THE ALCOTEST A NEW BREATH ALCOHOL ANALYZER. Lutz Grambow, Rudolf GroBkreutz, Roland Neuhaeusser, Hansjochen Schuck

Transcription

1 THE ALCOTEST A NEW BREATH ALCOHOL ANALYZER Lutz Grambow, Rudolf GroBkreutz, Roland Neuhaeusser, Hansjochen Schuck Draegerwerk AG, Liibeck The methods of measuring breath alcohol concentration provides major advantages such as - the possibility of quick and accurate results, - no injury to the human body, - better correlation to the effect of alcohol to the human in the invasion phase /1/. There are several measuring principles to measure breath alcohol concentration such as infrared absorption /2/, semiconductors /3/, electrochemical cells /4/, gas chromatography /5/ etc. The following paper describes an apparatus which uses the method of IR-absorption. The measuring principle of Alcotest 7010 The ethanol molecule is an elastic structure in which the individual atoms or whole atom groups vibrate against each other. Each possible vibration requires an exactly defined energy by which vibrating parts can be excited in resonance. 736

2 A simple way to transmit exactly dosed amounts of energy to molecules is by irradiation of the molecules with light of a uniform frequency (or wavelength), because as we know, light has an energy content associated with its frequency. he E = h V = y (1) in which E = energy of light h = Planck's constant \7 = light frequency = light wavelength c = velocity of light The excitation of molecular vibrations requires light in the wavelength range 0.8 um to 500 um, i.e. infrared radiation. The energy required within the molecule to excite the vibration is absorbed from the radiation field. The intensity of light is therefore weakened or gets absorbed on passing through a layer of molecules. If I is the intensity of the incoming light and k the concentration of the ethanol vapor, the intensity I remaining after having passed through distance d is given by I = I0e'Kkd (2) in which 0< = absorption coefficient If, with gaseous alcohol present, the light energy - or according to e.g. (1), its frequency, is varied continuously, then the illustrated absorption spectrum for ethanol, fig. 1, is obtained. In the near IR-range several absorption maxima are observable and can be related to characteristic vibrations of the ethanol or parts thereof (for instance the stretching vibration of the C-H bond). 737

3 When using the IR-measuring system Alcotest 7010 for breath alcohol analysis, the whole of the spectrum is not analyzed each time, analysis being confined to the examination of a sharply defined wavelength range around A = 3.4 nm. In selecting only this wavelength range it has been reasonably assumed that 1) no other gases present in the breathing air (N2, 02, C02, H2O-vapour, with smokers also CO) influence the alcohol determination (see fig. 2), 2) the absorption is large and 3) light sources, optical components and detectors are available as components in serial production with high accuracy and reliability. In order to determine the concentration, Alcotest 7010 measures in sequence the absorption of the light-rays in alcohol-free air (for standardization purposes) and in the sample containing alcohol. According to the absorption law, the alcohol concentration is the proportional difference between these two measurements. Construction and functioning of the Alcotest 7010 Alcotest 7010 (fig. 3) is a desk-top instrument, which can be operated from an AC power supply as well or from a 12 V battery. It consists of two functional elements connected by cable. These elements are the sensor and the display unit. The sensor (fig. 4) contains the measuring chamber necessary for determining breath alcohol; it also contains the inlet and outlet connections for exhaled air, a control arrangement for measuring the 738

4 sample gas flow rate, the heating elements for those parts which come into contact with exhaled air, a pump for sampling alcohol-free air for standardization purposes and an electronic unit essential for the measurement. All optical component parts are closely integrated within the measuring chamber. All parts that come into contact with exhaled air are heated to approximately 38 C. Thus not only: condensation of breath moisture on optical components is avoided, but also the dissolution of breath alcohol in the condensate. This part of breath alcohol would otherwise escape from the concentration determination of the remaining gas phase and would result in too low measured values of alcohol concentration. Such heating is therefore a prime necessity of all precision breath-alcohol measuring instruments. In order to reduce the problem of condensation of moisture to a minimum, the sensor of Alcotest 7010 was not integrated into the display unit. The construction chosen guarantees a very short path of exhaled air from the mouth to the measuring chamber. The chamber is constructed in a manner such that its volume is small in comparison with the whole exhalation volume. Consequently we can be sure that the alcohol concentration in the cell is identical to the actual breath-alcohol concentration. Radiation from a light source is collimated by an IR-lens having a sufficient light transmitting capacity in the near infrared range. The parallel light is reflected four times between two mirrors within the measuring chamber in order to obtain a long absorption path while keeping outer dimensions 739

5 small. Thereafter the parallel light goes through an interference filter which transmits only light of that wavelength range at which the infrared absorption is to be measured. After transmission through the intereference filter, the light is focussed on to an infrared detector by means of a second special lens. The detector consists of a PbSe-semi-conductor whose resistance alters slightly according to the amount of incident infrared radiation. The resistance alteration is converted into a voltage change which is then further processed by the electronics. Because small alternating voltage (AC) signals of a constant and known frequency are easier to process electronically than DC signals, the light is mechanically chopped by a rotating disc. The functioning of Alcotest 7010 is most easily understood by explaining the practical handling of the instrument. It is simple to operate, the number of functional and control elements having been confined to an absolute minimum. When the instrument is switched on by depressing the on/off push button, those parts of the sensor being in contact with breath are first heated to approximately 38 C. Until the sensor has reached its operating temperature a "Wait" instruction appears in the digital display. The instrument is ready for use, when the "Wait" instruction disappears and a lamp in the "Start" press button lights up. The warming up takes approximately 10 minutes at room temperature; in order to have Alcotest 7010 ready for use permanently, it can be left switched on continuously. The measuring cycle is initiated by depressing the "Start" push button for a short time. A miniature pump built into the sensor sucks in 740

6 ambient air and flushes it through the cell. In this way, alcohol-free air enters the measuring chamber. The intensity of the radiation arriving at the infrared detector is measured and serves as a relative "Zero" value for the following breath alcohol determination. The illumination of a control lamp on the sensor housing after completion of the purging procedure and determination of the base value (altogether approx. 30 secs.) shows that the subject may now blow into the sensor. A sterile mouthpiece with a saliva trap is now attached to the sensor gas inlet and the subject has to exhale into the Alcotest In order to ensure that deep lung air is used for the determination of blood alcohol concentration, the person to be tested has to blow continuously at a minimum volume rate for a minimum duration. Compliance with these three requirements is controlled automatically by the Alcotest 7010, thus preventing falsification of the test result by fraudulent blowing techniques. Furthermore a sufficient minimum volume rate is indicated by the control lamp on the sensor housing so that the person to be tested may, if necessary, be requested to blow harder. Only if a minimum volume rate is registered by the flow sensor of the Alcotest 7010 for approx. 7 secs, a measured value is indicated on the digital readout and printed out. The whole measuring cycle takes less than 1 minute. The need for these sampling conditions becomes apparent from fig. 5. After approximately 5 sec. the expired volume is about 1 1, i.e. deep lung air is entering the sensor. At about the same time the amount of breath alcohol reaches its maximum value. 741

7 The duration of blowing into the sensor, the minimum volume rate and the blowing resistance are correlated in such a way that the subject can blow without special effort, and achieve the best possible conditions for measurement. By operating the "Start" press button a new measuring cycle can be started. Simultaneously the reading of the previous recorded blood alcohol concentration value is cancelled. Proper functioning of Alcotest 7010 can be checked by means of a "Test" push button. When this push button is depressed continuously for approx. 7 secs, after the light up of the sensor's control lamp a sampling procedure is simulated. By depressing the "Test" push button the output of the IR-lamp in the sensor is reduced in intensity, thus - as in the presence of alcohol - the detector registers a decrease of light intensity and therefore a BAC-value is displayed. The test function is not a substitute for calibration of Alcotest Checks of calibration have to be performed twice a year in order to guarantee the high accuracy and reproducibility of the instrument. Finally the technical data of Alcohol 7010 are listed in table 1. Table 1 Technical data cf Alcotest 7010 Measuring principle Measuring range Accuracy in vitro in the range IR-absorption at 3.4 um 0...3% obac 742

8 0...1%oBAC 'I... 2 % obac 2...3%0BAC Sensitivity drift at 1%oBAC in vitro Atmospheric conditions operating temperature storage temperature humidity (environment) atmospheric pressure Warming up time Time between tests purging standardization ready for test Power supply Power consumption Weight Dimensions (L x W x H) Display of test results Check Calibration % 0BAC j; 5% of the measured value _+10% of the measured value _+ 0.05%oBAC within 6 month C C %r.h mbar approx. 10 min. at room temperature approx. 1 min. 30 sec. 3 sec. 40 sec V AC; 12 V DC approx. 40 W approx. 8 kg 480 mm x 420 mm x 140 mm 3-digit digital display built-in check twice a year Discussion The IR breath-alcohol measuring instrument Alcotest 7010 is easy to handle and can determine ethanol concentrations to great accuracy (tolerance equivalent to a few hundredths per mil). It is especially advantageous to police authorities, public health authorities, and in the field of protection of a work-place that in addition to ease of handling of the instrument, the quality of the test results remains constant for several months, i.e. checks of calibration are only necessary at intervals of several 743

9 months. Therefore, a minimum of maintenance and servicing is required. Any information about accuracy and reliability of a breath-alcohol measuring instrument can finally be achieved only by practical tests, from which a great number of correlated values of breath alcohol concentration and blood alcohol concentration must be available in order to obtain statistically significant data. On completion of the development work on Alcotest 7010, Dragerwerk carried out field trials in Hamburg, Liibeck and Munster. The Research and Development Department for Police Techniques, Munster, had supervised the trials. The evaluation of the tests was carried out in collaboration with the Institute for Forensic Medicine at the Medical University in Liibeck, Prof. Pribilla. Fig. 6 shows a record of the data gained. The correlation of the breath values with the blood values is convincing. The standard error of the estimate of the blood alcohol concentration from the breath alcohol concentration is about 0,3 %Q BAC with a confidence of 99,3 %. The histogram of the error rate for the concentration range ,00 %0 BAC (fig. 7 ) shows that the Alcotest 7010 under-reads a subjet s blood alcohol concentration very little. Conclusion The constructional features of the IR-Breath Alcohol Analyzer Alcotest 7010 are discussed. In vitro test prove that Alcotest 7010 has an excellent short term and long term stability. 744

10 About 400 BAC / BrAC correlation data were obtained in 3 field trials. The breath alcohol values were generally in good agreement with the certified blood results. References 1. Loos, U.; Heifer, U.: tiber den zeitlichen Verlauf von Atem- und venoser Blutalkoholkonzentration und von Alkoholwirkungen. Blutalkohol 1_6: (1979) 2. Harte, R.: An instrument for the determination of ethanol in breath in law enforcement practice. Journal of Forensic Science J_6: (1 y71) 3. Picton, W.R. : An evaluation of the alcohol level evaluation road side tester (ALERT) under laboratory and field conditions. Proceedings of the 7th International Conference on Alcohol, Drugs and Traffic Safety, Melbourne 1977, Huck, H.: Eine analytische Brennstoffzelle als Alkoholsensor Z. Anal. Chem. 270: (1974)

11 Penton, J.R.; Forrester, M.R.: A gas chromatographic breath analysis system with provisions for storage and delayed analysis of samples. Proceedings of the 5th International Conference on Alcohol and Traffic Safety, Freiburg (W. Germany) 1969, II II 86

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13 E wavelength Xl!SU0JUI 0Aj^D)9J 748

14 ^-Bre a t h «= o h o i a A 1 t e s t 7 0 i 0 749

15 mirror heater lens chopper lamp pump Schematic diagram of the se n so r 750

16 BREATH ALUUHUL CONCENTRATION EXSPIRED B R E A T H (ARBITRARY UNITS) VOLUME V [I] (N h cn r OO h CD h Ln -J co h CN n o Q) U) i_ j qj E at <u u > T> <D& H a 4) T 3 aaj C o al +» c <u o G O a o rc oa at <u 1 m in cn 751 a> 5 m 3 co b CD t> 0) s 3 ph o >

17 BrAC (%0 ) fig. 6 CORRELATION OF THE BAC AND BrAC VALUES. 752

18 Field Trial Alcytron Hamburg / Lubeck / Munster 76/79 No of tests : 92 A t BAC / B ra C 20 min rel. frequency of occurrence { % ) I I I 0.3 0, ,15 0,1 0,05 0 BAC «BrAC 0 0,05 0,1 0, ,25 0,3 BAC I BrAC Error rate at BAC %o ( abs. E rro r) H istogram of Error Rate 7 5 3