Gas alarm

A gas detector is disclosed which includes a CO sensor and a processor. The CO sensor senses an environmental CO concentration and provides a variable electrical output in response. The processor receives digitized samples of the sensor's electrical output and computes, with each measurement sample, a COHb level of a hypothetical person exposed to the CO concentration sensed. The computation involves determining an equilibrium COHb level for each measurement sample, which is compared with a previously computed COHb level associated with previously measured samples. The previously computed COHb level is updated based on the comparison. When the computed COHb level is above a predetermined danger threshold, the processor preferably causes an audible and visual alarm to be activated to alert any persons present. The gas detector can further include a CH.sub.4 sensor coupled to the processor for independently sensing CH.sub.4 levels such that an alarm is activated when the CH.sub.4 levels are above a predefined threshold. A hydrophobic zeolite filter is disposed on the sensor elements of one or both of the CO and CH.sub.4 sensors to prevent erroneous readings.

FIELD OF THE INVENTION 
This invention relates generally to carbon monoxide (CO) and methane (CH4) 
gas detectors and, more particularly, to a CH.sub.4 /CO gas detector and 
alarm suitable for residential use. 
BACKGROUND OF THE INVENTION 
Natural gases and liquefied petroleum gases are widely used as fuels for 
domestic, commercial and industrial purposes, e.g., for heating and/or 
cooking. As a result, there is often a danger that leakage of such gas 
from piping and other apparatus will contaminate the surrounding 
environment, creating a dangerous condition. For example, alkane gases 
such as methane are extremely combustible and furthermore could poison 
individuals if present at too great a level in enclosed surroundings. 
Moreover, small quantities of carbon monoxide can escape from such flowing 
gas streams into the environment, or could be generated by incomplete 
combustion of natural gas. Carbon monoxide is odorless and colorless, so 
that the contaminating levels are not readily observable by individuals. 
However, carbon monoxide is absorbed by an individual's lungs and reacts 
with the hemoglobin in the blood to form carboxyhemoglobin (COHb), 
reducing oxygen carrying capacity of the blood. Therefore, presence of 
carbon monoxide in an environment above certain levels is extremely 
dangerous and can easily poison individuals unaware of its presence. 
Accordingly, there is a need to provide for suitable detection and 
concomitant alarm of unwanted fluid, i.e., gaseous contaminants in order 
to prevent unsafe or dangerous conditions from developing, e.g., in an 
enclosed environment. In this regard, various detectors for carbon 
monoxide and alkanes such as methane gas have been developed. 
Various residential carbon monoxide gas detectors are found in the prior 
art and typically function to sound an audible alarm when a specified 
environmental CO concentration is detected. Carbon monoxide sensing 
elements employed within such detectors typically comprise a metal oxide 
layer on a ceramic substrate. When the ceramic substrate is heated to a 
high temperature, e.g., 200.degree.-400.degree. C., the resistance of the 
metal oxide varies as a function of the environmental CO concentration. 
The sensing element can thus be employed as a variable resistor in a 
calibrated detection/alarm circuit which continuously monitors the 
environmental CO concentration and sounds an alarm upon the detection of 
hazardous levels. 
Methane detectors which employ metal oxide semiconductor sensors are also 
in the prior art. As with CO sensors, many methane sensors also operate by 
maintaining the ceramic substrate at a high temperature such that the 
resistance of the metal oxide predictably changes as a function of ambient 
CH.sub.4 concentration. Methane sensors are available which are relatively 
insensitive to the presence of carbon monoxide, and vice versa. 
False alarms and/or gas detection inaccuracies due to the presence of 
airborne contaminants are often a problem with CO and CH.sub.4 detectors. 
The sensing elements are typically sensitive to the presence of airborne 
gases and vapors such as alcohols, solvents and water vapor. In the common 
household environment, the use of cleaning agents, paints, turpentine, 
solvents, etc., produce vapors which can alter the resistance of the 
sensing element, thereby causing false alarms in the detection of the 
target gas. 
When carbon monoxide enters a person's bloodstream, it reacts with 
hemoglobin (Hb) to form carboxyhemoglobin (COHb). A person is in danger of 
carbon monoxide poisoning when the person's COHb level exceeds a specific 
level, such as the levels promulgated by Underwriters Laboratories UL2034 
carbon monoxide exposure specification. An individual's COHb level is a 
function of their exposure time to the environmental CO concentrations. 
False alarms are particularly a problem in prior art CO sensors because 
these sensors do not accurately model the COHb level that would be present 
in the exposed person's bloodstream. For example, a relatively high 
environmental CO concentration that is present for only a very short time 
duration often causes an alarm to be sounded in prior art CO sensors when 
such exposure would actually produce a COHb level well below the danger 
level. 
U.S. Pat. No. 4,896,143 to Dolnick et al. discloses a CO sensor which is 
operatively connected to a microprocessor for calculating doses of carbon 
monoxide. The values of CO are accumulated over time, with the dose 
measurement being made by adding a measured value of CO concentration, 
which may be weighted, to a memory register each time the value is 
determined. An alarm is issued if the accumulated value reaches a 
predetermined value. A drawback of this technique is that the 
physiological response is not accurately modeled, which may lead to false 
alarms and/or under-detection of CO. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to improve 
reliability in detection of combustible, toxic or noxious fluids such as 
carbon monoxide and methane. 
It is a more particular object of the present invention to more accurately 
correlate sensing of toxic carbon monoxide gas in a closed environment 
with equilibrium levels existing in an individual's blood stream and 
adjusting alarm signals accordingly. 
It is also an object of the present invention to improve selectivity for 
sensing certain gaseous contaminants and vapors as opposed to other 
gaseous contaminants and vapors. 
It is a further object of the present invention to improve detection of 
dangerous threshold levels of fluid, i.e., gaseous concentration and at 
the same time minimize or eliminate unwanted generation of false alarms. 
These and other objects are attained by the present invention which is 
directed to a gas detector comprising a carbon monoxide (CO) sensor for 
measuring an environmental CO concentration and providing a variable 
electrical output responsive thereto, plus a processor responsive to 
samples of the electrical output for computing a carboxyhemoglobin (COHb) 
level of an individual as if exposed to the measured CO concentration and 
providing an output signal based upon the COHb level computed. The present 
invention is also directed to a gas detector comprising a filter of 
hydrophobic zeolite disposed upon a transducer composed, e.g., of metal 
oxide such as stannous or stannic oxide. The hydrophobic zeolite acts to 
filter out interfering fluid, i.e., gaseous particles such as isopropanol, 
ethanol and other high molecular weight vapors which might set off false 
alarm readings if detected by the metal oxide transducer which can ideally 
detect presence of methane gas. In a preferred embodiment, sensors for 
methane and carbon monoxide are incorporated into a single device which 
can be positioned within an enclosure to accurately sense for presence of 
both these gases. 
In an illustrative embodiment of the present invention, a gas detector is 
disclosed which includes a CO sensor and a processor. The CO sensor senses 
an environmental CO concentration and provides a variable electrical 
output in response. The processor receives digitized samples of the 
sensor's electrical output and computes, with each measurement sample, a 
COHb level of a hypothetical person exposed to the CO concentration 
sensed. The computation involves determining an equilibrium COHb level for 
each measurement sample, which is compared with a previously computed COHb 
level associated with previous measurement samples. The previously 
computed COHb level is updated based on the comparison. When the computed 
COHb level is above a predetermined danger threshold, the processor causes 
an audible alarm and visual indicator to be activated to alert any persons 
present. 
The gas detector can further include a CH.sub.4 sensor coupled to the 
processor for sensing CH.sub.4 levels. The CH.sub.4 sensor is insensitive 
to the presence of CO, and vice versa. An alarm is activated when the 
CH.sub.4 levels detected are above a danger threshold. A hydrophobic 
zeolite filter is disposed on the CH.sub.4 sensor to filter atmospheric 
gases and/or vapors such as alcohols and solvents from the sensing element 
and thereby prevent erroneous sensor readings. A similar zeolite filter 
can also be placed on the sensing element within the CO sensor to improve 
performance. When utilized in accordance with the present invention, the 
hydrophobic zeolite filter allows water vapor to pass therethrough. The 
adsorption sites on the filter are thereby not taken up with water 
molecules, so that the adsorption sites on the zeolite filter are 
available to adsorb the atmospheric gases/vapors described supra. 
Furthermore, even if water vapor has been adsorbed to a certain extent 
upon the hydrophobic zeolite filter, nevertheless contact with these other 
atmospheric constituents will displace the water molecules so that the 
filter is always available to adsorb such hydrocarbons over water.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, an illustrative embodiment of a carbon 
monoxide/methane alarm in accordance with the present invention is shown, 
designated generally as 10. Alarm 10 is particularly adapted for 
residential use and is preferably packaged for simple mounting on a wall 
or ceiling. 
CO sensor 14 and CH.sub.4 sensor 12 independently measure environmental CO 
and CH.sub.4 concentrations respectively, in the vicinity of the alarm. 
The sensors each provide an analog voltage output indicative of the 
respective gas concentration measured. Each sensor 12 and 14 includes a 
sensing element that changes its electrical characteristics in response to 
the associated target gas (CO or CH.sub.4) concentration, and electronic 
circuitry operatively coupled to the sensing element to provide the analog 
voltage output as a function of the target gas concentration. Preferably, 
the sensing elements are of the metal oxide/semiconductor type, e.g., tin 
oxide sensing element on a ceramic substrate. Such sensing elements are 
known in the art and are commercially available from various 
manufacturers. These sensing elements are operative to change their 
resistances in dependence on the incident CO or CH.sub.4 gas. The CH.sub.4 
sensor element of sensor 12 is designed to be relatively insensitive to 
the presence of CO, and the CO sensing element of sensor 14 is relatively 
insensitive to the presence of CH.sub.4. As such, each sensor output is 
substantially indicative of only the associated target gas CH.sub.4 or CO 
it is designed to detect. 
For proper operation of sensors 12 and 14, heaters 22 and 24 are 
respectively employed to heat the ceramic substrates of the sensing 
elements therein to a high temperature. Typically, this temperature is in 
the range of 300.degree.-500.degree. C. and is maintained during operation 
of the alarm. The sensing elements respond properly to their target gases 
only at specific temperature peculiar to the sensing element employed. 
Sensor element manufacturers typically specify recommended operating 
temperatures. The heaters 22 and 24 may include electronics to sense 
faults in the heaters, such as open circuited conditions. Logic level 
outputs indicative of heater faults are provided to processor 20, which 
responds by activating fault alarm/indicator 124 when faults are detected. 
Electronic circuitry (not shown) within gas sensors 12 and 14 provide the 
analogy output voltages as a function of the target gas concentration. 
Recommended electronics are typically specified by sensor element 
manufacturers and thus, are well recognized and available within the 
electronic circuitry art. Preferably, the sensor electronics include fault 
detection circuitry which provides logic level outputs on respective lines 
13 and 15 indicative of sensor fault conditions such as an open circuited 
sensor. Processor 20 is responsive to these outputs and activates fault 
alarm/indicator 124 when appropriate. 
A hydrophobic zeolite molecular sieve filter 16, 16' covers the sensing 
elements of each respective sensor 12 and 14. Filters 16, 16' function to 
filter out relatively large molecules such as alcohols while allowing the 
smaller molecules CH.sub.4 and CO to pass. In the household environment, 
gases and vapors are commonly generated by paints, solvents, cleaners, 
liquid alcohol, and so forth. As a result, the sensitivity of each sensor 
to the associated target gas is unaffected or only minimally degraded by 
the presence of undesirable gases and vapors. Consequently, the occurrence 
of false alarms is reduced, as well as the interference with detection of 
dangerous levels of the target gas. 
In particular, a hydrophobic zeolite molecular sieve filter is installed 
upon the sensor for methane gas, preferably over both respective sensors 
for methane and carbon monoxide gases. A description of zeolites can be 
found at Breck and Anderson, "Molecular Sieves", Kirk-Othmer Encyclopedia 
of Chemical Technology, Third Edition, Volume 15, John Wiley & Sons, Inc., 
New York (1981), pp. 638-669. More specifically, zeolites are constituted 
by hydrated silicates of aluminum and sodium, potassium, magnesium and/or 
calcium of the structural formula M.sub.x/n ((AlO.sub.2).sub.x 
(SiO.sub.2).sub.y)wH.sub.2 O where M is a cation and n represents its 
valence, w represents the number of water molecules per cell, and x and y 
represents the total number of tetrahedra per unit cell. 
Zeolites are especially suitable as molecular sieves, constituted by 
microporous structures composed of crystalline aluminosilicates. Such 
microporous structures present extremely small pore sizes, in the range of 
5-10 Angstrom units. The aluminosilicates forming these sieves possess the 
ability to undergo dehydration with little or no change in crystalline 
structure. The thus-empty cavities in the microporous structure possess a 
strong tendency to adsorb other particulate structures which come into 
contact with it. As a result, a sieving action is generated which allows 
for separation of smaller and larger molecules, e.g., in a fluid. As such, 
zeolites are capable of being extremely effective in gas filtering and 
separation. Such molecular sieve-oriented zeolites possess the specific 
chemical formula of M.sub.2/n O.Al.sub.2 O.sub.3 ySiO.sub.2 wH.sub.2 O. 
Molecular sieve zeolites can be naturally occurring or can be synthetically 
prepared. As noted supra, all zeolites have a high affinity for water and 
other polar molecules and can therefore be used, e.g., for drying gases 
and liquids. In the present invention, the zeolites are converted into 
hydrophobic form. For example, a synthetic zeolite such as synthetic 
mordenite can be dealuminated by acid treatment to increase the 
silicon/aluminum ratio. The SiO.sub.2 /Al.sub.2 O.sub.3 ratio can be 
increased to about 100 (e.g., the range of about 90-100 and above), so 
that water-adsorbing capacity is essentially eliminated and the molecular 
sieve becomes hydrophobic. Therefore, when such hydrophobic zeolite is 
incorporated as a filter on the sensing device of the present invention, 
water is not adsorbed and passes therethrough while molecules such as 
isopropanol, ethanol and other high molecular weight vapors are prevented 
from passing through the filter, with the result that only smaller organic 
compounds, i.e., methane gas, can pass through the filter and therefore be 
detected by the sensor. 
One benefit of utilizing a hydrophobic zeolite molecular sieve filter 
instead of activated carbon or other similar adsorbent media, is that 
hydrophobic zeolite does not adsorb water vapor so that adsorption 
capacity is not significantly reduced by water vapor adsorption. Molecular 
sieves such as activated carbon quickly lose their ability to filter when 
exposed to high relative humidity conditions. Additionally, the 
hydrophobic zeolite filter media can be effectively regenerated using 
simple resistive heaters which is not possible with activated carbon 
media. 
The filters applied on the sensors in accordance with the present invention 
are developed from high silica molecular sieves, e.g., "high silica" 
zeolites and are therefore hydrophobic. Such filters exhibit a high 
selectivity for organic vapors, even under humid conditions. High silica 
zeolites possess crystalline, inorganic silica-alumina structures, in 
addition to being non-combustible and nonreacted to most airborne gases. 
Especially preferred high silica molecular sieve for use with the present 
invention are marketed under the trade names HiSiv 1000 and HiSiv 3000 by 
UOP Molecular Sieves, DesPlains, Ill. HiSiv 1000 and HiSiv 3000 are high 
silica zeolites possessing pore structure for adsorbing molecules with 
critical diameters up to 0.8 and 0.6 nanometers, respectively. These 
molecular sieves are especially suitable for adsorbing relatively small 
molecules and low-boiling solvents such as alcohols, aldehydes, ketones, 
esters, aliphatics, aromatics and chlorinated hydrocarbons. In particular, 
HiSiv 1000 and HiSiv 3000 adsorb vapors from ethanol, acetone and 
methylene chloride extremely well, so that such vapors will not contact 
the sensors and activate the same to generate false alarms. Therefore, the 
zeolites effectively filter out all vapors except extremely small 
molecular vapors such as CO, CH.sub.4 and water. Accordingly, these 
molecular sieve zeolites perform excellently as filtering mechanisms upon 
CO and CH.sub.4 detectors in accordance with the present invention. 
The particular HiSiv products are available in powder form, with particle 
sizes of less than 200 mesh. Additionally, these products are also both 
available in the shape of approximately 1.5 mm. pellets in size, and 
Tri-Lobe 3 mm or 6 mm pellets under the trade name TriSiv by UOP. The 
product HiSiv 1000 possesses an average pore size of about 8 Angstrom 
units while the product HiSiv 3000 possesses an average pore size of 6 
Angstrom units. HiSiv 3000 is especially suited for adsorbing small 
molecules and low-boiling solvents such as ethanol, acetone and methylene 
chloride while HiSiv 1000 is especially suited for adsorbing larger 
molecules 0.6-0.8 nanometers in size, e.g., higher boiling solvents such 
as toluene and methyl isobutyl ketone. In a preferred embodiment of the 
methane sensor in accordance with the present invention, a blend of these 
two products is provided, with about 25% HiSiv 1000 and about 75% HiSiv 
3000 being blended together to provide the appropriate size filter. 
Any conventionally-available gas detection sensor, e.g., for low molecular 
weight alkanes or carbon monoxide, can be provided with a hydrophobic 
zeolite molecular sieve in accordance with the present invention to 
improve detection of the requisite gaseous contaminants. One such sensor 
is marketed by Figaro USA, Inc. Wilmette, Ill. under the name Figaro Gas 
Sensor TGS 813/813C. This sensor possesses excellent sensitivity to low 
molecular weight alkanes, e.g., methane, propane and butane, making this 
an excellent sensor for domestic gas leak detection. At the same time, 
this sensor exhibits very high sensitivity to "noise gases"; incorporating 
the hydrophobic zeolite molecular sieve supra reduces occurrence of false 
or nuisance alarming. The TGS 813 sensor comprises a sintered bulk 
semiconductor composed principally of tin dioxide (SnO.sub.2) and situated 
on a ceramic tube together with electrodes therefor. A heater coil, 
composed of 60 micron diameter core possessing a resistance of 30 ohms, is 
situated within the ceramic tube. A stabilized 5 volt heater supply 
provides current to the heating coil while a circuit voltage not exceeding 
24 volts is supplied to the tin dioxide semiconductor itself. Operation 
and calibration of the Figaro Gas Sensor TGS 813/813C is described in a 
specification brochure published by Figaro USA, Inc. 
Basically, the resistance of the tin dioxide semiconductor itself drops 
when contacted by a target gas, e.g., methane. As the concentration of gas 
increases, the resistance of the tin dioxide semiconductor drops, with 
concomitant flow of current therethrough increasing. Eventually, the 
current flow will increase to an alarm level and thereby activate an 
appropriate alarm coupled into the sensor circuit, e.g. a buzzer. 
A sensor especially suited for carbon monoxide gas detection is 
manufactured under the trade name "G" Series by Capteur Sensors & 
Analysers Limited, Abingdon Oxon, England. These sensors are also composed 
of a metal oxide sensing element and are provided in a casing with a 
resistor circuit in similar fashion to the methane gas sensor described 
supra. In particular, different levels of heater resistance and sensor 
resistance are provided so that the CO sensor does not respond to methane 
and hydrogen. For example, the sensor is heated to a temperature of about 
390.degree. C. by the heater which possesses a resistance of about 15 ohms 
at this temperature. The sensor itself possesses a resistance of about 52 
kilo-ohms at this temperature, this resistance increasing as the 
concentration of CO increases. 
More particularly, the sensor resistance under exposure to 100 ppm. CO 
concentration rises to 86 kilo-ohms, and to 105 kilo-ohms under 200 ppm. 
CO concentration. The current flow through the CO sensor will 
concomitantly drop and thereby a voltage representing ambient CO 
concentration will be generated. Additionally, the sensing element can be 
covered with a carbon filter to block out all vapor penetration except for 
carbon monoxide. Operation and calibration of the "G" series carbon 
monoxide sensors is described in a specification brochure published by 
Capteur Sensors & Analysers Limited. 
The two sensors can be separately incorporated into the invention system as 
illustrated in FIG. 7 or combined into a composite sensor unit as 
illustrated in FIGS. 8 and 9. More particularly, FIG. 7 illustrates a 
single gas sensor 1 disposed within a casing 2 that contains a zeolite 
filter or sieve 3 mounted a distance away from the sensor 1. The zeolite 
filter or sieve 3 is retained in position between two screens 4, 4' 
composed, e.g., of stainless steel. The casing 2 itself can be composed of 
any suitable material, e.g., molded plastic, and can be substantially 
cylindrical in shape as illustrated. The entire sensing unit can be 
mounted upon a suitable support 5 as illustrated, which can, in turn, be 
mounted upon a ceiling or wall panel. In this regard, the sensor 1 
comprises four leads 6, 6', 6" and 6'" which pass through the casing 2 and 
support 5 and are connected to appropriate terminals. 
Sensor 1 illustrated in FIG. 7 is a carbon monoxide sensor of the type, 
e.g., manufactured by Capteur Sensors & Analysers as described supra. In 
this regard, two of the four leads 6, 6', 6" and 6'" illustrated in FIG. 7 
are coupled to form a sensor resistance circuit while the other two leads 
are coupled to form a heat resistance circuit, i.e., to heat the sensor 1 
as described. The zeolite filter 3 positioned as shown in FIG. 7, operates 
to effectively filter out all vapors except extremely small molecular 
vapors such as CO, CH.sub.4 and water. While a CO sensor has been 
illustrated in FIG. 7, it is noted that a methane sensor, manufactured by 
Figaro U.S.A. as described supra, can be substituted in place of the CO 
sensor and function equally well to detect the presence of methane gas. 
Furthermore, methane gas and carbon monoxide sensors can be combined into a 
single sensing unit 100 (FIGS. 8 and 9). More particularly, carbon 
monoxide sensor 101 and methane sensor 102 are each mounted upon a support 
base 103 in turn mounted upon a support such as a ceiling or wall panel. 
Each sensor 101 and 102 is provided with appropriate electronic circuitry 
(not illustrated) as described supra. Casing 104 is designed to fit over 
both sensors 101 and 102 in similar fashion to the casing 2 shown in FIG. 
7, when the cover member 105 is lowered and secured upon support base 103 
from the raised position shown in FIG. 9. Raised wall members 106, 106', 
106", 106'" and 106"" are arranged to securely mate with an inner surface 
of cover member 105 when the cover member 105 is lowered upon the support 
base 103. 
Casing 104 is mounted upon an upper inner surface of cover member 105 as 
illustrated in FIG. 9. More particularly, two screens 107 and 108 are 
provided as shown in FIG. 9, between which hydrophobic zeolite filter 109 
is retained. Appropriate filtering of all undesired or "non-target" gases 
by the zeolite filter 109 in relation to both the carbon monoxide 101 and 
methane 102 sensors is effectively provided so that it is now possible to 
effectively combine said sensors 101 and 102 in a single gas sensing unit 
100 which can be conveniently mounted at any required location within an 
environment to accurately and swiftly detect presence of methane or carbon 
monoxide. It is no longer necessary to separately arrange, mount and 
couple carbon monoxide and methane sensors from one another. Additionally, 
the support base 103 and cover 105 can be each composed of the same 
material as casing 104, namely injected molded plastic. In particular, 
support base 103 is composed of plastic backing upon a base. 
Operation of the invention system for monitoring carbon monoxide gas 
detection will now be described with reference to FIGS. 1-6. 
The analog output voltages of sensors 12 and 14 are sampled and digitized 
by analog to digital (A/D) converters 18 and 19 respectively, with a 
typical sampling rate in the range of 1 to 5 seconds, for example. The 
digitized outputs are provided to processor 20 which has a software 
program stored therein to determine whether dangerous levels of either 
CH.sub.4 or CO have been detected. A memory 29 stores data used within 
calculations by the processor. Processor 20, memory 29 and A/D converters 
18 and 19 can be incorporated within a microcontroller or Application 
Specific Integrated Circuit (ASIC). 
If hazardous levels of either CO or CH.sub.4 gas are detected, processor 20 
provides a specific logic level output to CO alarm/indicator 26 or 
CH.sub.4 alarm/indicator 28, or both, as the case may be, to activate the 
respective alarms and visual indicators. Alarm/indicators 124, 26 and 28 
may have a common audio alarm element with separate light emitting diode 
(LED) indicators to indicate which hazardous condition or fault is 
present. Electronics are included therein to drive the alarm element and 
indicators responsive to the logic level outputs provided by the 
processor. A power supply 30 is employed to power the alarm as well as the 
other electronic components within the overall CH.sub.4 /CO alarm 10. 
Optionally, the CH.sub.4 /CO alarm 10 can be linked via line 23 to a 
centralized alarm center such as a fire station. When dangerous gas levels 
are detected, the alarm center would be alerted. The link to the alarm 
center could be a wireline or wireless communication link. Memory 29 would 
store an identification code identifying the location that is being 
monitored by the alarm. When danger levels are detected, processor 20 
would transmit the ID code to the alarm center along with information 
pertaining to the gas levels detected. Personnel at the alarm center would 
be equipped to quickly respond to the emergency situation. 
The memory 29 could also be used to store measured CO and CH.sub.4 
concentration data in a log for subsequent retrieval. Computed COHb data 
can also be stored in a log, as will be described infra. An external 
computer would retrieve the log data via appropriate connection to I/O 
port 27 along with suitable commands. 
Processor 20 determines whether a hazardous methane condition exists by 
comparing the measured CH.sub.4 concentration to a predetermined threshold 
level, e.g., 3,000 ppm., stored within memory 29. Once the threshold is 
met or exceeded, the CH.sub.4 alarm is sounded. 
The processor determines whether a hazardous environmental CO concentration 
exists by computing a carboxyhemoglobin (COHb) level that would exist in a 
person exposed to the CO concentration. The COHb level that would exist is 
a function of the environmental CO concentration and the exposure time to 
that CO concentration, as well as the COHb level, if any, that existed 
previously. COHb danger levels are promulgated by Underwriters 
Laboratories specification UL 2034. Once processor 20 determines that a 
predetermined COHb danger level COHb.sub.DL is reached or exceeded, e.g., 
a 10% COHb level, processor 20 activates CO alarm 26. Processor 20 also 
automatically activates CO alarm 26 even if COHb.sub.DL is not yet reached 
whenever very high concentrations of CO.sub.ppm are detected, e.g., 800 
ppm or above. 
The blood's COHb level, represented by the percentage of COHb relative to 
all hemoglobin in the bloodstream, increases as a function of 
environmental CO concentration and exposure time. Hereafter, the 
environmental CO concentration in parts per million (ppm) will be referred 
to as CO.sub.PPM. For any value of CO.sub.PPM that a person is exposed to, 
there is an equilibrium COHb level COHb.sub.EQ that will be eventually be 
reached with time. In FIG. 2, curve 32 represents %COHb level at 
equilibrium (COHb.sub.EQ) vs. environmental CO concentration (CO.sub.PPM) 
for an average person found in the text entitled "Medical Toxicology: 
Diagnosis and Treatment of Human Poisoning" by Ellenhorn and Barceloux and 
published in 1988 by Elsevier Science Publishing Company, Inc., New York, 
N.Y. This curve can be linearly approximated by the following equations 
(1) and (2): 
EQU COHb.sub.EQ =0.137.times.CO.sub.PPM, for CO.sub.PPM &lt;.about.264 ppm(1), 
EQU COHb.sub.EQ =22.2+0.053.times.CO.sub.PPM, 264 ppm&lt;CO.sub.PPM &lt;800 ppm(2). 
The dotted lines 36 and 34 of FIG. 2 are graphical representations of 
equations (1) and (2), respectively. It is understood that other linear or 
nonlinear approximations for curve 32 may alternatively be made. 
Referring to FIG. 3, %COHb level is shown plotted as a function of exposure 
time of an average person for various CO.sub.PPM concentrations. Curves 
40-43 show %COHb vs. exposure time for CO.sub.PPM ranging from 100 to 400 
ppm in increments of 100 ppm; curves 44 and 45 show the same for 
CO.sub.PPM equal to 600 and 800 ppm, respectively. The curves are based on 
data published in Underwriters Laboratory specification UL2034. For each 
curve, it is assumed that the person had a COHb level of zero percent in 
his or her bloodstream prior to being exposed to the respective CO 
concentration. 
Each curve 40-45 can be approximated by a straight line for relatively low 
values of %COHb. For example, dotted line 46 is a linear approximation for 
curve 40 and is a plot of the function 
EQU %COHb=0.1111t, (3) 
where t is exposure time. 
As shown in the table of FIG. 4, each of curves 40-45 can be represented by 
a different linear function. From these linear functions, a plot of the 
rate of COHb increase as a function of environmental CO concentration 
(CO.sub.PPM) can be constructed, as shown in FIG. 5. Curve 47 represents 
an approximated rate of COHb increase in %COHb/minute as a function of 
CO.sub.PPM, for an average person, and for relatively low values of %COHb, 
e.g., below 10%. 
Curve 47 may be approximated by a piece-wise linear approximation as 
follows: 
##EQU1## 
FIG. 6 is a flow chart illustrating software routine steps running on 
processor 20 to determine COHb levels from the measured environmental CO 
concentration CO.sub.PPM. When the CH.sub.4 /CO alarm 10 is first turned 
on, a "system calculated" %COHb level, designated as COHb.sub.SYS, is set 
to a predetermined minimum level, e.g., zero percent. (Step 52). Next, the 
environmental CO concentration CO.sub.PPM is measured in step 54. As 
explained above in reference to FIG. 1, processor 20 receives digitized 
samples of CO.sub.PPM from A/D converter 19. The samples may be received 
periodically at a sampling rate on the order of 1-5 seconds. Each sample 
represents a new CO.sub.PPM measurement. In step 56, the CO measurement 
sample is compared to a predefined maximum CO level CO.sub.MAX, e.g., 800 
ppm. If CO.sub.PPM equals or exceeds CO.sub.MAX, then COHb.sub.SYS is set 
to 80% in step 58, compared to COHb.sub.DL in step 70, and the CO alarm is 
automatically turned on in step 60. The 80% level represents a safe value 
at which to set COHb.sub.SYS when CO.sub.ppm is above full scale of the 
A/D converter 19. Notwithstanding the alarm activation, steps 54 and 56 
are repeated. Hence, if the CO.sub.PPM level drops, the alarm may 
eventually be turned off, as will become apparent below. 
If in step 56, CO.sub.PPM is less than CO.sub.MAX, the %COHb at equilibrium 
(COHb.sub.EQ) associated with the measured CO.sub.PPM is determined in 
step 62. This determination is based upon the values corresponding to 
curve 32 of FIG. 2. Thus, COHb.sub.EQ can be obtained either from a ROM 
look up table within the memory, or from an algorithm such as a linear 
algorithm based on equations (1) and (2) supra. In any case, COHb.sub.SYS 
is then compared to COHb.sub.EQ in step 64. If COHb.sub.SYS is less than 
COHb.sub.EQ, then COHb.sub.SYS is increased in step 66 by an amount which 
is based upon the %COHb vs. exposure time function associated with 
CO.sub.PPM (FIG. 3). For example, the graph of FIG. 5, which is derived 
from the curves of FIG. 3, may be stored in another ROM look up table 
within the processor, such that an approximate rate of %COHb increase can 
be readily retrieved. A linear or nonlinear algorithm that approximates 
the curve of FIG. 5 could also be used to compute the rate of increase. A 
new value for COHb.sub.SYS is then obtained as follows: 
EQU COHb.sub.SYS (new)=COHb.sub.SYS (old)+(rate of increase).times.(sampling 
interval), (5) 
where COHb.sub.SYS (old) is the value of COHb.sub.SYS prior to the update 
in step 66. 
If in step 64, COHb.sub.SYS is greater than COHb.sub.EQ, then COHb.sub.SYS 
is decreased exponentially between COHb.sub.SYS and COHb.sub.EQ (step 68) 
using a half life of about six hours, for example. The exponential 
decrease is based on the following equation: 
##EQU2## 
where COHb.sub.SYS (old) and COHb.sub.SYS (new) are the values for 
COHb.sub.SYS prior to and after step 68, respectively; COHb.sub.EQ was 
determined in step 62; .DELTA.t is the sampling interval (in minutes); and 
Y is the half life for elimination of CO in minutes. It has been found 
through experimentation that a half life of six hours (360 minutes) is 
preferable, since this approximates the decay in the COHb level of an 
average person when environmental CO.sub.PPM concentrations drop. It is 
understood, however, that other values can alternatively be used for the 
half life. For example, if an additional safety factor is desired in the 
alarm, a longer half life would be selected. 
With continuing reference to FIG. 6, once an updated value of COHb.sub.SYS 
is obtained in steps 66 or 68, the updated value is compared in step 70 to 
the COHb danger level COHb.sub.DL, e.g., 10%. If in step 64, COHb.sub.SYS 
equals COHb.sub.EQ, steps 66 and 68 are bypassed, as indicated by flow 
line 69, and step 70 is proceeded to directly. If the danger level is 
exceeded, the processor will command the CO alarm and indicator to be 
turned on in step 60 by supplying the appropriate logic level output 
thereto. If COHb.sub.SYS is below the danger level, the processor will 
supply the opposite logic level to the CO alarm and it will remain off in 
step 72. In any case, the updated value for COHb.sub.SYS is stored in the 
memory and replaces the previous value, and a new measurement is taken in 
step 54, whereupon the process is repeated. 
The values computed for COHb.sub.SYS, as well as the measured CO.sub.PPM 
values and a time reference, can be stored in step 74 as a log in the 
memory 29 (FIG. 1) to record the levels over a length of time. The stored 
values can be subsequently retrieved by appropriate connection of an 
external computer to the processor. The log period could be on the order 
of days or weeks, depending on the storage capacity of the memory and on 
the sample time between stored data. When the storage capacity is reached, 
the processor would typically purge the memory of the oldest data. The 
external computer, when connected, could similarly purge portions of the 
memory by appropriate commands. It is noted that the measured methane 
levels could also be stored in a log for retrieval by the external 
computer at such time. 
The following example is presented to illustrate the computation for the 
system calculated COHb level, COHb.sub.SYS, in a varying CO.sub.PPM 
environment. At the start, it is assumed that the environmental CO 
concentration CO.sub.PPM has been zero for a long period of time so that 
the subject's COHb level is zero. CO.sub.PPM then instantly becomes 200 
ppm for one hour. At the end of the hour CO.sub.PPM becomes and remains 50 
ppm. 
EXAMPLES OF OPERATION 
For the first hour, the human subject's COHb level will increase along the 
CO.sub.PPM =200 ppm function of FIG. 3 (curve 41). This function 
intersects the 10% COHb level after 35 minutes have elapsed. At the end of 
the hour, the subject's COHb level is at 15.3%. At this point, CO.sub.PPM 
becomes 50 ppm and the subject's COHb level decreases exponentially with a 
six hour half life from 15.3% towards 8.2%, which is the COHb equilibrium 
value COHb.sub.EQ associated with a CO.sub.PPM of 50 ppm. (The 8.2% level 
can be interpolated from the curves of FIG. 2). After 50 minutes of this 
exposure, the subject's COHb level decreases to 14.7% and after 6 hours it 
becomes 11.75% (half way between 15.3% and 8.2%). 
The accuracy in the computation of COHb.sub.SYS by the CH.sub.4 /CO alarm 
depends on the approximations used for the curves of FIGS. 2 and 3. For 
example, if linear functions are used to approximate curves 32 and 47, 
then in the example just presented, COHb.sub.SYS would reach 10% in about 
34.5 minutes and will be about 17.4% at the end of one hour. COHb.sub.SYS 
would then decrease exponentially with a 6 hour half life from 17.4% 
towards 6.9%. After 50 minutes of exposure at 50 ppm, COHb.sub.SYS 
decreases to 16.4% and after 6 additional hours becomes 12.15% (half way 
between 17.4% and 6.9%). It should be noted that COHb.sub.SYS accuracy 
superior to that achieved by the linear approximation can be realized with 
closer analytical modeling of the above-mentioned exposure curves, whether 
via nonlinear algorithms or by means of look up tables corresponding to 
the curves of FIGS. 2 and 3. A small microcontroller of e.g., eight bits, 
can accomplish all these calculations. 
It is to be understood that the embodiments described herein are merely 
exemplary and that one skilled in the art can make many modifications and 
variations to the disclosed embodiments without departing from the spirit 
and scope of the invention. All such variations and modifications are 
intended to be included within the scope of the invention as defined by 
the appended claims.