The present invention relates to an environmental monitor and to a method for monitoring environmental conditions. In particular, the present invention relates to a device and a method for sensing the presence of gaseous substances within an environment.

Specifically, the present invention relates to a carbon dioxide (C0 ₂ ) measurement device and method which may be utilised separately or in conjunction with an environmental monitor for measuring the concentration of CO, in an environment.

The present invention also relates to a methane (CH.) measurement device, which may also be utilised separately or in conjunction with the composite environmental monitor.

Presently, a variety of devices and methods exist for the measurement of specific parameters of interest from the physical environment. Typical parameters which are measured by such devices and methods include temperature, pressure, air velocity and gas concentration.

In a number of occupations, and particularly in that of underground coal mining, safety risks exist to personnel due to the possible presence of a variety of hazardous gases in the air. Typical gases which can be life threatening are C0 ₂ and CO. Also simple oxygen deficiency is, on its own, life threatening regardless of the gas which may dilute the oxygen content of the air. Methane, an explosive gas, is also a further risk in coal mining. Other gases including Carbon monoxide and hydrogen are generated during fires in mines. The invention detailed herein relates to complementary gas detection techniques which are interdependent although each is in itself a novel idea. The techniques are aimed particularly at solving two particular problems, those of the lack of a suitably compact and inexpensive personal

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carbon dioxide sensor and the fact that adequate methane detectors consume a lot of energy and are a battery life and bulk problem. The technique embodied herein also include a cost effective calibration technique which makes viable the proposed improvement in methane detection and the proposed novel carbon dioxide measuring technique.

At the present time there is little availability of a compact, hand-held device to measure Carbon Dioxide except for Non Dispersive Infra Red (NDIR) analysers which are complex, expensive and bulky, and interferometers which are not continuous monitoring devices. This invention discloses a means of measuring C0 ₂ in the atmosphere directly optionally using already existing oxygen sensors available for purchase from sensor manufacturers.

The oxygen sensors which are available and to which the invention may utilise are in general self-powered, diffusion limited, metal-air batteries. They contain an anode, electrolyte and an air cathode to which the diffusion of oxygen is limited by a diffusion barrier. At the air cathode, Oxygen is reduced to Hydroxy1 ions which in turn oxidise the metal anode. The electrical current between the anode and cathode is proportional to the rate of consumption of oxygen. The rate at which oxygen can get to the electrode through the diffusion barrier is proportional to the concentration of oxygen in the atmosphere. The current through, or voltage across a suitable resistor is therefore directly related to the concentration of oxygen in the air.

The electrochemistry of the metal-air battery oxygen sensor is well understood by a number of manufacturers of this type of device, however there is significant difference between the performance of various types and brands of sensor due to differences in diffusion barrier technology. The traditional diffusion barrier has been a very thin plastic membrane. Plastic membranes have a very high temperature and pressure coefficient, with suitable temperature compensation, sensors fitted with such a barrier are sensitive primarily to the partial pressure of

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oxygen. Such sensors are termed "Partial Pressure" sensors and it is understood that such sensors will have an output which varies not only with the atmospheric concentration of oxygen but also with changes in atmospheric pressure which may occur. There have been recent inventions in the area of diffusion barrier technology which have produced metal-air oxygen sensors which have markedly different characteristics due to innovative new diffusion barrier concepts. One such improvement, known as the "Gaseous Diffusion Barrier", takes the form of a simple capillary. The Gaseous Diffusion Barrier has the characteristics of making the sensor insensitive to atmospheric pressure as well as greatly reducing the temperature sensitivity of the device. This device has become known as a "Volumetric" or "Concentration" sensor, its main characteristic is that of insensitivity to atmospheric pressure.

The volumetric oxygen sensor has another characteristic which is a by product of the Gaseous Diffusion Barrier technology this characteristic is that acid gases are absorbed by the electrolyte and cause an "enhanced oxygen signal" which is proportional to the concentration of the acid gas. This side effect, namely that the oxygen reading from this type of sensor will be artificially high in the presence of acid gases such as CO, is the basis of this invention.

In one aspect, the present invention seeks to provide a CO, sensor using essentially two oxygen sensors, one which exhibits a CO, or acid gas enhancement effect and another which does not exhibit this effect. The difference in derived signal from the two sensors can be calibrated to give a reading of CO, concentration. This type of sensor may be described, and is described throughout the specification as having a "CO, enhancement effect", or alternatively, as having "cross-sensitivity to CO ".

In a further aspect, the present invention seeks to provide a device and a method for monitoring environmental

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conditions which overcomes the disadvantages of prior art monitoring devices by providing a device and method which is adapted to sense a plurality of environmental conditions and, by utilisation of a powerful microprocessor together with extensive memory capacity, is able to correlate the data from said sensors, compute and indicate to an operator, relevant information relating to the environmental conditions.

The present invention also seeks to provide a device for monitoring environmental conditions which is compact and functional, lending itself to be worn on the belt of an operator, for instance, in mines and chemical plants, to provide an alarm to said operator in the event of an emergency or hazardous situation.

The present invention also seeks to provide a device and a method for monitoring environmental conditions, and in particular, the presence of gases such as CH., O,, CO, CO, NO, H,S and H, to correlate data relating to such gases, and to provide information to an operator to indicate such conditions explosibility, toxicity and O- deficiency, together with information relating to temperature, pressure, etc, of the environment.

The present invention also seeks to provide a device and a method for monitoring environmental conditions wherein, by undergoing a calibration procedure, the parameters of various environmental conditions of a test environment may be monitored and stored in memory for comparison and correlation with the environment conditions of an unknown environment, such that, utilising regression analysis, information may be supplied to an operator to indicate qualitative and/or quantitative indications relative to said environmental conditions.

In one broad form the present invention provides a CO, concentration measurement device, comprising: a volumetric sensing means adapted to measure volume % of O, in an environment, said volumetric sensing means having a CO, enhancement effect; partial pressure sensing means adapted to measure the

partial pressure of 0 ₂ , said partial pressuring sensing means having substantially non C0 ₂ enhancement effect; and, processsing means adapted to: calculate the true 0 ₂ concentration; calculate the C0 ₂ enhanced 0 ₂ concentration; calculate the difference between said C0 ₂ enhanced 0 ₂ concentration and said true 0 ₂ concentration; and provide an appropriately calibrated indication of the CO, concentration in said environment. In a further broad form, the present invention seeks to provide a CH. measurement device consisting of: a catalytically active filament; and a thermistor thermal conductivity sensing filament; whereby, each of said filaments are individually powered such that the concentration of CH ₄ is calculated from the change in electrical properties of the filaments.

In yet a further broad form, the present invention provides an environmental monitor, comprising: at least one sensor adapted to sense a gaseous substance and to provide sensor data indicative of the concentration of said gaseous substance; memory means adapted to store calibration information obtained from said sensor data during a calibration proce _d ure of said device after monitoring a plurality of test environments; and, processing means adapted to process said sensor data obtained during monitoring of an environment, and perform correlation and regression analysis techniques on said sensor data and said calibration information to provide an indication to an operator regarding the nature and concentration of gaseous substances and other environmental conditions with said environment.

In still a further broad form, the present invention provides a method for monitoring the nature and concentration of gaseous substances and other environmental conditions within an environment, comprising

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the steps of : performing a calibration step wherein an environmental monitor is exposed to a plurality of test environments such that conditions relative to each of said test environments may be recorded and correlated to provide calibration parameters; and performing a monitoring step wherein said monitor is placed in an unknown environment such that the environmental conditions therein are adapted to be sensed, and data corresponding thereto is compared and correlated to said calibration parameters, such that an indication relative to the nature and concentration of gaseous substances and other environmental conditions within said unknown environment may be indicated to an operator.

The present invention will become more fully understood from the following detailed description of a preferred embodiment of the present invention.

In order to facilitate the ready understanding of the invention, the invention will be hereinafter described with reference to the accompanying drawings, in which:

Fig. 1 shows an environmental monitoring device in accordance with the present invention;

Fig. 2 illustrates a system diagram showing the calibration step of basic operational procedure of the environmental monitoring system;

Fig. 3 illustrates a block diagram of the CO, measurement technique in accordance with the present invention; and.

Fig. 4 illustrates a schematic representation of the CH. measurement technique in accordance with the present invention.

In Fig. 1, is shown a preferred embodiment of an environmental monitoring device, in accordance with the present invention. The monitoring device, generally designated by the numeral 1, is of cylindrical shape with a LCD display panel 2, a plurality of push buttons 3, and a plurality of warning LED's on one end of the cylindrical device 1. Intermediate the end of the cylindrical device,

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is provided a sensor sealing cover 4 which surrounds the main body 5. The sealing cover 4 is partially slidable around the main body 5, such that, in a first position, the cover 4 exposes a slot which provides environmental access to the sensors contained within the main body 5, allowing remote sampling with sample tube entry and gas exit revealed, in a second position, the slot is fully open and operational to expose the sensors to ambient gas diffusion, and, in a third position, the monitor is switched off and sealed to watertight standard. Within the device 1, the monitor is preferably constructed in a cylindrical shape, allowing a compact construction which enhances performance due to close proximity of sensors, and allows flexibility of expansion by increasing the height thereof. Consequently, by such construction, no separate attachments are required to operate the device. The monitoring device 1 is therefore of lightweight construction and is provided with its own power supply sufficient to operate the LED and LCD displays 4 and 2 together with audible alarms. The monitoring device 1 is also adapted to be connected to a serial data link interface.

In Fig. 2, is illustrated a system diagram showing the calibration step and basic operational procedure of the environmental monitoring system in accordance with the present invention. The system comprises a computer processing means 6, which is adapted to control the remainder of the system components, record the data derived therefrom, correlate said data and perform regression analysis thereon to obtain information indicative of the monitored environmental conditions, and supply same for indication to the operator.

The method of monitoring the environmental condition, in accordance with the present invention, is comprised of two core steps. The first step is the calibration step wherein the monitoring device undergoes a learn or calibration procedure in a test environment and is exposed to an array of physical conditions such that parameters

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representing such physical conditions may be recorded. The second step is the monitoring step, wherein the monitoring device is placed in an unknown environment, and, utilising the parameters previously recorded in the calibration step, the environmental conditions are sensed and indicated to the operator.

The calibration step, as mentioned hereinabove, may be described with reference to Fig. 2. The processing means 6 is adapted to control the emission of a plurality of gases from a gas supply means 7 into a gas blender 8, the output of the gas blender being supplied to an enclosed test environment 9, in which the environment monitor 1, under test, is provided. At this time, the environmental monitor 1 is connected by suitable interfacing means, for instance, an RS232 data link, to the processing means 6. The processing means 6 is also adapted to control other environmental conditions of the test environment 9, for instance, the temperature by means of a heating/cooling device 10.

With the system configured as shown in Fig. 2, the computer processing means 6 is adapted to control the physical conditions set up in the test environment, including the concentrations of various gases, temperature, pressure, etc. A plurality of conditions are separately established in the test environment 9 whilst the monitoring device 1 senses data corresponding thereto. This sensed data is recorded in the memory of the computer processing means and correlated with data pertaining to the established environmental test condition. After a plurality of conditions are separately established and recorded, the raw data may be processed by the processing means utilising regression analysis, such that mathematical formulae may be devised characteristic of the monitoring device 1, and in particular, to the actual sensors provided therein. Consequently, the monitoring device 1 is thus calibrated, the mathematical function then being stored in memory within the device 1. The monitoring step, wherein the parameters of an

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unknown environment may be determined, may then be performed. The monitoring device 1 is able to be provided within any environment, and, utilising the mathematical function stored in memory within the device 1 during the calibration step, the parameters, such as the concentrations of gases and the temperature and pressure of the environment, may be determined and indicated to the operator.

This technique utilising the calibration and monitoring steps, enables the use of any available electronic sensor in the monitoring device, instead of being confined to specific tolerances a with prior art gas sensors.

The technique of the present invention not only allows monitoring of a plurality of physical parameters by utilisation of a plurality of sensors, but enables the correlation or cross-sensitivity of the physical parameters to be monitored. Therefore, from the measurement of various independent parameters, other dependent parameters, dependent on one or more of the independent parameters, may be calculated. Also, other environmental conditions, for instance, humidity, wind velocity, etc, may be taken into effect when calculating the parameters. Thus, a composite environment can be monitored by the cross-correlation of the individual data.

As previously mentioned, the present invention may be applied to monitor the environmental conditions of any environment, however, a particularly useful application is that of monitoring the environment in the coal mining industry. That is, the present invention may be utilised to measure the concentration of various gases such as

°2' ^C0 2' ^CH 4' ^{0, N0, H} 2 ^{and H} 2 ^S ' ^{and correla} te such data with other environmental parameters such as temperature, pressure, humidity and wind velocity to provide composite data and indicate to a miner the occurence of an emergency condition when the environmental conditions become unsafe for human occupance. Such composite information is important, as the parameters ^*

measured separately by known techniques do not necessarily indicate such unsafe conditions. Such a situation, indicating the advantages of the present invention will be hereinafter illustrated.

A range of toxic gas sensors can be included in the monitoring device, for example, to sense CO, NO, H, and H2S,statistical data correlation techniques are utilised wherein the derived data is based on all of the significantly correlated variables obtained from said sensors. In this case, temperature and to a much lesser extent atmospheric pressure are likely to be significant. The approach will yield unprecedented temperature and pressure stability.

As will now become apparent, a variety of sensors will be required to be provided within the monitoring device, each sensor being provided to sense the presence of a particular gas or other environmental parameter. In this respect, particular sensors may be embodied to suit the parameter to be monitored.

In Fig 3 is illustrated a schematic diagram of a measurement technique utilised for measuring CO, concentration in an environment.

To exploit the C0 ₂ enhancement effect for the purposes of measuring C0 ₂ concentration it is necessary to know the actual concentration of oxygen in the air as a reference against which to compare the enhanced signal from the volumetric sensor. At the present time available sensors without a C0 ₂ enhancement affect, which could be used as such a reference have imperfections such as sensitivity to atmospheric pressure and temperature, generally these devices are known as "partial Pressure" devices. This invention does not however depend solely on partial pressure sensors as a reference and may in fact use any future available oxygen sensing device or technique which does not exhibit the "C0 ₂ enhancement effect".

The way the invention has been implemented is to use anavailable partial pressure oxygen sensor 11 which is

sensitive also to temperature and pressure, and compensate its output for temperature and pressure by providing separate sensors 12 and 13 to measure temperature and pressure, respectively. An oxygen reference subsystem 15 is therefore provided which gives a sufficiently accurate indication of oxygen over a sufficiently wide range of temperature and pressure. While this is the way the invention has been implemented at present it should be considered obvious to substitute an oxygen reference of a different type which may not require temperature and or pressure compensation. Such should therefore also be considered to fall within the scope of the present invention.

The implementation of this invention in its broad form is shown in Figure 3. A volumetric oxygen sensor 14 ' which is subject to the said C0 ₂ enhancement affect, a partial pressure oxygen sensor 11 which is not subject to or has a significantly smaller C0 ₂ enhancement effect, an absolute pressure sensor 12 and a temperature sensor 13 are each provided. Each of the sensors is provided with a basic electronic circuit which is designed to amplify the signal and offset any undesirable baseline outputs from the sensor to a level which is suitable for input to a microprocessor based data acquisition system 16. The exact electronic circuit used is unimportant and should in* general be based on suggested circuit layouts proposed by the suppliers of the individual sensors used. Each of the signals, namely oxygen volumetric, oxygen partial pressure, temperature and pressure are input to the microprocessor based data acquisition system. The microprocessor applies equations and algorithms to calculate, firstly the reference or true oxygen concentration in the air, then secondly, the enhanced oxygen concentration, then, the difference between the two which is then appropriately zeroed and span adjusted to read in units of C0 ₂ concentration.

The equations used in the calculations referred to

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above can be determine, ^, from a thorough examination of the performance of each device and the application of stan _d ard ana well known proportioning ana rationing techniques. An alternat ⁱ ve method and the one use _d in our basic ⁱ mplementation of this invention is the use of regression analys ⁱ s of the data from the four sensors, this method be ⁱ ng explained in more detail hereinafter.

The present invention, as previously mentioned does not however depend on the use of microprocessor technology. The invention could optionally be implemented w ⁱ th a future available oxygen sensor which could conceivably as a result of future innovation provide an adequate oxygen standard without temperature or pressure compensation. An implementation of this invention could then be done with a simple analog circuit utilising operational amplifiers. Even in the case of the partial pressure oxygen sensor as an oxygen reference which requ ⁱ res temperature and pressure compensation, the entire ⁱ nvent ⁱ on couZd be implemented with a suitable arrangement of operational amplifiers. .ement

For the measurement of methane (CH,) gas a specific of tn! !" ^{an9emen has been} •*»« ^■ -*. This embodiment of the methane sensor win _no „ be detailed hereinafter however, it should be understood that any other sensor' arrangement for the measurement of CH, or other gas for ut ⁱ l ⁱ sat ⁱ on with the environmental monitoring device and rethod as herein described, should also be considered o be w ⁱ th ⁱ n the ambit of the present invention gases' " ^la » ^l «»»"-»»«: for detecting flammable «ses ⁱ s a pa ⁱ r of electrically heated filaments in a wheatstone bridge. The filaments are, firstly the detecting filaments which react catalytically with the flammable gas which burns the gas to produce heat and a corresponding change in resistance, and secondly the compensating fϋament upon which no gas combusts but which compensates for the temperature and thermal conductivity of the gas mixture. The output of the bridge is a m ⁱ llivolt signal which is proportional to the gas

concentration over the 0 to 2 or 3 percent level. Above 5% at concentrations as high as 10%, the output of the bridge becomes unstable and will start to give a reverse output for increasing concentrations. Also the active catalytic filament is damaged if powered for any appreciable period of time at concentrations greater than 5%. For high concentrations, the thermal conductivity principle is normally used where two identical filaments, one in a sealed reference atmosphere, the other in contact with the atmosphere are arranged in a wheatstone bridge. The measurement is based on the difference in thermal conductivity of the gas mixture compared to the reference (air) mixture.

In Fig. 4 is illustrated how this invention provides a way of applying the catalytically active filament or pellistor only and hence dispensing with the prior art wheatstone bridge with its heavy power consuming compensation filament. The invention employs a catalytically active filament 17 for measurement in the range of 0-5% and a thermistor thermal conductivity sensing filament 18 which is operative over the range of 0-100% methane. Both filaments are individually powered and signals accurately monitored. To calculate the difference between the catalytically active and the compensating detector either an operational amplifier is used or the signals are individually amplified and conditioned and applied to a data acquisition microprocessor subsystem.

One interfering gas is C0 ₂ which has a significant thermal conductivity affect. For the purposes of redundancy it is an advantage to have a knowledge of any interfering gases which may occur and hence the inclusion of methane detecting filaments in a combined gas sensing product with a C0 ₂ sensing capacity is appropriate to take full advantage of this technique. This invention is therefore a companion of the preceding CO., sensing

-.hnique and the Automated Calibration Environment which _ids in its calibration, and therefore, the relationship

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of the CH. sensor with the C0 ₂ sensor of Fig. 3 is illustrated in Fig. 4.

The pair of thermistors 17 and 18 whose combined energy dissipation is well under that of the conventional compensating filament, is each separately input to the microprocessor 16 or operational amplifier subsystem. One of the thermistors 18 is self heating, that is it dissipates energy and operates at an elevated temperature, its temperature varying with its ability to dissipate energy which in turn depends on the thermal conductivity of the gaseous environment. The other thermistor 17 is not self heating and is designed to measure the ambient temperature. Rather than relying on complex circuits, the data is imputted from the individual thermistors and the catalytically active methane filament to the microprocessor based data acquisition system and regression anaylsis is used to extract the data and hence the calibration in volumetric units which is required. Another means is to not use a self heated thermistor but rather to rely on data from other sensors regarding significant background gas concentrations for example the C0 ₂ measurement technique based on the interdependency between a number of individually novel ideas which further enhances the usefulness of the whole, in this case the thermistor used to measure ambient temperature for the CO, technique can double as an output to the CH. technique and the heated thermistor if provided provides redundancy as it can be used to check the C0 ₂ result if the self heated thermistor were to fail the CO 2. subsystem could provide the data to compensate for the thermal conductivity effects of this most likely high concentration of diluting gas.

Fig. 4 therefore shows the interdependency of the

CH. and C0 ₂ techniques, as well as the redundancy of the self heated thermistor. It will be understood to persons skilled in the art that the actual circuit is unimportant and should be based on circuits proposed be sensor suppliers.

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The advantages of this scheme are, firstly, measurement of the full 0-100% methane range with only one catalytically active filament thus significantly improves battery efficiency when compared to previously available equipment, and, secondly, the invention offers improved fail safety through independent powering of filaments. In normal bridge arrangement, loss of one filament incapacitates the instrument while any device incorporating this invention is able to infer methane concentration below 5% on basis of thermal conductivity in the event of loss of catalytic filament.

Typical gas detecting instruments designed in the past included, at best, a form of temperature compensation which was usually designed at circuit level. This technique involved a laborious design and testing cycle and depended for success on each circuit, each gas sensor and each temperature compensation element being within a certain tolerance band of normal performance over the entire gas or temperature range. As a further complication, the temperature compensation was often compensating temperature effects in the circuit as well as in the sensor. Given the normal tolerance of available components and sensors, it is difficult to achieve very high precision of measurement for such instruments. An accuracy, taking into account linearity and temperature affects, of 5% is considered good, greater accuracy than this adds significant cost as a degree of customisation of each manufactured item is required.

This invention also provides a method of manufacture which employs a radical new approach to deriving accurate data from the sensors. The technique is based on the following.

Firstly, each sensor is surrounded by the simplest possible circuit without any attempt to achieve temperature compensation or linearisation. The circuits are designed to optimise repeatability and long term drift stability.

Secondly, data from each sensor, is input to an

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onboard microprocessor. The microprocessor thus has the raw data from a number of sensors as input.

Thirdly, the final stage of manufacture involves a fully automated, systematic test of each mechanically and electrically complete gas sensing product or subsystem over the gas concentration and temperature ranges in which it is expected to operate. This is carried out in the "Automated Calibration Environment".

Fourthly, data which is collected by the gas sensing product or subsystem during the automated test is transmitted to the ACE computer where it is stored until the test is complete along with the actual test gas concentrations and any other controlled parameters.

Fifthly, the ACE computer carries out a systematic regression analysis of the applied (independent) and derived (dependent) variables contained in the body of data resulting from the automated test, computes appropriate numeric constants for the equations used in the sensor product or subsystem and downloads them back to the sensor or subsystem.

Sixthly and finally, the sensor product or subsystem being now complete, the numeric constants which it contains are as much a part of the unit as are the electronic and mechanical components.

The advantages of this technique are as follows.

Firstly, the final performance of the instrument is totally independent from any differences in performance of any electronic component or sensor element due to manufacturing tolerance.

Secondly, this afford significant economies in manufacture allowing the customisation of each gas sensing product to be carried out to an unprecedented level in a completely automated fashion.

Thirdly, the technique offers the simplest possible circuit design therefore leading to low cost of manufacture and greater reliability due to lower component count.

Fourthly, the technique allows effects such as the

cross sensitivity of an oxygen sensor to carbon dioxide to be efficiently converted into a real data output without any special circuit design. It further makes such a cross sensitivity based measurement feasible where conceivably the effect is particularly variable between different sensors or is particularly temperature sensitive.

Finally, the technique is also of great value as it allows a fast product development path due to circuit simplicity while allowing the complex interrelation problems to be solved later by standard, systematic mathematical techniques.

The implementation of the present invention consists of a temperature controlled chamber, whose temperature may be remotely programmed by computer, a gas mixing device, capable of mixing at least two gases with air which is also able to be remotely controlled by computer, and a computer which has analog outputs with which to control the temperature and gas concentrations applied to the instrument under test and a means of accepting data from the instrument under test. Data is able to be received (and returned) over a single serial data link, and software.

Software in the ACE computer is adapted to program a systematic, time sequenced array of varying gas concentrations and temperature and pressure conditions, accept from the instrument under test all available data output applicable at the programmed temperature and gas concentration conditions, and log it to disk along with the programmed environmental conditions, if appropriate, run linear regression analysis software, calculate appropriate numeric constants, check the resulting equations for adequate fit to actual response curves and download suitable data back to instrument under test, and, finally, issue calibration report summarising performance of the instrument.

Regression analysis as a calibration technique is a means whereby a multiplicity of observations consisting of sets of raw data from one or more sensors in a measuring AMD/0170a

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product are recorded. In general one set of observations of raw data is recorded for each set of applied known conditions. For example, in the case of the C _O , measurement technique, precalibrated, accurately known C0 ₂ concentrations of 0,1,2,3,4,5,6,7,7,9 and 10 percent may be applied to the device and for each CO, concentration the raw output data from the volumetric and partial pressure sensors is recorded. The resulting data could be presented in the form of a table as follows, note that 0 ₂ data is not calibrated, it may in fact have no immediately recognisable relationship to the applied CO.

'2 concentration.

C0 ₂ 0 ₂ pp 0 ₂ vol Difference

y xl x2 χ3

0 930 950 -20

1 900 900 0

2 870 850 20

3 840 800 40

4 810 750 60

5 780 700 80

6 750 650 100

7 720 600 120

8 690 550 140

9 660 500 160

10 630 450 180

The regression technique is a means whereby an equation is proposed which may relate to observed dependant variables xl,x2 to xn which may include functions of xl and _/ or x2 such as

2 xl or xl*x2/(l-x2) to the independent, known variable "y" which is the true applied gas concentration. The equation, for example; y=a+b*xl+c*x2+d*x3+e*xl ²

is then optimised by standard mathematical regression analysis to discover values of a, b, c, d and e so as to minimise the sum of the squares of the errors between the predicted independent variable y=f(xl,x2..xn) and the actual independent variables. All this is done during the calibration phase.

The form of the equation and the values of the coefficients are then all that it is necessary to store in the gas detecting product to permit it to operate as a gas measuring instrument in the field.

The present invention provides a device wherein the calibration environment be built and used to apply a programmed array of independent variables to each manufactured product and that each product carry with it those unique equations and coeffiecients which are developed for it by means of regression analysis.

The present invention therefore provides a device and a method for monitoring environmental conditions wherein, by initially undergoing a calibration procedure, the parameters of various environmental conditions of a test environment may be monitored and stored in memory, such that they can then be compared and correlated with the parameters of the environmental conditions of an unknown environment, whereby, using regression analysis, information may be supplied to an operator to indicate qualitative and/or quantitative indications relative to said environmental conditions.

The present invention also provides specific novel arrangements for measuring concentrations of CO, and CH..

Whilst the present invention has been described with

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reference to particular methods and devices for performing the above techniques, numerous variations and modifications will be envisaged to persons skilled in the art. Such variations and modifications should be considered to be within the scope of the present invention as described hereinbefore as claimed hereinafter.