How to Understand Non-Invasive Breath Analyzers

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Breath Analyzers are used to descry and quantify the alcohol content in the subject’s breath non-invasively. These are invented by Robert Frank Borkenstein. These devices generate an estimated Blood Alcohol Concentration (BAC) in some acceptable units. Although the observed air/breath has small amount of alcohol concentration, though the results are relatively true to the actual values.

Evaporation of alcohol from the circulating blood to the lungs air is the basis of Breath Analyzer devices. After absorption of alcohol by the digestive organs, it enters the blood stream and travels through the body. During the breathing process, the oxygen from the lungs air enters into the blood and carbon-dioxide from the blood stream evaporates into the breath. When a person is drunk, in addition to carbon-dioxide, a modest quantity of alcohol also gets released into the breath through gaseous exchange process. As per The Henry’s law, the quantity of alcohol available in the person’s breath depends on its concentration in the blood stream. In equilibrium, the ratio of concentration of alcohol in blood (Blood Alcohol Concentration-BAC) to that in breath (Breath Alcohol Concentration-BrAC) is 2300:1 and it is almost constant. Statutory BAC limits are reported in concentration units of mg/100 ml, mg/g, g/l or mg/ml and the corresponding BrAC limits are reported as mg/l, g/l and g/100 ml, depending on the country.

Figure 1. Gaseous exchange (a) Normal person; (b) Drunken person
Figure 1. Gaseous exchange (a) Normal person; (b) Drunken person

To get a reliable BAC measurement, the sampled breath must be nearest to the blood vessel. Figure 1 shows the gaseous exchange profile of a normal person and a drunken person. In lungs there present a small deep capillaries viz. alveoli which are closest to the blood vessels by a thin membrane. Carbon-dioxide from the blood vessels along with alcohol content is exhaled through the alveoli.

Breath alcohol concentration measures how intoxicated a person is at a given time. The results can fluctuate even within the course of a day. The more alcohol a person consumes, the higher his BAC should be. If he stops consuming alcohol, his BAC may still continue to rise because some of the alcohol consumed earlier may not have been absorbed into the bloodstream at the time of the initial testing. Eventually, if consumption is discontinued, the level of alcohol in the blood should begin to drop, resulting in a lower BAC.

The ongoing demand to create a device for Breath Analysis which should be portable, accurate, simple to operate & calibrate, provide data safety and integrable with modern smartphones prompted STMicroelectroincs to develop “Breath Analyzer” using embedded technology. The design is compact and easy to calibrate at user level using NFC link. The high quality solenoid air-pump along with Fuel-Cell sensor makes these devices veracious.

Solenoid Air Pump
Solenoid air pump is used to collect the sample, then throwing it on-to the sensor wafer (Fuel-Cell). The sensor is then activated for a predefined interval; say 200 mS. The collected sample is then blown-out through solenoid air-pump to reset the sensor wafer so that the new sample could be collected. This way, the procedure is triparted – Sampling; Holding and Reset. The solenoid air-pump area is directly proportional to the sample volume. The typical values of solenoid diameter and sample volume are given in the Table-1 below:

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Table-1: Solenoid air-pump diameter vs. sample volume

The sample area can be decimated in tune with the sample volume without compromising the sensor reading accuracy. A sample volume in the range of 0.25 cm3 to 0.50 cm3 is sufficient for precise measurement. The smaller diameter of Solenoid air pump is preferable, however, it burdens the associated electronics as the signal amplitude reduces which in turn reduces SNR (signal-to-noise ratio).

Breath Analyzer Sensors
The sampling of breath is done by Breath Analyzer sensor. Various types of these sensors are available viz. fuel-cell sensor, semiconductor sensor and spectrophotometer sensor each having some advantages and limitations over the others.

Electrochemical fuel cell breath analyzers are devices in which an electrical current is produced as a result of a chemical reaction taking place on the surface of an electrode system. In Fuel cell sensors alcohol (ethanol), undergoes a chemical oxidation reaction at a catalytic electrode surface (platinum/gold) to generate a quantitative electrical response. These sensors are highly specific and sensitive to alcohol and measurement cannot be influenced by endogenous substances such as acetone (produced by diabetics), Carbon Monoxide or Toluene. They have high calibration stability. These have an average life span of 5 years. These sensors cannot detect if breath sample is alveolar (deep lung air). As a result it may produce a falsely high reading if a subject has recently drank and still has alcohol in his mouth (highly unlikely as mouth alcohol evaporates very quickly).

Semiconductor sensors offer an affordable means to measure their breath alcohol with some compromises on reliability and accuracy. Semiconductor technology uses an oxide sensor to measure the reactivity between the tin dioxide (SnO2) in the sensor and the ethanol molecules in the breath sample. When the ethanol molecules come in contact with the tin dioxide the reaction changes the electrical resistance of the sensor. The semiconductor measures this difference and calculates an estimate of the BAC of the sample. These sensors are of affordable cost because of lower cost of manufacturing and supports low power portable systems. On the flip side, these are unstable and highly sensitive to the atmosphere, altitude and elevation.

Spectrophotometer technology is used in large, table-top breath analyzers. Spectrophotometers work by identifying molecules based on the way they absorb infrared light. The level of ethanol in a sample is singled out and measured, and a subject’s alcohol level can then be determined. These devices are expensive and are normally available on request.

Considering the higher calibration stability, longer life span and higher accuracy, electrochemical fuel cell sensors are used in STMicroelectronics “Breath Analyzer” design.

ST Microelectronics Breath Analyzer Architecture
Breath Analyzer hardware architecture as shown in Figure 2 is a portable battery operated design based on 16 MHz proprietary STM8L core. STM8L includes an integrated debug module with a hardware interface (SWIM – Single wire interface module) which allows non-intrusive In-Application debugging and ultra-fast Flash programming. The ultralow power STM8L152R8T6 operates from a 1.8 V to 3.6 V power supply. A comprehensive set of power-saving modes allows the design of low-power applications.

Figure 2. Breath analyzer system architecture
Figure 2. Breath analyzer system architecture
Breath analyzer system reference design
Figure 3. Breath analyzer system reference design

Breath analyzer system reference design is shown in Figure 3. The front and back panels of the reference design PCB are shown in Figure 3(a) and Figure 3(b) while Figure 3(c) depicts the packaged product. It is powered by 3.7V Lithium ion battery which can be charged by a battery charging IC STC4054GR using a wall adapter and low battery is indicated using a voltage detector IC STM1061N31WX6F. LED backlight for LCD display is switched on, automatically by sensing the ambient light intensity using ALS (Ambient Light Sensor). A condenser microphone is used to detect if a person has blown into the mouthpiece in order to actuate the pump and collect a precise, fixed volume (0.25ml) of air sample. Air sample containing alcohol interacts with fuel cell sensor which produces current proportional to alcohol concentration.

The current produced by the sensor is converted into voltage using op-amp TS507ILT configured as a trans-impedance amplifier. Integrated 12-bit ADC of STM8L is used to sample this voltage proportional to the sensor current and integrated over time. The resulting output is converted into BAC using first order linear equation coefficients derived during calculation. This information is displayed on LCD and user can optionally save this reading into Dual interface EEPROM.

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