materials used exhibit photoluminescence, such as fluorescence or phosphorescence, when irradiated by light emitted from a source such as a laser, light emitting diode, or incandescent source. The radiation subsequently emitted by the optically sensitive material is detected (using a photodiode, for example). Characteristics of this detected radiation can then be used, using techniques known in the art, to determine the level of the target analyte in the surroundings.

The known optically sensitive materials suffer from photobleaching which is the photochemical destruction of the material due to radiation exposure. The rate of photobleaching is proportional to the intensity of light incident on the material. An effect of photobleaching is the reduction in intensity of the radiation re-emitted by the material over time. Eventually, the intensity of the re-emitted light is reduced to the extent that a meaningful detected signal can no longer be obtained. The effects of photobleaching therefore limit the lifetime of monitors which use such optically sensitive materials.

For some materials, the intensity of the light re-emitted by the material reduces exponentially for a constant excitation light (i.e. light incident on the material) intensity. To accommodate for measurement of re-emitted radiation with an intensity varying over a wide range, it is currently required that amplification of the detected signal which is used to determine the level of a target analyte also operates over a wide range. This reduces the accuracy of the detected signal. For efficient and satisfactory operation of the monitor, such inaccuracies can be unacceptable beyond a certain value. This is particularly relevant in gas safety products which are often used to detect the presence of harmful or even lethal gases. The effects of photobleaching are therefore detrimental to the performance of the monitor. It is an aim of the present invention to mitigate some of the problems of existing gas sensors as discussed above. Summary of the invention

According to a first aspect of the invention, there is provided a safety monitor for determining the level of an analyte in a fluid, the safety monitor comprising optically sensitive material arranged to re-emit radiation incident on it, a radiation source arranged to emit radiation incident on the optically sensitive material, a detector arranged to detect radiation emitted by the optically sensitive material, a controller comprising a processor configured to control emission of radiation from the radiation source using a pulse-modulated and amplitude - modulated square waveform incident signal, and to generate a detected signal based on the radiation detected by the detector, wherein the controller is configured to vary the characteristics of the incident signal based on characteristics of the detected signal so as to mitigate the effects of photobleaching of the optically sensitive material on the detected signal, thereby extending the lifetime of the optically sensitive material.

According to a second aspect of the invention, there is provided a method of monitoring the level of an analyte in a fluid, comprising: generating a pulse-modulated and amplitude modulated square waveform incident signal having adjustable characteristics; converting the incident signal to incident radiation by controlling a radiation source using the incident signal; irradiating an optically sensitive material with radiation emitted from the radiation source; detecting radiation re-emitted by the optically sensitive material; and converting the detected radiation to a detected signal.

According to a third aspect of the invention, there is provided a safety monitor comprising optically sensitive material arranged to re-emit radiation incident on it, a radiation source arranged to emit radiation incident on the optically sensitive material, a detector arranged to detect radiation emitted by the optically sensitive material, wherein the controller is further configured to calibrate the initial intensity of the incident radiation and the initial intensity of the detected radiation and to calibrate the intensity of the incident radiation and the intensity of the detected radiation after the lapse of a known time period and characterize performance of the monitor using the calibrated values. Preferable features of the invention are defined in the dependent claims.

Brief description of the drawings

Embodiments of the invention will be described with reference to the accompanying Figures in which:

Figure 1 is a schematic diagram showing components of a gas safety monitor in accordance with an embodiment; Figure 2 is a graph illustrating decay of a detected phosphorescence signal with time;

Figure 3 is a flow diagram showing operational steps in accordance with a preferred embodiment. Detailed description

Referring to Figure 1, components of a gas safety monitor 10 are a control system 12, light source 14, light emitting diode 16, optical sensor 18, photodiode 20 and light detector 22. Control system 12 comprises processor 13 and memory 15 and is in communication with light source 14 and light detector 22, such as via an electrical connection. The light source 14 comprises a light emitting diode 16 which is configured and positioned to emit radiation incident on the optical sensor 18 in accordance with an incident signal received from the control system 12. Optical sensor 18 comprises an optically or chemically sensitive material, which in some embodiments may be a phosphorescent material and may be oxygen sensitive complexes such as platinum or ruthenium complexes, including Ruthenium dioxide The optical sensor 18 is disposed between the light source 14 and light detector 22 and is arranged to absorb at least some of any radiation emitted by the light source 14. The optically sensitive material is further configured to re-emit at least some of the radiation it has absorbed. This re- emitted radiation is incident on light detector 22. The light detector 22 comprises a photodiode 20 (which is preferably a silicon photodiode) which detects the radiation re- emitted by the material. The control system 12 generates a detected signal based on the detected radiation.

The processor 13 may be any suitable microprocessor. The memory 15 is a non- volatile memory (preferably flash memory) and the control system 12 is configured to store data relating to the level of target analyte determined, the performance of the sensor, calibrated values of incident light intensity and detected light intensity, remaining lifetime of sensor, user preferences and normal/safe levels of a target gas, for example, in the memory 15. In use, the control system 12 generates a signal which is input to the light source 14. The light source 14 coverts the digitized incident signal to a voltage which drives the LED 16 and determines its output accordingly. The light incident on the optical sensor 18 excites the optically sensitive material such that the material absorbs some of the energy of the incident photons. The optically sensitive material 18 subsequently re-emits some of the absorbed energy as radiation which is incident on the photodiode 20. The photodiode generates a voltage based on the detected radiation which is input to the light detector 22. The light detector 22 coverts the voltage into an analogue and then digital signal which is then input to the control system 12. The control system 12 is arranged to generate an incident signal periodically, such that the signal has a defined duration. Thus, the control system 12 operates a sampling technique having a defined sampling rate. The duration of the applied incident signal is typically in the order of 50μβ and the sampling rate is typically in the order of 20 kHz. The duration and/or sampling rate of the incident signal can be adjusted either automatically by the controller in order to generate a minimum number of sample cycles with a specified time period (as explained in further detail below) or directly in response to a user input.

The control system 12 is arranged to generate an incident signal which has defined characteristics. The incident signal is characterized by a square waveform and is pulse- and amplitude modulated so as to have a defined frequency, duty cycle and amplitude. The duty cycle is the ratio between the duration of a pulse (i.e. the duration of the active part of the signal) and the duration of the period. The duty cycle can be adjusted by the control system 12 automatically (for example, to reduce it to a minimum) or in response to a user input or so as to reduce the time in which the light source 14 spends in an active state, without affecting the frequency of the signal. The less time the light source 14 spends in an active state, the less power the light source 14 will consume, since the incident signal drives the LED accordingly. Additionally, minimising the duty cycle minimises light exposure to the optically sensitive material thus also minimising photobleaching of the optically sensitive material. By reducing the number of photons absorbed by the sensor photobleaching is reduced. Use of a square waveform also allows the frequency of the incident signal to be adjusted easily. This is advantageous when the detected signal is affected by noise. By increasing the frequency, a higher measurement rate results, and thus more detected signal measurements can be generated without affecting the time delay between the incident signal and detected signal. The detected signal may also be distorted due to such conditions.

The LED emits light in accordance with the characteristics of the incident signal, such that the amplitude is translated as intensity and the frequency as pulsed operation of the LED. The length of time between the absorption of the incident radiation by the optically sensitive material and subsequent re-emittance varies depending on the composition of the material and the composition of the surrounding atmosphere. The length of time between absorption and re-emission is known for particular materials. Consequently, any change in time between absorption and re-emission will be caused by a variation in the composition of the surrounding atmosphere, and more specifically, is dependent upon the relative amount of a particular gas in the surrounding atmosphere or dissolved gas in fluids. For example, the length of time between absorption and re-emission for Ruthenium dioxide is known to vary in accordance with the relative levels of oxygen. Depending on the material and the length of time between absorption and re-emission, the phase shift between a pulse modulated incident signal and the corresponding detected signal, or the time delay between the pulse-modulated incident signal and the corresponding detected signal, can be used to determine a change, and the relative extent of the change, in the amount of a particular gas present in the surrounding atmosphere. When the optically sensitive material re-emits radiation, the phase shift and or time delay between the incident signal and the detected signal is used by the processor 13 to determine the level of a target gas in the atmosphere. The processor 13 can then determine the amount or level of a gas in the surrounding area relative to a known amount or level to determine whether the proportion of a target gas has exceeded or dropped below a set limit, which may be based on safety considerations. The result of each determination made by the processor is stored in memory 15 and is compared against a stored value of an acceptable level of the target analyte. In one embodiment, the processor is arranged to transmit the result of each determination to a PC or monitor, and may be further configured to generate a user notification signal for output to an alarm so as to indicate that a level of a target analyte has fallen above or below the set level.

The control system 12 also comprises a circuit (not shown) which is configured to operate an automatic feedback control mechanism in which the amplitude of the incident signal is varied automatically in order to maintain the amplitude of the detected signal. Use of a square waveform digitized incident signal allows its amplitude to be easily adjusted and requires little additional power. Amplitude modulation is achieved using an electronic drive system (not shown). A PWM (pulse width modulated) control signal is converted using a DAC (digital to analogue) converter to achieve a modulated voltage controlled current source which then drives the light source 14.

The processor 13 conducts an initial calibration of the incident signal and detected signal. Because the rate of photobleaching increases as the incident light intensity increases, the amplitude of the incident signal is kept to a minimum. The minimum of the amplitude of the incident signal will correspond to an optimum amplitude of the detected signal, and thus intensity of the incident radiation is initially set to result in an intensity of detected radiation which provides an optimum amplitude of the detected signal. A minimum value of the amplitude of the detected signal (below which no accurate measurements can be derived) is stored in the memory 15 following calibration. The photodiode and the composition of the optically sensitive material may also affect the optimum value of the detected signal. The amplitude of the incident signal which produces this optimum detected signal is stored in memory 15. The automatic gain control circuit of the control system 12 will then generate later incident signals based on this incident signal amplitude. As discussed above, the optically sensitive material will degrade over time and the amplitude of the detected signal will reduce to reflect this. For each sample, the control system determines the amplitude of the incident and detected signals and compares them against the values of the amplitude of the incident and (optimum) detected signals stored in the memory 15. When the control system 12 detects that the amplitude of the detected signal has dropped (due to photobleaching) below the optimum value, the amplitude of the next incident signal sample will be increased by the amount necessary so as to maintain, in the next sample cycle, the optimum amplitude of the detected signal. The amplitude of the detected signal of the next sample will then be used to determine the relative extent of adjustment to the amplitude of the next incident signal. Accordingly, depending on how far below the optimum value of the detected signal the actual amplitude of the detected signal is, the amplitude of the incident signal may be increased. This automatic feedback adjustment ensures that the amplitude of the detected signal is maintained within set limits. The lifetime of the monitor is extended since the effects of photobleaching on the intensity of the re-emitted radiation are mitigated by irradiating the optically sensitive material with incident light of increasing intensity.

For optically sensitive material of a particular composition, the intensity of the detected light may be affected by changes in the proportion of a particular gas dissolved in a fluid or in the surrounding atmosphere. In the case where the gas safety monitor comprises Ruthenium dioxide, for example, the intensity of the detected radiation is generally greater for a lower level of oxygen in the surrounding atmosphere. If the monitor is used during its lifetime in such as environment to monitor unexpected changes in the proportion of oxygen present, the feedback adjustment loop compensates for this increased value of detected light intensity by reducing the intensity of the incident light intensity appropriately (so as to minimise the effects of photobleaching) in order to reduce the intensity of the detected radiation to a lower average value. A graph showing the typical variation in amplitude of the signal detected by the light detector 22 is illustrated in Figure 2, which is based on experimental data generated by the inventors of the present application. The amplitude (indicative of the intensity of the re-emitted radiation) decreases exponentially due to photobleaching of the optically sensitive material when the intensity of the radiation output from the light source 14 is constant.

The relationship between the energy of the incident light and energy of the re-emitted light can be modelled by the following formula:

E = Ae"Bt

where E is the minimum amplitude of re-emitted light, A is the amplitude of incident light, B is a constant (specific for each monitor), e is natural logarithm constant, and t is time elapsed since first use of the monitor.

It can be seen from the form of the equation that, as t increases, E must decrease. It has been advantageously found that a decay profile (indicative ofthe rate of degradation of the optically sensitive material) for a particular monitor can be characterized. Following the initial calibration of the incident and detected light intensities, a further calibration at a later time is conducted where the intensity of the incident radiation necessary to result in the optimum amplitude of the detected signal is stored in memory 15. These two correlated pairs of readings for A and t are used by the processor 13 to determine the constant B for a particular monitor. Determination of the constant B thus characterizes the decay profile for a particular monitor. This allows the remaining lifetime of a particular monitor to be calculated. Knowledge of the remaining time for which a particular monitor will be operating within set safety limits provides advanced warning of monitor failure due to degradation of the optically sensitive material. This is particularly useful when the monitor is used in remote locations, for example, where knowledge of when a sensor is likely to fail can be used in order to determine when to replace a sensor as part of the maintenance cycle. Once constant B has been determined for a particular monitor, the processor 13 is able to calculate the value of E (i.e. the emitted light amplitude as a function if time. During the lifetime of the sensor, the processor, using a current value of A and the minimum permissible value of E, can calculate t, thus providing the sensor lifetime - the time at which E falls below a minimum value (determined during calibration and stored in the memory 15) is the sensor lifetime.

Whilst increasing the intensity of the incident radiation increases the lifetime of the monitor, the power consumption of the light source 14 will increase to generate the required increasing intensity. It has been advantageously discovered that the power consumption of the light source 14 can be minimised by the utilisation of a pulse-modulated square waveform to define the incident signal, since the time the source spends in an active state can be minimised. The duty cycle is therefore initially set at a minimum value by the control system 12.

It has been beneficially further found that information indicative of a monitor's performance can be derived by periodically sampling with the incident signal with an amplitude which is significantly larger than the amplitude of the preceding or following sample. Such a sample is used to generate a detected signal having a lower associated error value than a regular incident signal. The amplitude and phase of the incident and detected signal are used to recalibrate the monitor and/or are analysed by the processor 13 to provide information relating to the photobleaching process, changes due to pressure fluctuations (since pressure may affect the time delay between the incident and detected signal) and optical scattering.

The delay in the re-emission of radiation by the material is known to depend on the composition of the surrounding atmosphere. The rate of photobleaching and therefore the phosphorescence efficiency is also affected by variations in thickness of the material. The material is usually in the form of a coating on a substrate, and inhomogeneities often occur. By calibrating the incident and detected intensities and applying automatic amplification of the incident signal, any such inhomogeneties will not significantly affect the detected radiation intensity.

Figure 3 shows a flow diagram outlining some operational steps of an embodiment. The control system 12, at starting step 32, calibrates values of the incident and detected radiation intensity based on an optimum amplitude of the detected signal. An appropriate drive signal (incident signal) is then generated by the control system 12 and input to the light source 14 to drive the LED accordingly. The intensity of the radiation re-emitted by the optical sensor is detected by the photodiode and the control system 12 generates a detected signal at step 35.

For each sample cycle, the phase between the incident and detected signals, or the time delay between the incident and detected signals, are compared to determine the level of a target analyte at step 36. Additionally, characteristics such as the amplitude of the incident and detected signals may be compared to provide diagnostic information relating to the monitor as described above.

At step 38, the automatic gain control circuit of the control system 12 determines a decrease or increase in the amplitude of the detected signal of the previous sample cycle from the stored optimum amplitude to determine the amount of amplification of the incident signal of the previous sample cycle required to maintain the optimum amplitude of the detected signal for the next sample cycle. The control system 12 generates an appropriately amplified incident signal for input into the light source 14 at step 34. At step 40, the degradation of the optically sensitive material is characterized in a decay profile for the monitor as described above. This is used to periodically calculate the lifetime of the monitor at step 42.

Whilst the above description has been made with particular reference to safety monitors for measuring the level of an analyte in a gaseous phase, the invention is equally applicable to monitoring the levels of an analyte dissolved in a liquid. Therefore, the invention can be used to determine the levels of gas in fluids (i.e. liquids and gases) and to predict the lifetime of such sensors based on the above described methods.

Claims

Claims
1. A safety monitor for determining the level of a target analyte in a fluid comprising optically sensitive material arranged to re-emit radiation incident on it, a radiation source arranged to emit radiation incident on the optically sensitive material,
a detector arranged to detect radiation emitted by the optically sensitive
material,
a controller comprising a processor configured to control emission of radiation from the radiation source using a pulse-modulated and amplitude -modulated incident signal, and to generate a detected signal based on the radiation detected by the detector, wherein the controller is configured to automatically vary the amplitude of the incident signal based on the amplitude of the detected signal, and is further configured to determine a level of analyte using information derived from a comparison of the incident signal and detected signal.
2. The monitor of claim 1, wherein the controller is configured to automatically increase or decrease the amplitude of the incident signal to maintain a constant, or near constant amplitude of the detected signal.
3. The monitor of any preceding claim, wherein the controller is further arranged to reduce the duty cycle of the pulse-modulated incident signal.
4. The monitor of any preceding claim, wherein the controller is configured to determine the signal to noise ratio of the detected signal.
5. The monitor of claim 4, wherein the controller is further configured to vary the frequency of the incident signal based on the signal to noise ratio of the detected signal.
6. The monitor of any preceding claim, wherein the processor is further arranged to determine the relative phase shift or time delay between the detected signal and incident signal.
7. The monitor of claim 6, wherein the processor is arranged to output a signal if the phase difference or time delay calculated by the processor exceeds or falls below a value.
8. The monitor of claim 7, wherein the signal indicates that the proportion of a particular gas in the surrounding atmosphere exceed a certain level.
9. The monitor of any preceding claim, wherein the controller is configured to generate the incident signal periodically in sample cycles, and wherein the generation of the sample cycles occurs at a defined sampling rate.
10. The monitor of claim 9, wherein the controller is further configured to determine the amplitude of the detected signal for each sample and automatically adjust the amplitude of the incident signal of the next sample so as to maintain the amplitude of the detected signal.
11. The monitor of claim 9 or claim 10, wherein the controller is configured to periodically generate a sample of the incident signal having an amplitude substantially larger than the amplitude of the incident signal of the sample immediately preceding it.
12. The monitor of claim 11, wherein characteristics of the detected signal corresponding to the incident signal having a substantially larger amplitude are compared with characteristics of said incident signal to derive information relating to the performance of the monitor.
13. The monitor of any preceding claim, wherein the processor is arranged to calibrate the incident signal and detected signal so as to characterise the rate of decay of the monitor, and wherein the calibrated values are stored in a memory.
14. The monitor of claim 13, wherein the processor is arranged to calibrate the incident signal and detected signal periodically.
15. The monitor of Claim 14, wherein the processor is arranged to calculate a value indicative of the estimated lifetime of the monitor based on calibrated values of the incident signal and detected signals.
16. The monitor of any preceding claim, wherein the sample of optically sensitive material comprises a phosphorescent material, and preferably wherein the phosphorescent material is a platinum or ruthenium complex, such as Ruthenium dioxide.
17. The monitor of any preceding claim, wherein the radiation source comprises an LED.
18. A method of monitoring the level of an analyte in a fluid, comprising:
generating a pulse-modulated and amplitude-modulated incident signal having adjustable amplitude;
converting the incident signal to incident radiation by controlling a radiation source using the incident signal;
irradiating an optically sensitive material with radiation emitted from the radiation source;
detecting radiation re-emitted by the optically sensitive material; converting the detected radiation to a detected signal;
automatically varying the amplitude of the incident signal based on the amplitude of the detected signal; and
calculating a level of an analyte using information derived from the incident and detected signals.
19. A safety monitor for determining the level of a target analyte in a fluid comprising optically sensitive material arranged to re-emit radiation incident on it, a radiation source arranged to emit radiation incident on the optically sensitive material,
a detector arranged to detect radiation emitted by the optically sensitive
material, wherein
the controller is further configured to calibrate the initial intensity of the incident radiation and the initial intensity of the detected radiation and to calibrate the intensity of the incident radiation and the intensity of the detected radiation after the lapse of a known time period and characterize performance of the monitor using the calibrated values.
20. The monitor of claim 19, wherein the controller is configured to determine the remaining lifetime of the monitor based on a minimum value of the intensity of the detected radiation.
21. The monitor of claim 19 or claim 20, wherein the controller is configured to determine the remaining lifetime of the monitor based on a maximum value of the intensity of the incident radiation.
22. The monitor of any of claims 19 to 21, further comprising a processor configured to control operation of the radiation source to automatically increase or decrease the intensity of the incident radiation in order to maintain a constant, or near constant intensity of the detected radiation.
23. A monitor as substantially described herein with reference to one or more of the accompanying drawings.
PCT/GB2012/052849 2011-11-18 2012-11-16 Long lasting gas and liquid sensor WO2013072700A1 (en)

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GB1119924.7 2011-11-18

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GB201119924D0 (en) 2012-01-04 grant

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