Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For ph
Patent
1998-09-17
2000-09-26
Tung, T.
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For ph
204415, 204432, 2057825, G01N 27404
Patent
active
061238287
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of measuring ethanol vapour concentration and is particularly, but not exclusively, directed to the measurement of breath alcohol concentration (BAC).
Fuel cells are extensively used in equipment for detecting and/or measuring ethanol vapour concentrations. As is well known the oxidation of the volatile component in the fuel cell results in a potential difference being developed between a working electrode and a counter electrode, the potential difference being proportional to the concentration of the volatile component. This potential difference can be used to give a quantitative ethanol vapour measurement, either by monitoring the voltage directly or the resulting current, and typical existing methods of developing that measurement utilise either the peak height of the curve or a calculation of the area under the curve. To obtain meaningful results from either the measurement of the peak height or the area under the curve, a fixed volume of sample must be supplied to the fuel cell and hence a sampling system is required. The time taken between the delivery of the sample to the fuel cell to the display of the measurement is typically of the order of 20 to 30 seconds. Sampling systems can make the apparatus quite bulky and expensive, whilst response times quickly mount up when extensive screening programmes are in operation.
From one aspect the invention consists in a method of measuring ethanol vapour concentration comprising: proportional to the vapour concentration, said output rising to a peak at a substantially steady rate after an initial non-linear portion,
Thus the applicants have determined that the gradient of the steady state part of the graph is directly proportional to the vapour concentration and so a reading can be obtained within a very short time (typically under 5 seconds), simply by exposing the fuel cell to the ethanol vapour, i.e. there is no need for a sampling system of the sort previously utilised.
The gradient may be conveniently determined in a predetermined time window after the start of the output. As has been indicated earlier this is less than 5 seconds, but not immediate due to an initial non-linear portion of the output. Additionally or alternatively the method may include comparing successive gradient determinations and selecting a gradient determination for generating the concentration signal after at least a pair of successive determinations are substantially equal. This equality should indicate that the determination is being made in relation to a linear portion of the output.
From a further aspect the invention consists in apparatus for measuring ethanol vapour concentration including: concentration of the vapour to which it is exposed, said output rising to a peak at a substantially steady rate after an initial non-linear portion, and, determined gradient.
In arrangements such as BAC measurement, it is often desirable to sample a flow of the gas bearing the vapour and it has been found that such fuel cells can be made flowrate insensitive, if a gas permeable membrane or some other structure which prevents flow induced local gas movement, overlies the working electrode. The apparatus therefore includes such a membrane or structure. If a membrane, it may be hyrophobic and, for example, it may be a PTFE membrane.
The apparatus may have a body defining a sample path (e.g. a breath tube) and the apparatus may then further comprise means for exposing the fuel cell to vapour in the sample path.
In one arrangement the fuel cell may lie adjacent to the flow part, but separated there from by a shutter, such as an iris shutter. Alternatively a sample may be drawn to the working electrode of the fuel cell by a change in relative pressure between the vicinity of the fuel cell and the sample part. As no specific volume has to be drawn this could be achieved, for example, by introducing a restriction into the sample path.
Although the invention has been defined above it is to be understood that it includes any inventive combination of the features s
REFERENCES:
patent: Re31915 (1985-06-01), Oswin et al.
patent: 3342558 (1967-09-01), Reinecke
patent: 4127780 (1978-11-01), Kimbell
Jones Dylan
Williams Paul M.
Lion Laboratories Limited
Tung T.
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