The present invention relates to a flow transducer or hot wire sensor useful for measuring bi-directional flow of gas, for example during the ventilation of patients.

During assisted ventilation of patients, especially neonates, it is highly advantageous to obtain an accurate reading of the flow and volume of gas delivered to the patient. There is no direct way of checking the condition of the lung and the management of the patients relies heavily on regular blood gas analysis and the continuous measuremenr of blood gas oxygen saturation and carbon dioxide partial pressure.

Flow and volume information coupled with a pressure transducer will provide useful information, such as the detection of a leak in the system, the compliance of the circuit, the effect of the drugs on compliance and the synchronisation of the patient with the ventilator. Such information is particularly critical in patients ventilated with high frequency ventilators, since in general they have the most severe respiratory problems.

The hot-wire principle is well known, and the technique of measuring fluid flow with a hot wire is used in many different applications, ranging from measuring wind speed in a wind tunnel to the quantity of air injected into a fuel injection carburettor. The principle is that passage of a moving fluid over a hot surface tends to cool the surface, the rate of the heat loss from the hot surface being proportional to fluid velocity.

Figure 1 shows a common arrangement for hot-wire flow measurement. In this configuration a sensing wire S,

commonly of platinum, inserted in one leg of a Wheatstone Bridge W is kept at a constant temperature by the action of a servo amplifier R the output of which provides the bridge supply (see, for example, "Evaluation of a Constant Temperature Hot Wire Sensor for Respiratory Gas Flow Measurement", J S Lundsgaard, Med. Biolog. Eng. Comput. , vol. 17, pages 211-215) . A control T, such as a variable resistor, in a second leg of the bridge, is adjusted to pre-set a required temperature. When the flow velocity increases the sensing wire becomes cooler and its resistance decreases, forcing the output of the amplifier to rise and bring the wire temperature back to the pre-set value. Because the output of the bridge is dependent on convected heat loss, the result is a true mass flow measurement.

To differentiate the flow direction, for example between inspiration and expiration, a bi-directional sensor is needed, but the basic hot-wire apparatus fails to distinguish between the two directions of flow.

In a modification, a vane is used to enable the measurement of flow direction. US Patent No. 4 363 238 (Willam) discloses another arrangement in which two hot-wires A, C are used, with an interposed bar B to shield one of the wires in one direction, as illustrated schematically in Figure 2. It is necessary for wire A and bar B to be accurately positioned relative to each other for reliable results.

The present invention provides a hot-wire sensor comprising a flow conduit having an inlet portion and an outlet portion and arranged such that in use the majority of fluid flowing from the inlet to the outlet follows a first flow path and the majority of fluid flowing from the outlet to

the inlet follows a second flow path, the first and second paths being spatially separate along at least a portion of their respective lengths, a first sensing wire placed in said portion of the first flow path and a second sensing wire placed in said portion of the second flow path.

Preferably, spatial separation of the first and second flow path portions may be produced by making the inlet and the outlet of the sensor non-collinear.

Preferably, the inlet portion and outlet portion are parallel, and they may be of generally uniform internal cross section, e.g. circular, square or rectangular. The conduit may have an enlarged central portion where the spatially separate portions are located, and the inlet, outlet and central portion may have common (continuous) upper and lower wall portions.

The sensor could include other transducers, for example a pressure transducer.

An island may be located within the sensor, for example between the first and second wires, to provide the spatially separate flow path portions laterally either side thereof, by its position and/or shape. This island could extend fully or partially between the upper and lower walls of the flow conduit, and could include the other transducer(s) .

A sensor according to the present invention used as a bidirectional flow sensor can have a fast response time and a high dynamic range enabling it to work with HFO (high frequency oscillation) ventilators which could produce a sinusoidal flow of 20 Hz. By increasing the dimension of the sensor, and so reducing the average velocity of the

flow, it is possible to measure the very high flow rate encountered with adult patients.

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

Figure 1 shows a known circuit arrangement for a hot wire anemometer;

Figure 2 shows a known hot wire anemometer employing two wires for distinguishing oppositely directed flows;

Figure 3 shows a schematic view of a hot wire sensor arrangement incorporating a sensor according to the present invention;

Figure 4 is the elevation of a first embodiment of sensor according to the invention;

Figure 5 is the cross-sectional plan view of the sensor of Figure 4;

Figure 6 is the outlet end view of the sensor of Figure 4;

Figure 7 is the inlet end view of the sensor of Figure 4;

Figure 8 is a cross-section view of the sensor of Figure 5 along the line A-A;

Figure 9 is a second embodiment of sensor including flow separation barrier; and

Figure 10 is a third embodiment of sensor including pressure transducer;

As shown schematically in Figure 3, a sensor 1 according to the present invention (for example, as described in more detail with respect to Figures 4 to 8, Figure 9, or Figure 10) comprises two hot wires 2, 3 which are connected, by way of terminals 2a, b and 3a, b respectively, to their respective circuits in a control module 100 acting to keep their respective temperatures at a constant level, well above the ambient temperature. When the flow is in the direction indicated by the arrow 101, the momentum of the flow will tend to carry it in a path straight to the wire 2 so that wire 2 will experience considerably more fluid flow than wire 3. At flow rates above 0.2 L/min the differential output from the wires is quite large. When the flow is smaller, it tends to be less focused, and therefore a relatively large fraction of the flow will also be experienced by wire 3, tending to reduce the differential output from the wires.

In the other flow direction the role of the wires 2, 3 is reversed and wire 3 will experience more fluid flow than wire 2. Wire 3 can now be regarded as the sensing wire and the information from wire 2 could be discarded.

The control module compares the information from wire 2 to that from wire 3. If the difference between the flow indicated by wire 2 and that indicated by wire 3 is less than a predetermined level (the level is dependent on the sensor application) the flow is displayed as being zero. Otherwise, if the comparison indicates the higher flow is experienced by wire 2, the flow is taken as having the direction 101 and its magnitude is taken as that indicated by wire 2; and if the comparison indicates the higher flow is experienced by wire 3, the flow is taken as having the direction opposite that of direction 101 and its magnitude is taken as that indicated by wire 3.

In the first embodiment shown in Figure 5, the sensor 1 comprises a flow conduit having an enlarged central portion 6 between an inlet 4 intended to connect to a ventilator Wye piece and an outlet 5 intended to connect to a endotracheal tube connector (as should be clear, the terms inlet and outlet are used merely to distinguish the two ends of the sensor, and are not definitive of flow direction) . Inlet 4 and outlet 5 are parallel but offset, such that a side wall portion 7 of inlet 4 and a side wall portion 9 of outlet 5 continue as opposed side wall portions 8, 10 respectively of the enlarged portion. Wall portions 11, 12 serve to connect the wall portions 10 and 8 to the other (opposed) side walls 13, 14 respectively of the inlet and outlet.

As shown, the inlet and outlet are of circular internal cross section, so that their upper and lower wall portions of the inlet and outlet are curved, and merge smoothly and continuously with the planar upper and lower wall portions of the enlarged portion. Furthermore, the upper and lower wall portions of the inlet and outlet could be of another shape (e.g. planar in the case of a rectangular or square cross section, thereby providing a better transition to the enlarged portion) . Additionally, or alternatively, the walls of the enlarged portion may be non-planar, and could be shaped to encourage separation of the two flow paths, such as by providing smooth ridges therebetween. The overall consideration should be that the boundaries of the flow conduit are smooth and do not exhibit discontinuities capable of disturbing the smoothness of the flow paths.

Thus, it is also preferable for the joins between the side wall portions (8, 12; 12, 14; 9, 11; 11, 13) to be smooth, rather than angular.

Sensing wires 2 and 3 are located in lateral spaced relation and at the same height, e.g. mid-height, within the enlarged portion 6, wire 2 being in line with the inlet 4 and wire 3 being in line with the outlet 5.

It will be appreciated that, as shown, both the flow conduit and the transducer as a whole have a two-fold rotational axis of symmetry at right angles to the plane of Figure 5. It may be necessary to provide some means of distinguishing the input and output in order that the outputs from the two hot wires can be correctly identified.

During inspiration, flow is from inlet 4 to outlet 5. By its momentum the majority of the gas will flow over wire 2 and therefore, the electronic control unit connected to wire 2 will indicate considerably higher flow than that connected to wire 3. Consequently readings from wire 3 are discarded and readings from wire 2 will be used to derive the indicated flow. In the other flow direction (expiration) the roles of the wires are interchanged and readings from wire 2 are discarded and readings from wire 3 are used to indicate the actual flow.

At zero flow or low flow, readings from the two wires will be substantially the same or there will be a very small difference.

Figure 9 shows views similar to those of Figures 4 to 8 for a second embodiment of sensor la wherein like parts have like references. In the sensor a flow separation barrier 16 is arranged between the wires 2, 3 and extends fully between the upper and lower walls of the flow conduit. Its position and shape assist in flow separation and so enhance flow direction detection at low fluid flow rates by production of a higher differential output from the sensor

wires 2, 3. The flow separation barrier 16 could comprise a pressure transducer.

Figure 10 shows views similar to those of Figures 4 to 8 for a third embodiment of sensor lb in which like parts are given like references in which a pressure transducer 15 is disposed centrally between the two hot wires 2, 3 for measuring the pressure delivered to the patient. This will allow the control module to determine the compliance and the airway resistance of the respiratory circuit. To a certain extent it will also assist in lateral separation of the differently directed flows as in the embodiment of Figure 9.

In variations of this embodiment, the pressure transducer can be placed at other locations, such as in the inlet or outlet, which is more likely to give an accurate pressure measurement, or at another position within the enlarged portion 6, as desired. Furthermore, the degree to which the transducer extends into the sensor may be altered, between extending fully to the opposed wall (cf Figure 9) to a position flush with the wall (or even recessed below the wall) in which it is mounted.

The sensor may be operated at constant temperature using a circuit of the type outlined in Figure 1.