Apparatus for Detecting or Measuring a Constituent of a Gas

The present invention relates to apparatus for detecting or measuring a constituent of a gas and is particularly but not exclusively concerned with apparatus for detecting or measuring alcohol in a gas, such as exhaled breath.

According to the present invention there is provided apparatus comprising means for introducing a sample of the gas to a sensor for providing a voltage or current output dependent upon the amount of the constituent in the gas sample and means for measuring the voltage or current generated by the sensor. Preferably the measuring means is arranged to measure the peak potential or current or some other parameter which is a function of the peak potential or current so generated.

An embodiment of the present invention will now be described by way of example and with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of apparatus for measuring blood alcohol constructed in accordance with the present invention, and

FIG. 2 is a section through a fuel cell sensor suitable for use in the meter of FIG. 1.

The apparatus shown in the accompanying drawing comprises a detachable tube 10 of plastics material through which a person exhales air.

The tube 10 has leading radially into it one end of a pipe 12. The other end of pipe 12 leads to one side of a sensor in the form of a fuel cell generally indicated at 14 and shown in greater detail in FIG. 2. The fuel cell 14 consists of a thin sheet 16 of a porous material such as glass, ceramic or plastic which is coated on both sides with layers 18, 18' of one or any combination of the following metals; namely gold, silver, platinum, palladium or their alloys. The porous sheet 16 contains an electrolyte of phosphoric acid and/or sodium hydroxide. The layers 18, 18' act as the anode and the cathode of the cell 14 and the cell is contained in a housing which has an opening 20 so that the layer 18' is in constant contact with the atmosphere. The layers 18, 18' are also connected to electrodes 22, 22' from which the potential generated by the cell can be measured.

The surface of the layer 18 is connected via a pipe 24 to a piston and cylinder arrangement indicated at 26. The piston 28 of this piston and cylinder arrangement 26 is biased by resilient means, here a compression spring 30. The piston 28 also has a groove 32 arranged to co-operate with a latch 34 so that the piston 28 can be held in a depressed position with the spring 30 compressed by the latch 34. A further press switch 36 is connected to the latch 34 and is also arranged to act upon a pair of electrical contacts 38 so that depression of the switch 36 releases the latch 34 and closes the contacts 38 thus completing an electrical circuit between a battery 40 and an operational amplifier 42 also connected to the electrodes 22, 22'. The output of the operational amplifier 42 is taken via a potentiometer 44 to a microameter 46. The microameter 46 has a taut suspension movement, the relatively small angle of deflection being amplified by means of a drum partly shown at 46 which has a spiral line 48 marked on it, thus giving an analogue display of the output from the operational amplifier. The meter 46 can be calibrated to indicate the amount of alcohol in the gas sample, or it may be arranged to indicate whether the amount of alcohol in a sample exceeds a predetermined value.

When in operation, the piston 28 is depressed and held in this position by the latch 34. The tube 10 is fitted to the pipe 12 and the subject asked to blow through it. Because of the open end of the tube 10 and the aperture 10a, the tube acts as a whistle thus providing an indication that the subject is blowing. It has been found that with a low resistance system of this kind, it is easy to get a discard volume of a liter in practically all cases and in most cases 2 liters or more can be obtained. As the subjects level of exhalation starts to decrease as indicated from the sound of the whistle the switch 36 is pressed releasing the latch 34. The compressed spring 30 pushes the piston 28 thus drawing air from the tube 10 via the pipe 12 into the fuel cell 14 so that this sample of air comes into turbulent contact with the layer 18 of the fuel cell 14. A potential is thus generated in the cell by the oxidation of any alcohol at this electrode and the reduction of oxygen at the other electrode. At the same time as the release of the piston 28 the switch 36 closes the contacts 38 so that a reading of the potential generated can be obtained from the microameter 46. The output from the fuel cell 14 rises to a peak in a few seconds but takes about 3 minutes to decay to a point at which the rate of change is slow enough for another reading to be taken. The tube 10 is not essential, the same effect could be obtained by asking the subject to breathe over the open end of the pipe 12.

If necessary a zeroing device can be fitted for the display, such as by providing a switch which is arranged to short circuit the electrodes 18, 18' and to open circuit the electrodes before a reading is taken. Such a switch could be operated by the switch 36. The sensitivity can be adjusted and is normally set by the calibration potentiometer 44 so that the microameter reads 50% of full scale at the limit level. This makes readings of sufficient precision quite easy and enables readings up to twice the limit to be measured. If it is felt desirable to make any different readings above this level a divide by two button can be supplied.

In the cell described the air side of the fuel cell, i.e., that formed by layer 18' acts as an oxygen electrode which imposes a constant power potential, so that oxidation of alcohol does not proceed beyond acetic acid making the system reasonably specific for alcohol in breath. In particular it is not sensitive to acetone or hydrocarbons such as petrol.

While apparatus has been described using a sensor in the form of a fuel cell other suitable electrochemical sensors can be used such as a polarographic cell or a voltaic cell, or semiconductor or catalytic sensors.

The valve system has the important advantage that it introduces to the sensor a small, precise and reproducible volume of gas to the sensor. In addition the apparatus is simple in operation merely requiring the operation of the push button 28a to set the apparatus and the operation of switch 36 to sample the gas and provide the output indication on meter 46.

Furthermore the apparatus can be made such a size that it can easily be carried in an inside pocket or better still in a small holster next to the skin. This ensures that temperature is maintained at a sufficiently high and constant level to ensure constant calibration and avoid excessive condensation or moisture in the air passages. There is no need for a heated sampling system as breath is sucked directly into the sensor. Although the apparatus has been described embodying an analogue display device 46, it is possible to substitute a digital display device for the analogue device.

Finally, current is only drawn from the battery 20 when actually taking a reading so that the latter has virtually its shelf life and provided it is renewed regularly there is no need for a voltage checking system.

Although the description hereinbefore has been solely concerned with the detection of alcohol in breath naturally it may be possible to have other fuel cells which could detect other contaminants in other gasses, thus extending the utility of the meter.