This project, a breath alcohol analyser from
1992, shows exactly how blood alcohol level is measured without taking a blood sample. It uses a fuel cell which is a chemical device that converts hydrogen and oxygen into electrical energy.
A method and apparatus for measuring the concentration of breath alcohol or other reactants is provided in which a breath sample is introduced to a fuel cell, wherein the number of electrons from the fuel cell resulting from oxidation of the alcohol in the breath rises to a peak and thereafter falls to a substantially steady minimum base to form a curve.
The present method is based on the discovery that this curve, regardless of reactant concentration or age of fuel cell, is a log-normal distribution curve.
This design relates to devices and methods for the quantitative determination of the concentration of a chemical constituent in a gaseous mixture. It has particular but not exclusive application to breath alcohol testing devices such as the ones sold by Intoximeters, Inc., 1901 Locust Street, St.
In breath alcohol testing devices presently used commercially, in which fuel cells are employed, the conventional way of determining breath alcohol is to measure a peak voltage across a resistor due to the flow of electrons obtained from the oxidation of breath alcohol on the surface of the fuel cell.
There are a number of problems. The peaks become temporarily lower with repeated exposure to alcohol. The peaks also vary with temperature. In order to produce a high peak voltage, it is customary to put across the output terminals of the fuel cell a high external resistance, on the order of a thousand ohms, but the use of such a high resistance produces a voltage curve which goes to the peak and remains on a high plateau for an unacceptably long time.
To overcome that problem, present systems provide for shorting the terminals, which drops the voltage to zero while the short is across the terminals. However, it is still necessary to let the cell recover, because if the short is removed in less than one-half to two minutes after the initial peak time, for example, the voltage creeps up.
Peak values for the same concentration of alcohol decline with repeated use whether the terminals are shorted or not, and require 15-25 hours to recover to their original values.
Individual fuel cells differ in their characteristics. All of them slump with repeated exposure to alcohol in quick succession. Over time, their sensitivity decreases to a point at which they must be re-calibrated or replaced. Presently, the cell is replaced when it peaks too slowly, when it returns too slowly to a base line output, when the output at the peak declines beyond practical calibration, or when the background voltage begins creeping excessively after the short is removed from the cell terminals.
The present design enhances the analytical capabilities of the device by providing a new method for determining the level of breath alcohol or other gaseous constituent of a mixture. The improvement is applicable to a wide variety of other electronic analysis circuits associated with fuel cell detectors and to instruments for measuring a wide variety of reactive volatiles.
One of the objects of this design is to reduce the time required for determining the level of breath alcohol or other reactive gases.
Another object is to reduce the computational requirements for such analysis.
Another object is to reduce the length of time required between successive such analyses.
Another object is to eliminate any error in a breath alcohol determination created by the residual effects of a previous test.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawing.
In accordance with one aspect of the present design, generally stated, an improved method of measuring breath alcohol concentration is provided in which a breath sample is introduced to a fuel cell, wherein the number of electrons from the fuel cell resulting from oxidation of the alcohol in the breath rises to a peak and thereafter falls to a substantially steady minimum base to form a curve. The present design provides a greatly simplified method for determining the area under the curve to a high degree of accuracy.
The present method is based on the discovery that this curve, regardless of reactant (e.g. alcohol, carbon monoxide, hydrogen, or other chemical compound for which the fuel cell is designed to react), concentration of reactant, or age of fuel cell, is a log-normal distribution curve. In the present method, the entire area under the curve is determined by identifying two points on the curve and calculating the parameters that define the entire curve as well as the entire area under that curve, thereby providing a measure of substantially all of the electrons generated by the oxidation (or reduction) of the alcohol or other reactant, and an intelligible signal representing that area is generated.
The preferred method includes two additional steps: first, a step of establishing an absolute base line output of the cell (if any) and identifying points on the curve relative to that base line, and second, a step of establishing a secondary base line output immediately previous to introducing a sample to the fuel cell in order to determine the presence of residual effects from a previous test (if any), the value of which is used to mathematically determine a correction value for the subsequent test.
The correction value is preferably based on the square of the secondary base line valued to take into account the area under the tail of the previous curve.
Apparatus in accordance with the present design is provided for measuring a reactant in a gaseous sample by reacting the reactant in a fuel cell which produces a current that flows via output terminals through an external circuit. The current consists of those electrons generated at any point in time by the conversion of the substance to be analyzed, the current rising in response to the presence of the reactant in contact with the fuel cell and falling again to a base level to establish a current-time curve, the apparatus comprising means for calculating the parameters of the extrapolated log-normal curve, means for determining the area under the curve, means for adjusting for the residual effect of a previous test, and means for displaying a value indicating reactant concentration as a function of the area.
In the preferred apparatus of the present design, an external resistor across the output terminals of the fuel cell has a resistance high enough to avoid bypassing significant current from the current amplifier, but low enough to maintain the stability of the cell between tests.
Referring now to the drawings, and particularly to FIG. 1, for a circuit illustrating one embodiment of apparatus of this design, reference numeral i indicates a fuel cell with terminals 2, 3 and 4. Terminals 3 and 4 are output terminals and terminal 2 is a biasing electrode which may or may not be included depending on the type of fuel cell configuration, in accordance with well-known practice.
A resistor 5 is connected across the terminals 3 and 4. The resistor 5 illustratively has a resistance of 1.5 ohms. In practical usage, this value may vary widely, say from 1.5 ohms to 1000 ohms. A capacitor 6 is also connected across terminals 3 and 4. The capacitor 6, in this embodiment, has a capacitance of 0.1 μfd.
Terminal 3 is connected to the negative input of an operational amplifier (op amp) 7. Terminal 4 is connected to a common or ground 13 as is the positive input of the op amp. In this embodiment, a 25k ohm potentiometer 8 provides feedback for gain control and a 2.2 μfd. capacitor 7 provides smoothing of the output.
In this embodiment a potentiometer 10 connects to the offset terminals of the op amp providing zero offset for the op amp output. The output of the op amp 9 is electrically connected to the input of an analog to digital converter 11. The output of the analog to digital converter 11 is then electrically connected to a peak point and second point detector 12.
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