This invention relates to gas analysers and in particular analysers of gas mixtures containing a gas with non-zero magnetic susceptibility.

Molecular oxygen (0 ₂ ) is a paramagnetic molecule, having a small non-zero magnetic susceptibility (χ) which enables it to be distinguished from most other common gases. Most existing paramagnetic oxygen analysers use a magnetic field to physically move the oxygen molecules and a resulting force, pressure or flow is measured. This principle is described in US 2,416,344 (Pauling). Such analysers tend to be complex and mechanically fragile, and are subject to interference, for example from external shock and vibrations.

Other oxygen analysers have been described previously that measure the AC susceptibility of oxygen using alternating currents to generate alternating magnetic fields. US 3,076,929 describes a system using four coils in a differential transformer arrangement. In this system, the transmit coils are axially displaced from the receive coils, making them subject to thermal drift effects. The sensor and compensator subassemblies are also in separate locations, permitting further drift effects. US 3,539,913 describes a differential measurement using a rotating sample, which is periodically moved into and out of the field. Prival's system relies on air gaps in a permeable yoke, which are subject to change with thermal expansion, shock and vibration. US 4,464,926 describes an analyser with four inductors in a bridge arrangement, as opposed to the differential transformer arrangement. US 4,763,509 describes another 4-coil arrangement with a rotating gas chamber, which may be driven in a transformer or bridge configuration. The requirement for a motor or similar driving source (such as compressed air) is a significant disadvantage of the methods employing rotating sample chambers, since it introduces additional cost and complexity, along with mechanical vibration and/or a local source of electromagnetic interference.

Although various paramagnetic oxygen analysers have been described in previous publications, all or almost all of them suffer from one or more disadvantage. Hence, there is a need for a paramagnetic oxygen analyser that is drift free, simple in design and construction, is resistant to interference and damage, for example from vibrations, has small size and is lightweight, has fast response, has a wide operating temperature range and can be manufactured at low cost. According to the present invention there is provided an analyser system and method as defined in the claims. In the present invention, apparatus and methods are provided in which the AC susceptibility of oxygen is detected by a single pair of transmit and receive coils with low mutual coupling.

This invention uses the AC magnetic susceptibility of oxygen to inductively couple energy in an alternating magnetic field from a transmit coil to a receive coil. The transmit and receive coils are nominally balanced with respect to one another, such that their mutual inductance M is close to zero. The susceptibility of the test gas, or the difference in the susceptibility between the test gas and a reference gas causes an imbalance between the transmit and receive coils, i.e. a change in M. An AC current in the transmit coil induces a voltage in the receive coil and the ratio of the voltages or currents in the two coils can be used as a measure of the test gas susceptibility and hence, for example, oxygen partial pressure.

By the term "substantially balanced" we mean that the coupling factor, k, between the transmit and receive coils is small, i.e. \k\ « 1 , preferably |k| < 0.01. The coupling factor is defined in relation to the coil inductances Li and L ₂ and mutual inductance M as M = and can take values from -1 to 1 in general. For the designs described herein, |k| < 0.01 is readily achieved by conventional coil fabrication techniques. Advantageously, coil balancing is achieved to |k| < 10 ^"4 , or more preferably to |k| < 10 ^"6 . Trimming or active nulling is typically required to achieve the better balancing values of |k| < 10 ^"4 . This high degree of balance allows a high sensor sensitivity, since a 1 % oxygen concentration induces a coupling of the order 10 ^"8 . Hence, the signal from this is -120dB below the background breakthrough if the coils are balanced to k=0.01 , but only -40dB if the coils are balanced to k=10 ^"6 .

The receive coil may be balanced with respect to uniform fields so as to have a low tendency to pick up interfering electromagnetic fields from other devices.

The coil arrangements in this invention, the transmit and receive coils share a common centre point. This allows improved balancing and stability compared to previously published configurations. For example, thermal expansion effects can be arranged to be symmetric about this point, resulting in greatly reduced thermal drift. In its simplest form, the analyser detects the presence of a paramagnetic gas, or a change in the oxygen concentration from its usual value, whereas more sophisticated implementations may provide an output indicative of the total amount of oxygen present.

The receiver coil may float with respect to the transmit coil, reducing the pickup of electric fields (capacitive coupling).

Examples of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 illustrates key elements of the gas analyser,

Figure 2 illustrates embodiments of the gas analyser,

Figure 3 illustrates an embodiment of the gas analyser using perpendicular transmit and receive coils,

Figure 4 illustrates embodiments of the gas analyser using a split receive coil, Figure 5 illustrates embodiments of the gas analyser using planar coils,

Figure 6 illustrates further embodiments of the gas analyser using substantially planar coils,

Figure 7 illustrates drift compensation methods for the gas analyser,

Figure 8 illustrates drive and sensing circuits for the gas analyse, and

Figure 9 illustrates the response of the gas analyser to varying oxygen concentration.

Figure 1 illustrates elements of a gas analyser system according to the invention. An excitation and detection circuit 105 drives an alternating current through a transmit coil 101. A receive coil 102 is located within the strong-field region of the transmit coil 101. By the "strong-field" region, we mean within the region where their separation distance is less than their size, and advantageously, the receive coil 102 and transmit coil 101 occupy the same volume, as detailed later. The receive coil 102 is substantially balanced with respect to the transmit coil, such that the induced emf in the receive coil 102 is small. A volume of test gas 103 and a volume of reference material 104 are present within the strong-field regions of the transmit coil 101 and receive coil 102. The test gas volume 103 and reference material volume 104 are located such that a change in magnetic permeability of the two materials results in a change in the mutual coupling of the transmit 101 and receive 102 coils. The change in mutual inductance results in a change in the induced emf and hence the voltage or current in the receive coil 102. This is measured by the excitation and detection circuit 105. Figure 2(a) illustrates an embodiment of the gas analyser. A solenoidal transmit coil 201 with N1 turns surrounds a receive coil 202 with N2 turns, in which two halves are wound with opposite senses, as illustrated using the convention of an x for winding direction into the page and a dot for winding direction out of the page. The coils are wound around a common rotational symmetry axis 208. The transmit coil 201 is longer than the receive coil 202, so that the receive coil 202 is located substantially within a constant field region. This has the benefit that thermal expansion effects have a smaller effect on the mutual coupling between coils. Preferably the transmit coil 201 is between 1.5 times and 6 times the length of the receive coil 202. More preferably the transmit coil 201 is approximately three times the length of the receive coil 202. There is a radial spacing between the transmit coil 201 and receive coil 202 to reduce undesirable capacitive (electric field) coupling between the two coils. The coils are wound on a coil former assembly 206, which may be made primarily of a low thermal expansion material, such as fused silica. A test gas volume 203 and a reference material volume 204 are located so as to at least partially fill the two halves of the receive coil 202. There may be an electrostatic shield 209 in the space between the transmit 201 and receive 202 coils, to reduce electrostatic coupling between the coils. The electrostatic shield 209 may be comprised of a low conductivity conductor, such as graphite, or may be a patterned metallic conductor, patterned to avoid eddy currents from being induced in the electrostatic shield (for example dividing the electrode into multiple parallel "fingers" to avoid closed loops), the key function being blocking the electric field and allowing the magnetic field to penetrate. A shim coil 207 can be used to dynamically balance the coils, by injecting a current at the drive frequency, but with an adjustable magnitude and phase. This "active nulling" process can be used to achieve a more precise balance than can be made by standard manufacturing methods, and variation of the injected balancing current may be used to adjust for sensor drift over its lifetime. Alternatively, fine- tuning of the balancing of the transmit 201 and receive coils 202 may be achieved by adding a balancing coil 210 in series with the receive coil 202. A single turn balancing coil 210 at a reduced radius allows a greater degree of balancing than can be achieved using fixed manufacturing processes, since the additional turn may have a smaller area (capturing less flux) and its precise dimensions may be selected (e.g. by laser trimming) during a calibration procedure following manufacture of the rest of the coil assembly. A thermal shield 200 comprising a tube of high thermal conductivity material such as aluminium or copper may be used to help ensure that the entire assembly is at a uniform temperature. The thermal shield 200 has a combined function as an electromagnetic shield, reducing unwanted pick-up of external electromagnetic fields. The thermal shield 200 thickness may be greater than the skin depth at the operating frequency of the sensor, in order to effectively shield magnetic fields. One or more temperature sensors 220a, 220b are included on the outside of the coil to compensate for overall temperature changes and temperature gradients within the assembly. The electrical resistances of the coils may also be measured to assist with temperature compensation, using the temperature coefficient of resistance of the conductor. The coils may be constructed of copper wire or litz wire, wound on cylindrical glass/ceramic formers and subsequently potted, or they may be a patterned metal film deposited onto one or more glass/ceramic supports. For example, sputtered or evaporated silver, patterned by laser etching may be used in order to minimise thermal expansion effects.

The operating frequency and turns ratio affect both the magnitude of the received signal and the ratio of inductive to capacitive coupling. In order to minimise unwanted capacitive coupling, N2 is approximately twice the value of N1. However, a larger N2 provides a higher received voltage, so preferably N2 is in the range 2*N1 to 10*N1. A transmit coil length of five to twenty times its diameter is preferred for producing a highly uniform field. An operating frequency of 10-100kHz is preferred for a sensor of 20mm transmit coil diameter, with correspondingly higher frequencies preferred for smaller sizes (frequency inversely proportional to diameter). Sensing at two or more frequencies may be used to cancel out capacitive coupling effects. A trimming capacitor can be used to cancel out the capacitive coupling, by adding capacitance between one end of the transmit coil and the (physically) opposite end of the receive coil.

The test gas volume 203 may have an inlet or outlet port that extends through the reference volume 204, provided that its cross-sectional area is comparatively small in this region. This enables a flow-through sensor format with a port at each end of the sensor.

Figure 2(b) illustrates a further embodiment of the gas analyser. In this embodiment, the transmit coil 21 1 is configured to produce a highly uniform magnetic field across the volume of the receive coil 212, particularly at the ends, where movement due to thermal expansion might otherwise affect the mutual coupling. A Barker coil is illustrated (this particular variant sometime being referred to as a Lee-Whiting coil in the academic literature), having the advantage of fixed radius R on the coil former 216, although other coils for generating uniform fields are well-known, such as Helmholtz, Maxwell, Garrett, Rubens and Braunbek coils. The coils are wound around a common circular symmetry axis 218. Preferably, a shim coil 217 for active nulling and electrostatic shield 219 are included. Test gas volume 213 and reference material volume 214 fill the two halves of the receive coil 212, as far as is practical.

Figure 3 illustrates an embodiment of the gas analyser using perpendicular transmit and receive coils. In this embodiment the transmit coil 301 is oriented at right angles to the receive coil 302 and aligned to ensure low mutual coupling. As shown, the transmit coil 301 produces a field along the y-axis and the receive coil 302 is sensitive to fields along the x-axis. The test gas volume 303 is oriented between these two axes, so that any non-zero susceptibility of the test gas results in enhanced mutual coupling between the transmit and receive coils.

Figure 4(a) illustrates an embodiment of the gas analyser using a split receive coil. The receive coil comprises an outer half 402b and an inner half 402a, wound with opposite senses, so that the whole receive coil (i.e. 402a in series with 402b) is substantially balanced with respect to the transmit coil 401. The test gas volume 403 has an inlet 406 and outlet 408, allowing flow-through operation of the sensor. The different cross-sectional areas of the two halves of the receive coil 402a and 402b, means that they have differing sensitivity to the susceptibility of the material in the test volume.

Figure 4(b) illustrates an embodiment of the gas analyser using a split receive coil. The transmit coil 41 1 surrounds two halves of the receive coil 413 and 414. The first half of the receive coil 413 is wound with the opposite sense to the second half of the receive coil 414, as indicated by the arrows showing winding direction 410. The test and reference volumes are within the first 413 and second 414 halves of the receive coil respectively. Figure 5(a) illustrates a cross-section of an embodiment of the gas analyser using planar coils in cross-section. The planar coil construction has advantages of precision and low manufacturing cost. Transmit 501 and receive coils 502a & 502b are formed on a ceramic substrate 509 by printed circuit methods. The upper part of the receive coil 502a is more sensitive to the susceptibility of the test gas volume 503, whereas the lower part of the receive coil 502 is more sensitive to the reference material volume 504. Circular coils wound around a symmetry axis 508 are shown, although square or other shaped coils would show similar performance. It can be seen that the transmit 501 and receive 502a & 502b coils are substantially balanced by design. Advantageously, the coils are balanced more precisely during fabrication by laser trimming of the coil geometry. Figure 5(b) illustrates an embodiment of the gas analyser using planar coils. The illustration is shown in cross-section with coils wound around a symmetry axis 518. Transmit 511 and receive 512 coils are formed on a ceramic substrate 519 by multilayer printed circuit methods. The upper part of the receive coil 512 is more sensitive to the susceptibility of the test gas volume 513, whereas the lower part is more sensitive to the reference material volume 514. Precise balancing of the transmit 511 and receive 512 coils is performed by laser trimming.

Figure 6 illustrates further embodiments of the gas analyser using substantially planar coils. The planar coil construction has advantages of precision and low manufacturing cost. Figure 6(a) shows a loop transmit coil 61 1 and quadrupole receive coil 612 located above test gas 613 and reference material 614 volumes. Figure 6(b) shows a loop transmit coil 621 and a magnetic field sensor receiver 622, such as a GMR or Hall effect sensor, with in-plane sensitivity. These are located above test gas 623 and reference material 624 volumes. The magnetic field sensor receiver 622 replaces the receive coil in the system and performs the same function. The magnetic field sensor 622 is balanced with respect to the transmit coil and they share a common centre point.

Figure 7(a) illustrates a drift compensation method for the gas analyser. The transmit coil 701 and receive coil 702 are concentrically wound around a common axis 708, as described in figure 2(a). The test gas chamber 709 has first 705 and second 706 gas ports and contains a moveable reference element 707 of known susceptibility and low dielectric constant. In operation pressure pulses or jets of gas from the two ports are used to move the moveable reference element between the two ends of the receive coil, hence inverting the polarity of the coil imbalance due to gas susceptibility. This allows any zero drift to be removed.

Figure 7(b) illustrates a drift compensation method for the gas analyser. The transmit coil 711 and receive coil 712 are concentrically wound around a common axis 708, as described in figure 2(a). The test gas chamber 719 has a least one gas port 715 and contains compressible reference material 717 of known susceptibility and low dielectric constant. The compressible reference material may, for example, be a closed-cell foam. In operation, the pressure of the test gas is varied and the size of the reference element 717 changes in response to this. Since the sensor is sensitive both to the partial pressure of oxygen and to the position of the dividing line between test and reference volumes, the signal vs. pressure will increase from zero in vacuum, through a maximum and back to zero when the reference block is highly compressed. This allows any zero drift to be removed.

Figure 7(c) illustrates a drift compensation method for the gas analyser. The transmit coil 721 and receive coil 722 are concentrically wound around a common axis 728, as described in figure 2(a). The test gas chamber 729 includes an inlet gas port 725, an outlet gas port 726 a moveable volume of reference material 727 and a spring 728, configured so that the position of the block of reference material 727 depends on the gas flow rate between inlet 725 and outlet 726 ports. The reference material 727 may be made to move to provide between positive, neutral and negative sensor gains in the left, central and right positions respectively. This variation of gain allows any zero drift of the sensor to be eliminated.

Figure 8 illustrates drive and sensing circuits for the gas analyser. The transmit coil 801 is inductively coupled to the receive coil 802a &802b. The first half of the receive coil 802a is exposed to the test gas and the second half of the receive coil 802b is exposed to the reference material. The AC signal source 803 may include some filtering and matching networks and generates an AC signal at one or more frequencies. The current or voltage supplied to the transmit coil 801 is monitored, preferably by a current shunt resistor and amplifier 804. Advantageous, the receive coil is floating with respect to the transmit coil, to minimise capacitive pick-up. In order to balance the received signal and to maximise signal to noise ratio, a pair of resonant capacitors 805a & 805b are used to feed into an impedance-matched differential amplifier 806, which preferably has an input impedance comparable to the AC resistance of the receive coil. An analogue to digital converter (ADC) 807 digitises signals from the transmit and receive coils and digital signal processing (DSP) 808 is performed, for example in a microprocessor, to compare the magnitude and phase of the transmitted and received signals and interpret the signal in terms of oxygen concentration. An electrostatic shield 809 may be included to reduce capacitive coupling. A trimming capacitor 800 may be included to cancel out any unwanted capacitive coupling. Different resonant capacitors 805a & 805b may be switched into the circuit to allow resonant operation for different frequencies. This multi-frequency operation may be use to compensate for sensor drift effects. Figure 9 illustrates the response of the gas analyser to varying oxygen concentration. The horizontal axis plots the oxygen concentration in a gas sample at atmospheric pressure. The vertical axis shows the sensor output in arbitrary units. The sensor has benefits of a linear response vs. partial pressure of oxygen and a low cross- sensitivity to other gases.