Abstract:
A breath instrument using a combination of breath pressure, temperature, and humidity measurements to determine whether or not breath samples are human and properly delivered. Valid samples require maintenance of a threshold dynamic pressure of a sample being introduced for a predetermined time, a valid temperature of the sample and a valid humidity of the sample, wherein the range of valid sample temperatures is adjusted according to a measured sample humidity. Also disclosed is a breath alcohol instrument having an alcohol-specific fuel cell and a fuel cell circuit for generating a breath alcohol signal. Further disclosed is an interlock system for inhibiting operation of machinery such as a vehicle, and a method for screening breath samples and determining an alcohol content thereof.

Description:
BACKGROUND  
         [0001]    The present invention relates to breathalyzers, and breath alcohol interlock devices for preventing operation of vehicles and other machines by intoxicated persons.  
           [0002]    In the 19th century, law enforcement officials dealt with the problem of alcohol abusers by imprisoning them until they were sober. In the 20th century, the advent of high-speed transportation and complex machinery gave high priority to alcohol testing and screening. Automobiles traveling at 90 feet per second on the freeway are unforgiving of drivers with alcohol impairment. The same is true for a 300-passenger aircraft guided by an alcohol-impaired pilot attempting to land under minimum-visibility conditions. There is very little margin for error. People who operate complex equipment with their judgment impaired by alcohol may not only be a danger to themselves, but impact the safety of others.  
           [0003]    Until recently, the main application of alcohol testing was to traffic law enforcement. The intent was to identify people suspected of driving under the influence of alcohol and remove them from the road. After arrest, law enforcement officers gave the subject a chemical test to determine his blood alcohol level. Subjects were either released or incarcerated and prosecuted, depending on what alcohol levels were illegal as dictated by state law. Until the mid-1940&#39;s, the primary means of measuring blood alcohol levels involved either blood or urine sample testing, both of which were time-consuming and expensive procedures. In the late 1940&#39;s, alcohol breath testing replaced blood and urine sample testing as a means of screening subjects and producing evidentiary results for prosecution.  
           [0004]    In the 1980&#39;s, railroad, nuclear, Department of Defense, and maritime employees came under Federally-mandated testing requirements. In each case, new laws followed a major, alcohol-related disaster. In 1991, the United States Congress passed the Omnibus Transportation Employee Testing Act. This legislation mandated alcohol testing for transportation personnel involved in safety-sensitive jobs. This mandate included airline pilots and cabin attendants, truck drivers, railroad crews, and gas pipeline workers. The US Department of Transportation further defined unacceptable maximum alcohol levels.  
           [0005]    Since the mid-1980s, infrared (IR) technology has been the primary means of breath alcohol testing in the United States. Current technology uses infrared measurement systems that are made more specific for alcohol by using several optical filters. Breath alcohol levels are measured this way by passing a narrow band of IR light, selected for its absorption by alcohol, through one side of a breath sample chamber and detecting emergent light on the other side. The alcohol concentration is then determined by using the well-known Lambert-Beers law, which defines the relationship between concentration and IR absorption. This IR technology has the advantage of making real-time measurements; however, it is particularly difficult and expensive to achieve specificity and accuracy at low breath alcohol concentration levels. Also, the IR detector output is nonlinear with respect to alcohol concentration and must be corrected by measurement circuits. A more favored technology uses electrochemical cells, also known as fuel cells.  
           [0006]    The fuel cell effect was discovered in the early 1800&#39;s when a British scientist immersed two platinum electrodes in sulfuric acid electrolyte and supplied hydrogen at one electrode and oxygen at the other. The resulting reaction created a current flow between the electrodes. There was no practical application of fuel cells at that time because of high cost and technological problems. In the 1960s, researchers at the University of Vienna demonstrated a fuel cell that was specific for alcohol. This evolved into the present-day cell used in all fuel cell-based breath alcohol measurement instruments. In its simplest form, the alcohol fuel cell consists of a porous, chemically inert layer coated on both sides with finely divided platinum (called platinum black). The porous layer is impregnated with an acidic electrolyte solution, and platinum wire electrical connections are attached to the platinum black surfaces, and this assembly is mounted in a plastic case having a gas inlet that allows a breath sample to be introduced as shown in FIG. 1.  
           [0007]    The exact chemistry of the reaction that takes place in an alcohol fuel cell is open to some conjecture. Researchers assume that the reaction converts alcohol to acetic acid. In the process, this conversion produces two free electrons per molecule of alcohol. This reaction takes place on the upper surface of the fuel cell. H+ ions are freed in the process, and migrate to the lower surface of the cell, where they combine with atmospheric oxygen to form water, consuming one electron per H+ ion in the process. Thus, the upper surface has an excess of electrons, and the lower surface has a corresponding deficiency of electrons. When the two surfaces are connected electrically, a current flows through this external circuit to neutralize the charge. This current is a direct indication of the amount of alcohol consumed by the fuel cell. With appropriate signal processing, breath alcohol concentrations directly can be displayed. Commercial fuel cell instruments, introduced in the mid-1970s and initially suitable for non-evidential alcohol breath testing, were improved sufficiently by 1980 to be certified for evidential use by the US Department of Transportation, and by a number of state agencies and foreign governments. The fuel cell has established a reputation for specificity and linearity of response over the complete range of alcohol concentration expected in the breath. This range is from 5 to 900 ppm or its equivalent in other units of measurement.  
           [0008]    When a precise volume of breath sample is quickly introduced into a fuel cell, the output current from the cell rises from zero to a peak, and then ultimately decays back to zero. The rate at which this happens is highly dependent on the loading across the sensor terminals. FIG. 2 illustrates this effect, for loadings of 100 and 300 ohms, and for a shorted condition (0 ohms). Traditional fuel cell measurement instruments of the prior art have load resistors of several hundred to one thousand ohms, and the height of the voltage peak across the resistor is used as the measure of alcohol content of the sample. Although this technique produces good linearity, significant time elapses before an acceptable measurement can be obtained, and the measurement cycles are objectionably long because complete conversion of alcohol to electric current must occur prior to a new cycle, the current being limited by the load resistance of the measurement circuit.  
           [0009]    More recent instruments have utilized lower load resistance to shorten the time to reach the peak output and speed up the recovery time, and they integrate the output signal to obtain enhanced accuracy. However, the number of positive samples analyzed in rapid succession with these prior art instruments still had to be strictly limited. Successive readings might be in error as peak fuel cell output decreased because of the time required for the cell to complete the alcohol conversion reaction. This could conceivably give readings beyond the acceptable limits for evidential measurement. In a typical unit, ten successive measurements of 0.100 gm/dl gas at 3 minutes between readings might result in the tenth reading being 0.095 or 0.094. Accordingly, these instruments have unfortunately been limited to no more than five positive tests per hour for maintenance of evidential accuracy. Consequently, only one subject could be tested per hour with evidential accuracy in those jurisdictions requiring two tests per subject, and a third test if the first two differed by more than a given amount, and an additional a test reading on a standard to verify calibration of the instrument. In addition, once the fuel cell output of these instruments decreases due to repeated testing, an extended period of time (up to 16 to 24 hours) is required before there is full recovery to the initial output.  
           [0010]    Another problem with operating the fuel cell in the conventional mode with a load resistor is that although the output is very linear up to alcohol levels of about 0.150 gm/dl, the readings are increasingly low at higher levels. Thus the cell reads 2-3% low at 0.200 gm/dl, and 5-6% low at 0.300 gm/dl. This was of very little practical significance where legal maximum blood alcohol levels were fixed at 0.100 or 0.080, but it was a subject of criticism.  
           [0011]    In early 1986, in a further investigation of fuel cell output measurement, based on a supposition that the entire signal from the fuel cell, rather than just the peak value, might be useful. The integrated output might contain enough information so that when the signal was analyzed properly, the effects of memory and high alcohol level nonlinearity might be minimized. Table 1 shows the results of a study made early in 1994. The investigators used a compressed gas standard with 0.100 ethanol concentration. The investigators made tests 3 minutes apart, at an ambient temperature of 23° C. While the peak value varies by 13%, the calibrated fuel cell output integral remains constant.  
                                           TABLE 1                           Normalized Peak Value vs. Calibrated Integral                Peak Value   Integral                            0.100   0.1009           0.0971   0.1008           0.0950   0.1006           0.0934   0.1004           0.0921   0.1010           0.0909   0.1009           0.0899   0.1008           0.0889   0.1006           0.0881   0.1004           0.0873   1.0004                      
 
           [0012]    It is also known to operate the fuel cell with the output essentially short-circuited, which gives the fastest response as shown in FIG. 2. With this configuration, cell output peaks in 2-5 seconds and typically returns to zero by the time that a cell with a 300-ohm load is reaching its peak value. In this mode, the fall-off in peak values from test to test is much worse than in the mode with a resistor; however, by integrating the entire area under the curve, the slump in reading from test to test is virtually eliminated. Because the cell has already returned to zero output, it is ready for another test without an additional waiting for a cleanup period to complete the reaction. The readings also recover much more quickly after a series of tests. For practical purposes, the number of tests per hour is limited by the recycling time of the test instrument and test protocols rather than the performance of the fuel cell. Research also established that a cell used in this mode is capable of linear response out to 0.400 gm/dl with an error no greater than 2%. For example, the linearity of a cell that was linear up to 0.150 gm/dl in a conventional voltage mode is preserved out to 0.400 gm/dl or more. A prior art circuit that provides an analog output in response to a fuel cell being loaded selectively with 330 ohms of resistance or essentially a short circuit is shown in FIG. 3, the short circuit being applied in response to an external signal.  
           [0013]    Studies made by the Transportation Research Board concluded that blood alcohol concentrations (BAC) below 0.050% may impair driving-related skills. Further testing has shown that instruments using fuel cells showed greater accuracy at low BACs than the instruments using infrared techniques. In yet additional tests, investigators at the University of Tennessee at Memphis measured the response of fuel cell-based alcohol breath testing instruments to various substances including many that might be expected to be present in the breath of individuals being tested. In addition to separate responses to various non-alcoholic substances, ethanol, methanol, and isopropanol were separately introduced at alcohol concentrations of 0.1 gm/dl. The results indicate generally that the sensitivity to the non-alcoholic substances was from zero to about 2%; for ethanol the response was 100%; and for methanol and isoproponal the response was about 45%.  
           [0014]    Notwithstanding the above developments, the breath alcohol instruments and interlock devices of the prior art are not entirely satisfactory, typically exhibiting one or more of the following disadvantages:  
           [0015]    1. They are ineffective in distinguishing human breath samples, properly delivered, from non-human and/or improperly delivered samples; and  
           [0016]    2. They are unreliable in that they are adversely affected by variations in ambient pressure (altitude) and temperature, as well as by the temperature and/or humidity of breath samples.  
           [0017]    Although it is known to require a threshold dynamic pressure of the sample as well as to require a valid range of temperature and humidity of the sample, calibration of the instruments of the prior art to reject all non-human samples often results in unwanted rejection of proper human samples, and vice-versa. Also, even with breath samples properly accepted as human, there is still a likelihood of significant error in measured breath alcohol levels due variations in ambient conditions as well as the temperature and humidity of breath samples, even when the temperature and humidity are within prescribed limits and dynamic pressure maintained over a full required duration of time.  
           [0018]    Thus there is a need for a breath alcohol interlock device that avoids the disadvantages of the prior art.  
         SUMMARY  
         [0019]    The present invention meets this need by providing a breath alcohol interlock device that is particularly effective in distinguishing permitted from disallowed alcohol levels, having quick response characteristics, and is both easy to operate and inexpensive to provide. In one aspect of the invention, a breath measurement instrument includes means for receiving a breath sample; and means for validating the sample, including means for determining a sample temperature of the sample, means for determining a moisture content of the sample, and means for comparing the determined sample temperature and moisture content with a predetermined profile of valid temperatures and moisture contents, validation being blocked unless the determined temperature and moisture content is within the predetermined profile, wherein the predetermined profile includes a valid range of the determined sample temperature that is dependent on the determined sample moisture content. The means for receiving the breath sample can include a tubular conduit having a mouthpiece extremity, the conduit defining a sample passage. The means for determining the sample temperature can include a temperature sensor supported relative to the tubular conduit and projecting into the sample passage, a sample temperature circuit having a sample temperature output, and means for signifying at least an out-of-limit temperature, and an in-limit temperature of the breath sample. The means for determining humidity can include a humidity sensor supported relative to the tubular conduit and projecting into the sample passage, a humidity circuit having a sample humidity output, and means for signifying at least an out-of-limit humidity and an in-limit humidity, the validation being blocked unless the temperature and humidity outputs are both in-limit. The in-limit humidity can be a first in-limit humidity, the humidity output also being capable of signifying a second in-limit humidity, the in-limit temperature also being a first in-limit temperature, the temperature output also being capable of signifying a second in-limit temperature, at least one combination of in-limit temperature and in-limit humidity being outside of the predetermined profile, that combination blocking the validation. In this way the range of acceptable sample temperatures is advantageously adjusted according the moisture content of the sample for facilitating effective distinguishing of non-human and/or improperly delivered breath samples from valid ones.  
           [0020]    The invention also provides a breath alcohol instrument including the breath measurement instrument in combination with means for determining an alcohol content of the sample. The means for determining the alcohol content can include an alcohol-specific fuel cell, and further, a fuel cell circuit for producing a breath alcohol signal, and means for compensating the breath alcohol signal in response to variations in one or more variables of the set consisting of ambient temperature, ambient pressure, sample temperature, and sample humidity.  
           [0021]    The invention further provides a breath alcohol interlock device for preventing use of a machine by an intoxicated operator, the device including the breath alcohol instrument in combination with an interlock circuit for disabling the machine except upon validation of a breath sample having an alcohol content below a predetermined amount.  
           [0022]    In another aspect of the invention, a method for screening breath samples and determining an alcohol content thereof includes receiving a breath sample; validating the sample by determining a sample temperature of the sample, determining a moisture content of the sample, comparing the determined sample temperature and moisture content with a predetermined profile of valid temperatures and moisture contents, and blocking the validation unless the determined temperature and moisture content is within the predetermined profile, wherein the predetermined profile includes a valid range of the determined sample temperature that is dependent on the determined sample moisture content; determining an alcohol content of the sample by producing a breath alcohol signal responsive to the alcohol content of the sample; and compensating the breath alcohol signal in response to variations in one or more variables of the set consisting of ambient temperature, ambient pressure, sample temperature, and sample humidity. 
       
    
    
     DRAWINGS  
       [0023]    These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings, where:  
         [0024]    [0024]FIG. 1 is a sectional diagram view of a prior art alcohol-specific fuel cell sensor;  
         [0025]    [0025]FIG. 2 is a dynamic electrical response profile of the fuel cell sensor of FIG. 1 for various loadings thereof;  
         [0026]    [0026]FIG. 3 is a schematic diagram of a prior art circuit for interfacing the fuel cell sensor of FIG. 1;  
         [0027]    [0027]FIG. 4 is a partly pictorial, partly schematic overall system block diagram of a breath alcohol interlock system according to the present invention, the system being installed in equipment having a power plant (such as a vehicle) being protected thereby;  
         [0028]    [0028]FIG. 5 is a schematic diagram of a breath pressure circuit portion the system of FIG. 4;  
         [0029]    [0029]FIG. 6 is a schematic diagram of a breath temperature circuit portion of the system of FIG. 4;  
         [0030]    [0030]FIG. 7 is a simplified schematic diagram of a breath humidity circuit portion the system of FIG. 2;  
         [0031]    [0031]FIG. 8 is a schematic diagram of a breath sample pump circuit portion the system of FIG. 4;  
         [0032]    [0032]FIG. 9 is a schematic diagram of an ambient temperature circuit of the system of FIG. 4;  
         [0033]    [0033]FIG. 10 is a simplified block diagram of a control module of the system of FIG. 4;  
         [0034]    [0034]FIG. 11 is a schematic diagram of a power section circuit of the system of FIG. 4;  
         [0035]    [0035]FIG. 12 is a timing diagram of a breath alcohol measuring cycle of the system of FIG. 4; and  
         [0036]    [0036]FIG. 13 is a schematic diagram showing an alternative configuration of an engine run circuit portion of the power section circuit of FIG. 9. 
     
    
     DESCRIPTION  
       [0037]    The present invention is directed to a breath alcohol interlock system that is particularly effective in protecting machinery such as vehicles from unauthorized operation by persons having excessively high alcohol intake. With reference to FIGS.  3 - 12  of the drawings, an interlock system  10  for a vehicle  12  includes a base unit  14  that is connected for conditionally inhibiting operation of the vehicle, and a sample unit  16  that is connected to the base unit and being adapted for receiving through a mouthpiece  18  breath samples from a person intending to drive and/or actually driving the vehicle, as shown in FIG. 4.  
         [0038]    The vehicle  12  is shown as having an exemplary power plant  20  including an engine  22  that drives an alternator  24  for charging a vehicle battery  26  through a regulator  28 . The engine  22  also has an ignition  30 , and a starter motor  32  having a main terminal  34  and an associated starter relay  36 . Conventionally, an ignition switch  38  which is powered by the battery  26  has a momentary start terminal  40  for activating the starter relay  36  via a start circuit  41 , and a run terminal  42  for powering the ignition  30  through a run circuit  42 . Installation of the interlock system  10  typically (but not necessarily) involves breaking the start and circuit  41  to prevent operation of the starter motor  32 . It will be understood that other means for preventing (or restricting) operation of the engine  22  are contemplated, such as by breaking the run circuit  42 , inhibiting operation of a fuel system (not shown) of the vehicle  12 , or any suitable means by which other equipment to be protected by the present invention may be inhibited. Normally, however, preventing continued operation of the engine  22  is sometimes to be avoided, such as when forced termination of such operation has hazard potential. In such situations it is contemplated that the engine  22  (or other motive means) would be disabled only after it was first parked or otherwise stopped subsequent to a failed breath alcohol test.  
         [0039]    The base unit  14 , which is connected to the sample unit  16  by a signal cable  44 , has a housing  45  and circuit interrupting elements therein in the form of first second, and third relays  46 ,  47 , and  48 , the first relay  41  (which itself may be located external to the base unit  12  as shown in FIG. 4) being interposed in the start circuit  41  for selectively disabling the starter motor  32  and its starter relay  36  as further described below. The second relay  47  controls connection of a battery charging circuit to a rechargeable battery as described below. The third relay  48  is optional, being connected for activating a horn  49  of the vehicle  12  under certain conditions (without preventing other and conventional activations of the horn), also as further described blow. In the exemplary configuration of the system  10  shown in the drawings, the relays  46 ,  48 , and  50  are connected in a power circuit  50  of a power module  51 , the electrical connections between the interlock system  10  and the vehicle  12  being through an interface connector J 5 . The signal cable  44  has a cable plug  52  being connected to a socket J 3  of the base unit  14 , and a cable socket  53  being connected to a plug J 1  of the sample unit  16 , the power circuit  50  being further described below in connection with FIG. 11.  
         [0040]    The sample unit  16  has a sample tube  54  forming an inlet  55  of a sample chamber  56 , the inlet  55  being in the form of a resilient grommet  57  that is adapted for sealingly extending the inlet through one wall of a case  58  of the sample unit  16 , receiving a projecting outlet  59  of the mouthpiece  18 . According to the present invention, the mouthpiece  18  also has a labyrinth chamber  60  for trapping entrained moisture of a breath sample that is introduced through a projecting inlet  62  of the mouthpiece  18 . A breath pressure transducer  64  is supported in the case  58  and having a small strain-gage actuator  66  projecting into the chamber  56  a short distance from the inlet  55 . A breath temperature sensor  68  also projects into the chamber  56  a short distance from the inlet  55 , preferably down stream of the breath actuator  66 . Further, a humidity sensor  70  also projects through the sample tube  54  and into the sample chamber  56 , the sample tube also having a sample port  72  forming a side exit from the chamber  56  down stream of each of the above-described sensing elements. Downstream of the sample port  72 , the sample tube  54  projects through a heat shroud  74  having a tubular heating coil  75 , and through an opposite wall of the case  58  to a chamber exhaust outlet  76 . The sample tube  54  can be formed by modifying a commercial breath sample tube that is available as Part No. 95-00130, from Alcohol Countermeasure Systems Corp., of Ontario, Canada. The modifications to the sample tube  54  consist of forming openings for the humidity sensor  70 , and for any of the breath actuator  66 , the breath temperature sensor  68 , the humidity sensor  70 , and the sample port  72 , and installing the heat shroud  74  and its associated heating coil  75 .  
         [0041]    The sample port  72  is fluid-connected through a flexible inlet conduit  78  to the inlet  79  of an alcohol-specific fuel cell alcohol sensor  80 , another flexible outlet conduit  82  being fluid-connected from an outlet  83  of the alcohol sensor to the inlet  84  of a piston pump  86  having a solenoid coil  87 . Activation of the solenoid coil  87  moves an internal piston (not shown) of the pump  86  away from the pump inlet  84  so as to draw a gas sample from the sample chamber  56 , through the inlet conduit  78  and into the alcohol sensor  80 , a portion of the gas being drawn into the pump  86 . The piston is biased toward the pump inlet so as to return the piston to a rest position when the solenoid coil is deactivated, the pump being equipped with a suitable one-way valve (not shown) and exhaust port (represented at  88  in FIG. 4) for permitting return of the piston to its rest position without requiring the sampled gas to be expelled through the sample port  72  back into the sample chamber  56 .  
         [0042]    The sample unit  16  also includes an electronic interface module  90  having the plug J 4  mounted thereto for receiving the cable socket  53  of the signal cable  44 , the interface module  90  having circuits described herein for interfacing the various breath sensor elements described above, and preferably one or more ambient sensors. As shown In FIG. 4, the interface module  90  includes an ambient temperature circuit  93  (further described below in connection with FIG. 9) for interfacing an ambient temperature sensor  94 . There is also a heater driver  95  (which can be simply a power FET driver transistor and input biasing resistor) for activating the heating coil  75 , and a pump driver  96  (further described below in connection with FIG. 8) for powering the solenoid coil  87 . The sample unit  16  also includes a keypad  97  for receiving operator input, and an LCD display  98  for indicating various responses and operating states of the interlock system. The display  98  is connected by a separate plug (DO, not shown) through the cable  44  to another separate socket (J 2 , not shown) of the base unit  14 . If desired, the display  98  can also be located separately or remotely from the sample unit  16 .  
         [0043]    More particularly, the breath pressure transducer  64  is connected in a breath pressure circuit  100  as shown in FIG. 5, the transducer  64  including a strain-gauge bridge  102  that feeds a first stage amplifier  104  having a gain of approximately 1000. The first stage amplifier feeds a second stage amplifier  106  of approximately unity gain to produce a BPRESSV output, the amplifier  106  having a baseline adjustment  107 . It will be appreciated that the exact gain of the pressure circuit  100  is not critical, in that the primary purpose of the pressure transducer  64  is to detect a threshold dynamic blowing pressure of a breath sample and maintenance of at least that pressure during a sampling interval of the apparatus  10 . Thus it is important that the gain be sufficiently high for adequate sensitivity, and that the dynamic range be sufficiently broad to encompass that threshold. Further, for reasons developed below, it is also important that the gain be reasonably linear and that the dynamic range extend above the threshold to encompass any reasonably expected maximum dynamic pressure to be encountered during the sample interval and, preferably, to allow for programmable adjustment of the threshold such as for accommodating users with medical conditions (asthma, for example) that would affect normal breathing. A device suitable for the breath pressure transducer  64  is commercially available as Model FPM-02PG also available from Alcohol Countermeasure Systems Corp. An integrated circuit device suitable for use as active portions of the first and second amplifiers  104  and  106  is available as an LM2902 Quad Operational Amplifier from National Semiconductor of Santa Clara, Calif., and a variety of other sources (half of the device being used in the pressure circuit  100 ).  
         [0044]    The breath temperature sensor  68  is connected in a breath temperature circuit  110  as shown in FIG. 6, variably shunting the feedback path of a third amplifier  112  having an offset adjustment  113 . A device particularly suitable for use as the sensor  68  is a negative temperature coefficient (NTC) chip thermistor, which has a high sensitivity response to temperature, one preferred device of this type being available as Part No. 56A1002-C1 from Alpha Sensors, Inc. of San Diego, Calif. The output of the third amplifier  112  is connected to a voltage amplifier  114  that feeds an analog output buffer  115  (having a BTEMPV output) and a pair of comparators  116  that are connected for activating an output transistor  118  to illuminate a light-emitting diode (LED)  119  when the output of the voltage amplifier  114  (and the BTEMPV output) is between approximately 2 volts and approximately 3 volts, those values corresponding to a relatively narrow range of temperatures that are associated with human breath samples. The third amplifier  112  and the voltage amplifier  114  can be implemented using elements of a quad integrated circuit amplifier such as the LM2904 device described above. The comparators  116  can be elements of a quad integrated circuit comparator such as the LM139 device that is also available from a variety of commercial sources. The output of the analog buffer  115  is used in combination with the humidity sensor  70  for enhanced verification (screening) of human breath samples, and for enhanced accuracy in measuring breath alcohol (BAC) levels as described below.  
         [0045]    The humidity sensor  70  is connected in a breath humidity circuit  120  as shown in FIG. 7. An inexpensive sensor suitable for use as the humidity sensor  70  has been available as Model PCRC-55, from Phys-Chemical Research Corp. (PCRC) of New York, N.Y.; a similar device, No. HC-800, is available from Ohmic Instruments, Co. of Easton, Md. The humidity sensing portion of this sensor, an outer surface layer of a styrene copolymer, advantageously produces a very quick response time on the order of a few seconds. It does, however, have a positive temperature coefficient of 0.36 RH unit/degree Celsius. Accordingly, the humidity circuit  120  includes a temperature-compensated square-wave oscillator  122  that excites the sensor  70  with a current that is feed into a logarithmic amplifier  124 , the amplifier compensating significant non-linear behavior of the sensor  70 . The output of the logarithmic amplifier  124  feeds a conditioning amplifier  126  which drives a breakpoint amplifier  127  and an operational amplifier  128 . The output of the breakpoint amplifier  127  is fed into the feedback path of the operational amplifier  128  to provide further non-linear compensation of the sensor  70  below 40% relative humidity (RH). The operational amplifier  128  feeds a scaling buffer amplifier  129  to provide a 5-volt full scale BHUMV output, the operational amplifier  128  having a full scale output of 10 volts for facilitating calibration of the non-linear compensation for the sensor  70 .  
         [0046]    The oscillator  122  produces a square wave of fixed frequency and amplitude and having no DC component, being symmetrical about ground potential for prevention of detrimental electrochemical migration. Further details and appropriate calibration and adjustment of the humidity circuit  120  are described in Application Note 256, entitled “Circuitry for Inexpensive Relative Humidity Measurement” (National Semiconductor, 1981).  
         [0047]    The alcohol sensor  80  is connected in a breath alcohol circuit  130 . As shown In FIG. 3, the sensor is loaded with 330 ohms by R24, and selectively subjected to a virtual short circuit in response to an external signal FCSHRT by a FET transistor Q 5 . An operational amplifier  132  provides voltage gain, the output of the amplifier  132  feeding an integrating voltage follower  134  having a BACV output that has time constant of approximately 10 mS. The voltage follower  134  produces approximately 0.8V when the BAC is 0.05% and approximately 1.6V when the BAC is 0.1%.  
         [0048]    The solenoid coil  87  of the pump  86  is driven by a pump driver circuit  96  as indicated above and shown in FIG. 8, the circuit  96  having a voltage multiplying DC to DC converter  136  for generating approximately 24V from the vehicle battery  26  or, as shown in FIG. 8, a rechargeable battery  137  (which was introduced above and is shown in FIG. 4 as external to the both the base unit  14  and the sensor unit  16 , it being understood that the location of the rechargeable battery is no critical). The converter  136  feeds a power inverter  138  that selectively applies the 24V to the coil  87  in response to an external PMP signal.  
         [0049]    The ambient temperature circuit  93 , shown in FIG. 9, includes a resistive bridge circuit  140  having the ambient temperature sensor  94  in one leg thereof, respective output nodes of the network  140  being resistively coupled to complementary inputs of a differential operational amplifier  142  having an ATEMPV output. Devices suitable for use as the ambient temperature sensor  94  are available as TD2A (and TD5A) negative coefficient thermistor from Microswitch Corp. of Morristown, N.J. With the gain of the operational amplifier set at  10  (R8/R6+1), and using resistors having 1% tolerance, the output of the amplifier  142  at room temperature (20° C. or 68° F.) is approximately 2.7V, the sensor having a room-temperature resistance of approximately 2K ohms. The sensor  94  advantageously has a slow response time on the order of one minute so as to be insensitive to momentary power fluctuations within the sample unit  16 . Also, as further described below, the sensor  94  is located close to both the sample tube  54  (and the sample chamber  56 ) and the alcohol sensor  80 , for measurement of a “locally ambient” temperature which can be significantly higher than the temperature outside the sensor unit  16  at relatively cold temperatures calling for heating of the sample tube  54  as well as occasions of heating by the heating coil  75  for accelerated purging of the alcohol sensor  80 . At higher outside ambient temperatures relevant to adjustment of qualifying breath temperatures and moisture contents, the measured locally ambient temperature more closely matches the outside ambient temperature.  
         [0050]    In addition, the sample module  16  also has a power switch  146  that can be mechanically or magnetically coupled to a moveable cover or lid  148  that is displaced to uncover the keypad  97  and the LCD display  98  and produce an OPN signal when the interlock system  10  is to be actuated.  
         [0051]    The respective outputs of the ambient temperature circuit  93 , the breath pressure circuit  100 , the breath temperature circuit  110 , the breath humidity circuit  120 , and the breath alcohol circuit  130 , as well as inputs of the heater driver  95  and the pump driver circuit, are connected through the signal cable  44  to a control module  150  that is located in the base unit  14 , as further described herein in connection with FIG. 10. The control module  150  is situated in the base unit  14  together with the power board  51  and is connected thereto by a ribbon cable (not shown) between a connector J 1  of the control board  140  and a connector J 3  of the power board  51 . The connector SO to which the signal cable  44  is connected is located on the power board  51 , with connections to the control board  150  being completed via J 1  and J 3 . Similarly, the connector J 2  for signals to the LCD display  98  is located on the power board, the connections thereto being made through the connectors J 1  and J 3  and the interconnecting ribbon cable. It will be understood that the display decoder/driver  156  can be located on the power board  51  in order to conserve conductors of the connectors J 1  and J 3 , as well as of the ribbon cable. A control circuit  151  of the control board includes a microprocessor  152  having associated memory  154  (which can include EPROM, EEPROM, and SRAM), a display decoder/driver  156 , a keyboard interface  158 , and a buffer  160  (which can include several “type D” flip-flops, as they are commonly known, and associated logic.  
         [0052]    The microprocessor  152  is preferably of the type having on-board analog to digital (ADC) conversion of multiple signals as indicated at  153  in FIG. 10, such a device being commercially available as Model 68HCl  1  control processor, from Motorola Corp. of Phoenix, Ariz. As shown in FIG. 10, each of the analog outputs ATEMPV, BPRESSV, BTEMPV, BHUMV, and BACV from the above described circuits of the sample unit  16  are fed into the ADC  153  of the microprocessor  152 . In addition, the ADC receives an optional altitude pressure signal APRESSV from the power board  51 , as well as attenuated battery voltages VBATV and RBATV of the vehicle battery  26  and the rechargeable battery  137 , also from the power board  51 . As shown in FIG. 10, an attenuator  162  is interposed between the RBATV signal and the ADC  153  to scale the ADC input to within its 5V range; the attenuator can include a voltage divider that preferably feeds a voltage follower in a conventional manner. The OPN signal from the power switch  146 , and an engine run signal ERUN (described below) from the power board  51  are connected both to the microprocessor  52  and the buffer  160 , the buffer also being connected to the microprocessor and having a PWR output that is activated upon opening of the lid  148  to expose the keypad  97  and the LCD display  98 , the PWR output remaining active until terminated by the microprocessor  152 . The buffer  160  is also responsive to the microprocessor  152  for producing a CHG signal to activate charging of the rechargeable battery as described below. Other outputs of the microprocessor  152  include an ALWIGN. signal for enabling activation of the first relay  46  (see FIG. 4), a HEAT signal for activating the heating coil  75  in the sample unit  16  as further described below, and a BEEP signal for activating a suitable beeper  163  (which can be located externally of the base unit  14  as shown in FIG. 4). The beeper  163 , which can include a piezoelectric transducer, is activated with a series pulses for prompting operator response, such as at periodic intervals during operation of the vehicle when it may be requited to conduct a “rolling” breath test to verify that the driver of the vehicle  12  has not become intoxicated subsequent to starting the vehicle.  
         [0053]    As shown in FIG. 11, the power circuit  50  includes a vehicle battery attenuator  164  connected to the vehicle battery  26  (VBATT) for producing the VBATV signal within the 5V range of the ADC  153 . The vehicle battery (VBATT) feeds a charger circuit  165  having a conventional configuration, the exemplary form thereof shown in FIG. 11 having a Unitrode (Texas Instrument) UC3906N battery charger device which is representative of such circuits that are commercially available from a variety of sources. The charger circuit  165  is connected to the rechargeable battery  137  through contacts of the second relay  47 , the battery  137  powering a first regulator U 29  that always provides 5V (+5A), a second regulator U 26  that provides the HTR signal to the heating coil  75  in response to the PWR and HEAT signals from the control module  150 , and a third regulator U 30  that is activated by the PWR signal. A fourth regulator U 28  that is powered at 20V by a converter circuit  166  and filter  167  provides 10V for use as described below, the fourth regulator U 28  also feeding a voltage follower U 27  through a divider  168  to provide a 5V reference (A/D+5R) to the ADC  153 . An inverting buffer U 23  isolates the CHG, PWR, and HEAT signals from the control module  150 , as well as activating the first relay  46  in response to the ALWIGN signal, and activating a BEEPR output to the beeper  163  in response to the BEEP signal, the ALWIGN and BEEP signals also being from the control module  150 .  
         [0054]    The power board  51  also includes an engine run circuit  170  that is responsive to the voltage (IGNR) at the run terminal  42  of the ignition switch (see FIG. 4). The engine run circuit  170  has a voltage divieing filter  172  that feeds a comparator amplifier U 22  having an ERUN output. The ERUN output is activated for as long as the voltage IGNR is sufficient, allowing for negative spikes such as are blocked by the filter  172 , to indicate that the ignition switch  38  remains in an on condition.  
         [0055]    Optionally, the power board  51  additionally includes a pressure altitude circuit  180  having an ambient pressure transducer  182 , the outputs of which feed a differential amplifier  184  for producing the ambient pressure signal APRESSV for receipt by the ADC  153 . The APRESSV signal, when implemented, is used by software of the microporcessor  152  to compensate the BACV signal from the breath alcohol circuit  130 .  
         [0056]    Operation of the interlock system  10  in a breath alcohol measurement cycle is best understood with particular reference to FIG. 12, which is partly a timing diagram and partly a flow chart. With the lid  148  opened so as to activate the PWR signal as described above, a interlock cycle  200  is initiated by depression of a key of the keypad  97 , the LCD display  98  being written in response as indicated at  202  under control of the microprocessor  152  for prompting the user to enter a code of 4 digits during a code entry interval  204  such as 30 seconds in which a cursor of the display is blinking. Failing correct entry of the code in that interval, the system returns to a “sleep” mode  206  in which the PWR signal is inactive. Upon correct code entry the microprocessor causes an “active” prompt to be displayed as indicated at  208 , and the alcohol sensor  80  is purged of alcohol for an interval such as 2 seconds as indicated at  210  by activation of the FCSHORT input of the breath alcohol circuit  130 . If the BAC remains above 0.1% a heating interval  212  is initiated for approximately 5 seconds and the purge cycle  210  is repeated; if the BAC is less than 0.1% but above a predetermined baseline value, the only purge cycle  210  is repeated, until the BAC is less than the baseline value, signifying that the alcohol sensor  80  is sufficiently purged, ambient measurements are taken of temperature, humidity and, optionally, pressure, as indicated at  214 . Once the ambient measurements are taken, a breath test interval  216  is initiated in which the user is prompted to produce a breath sample by blowing into the mouthpiece  18 , ant a wait interval  218  is simultaneously activated for the user to produce a threshold breath pressure (nominally 0.5 PSI), whereupon the active prompt  208  is terminated and a collection delay  220  of approximately 1.4 second and a sample interval  222  of approximately 2 seconds (during which a breath sample is obtained as indicated at  224 ) are simultaneously initiated. An analysis interval  226  having a maximum duration of approximately 10 seconds is initiated at the conclusion of the collection delay  220 , in which the pump solenoid coil  87  is activated as indicated at  227  and the BACV signal from the breath alcohol circuit  130  is tracked for peak detection as indicated at  228 , at which point the BAC is computed as indicated at  230 , provided that, following completion of the sample interval  222 , the sample is verified as being human as indicated at  232 , the breath temperature and humidity both being within a predetermined profile that has been dynamically adjusted to compensate for variations in one or more of the ambient measurements obtained in the ambient measurement interval  214 , as indicated at  234 . The Solenoid coil  87  is deactivated upon completion of the BAC computation (and the alcohol sensor  80  as purged as previously at  210  for prompt readiness for a subsequent measurement cycle, if required.  
         [0057]    If the sample is determined to be non-human as indicated at  236 , if the breath test interval  216  times out before the sample interval  222 , or if the analysis interval  226  times out before completion of the BAC computation, an error condition  238  is reached, at which point the cycle must be repeated before operation of the vehicle  12  is enabled. Otherwise, if the measured (and compensated) BAC is within a predetermined limit, such as 0.08%, as indicated at  240 , operation of the vehicle  12  is enabled as indicated at  242 , by activating the first relay  46  as described above. However, if the BAC exceeds the predetermined limit as indicated at  244 , operation (starting) of the vehicle is not enabled as indicated at  246  and, preferably, the user is prompted (not shown) that the vehicle will be disabled once it is stopped. The microprocessor  152  is preferably also programmed for requiring initiating “rolling tests” at predetermined intervals during operation of the vehicle  12 , using means that are within the ordinary skill of those in the art.  
         [0058]    With further reference to FIG. 13, a preferred alternative configuration of th4e engine run circuit, designated  170 ′, includes an active filter circuit  190  that incorporates a monolithic active filter building block MF10, which is available from National Semiconductor and is more fully described in Application Note 307, dated 1995 and available form the same source. The active filter circuit  190  receives the IGNR signal from the ignition switch  38  as described above, and is clocked at 100 MHz as shown in FIG. 18 in any suitable manner, to produce a low-pass output  192 . The circuit  190 , requiring only a single supply voltage, provides a 1 KHz fourth-order Butterworth filter that detects the presence of alternator noise at the ignition switch  38 , the alternator noise having a rectified AC profile of approximately 0.3V amplitude at frequencies ranging upwardly from approximately 400 Hz. The supply voltage of 10V, indicated as being supplied by U 2  in FIG. 13, can be provided by the fourth regulator U 28  of FIG. 11. The low pass output  192  feeds a frequency to voltage converter U 3 , such as an LM 2917N which is available from National Semiconductor, the converter U 3  being preferably configured for saturation at a relatively low speed of the vehicle engine  22 , thereby to deliver the ERUN output as essentially a digital signal that indicative of the engine running, as opposed to being merely indicative of the ignition switch  38  being on as in the engine run circuit  170  of FIG. 11.  
         [0059]    Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, the power interlock circuit can be connected in any suitable way to a vital subsystem of the vehicle  12  or other equipment being protected by the system  10 , in order to inhibit operation of the equipment. Therefore, the spirit and scope of the appended claims should not necessarily be limited to the description of the preferred versions contained herein.