This invention relates to sensing using electromagnetic waves, in particular, but not exclusively, microwaves, for on-line analysis of flowing fluids such as substances produced by or during a reaction or other procedure, for example, or for monitoring water quality, or for biological monitoring.

GB2203553 discloses a gas sensor having a layer of semiconducting organic polymer such as a polypyrrole that can be exposed to a gas to be detected. An alternating electric signal of varying frequency is applied to conductors bridged by the polymer and the change in impedance characteristic of the sensor when exposed to the gas detected by an impedance analyser. A sensor unit may comprise a number of such sensors of different polymers reacting to different gases. The frequency range used is lMHz to 500MHz.

Known as an 'electronic nose', the gas sensor can be trained using a neural net to recognize different sets of changes in impedance of an array of sensors in response to different gases. GB2203553 suggests that it may be possible to detect particular gases by investigating changes in the impedance characteristic localised at particular frequencies, but notes that it is difficult to do this on account of noise, opting instead for a comparison system in which differences in the variation of impedance characteristics as compared with a reference gas such as nitrogen are determined over a range of frequencies and in particular, not using frequencies above 500MHz.

This is clearly complex and time consuming, and it would appear also that the impedance characteristics change with time, on a scale of minutes. The method appears suitable only for gases or vapours, and, more particularly, gases or vapours that react with a semiconducting polymer. It is not readily adaptable to flowing liquids or powders.

WO2006/054238 and WO2010/119380 disclose microfluidic devices for detecting substances in a capillary flow through liquid sample, using various active detectors such as giant magnetoresistors for detecting magnetically labelled molecules. The flow channels have depths typically of a few nanometers. The detection arrangements clearly depend on the nature of the molecules being detected, and the capillary flow is necessarily slow and the signal time dependent. These devices cannot be used for free- flowing fluids and are restricted in any event to liquids carrying the molecules to be detected.

The present invention provides a robust method and apparatus for monitoring flowing fluids generally, whether they be gases, vapours, liquids or flowing solids, such as powders, and to do so in a more convenient way that is not time-dependent.

The invention comprises a method for monitoring a flowing fluid comprising flowing the fluid through a space in a sensor with a fluid input and a fluid outlet and electromagnetic signal input means applying an electromagnetic signal within a given frequency range to the space, and measuring an output signal comprising a transmitted or reflected input signal, the space dimensions and the electromagnetic signal frequency range being such that the output signal will be measurably dependent on the characteristics of the fluid flowing through the sample space.

A fluid being monitored is likely to have characteristics that do not vary by much. The sensor will be designed and the frequency range selected for any particular fluid being monitored so that the measurement is sensitive to small variations in characteristics.

The signal may be a microwave signal and may be in the frequency range lMHz to 300GHz and particularly in the range 500MHz to 300GHz.

The power, phase and/or frequency of a reflected or transmitted input signal may be measured. An electromagnetic signal input may be connected as by one or more transmission lines to a microwave generator, and the reflected or transmitted signal fed to an analyser such as a vector network analyser or a spectrum analyser.

A useful technique involves sweeping the signal across a range of frequencies and detecting a resonance - a peak in the reflected or transmitted signal power. Small variations in fluid characteristics, such for example as density variations, concentration of a solution, quantity or particle size of suspended matter and so forth, may be reflected in measurable changes in a resonant frequency, or the amplitude of a resonant frequency.

Some fluids may exhibit more than one peak in a given space, and can be identified by their spectra, in the same way that elements can be identified by their optical spectra. The flow may be interrupted for a measurement that is not substantially instantaneous, or not, depending on the time for measurement, the flow rate and the likelihood that the fluid will change during the measurement time.

The invention also comprises a sensor for monitoring a flowing fluid sample comprising a space in a sensor having a fluid input and a fluid output through which a fluid can be flowed for analysis, electromagnetic signal input and output means connected to a microwave generator generating an electromagnetic signal in a given frequency range and output signal measuring means measuring an output signal comprising a transmitted or reflected input signal, such that the output signal will be measurably different depending on the characteristics of a fluid flowing through the space.

A signal input may be adapted for connection to a signal generator by one or more transmission lines such as coaxial cables, and a signal output may be similarly adapted for connection to an analyser. The signal input and output may be the same or different, a microwave generator and analyser being arranged to measure the power, phase and/or frequency of the reflected and/or transmitted input signal.

The sensor may comprise:

an electrically insulating substrate: an active conductor layer on the substrate comprising signal input and/or output electrodes; and a sample space having a sample input and a sample output adapted to accept a fluid sample adjacent the active conductor layer.

The substrate may be on a base conductor layer.

The sample space may have dimensions of the order of size of the electromagnetic wavelength. A 300GHz microwave has a wavelength of 1mm, and a space of, say, 5mm square and a few millimetres deep will give a good response to microwave interrogation up to this frequency. Water resonates at a frequency of about 22.2GHz giving a wavelength of about 13.5 mm, so for water based fluids, a sensor having a space 25mm square would allow measurements around the resonant frequency.

As a fluid will absorb a lot of energy at its resonant frequency, the signal power must be low enough not to raise the temperature of the fluid unduly, taking into account flow rate through the sensor.

The signal conductor layer may have intercalated or interdigitated input and output electrodes, which may have square, circular, spiral or stellate configuration.

A cell having a capacity for a known amount of the fluid may define the space. A sensor may comprise a plurality of sample spaces, each being differently dimensioned and or having a different signal input arrangement.

The active conductor layer may be in the space or on the cell outer wall.

If in the cell, the active conductor layer may be covered with a transducer coating,

The transducer coating may be selected or adapted to respond in known manner to electromagnetic waves when in contact with a sample so that the sensor transmits and/or reflects electromagnetic waves in a manner characteristic of the sample.

The frequency range may extend from 9kHz to 300GHz, and may comprise the microwave range 300MHz to 300GHz or any part or parts of it.

The base conductor layer, if present, may be of the same material as the active conductor layer, or of a different material, either comprising any well-conducting metal such as gold, silver, copper, platinum/gold alloy or a conductive carbon-based material. The electrically insulating substrate may comprise any printed circuit board material, a glass- reinforced epoxy material such as FR4, a glass reinforced PTFE, Duroid® high frequency circuit materials, glass, or alumina, and may be rigid or flexible. The material may have dielectric properties that influence electromagnetic signal decay. The cell may comprise polycarbonate or like material, preferably transparent or translucent to allow for visual check on the sample space.

The transducer coating, if present, may be selected from metal oxides, polymers, mixtures of oxide and polymer, polymers filled with nanoparticles for enhanced conductivity, and which may operate in the percolation region or near to the percolation threshold. Phosphate and nitrogen binding polymeric hydrogels, as well as cadmium phthalocyanines, may be used. Biological coatings such as enzymes, proteins or even living organisms such as E. coli 600 or Pseudomonas aeruginosa can also be used.

The selected transducer coating may be a stable, general-purpose coating that may survive multiple measurements or may be of limited utility, adapted for one or a small number of sample materials and/or reacting with or becoming contaminated by a sample. Different fluid analytes in contact with the transducer coating will exhibit different responses to microwaves, for example different levels of attenuation, different resonant frequencies, different reflection and transmission characteristics and so forth. Analytes may exhibit different responses when in contact with different transducer coatings.

The response may be measured from a signal reflected back along a transmission line supplying an interrogating signal or from radiated energy picked up by an aerial..

The applied electromagnetic signal may be controlled by the response, for example, in a feedback loop, to adjust the frequency, for example, to achieve resonance.

The method may be carried out as an element of process control or in a continuous monitoring role, in which samples are introduced robotically.

Methods and apparatus for monitoring flowing fluid samples in accordance with the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a view of one embodiment of sensor arrangement, connected to a microwave generator/analyser;

Figure 2 is a plan view of another embodiment of sensor arrangement;

Figure 3 is a section through another embodiment;

Figure 4 is a view of one embodiment of a different electrode configuration for the sensors of Figures 1 to 2;

Figure 5 is a view of another embodiment of electrode configuration for the sensors of Figures 1 to 3;

Figure 6 is a diagrammatic illustration of a sensor connected in a control arrangement; and Figure 7 is a graphical display of power against frequency for particular sensor configurations with a particular analyte.

The drawings illustrate a method for monitoring a flowing fluid sample comprising flowing the sample through a sample space 12 in a sensor 1 1 with a sample input 13 and a sample outlet 14 and electromagnetic signal input means 15 applying an electromagnetic signal within a given frequency range to the sample space 12, and measuring an output signal comprising a transmitted or reflected input signal, the sample space 12 dimensions and the electromagnetic signal frequency range being such that the output signal will be measurably dependent on the characteristics of the fluid flowing through the sample space 12.

The signal input means 15 comprise a base conductor layer 15a, an electrically insulating substrate 15b on the base layer 15a, an active conductor layer 15c on the substrate 15b having an external connector arrangement 17.

In Figure 1, the signal input means 15 are on an outer wall of the sensor 11, which comprises a transparent plastic, e.g. polycarbonate, cell.

The active conductor layer 15c is in the form of printed intercalated finger electrodes with contact pads constituting the external connector arrangement 17, better seen in Figure 2, which is a plan view of a sensor 11 with two sample spaces 12. The electrode pattern may comprise simple interdigitated fingers as illustrated or more complex patterns such as stellate, as shown in Figure 4, or circular, as shown in Figure 5.

The conductor layer 15c may be of gold, copper, silver, platinum/gold alloy, conductive carbon material or indeed any of the usual conductor materials, as may the base layer 15a, which may be of the same conductor material as the layer 15c, or different.

In the embodiment of Figure 2, the signal input means 15 are inside the sample spaces 12, which are of different sizes. This allows measurements to be made using signals in different frequency ranges using whichever space gives better sensitivity. In this embodiment, there is a transducer coating 15d on top of the intercalated conductor layer 15c. The transducer coating may be the same or different in the two spaces

The transducer coating 15d is selected or adapted to respond in known manner to electromagnetic waves in the said range when in contact with a sample so that the sensor transmits and/or reflects electromagnetic waves in a manner characteristic of the sample. The two sample spaces 12 of the embodiment of Figure 2 can have different transducer coatings, so that they give different outputs from the same signal input, or can have different signal inputs, for example inputs at known resonance frequencies for different substances.

The transducer coating 15d is selected from metal oxides, polymers, mixtures of oxide and polymer, polymers filled with nanoparticles for enhanced conductivity, and which may operate in the percolation region or near to the percolation threshold. Phosphate and nitrogen binding polymeric hydrogels, as well as cadmium phthalocyanines, may be used. Biological coatings such as enzymes, proteins or even living organisms such as E. coli 600 or Pseudomonas aeruginosa can also be used. The selected transducer coating may be a stable, general-purpose coating that may survive multiple measurements or may be of limited utility, adapted for one or a small number of sample materials and/or reacting with or becoming contaminated by a sample.

The transducer coating 15d may serve only to insulate the conductor layer from the sample, but different coatings may also influence the sensor response to the signal, without necessarily reacting or interacting in any physical or chemical way with the sample.

The frequency range may extend from 9kHz to 300GHz, and may comprise the microwave range 300MHz to 300GHz or any part or parts of it. The size of the sample space is commensurate with the signal wavelength. A 300GHz signal has a free-space wavelength of 1mm. A sample space a few millimetres, say five to ten millimetres, square and one or two millimetres deep, with a volume between 20 and 100 cubic millimetres is typical for higher frequency signals, larger spaces for lower frequency signals.

In one method, the signal generator sweeps through a selected frequency range to detect a resonance, and is controlled in a feedback loop to search for one or more resonances. The electrically insulating substrate 15b comprises any printed circuit board material, a glass-reinforced epoxy material such as FR4, a glass reinforced PTFE, Duroid® high frequency circuit materials, glass, or alumina, and may be rigid or flexible. The material may have dielectric properties that influence electromagnetic signal decay.

Figure 4 illustrates the sensor 11 in a control arrangement comprising an EM generator/analyser 16 connected via a cable 18 or by radiating microwaves or both. A fluid analyte produced by a chemical reactor 41 flows via an outlet pipe 42 to and through the sensor 11. The generator/analyser in turn controls the reactor 41, for example the temperature of pressure under which the reaction takes place, or a controllable variable such as pH or salinity, to produce a uniform product that will maintain a constant transmitted or reflected signal from the sensor 11.

Sensors 1 1 can be constructed to different designs in terms of dimensions, electrode pattern, transducer coating, if present, and connectivity and used as on-line sensors for monitoring flowing fluids, whether they be gases, vapours, liquids or powders, either in a monitoring role or a process control role.