This invention relates to hot wire anemometers suitable for measurements of gas flow in clinical applications. More specifically the present invention relates to a pulsed hot wire anemometer which may act as a time of flight sensor.

In the clinical assessment of patients, airflow measurements are extremely important. The instruments used for these measurements are known as pneumotachographs, pneumotachometers and anemometers. All have a common function though the mode of operation of each differs and their inherent disadvantages vary.

All such devices have to be able to respond accurately and rapidly over a wide range of dynamically changing flow rates. They must not impede the patient's breathing ability and the requirements of a device for use with adults are different to those for use with infants and neonates. The "dead space" of the sensors used must be as small as possible. In addition, the devices should, ideally, be capable of differentiating between inhalation and exhalation cycles. The response characteristics of these devices should be independent of gas temperature, gas density/composition, humidity and viscosity.

Hot wire anemometry exploits the cooling effect of air moving over a heated resistance element (usually a fine wire suspended in the gas flow). Since the rate of heat loss from the heated element will be directly influenced by the velocity of any gas moving past it, this can be used to determine the gas velocity. However, this is not independent of gas composition since the heat capacity and density of the gas have significant effects. Pulsing the heating element can remove the density dependence and enable true "time of flight" measurements to be made.

GB 2210983 describes an anemometer with a pulsed heating element. The anemometer described comprises at least two transmitting wires and at least two sensor wires (all of which are fixed in the flow of the fluid), means for supplying heat to the pulsed wire, means for detecting and monitoring the heat pulse arriving at the sensor wire, means for measuring the interval between successive pulses, and means for measuring the interval between initiation of the pulse and detection of the pulse at the sensor wire.

We have developed an improved hot wire anemometer in which the sensor chamber comprises an inlet for gas, an outlet for gas, and, intermediate between said inlet and said outlet, a single elongate resistance element and two elongate sensor elements, all said elements being fixed in a common plane parallel to gas flow, said resistance element being intermediate between said sensor elements.

By using the elongate resistance element in conjunction with a sensor element located downstream thereof, pulses of heat generated in the resistance element and transferred to the gas flowing past it may be sensed, enabling the true "time of flight" of the gas to be determined. The use of a further sensor element located upstream of the resistance element, acting as reference for the downstream sensor element, enables the direction of the gas flow to be determined.

The anemometer preferably comprises means for initiating a heat pulse in the resistance element, means for detecting and monitoring a heat pulse arriving at each sensor element, means for measuring the interval between successive pulses, and means for measuring the interval between initiating the pulse and detecting the pulse at the respective sensor element.

The resistance element and the sensor elements are preferably located symmetrically in the sensor chamber, which has an inlet/outlet for gases at each end thereof. Typically, the resistance element is spaced about 1mm from each of the respective sensor elements, and is generally parallel thereto.

The sensor elements are typically about 5 microns in diameter and composed of tungsten coated with gold; platinum can alternatively be used. The sensor elements are preferably connected in a balanced bridge circuit.

The resistance element is preferably of a noble metal. In this case platinum is preferred; iridium is an alternative. The diameter of the resistance element is typically about 20 microns.

A smooth plate may, in some embodiments be placed substantially parallel to the direction of flow and to the sensor elements, so that the respective sensor element will be in the boundary layer of the plate, where the flow will be more laminar. There are generally meshes at either end of the sensor chamber to make the flow pattern inside the sensor chamber substantially independent of the geometry of the connecting tubing, which also produces a more even flow.

Typically, the sensor chamber has a casing of a gamma radiation sterilisable polymer, such as polystyrene.

The present invention is exemplified with reference to the following drawings, in which:

Figure 1 is an illustrative schematic plan view of a sensor chamber employed in an anemometer according to the invention;

Figure 2 is an illustrative block diagram of electronic circuitry suitable for use with an anemometer according to the invention; and

Figure 3 is a diagram showing individual components of the sensor chamber and the lines over which they are connected to the circuit of Figure 2.

Referring to Figure 1, the sensor chamber 1 comprises a resistance wire 2, sensor wire 3, and a further sensor wire 4.

In use, a cylinder of heated gas is formed around the resistance wire

2 when a heating pulse is applied, the cylinder being carried in a direction dependent on gas flow. When the flow is in the direction of arrow A, wire 3 functions as a receiver and wire 4 as reference. As the gas flow carries heated gas towards wire 3, the temperature of wire

3 will increase. This results in an increase in its electrical resistance. The temperature and electrical resistance of reference wire 4, which is upstream of the heated gas, will remain unaffected by the heat pulses and it can thus act as a reference. This results in the resistance bridge between wires 3 and 4 becoming unbalanced towards the hotter wire.

When the gas flow is in the opposite direction, the roles of wires 3 and

4 are interchanged, wire 3 acting as the reference and wire 4 acting as the receiving element. Sensor wire 4 is thus heated by gas flown past the resistance wire 2, increasing its electrical resistance compared to that of wire 3. The resistance bridge imbalance between the two wires is the inversion of that caused by gas flow in the opposite direction. The direction of the gas flow can thus be determined, enabling respiratory exhalation and inhalation cycles to be differentiated. In cases where no flow is present, the cylinder of hot gas produced around the resistance element will diffuse outwards towards wires 3 and 4, and will reach both wires at the same time, causing no disturbance in the resistance bridge.

When the heating pulse is initiated (typically for about 10 microseconds), the resistance wire may reach a temperature of about 200°C. The temperature of gases entering the sensor chamber is generally between 35 and 38°C and the gases have a high water vapour content. Condensation of water vapour onto the surfaces of the sensor chamber or wire elements would impair the sensor function and produce erroneous gas flow measurements. To minimise condensation of this water vapour within the sensor chamber, the whole of the chamber is generally heated to about 40°C.

The sensor is preferably calibrated for flow rates in each direction; separate potentiometers are generally employed for each direction of flow.

The tension in the wires is critical in ensuring that the accuracy and full dynamic range of the sensor is maintained. The wires may be fixed in place by the use of microresistance welding or laser spot welding or similar techniques. Conductive adhesives may not ensure that constant tension is maintained and are therefore generally unsuitable.

Referring to Figure 2, the illustrated anemometer contains a sensor chamber 1 (as described above with reference to Figure 1), a signal generating and conditioning circuit 5 provided with a power supply 6 (which supplies a.c. across transformer 7) and an interface and video circuit 8. Circuit 8 drives various utilities such as display cathode ray tube 16, a chart recorder (analog) output 17 and a serial output 18; the circuit is provided with a keyboard input 19 under the control of an instrument control microprocessor 11.

Circuit 5 is arranged to transmit pulses over line 12 to resistance wire 2 in the sensor chamber 1, to receive over line 13 output signals from the bridge in which the wires 3 and 4 are connected, to provide heater control signals over line 14, and to receive calibration signals over line 15. Thus referring to Figure 3, resistance wire 2 is connected between line 12 and ground. The bridge which includes wires 3 and 4 is connected between a supply rail + V and ground and has its output comers connected to the inputs of a differential amplifier driving line 13. A heater coil H is connected between line 14 and ground and provides the background heating within the chamber: the actual temperature is sensed by thermistor TH which is connected between ground and line 14 and the signal on line 14 is used for thermostatic control. One or more calibration potentiometers are connected between ground and line 15. Circuit 5 is further connected to a flight time measurement and sampling rate generator 20, which provides two timers for respectively, measuring the flight time and generating sampling rate (i.e. transmit pulses 12).

These two parameters are passed to the sensor control microprocessor 21 for calculating flow-rate and the. volume. The flow-rate is calculated from:

distance between the wires

Flow Rate = x cross sectional area of the sensor chamber flight time

Sample Volume = Flow x Sampling Time.

A breath is registered when the flow changes from negative direction (expiration) to a positive flow (inspiration). To reduce the effect of spurious breath if the breath volume is below a certain level (say 0.1 ml or 10% of the average tidal volume) it is rejected.

Signals corresponding to above parameters and the state of the sensor are passed through the isolation barrier 9 (typically in the form of an opto isolator 10) to the interface and video circuits 8.