WO2006122548A1

WO2006122548A1 - Chemiluminescent gas analyser with a cooling unit for measuring oxides of nitrogen

Info

Publication number: WO2006122548A1
Application number: PCT/DK2006/000183
Authority: WO
Inventors: Carsten Hansen
Original assignee: INSTRUMATIC HOLDING APS
Current assignee: INSTRUMATIC HOLDING APS
Priority date: 2005-05-20
Filing date: 2006-03-31
Publication date: 2006-11-23
Anticipated expiration: 2007-11-20

Abstract

A gas analyzer (1) for determining the concentration of an oxide of nitrogen in a sample gas comprising- a reaction chamber (38) for chemiluminescent reaction of nitric oxide and ozone, - a gas collector (5) for collecting a sample of gas with an oxide of nitrogen, the gas collector being connected to the reaction chamber, - an ozone provider (54) connected to the reaction chamber for providing ozone to the reaction chamber, - a detector housing (49) containing a light detector optically connected to the reaction chamber (38) for detecting light (59) from the chemiluminescent reaction and for providing a signal representative of the concentration of said oxide of nitrogen, - a detector-temperature control unit with a cooling unit for controlling the temperature of the light detector, - wherein the detector-temperature cooling unit comprises a gas supply (14, 46) connected to a vortex cooler (47), where the cool gas exit of the vortex cooler is connected to a gas inlet (48) of the detector housing (49) for cooling of the light detector.

Description

Chemiluminescent gas analyser with a cooling unit for measuring oxides of nitrogen

FIELD OF THE INVENTION

The present invention relates to a chemiluminescent gas analyser for measuring nitric oxide, for example in exhaust gases from combustion engines.

BACKGROUND OF THE INVENTION

When nitric oxide NO reacts with ozone O₃ to nitrogen dioxide NO₂ , nitrogen dioxide is formed in an exited state giving off a photon during the immediate relaxation to the ground state. This effect has been utilised for measurements of the oxides of nitrogen NO_x in the exhaust gas of combustion engines, for example from automobiles, ships or power plants.

Typically, such chemiluminescence is measured by a light detector connected to a reaction chamber in which combustion gas is mixed with ozone for conversion of nitric oxide NO to nitrogen dioxide NO₂. The signal from the detector is indicative of the concentration of nitric oxide in the reaction chamber. Other oxides of nitrogen may be converted beforehand to nitric oxide with a carbon converter at 200°C as also disclosed in US patent No. 5,633,170 by Neti or with a steel converter at 600⁰C as disclosed in US 4,822,546 by Howard, such that an indication is given by the detector of the complete content of the oxides of nitrogen in the combustion gas.

When gas is sampled from a combustion engine, the gas sample may be diluted with dry air, as disclosed in US patent No. 4,822,564 and in US 5,976,889, in order to reduce the quenching effect from collision of the produced nitrogen dioxide in the reaction chamber and in order to reduce problems with condensation of water in the sys- tern. Furthermore, as reported in US patent No. 5,976,889 by Hirai and Miyai, the amount of by-products generated and the clogging of pipes is reduced due to the dilution. As recognised in US patent No. 4,822,564, it is generally difficult to provide an adequate air flow for cooling the light detector. In order to stabilize the output span of the light detector, it is disclosed in US patent No. 4,822,564 that a light diode maybe used in cooperation with a reference diode, where the two diodes are mutually thermally connected through a heat sink. The system with two detectors is disadvantageous in that a reference diode and the corresponding electronics makes the complete system more complicated and expensive.

A different cooling system with a circulating cooling medium for a gas detector is dis- closed in US 4,193,963.

It is known that detectors give a better signal, if they are kept cool. However, cooling of the light detector is a general problem, especially, if the light detector is close to the reaction chamber where the temperature is substantially elevated or if the light detec- tor is contained in a compartment at elevated temperature.

DESCRIPTION / SUMMARY OF THE INVENTION

It is therefore the object of the invention to provide an efficient cooling mechanism of the light detector which requires low cost and which is easily installed in existing systems.

This purpose is achieved with a gas analyzer for determining the concentration of an oxide of nitrogen in a sample gas according to the independent claim. The gas analyser comprises

- a reaction chamber for chemiluminescent reaction of nitric oxide and ozone,

- a gas collector for collecting a sample of gas with an oxide of nitrogen, the gas collector being connected to the reaction chamber,

- an ozone provider connected to the reaction chamber for providing ozone to the reaction chamber,

- a detector housing containing a light detector optically connected to the reaction chamber for detecting light from the chemiluminescent reaction and for providing a signal representative of the concentration of said oxide of nitrogen, - a detector-temperature control unit with a cooling unit for controlling the temperature of the light detector, wherein

- the detector-temperature cooling unit comprises a gas supply connected to a vortex cooler, where the cool gas exit of the vortex cooler is connected to a gas inlet of the detector housing for cooling of the light detector.

A vortex cooler that can be used in connection with the invention is commercially available in the Vortec product line of the Ohio, USA company ITW Air Management with Internet site www.vortec.com.

The gas analyser according to the invention provides a simple but efficient cooling mechanism for the light detector. This implies that such a cooling mechanism also can be installed in existing analysers without major change of the system. As experiments have shown, even small vortex cooler can supply cooling air with a temperature of between -5⁰C and -15°C, which is sufficient to cool the light detector, such as a photo diode, far below room temperature, for example down to -5°C. Therefore, for a gas analyser according to the invention, the gas supply and the vortex cooler are preferably dimensioned to keep the temperature of the detector at or below room temperature, preferably between -5°C and +2O⁰C.

Use of a vortex cooler implies a number of other advantages as well. This becomes clear when comparing with a thermoelectric cooler. A thermoelectric cooler requires a rather large power supply due to the required power for cooling. By using a vortex cooler, the required power by the detector system is much less such that a smaller power supply can be used. This is an essential advantage when the gas analyzer is mounted directly on the exhaust of an engine, especially when taking into regard that a gas analyzer may be exposed to vibrations from the engine and exhaust system causing acceleration of up to 4G.

Furthermore, if thermoelectric coolers are applied according to prior art, typically switch mode power supplies are used. However, such switch mode power supplies causes interferences with the detector signal with the result of noise in the measurements. Due to the lower power consumption of the gas analyzer according to the in- vention, a linear power supply may be used which reduces interferences with the detector signal such that the measurements with a gas analyzer according to the invention is more reliable.

A further advantage of the use of a vortex cooler is the fact that the air from the vortex cooler is dry air, which can be blown directly onto the detector for cooling in order to prevent condensation on the detector.

The air to drive the vortex cooler can easily be taken from the dilution air supply. Therefore, in a practical embodiment of the invention, the gas analyzer comprises an air intake for dilution of the sample gas before provision of the sample gas to the reac- tion chamber, wherein the gas supply for the vortex cooler is branched off the air intake.

An elevated temperature of the reaction chamber reduces the risk for condensation of water in the system. Thus, it may generally be desirable to keep the reaction chamber at a high temperature in order for the reaction to perform smoothly. Therefore, the gas analyser according to the invention may comprise a heating system for heating the reaction chamber to a temperature which is substantially higher than room temperature, preferably between 18O⁰C and 200°C.

Thermoelectric coolers according to prior art are difficult to use in connection with a detection chamber that is heated because the heat from the thermoelectric cooler cannot be emitted locally in the heated chamber. This requires additional cooling equipment which increases the volume and the weight of the gas analyzer. In contrast, the air flow, for example at a temperature of 120°C, from the vortex cooler can directly be fed into a heating unit of the reaction chamber.

Also, a sufficiently elevated temperature may reduce the need for a drying of the combustion gas before entering the reaction chamber. Therefore, in a further embodiment of the invention, the gas analyser does not have a pre-drying unit for the sample gas, but the compartment surrounding the reaction chamber is also heated to a sufficiently elevated temperature. In this case, the sample gas enters from the source, for example the exhaust system of an engine, into a heated compartment without the risk for condensation. However, this generally put some constraints on the possibility for where to place the detector, unless an efficient cooling system is provided. A vortex cooler as employed by the invention provides such necessary cooling power. It is therefore possible to use an enclosed compartment containing both the reaction chamber at elevated temperature for above room temperature and the light detector at cooled temperature at or preferably below room temperature. In case that the surroundings of the reaction chamber also should be kept at elevated temperature, as indicated above, the gas analyser may comprise a heating system for heating the compartment to a temperature substantially higher than room temperature, preferably between 180°C and 200°C.

The signal from the light detector is used for determination of the content of oxides of nitrogen in the sample gas. This requires electronics. A purpose of the invention is to provide a relatively small, preferably portable, gas analyser containing the reaction chamber, the light detector and the corresponding electronics. The latter may be contained in an electronic compartment as a module attached to the compartment for the reaction chamber and the light detector, hi addition, the gas analyser may contain an ozone producing unit. The compartment with the reaction chamber and the light diode, the compartment with the electronics and the ozone generator may be provided as one unit, optionally as different mutually attached modules in order to provide a portable gas analyser.

In this case, the cooling gas from the vortex detector may be used not only for cooling of the light detector but also for cooling of the electronics and the ozone generator. For example, the detector housing may have a gas outlet connected to a gas inlet of the electronics compartment for cooling of the electronics in the electronics compartment with the same cool gas. In addition the electronics compartment may have a gas outlet connected to a gas inlet of the ozone producing module for cooling of the ozone generator with the same cool gas. The described serial re-use of the cooling gas may ad- vantageously be employed in a system, where the electronics compartment is mounted closer to the compartment enclosing the reaction chamber than the module with the ozone generator. The cool gas temperature may be higher for the ozone generator, but the ozone generator is also provided farther away than the electronics compartment from the heated compartment with the reaction chamber, and thus requiring less cooling. The serial sequence for the transport of gas may be changed, if the ozone generator is closer to the reaction chamber than the electronics.

hi order to secure pure cooling gas to enter the ozone producing unit, a gas filter may be placed between the electronics compartment and the ozone producing unit for filtering of the cool gas.

A gas analyzer according the invention may comprise a pressure sensor for measuring the gas pressure in the reaction chamber. It may further comprise a compensator for electronically compensating variations of the detector signal in dependence of variations in the gas pressure in the reaction chamber. This feature is a great advantage to ensure an evaluation of detector signals without the risk of pressure variations influencing the evaluation. It may be used in connection with a gas analyser according to the invention, or it may be used independently in other gas analysers, where gas pressure may have influence on the chemiluminescent light signal.

hi a further embodiment, the gas analyser comprises a converter for converting oxides of nitrogen, such as NO₂, into nitric oxide, NO. The converter comprises steel flakes or steel granules and a heater for heating the steel to an elevated temperature, for example between 500⁰C and 700⁰C₅ preferably around 600⁰C.

The gas analyser according to the invention may be used for measuring combustion gas in engines, such as ship engines, in power plants, so-called DeNOx plants for re- moval of NOx, but also in medicine, for example for measurements of nitric oxide NO in exhalated air as an indication for illnesses.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, where FIG. 1 illustrates use of a gas analyser according to the invention FIG. 2 illustrates computerised connection of the gas analyser according to the invention FIG. 3 is a schematic with the gas analyser according to the invention, FIG. 4 shows a user interface graphically illustrating the NOx content FIG. 5 shows a possible embodiment for a connection flange.

DETAILED DESCRIPTION / PREFERRED EMBODIMENT

FIG. 1 illustrates the use of a gas analyser 1 according to the invention in connection with a diesel engine 2 of a ship. The diesel engine 2 is connected to a gas exhaust duct 3 into which combustion gas from the engine 2 - or several engines - is transmitted. The gas analyser 1 is connected to the duct 3 through a connection flange 4 such that a sampling pipe 5 from the gas analyser 1 is located inside the exhaust duct 3 for sampling gas to the gas analyser 1, where the amount of oxides of nitrogen is measured. The gas analyser 1 is portable and may be connected to the connection flange 4 when measurements are to be performed on the specific duct. When connected, the gas analyser is electrically connected to a power supply 6 and via an air inlet 14 through a precision pressure instrument 7 to a clean air supply 8 for dilution of the combustion gas and for cooling of the gas analyser 1. In addition, calibration gas 9, such as clean nitrogen gas with 2000 ppm NO or with 1% oxygen, may be supplied via a calibration gas inlet 15 in order to calibrate for nitric oxide and for oxygen to assure a control proper functioning of the gas analyser 1.

The distance 10 in height from the combustion gas entrance 11 at the engine 2 is normally chosen to be at least 3 times the diameter of the duct diameter. This is done in order to reduce the influence of turbulence in the duct.

Between the flange 4 and the measuring probe 12 of the gas analyser, a NOx converter 13 is located. Such a NOx converter 13 may be of the type, where carbon at around 200⁰C is used as conversion catalyst for converting nitrogen dioxide NO₂ to nitric oxide NO. However, this type of converter suffers from being consumed and must be replaced after some time of use. Therefore, advantageously, steel granules or flakes are used at 600⁰C performing the same function without any necessary replacement of the steel. As illustrated in FIG. 2, the gas analyser 1 measures the concentration of the nitric oxides and transmits the measured data as digital data to a computer 20 in the control room 21 for remote operation of the gas analyser 1. In addition in the control room 21, a data logging station may be provided. The gas analyser 1 may also transfer the digi- tal data to a computer 22 on the bridge 23 for remote operation, hi addition, it may be connected to a mobile computer 24 such as laptop computer or a PDA (personal digital assistant) via a suitable data link 25. This mobile computer 24 may be used in the vicinity of the gas analyser 1 for easy local set-up, data logging and calibration.

The gas analyser 1 may be connected to the various computers 20, 22, 24 by suitable databus cables. However, in order to ease mount and demount of the gas analyser, the data link may include a wireless datalink such as a Bluetooth connection. This wireless link may be a link between the gas analyser 2 and one or more of the computers 20, 22, 24, or it may be a link between the gas analyser 2 and a receiver that via da- tabus cables transmits the data to the respective computer 20, 22, 24. The latter may be used advantageously, if the gas analyser 1 is located so far from the bridge 23 or control room 21 such that the range of the wireless link does not cover the bridge 23 or control room 21.

The shown gas analyser is portable and comprises a handle 29 for easy transportation. In addition, a compartment 28 is located underneath the probe 12, the compartment containing an electronics compartment and an ozone generator.

In addition, the gas analyser may have an analogue output 26 for output modules 27 that may be used for various control and measurements. This is used when the data system requires analogue signals, for example as an electrical industry standard with a signal of 4 mA - 20 mA.

FIG. 3 is a schematic diagram of the gas analyser 1 according to the invention. A gas sample is provided through a sampling pipe 5. A NOx converter 13 converts oxides of nitrogen, primarily nitrogen dioxide NO₂, into nitric oxide NO. The NOx converter 13 utilises steel flakes at a temperature of 600⁰C. A temperature controller 30 is connected to a thermocouple 31 to measure the temperature and is connected to a heating element 32 to control the temperature of the NOx converter 13. Nitrogen dioxide NO₂ calibration gas may be provided to the NOx converter through a calibration gas inlet 33 in order to check the conversion efficiency.

After the conversion process in NOx converter 13, the gas sample is tested for oxygen in an oxygen sensor 34, for example a zirconium oxygen sensor. The result of this measurement is transmitted to a micro controller 35 in the electronics compartment 28 attached below the probe 13 as shown in FIG. 2. With reference to FIG. 3, the gas sample passes a zero velocity filter 36 to remove any particles in the gas sample. Through a sonic orifice 37, the gas sample is injected into the reaction chamber 38. The gas sample may be diluted with nitrogen gas or clean instrument air, provided from air inlet 14 and through sonic orifice 39. For example, such sonic orifices 37, 39, 46, 57 may be made of sapphire by laser drilling.

Part of the clean instrument air supplied through air inlet 14 is branched off at branch 40 to flow through an ejector pump 41 creating vacuum in vacuum pipe 42 connected to the reaction chamber 38. A vacuum sensor 43 measures the pressure in the vacuum tube 42 and transmits the corresponding data to the micro controller 35. After propagation of the clean air through the ejector pump 41, the air is released through release pipe 44.

Another part of the clean instrument air supplied through inlet 14 is branched off at branch 46 to feed vortex cooler 47. Cool air from the vortex cooler 47 is provided to an air inlet 48 of the detector housing 49 containing the light detector for detecting the light 50 from the conversion of nitric oxygen NO to nitrogen dioxide NO₂ in the reaction chamber 38. After cooling of the light detector in the detector housing 49, air is released through air outlet 51 of the detector housing 49 and through air exit 52 into the electronic compartment 28 and through air exit 53 into the ozone producing unit 54 with the ozone generator 55. Alternatively, the air may flood the electronic com- partment 28 for cooling of the electronics and be taken from this compartment 28 to be fed into the ozone producing unit 54. As the density of ozone gas is highly dependent on the temperature, the ozone content for the reaction in the reaction chamber 38 can be optimised by cooling the ozone generator 55 and by controlling the temperature of the ozone generator 55.

The ozone generator 55 produced ozone to be fed through piping 56 and through sonic orifice 57 into the reaction chamber 38. The micro controller 35 is electronically connected through a suitable interface 58 to the ozone generator 55 for control of the ozone production.

The micro controller 35 is also electronically connected 61 to the light detector in the detector housing 49 in order to retrieve the measured chemiluminescence signal from the detector due to the conversion in the reaction chamber 38. As the light signal from the light detector usually varies in dependence of the gas pressure in the reaction chamber 38, the gas sample pressure in the reaction chamber is measured by sample pressure sensor 59 through piping 60 in order for the pressure value to be used for compensation of the light detector signal in the case that this varies in line with pressure variations in the reaction chamber 38. This is performed by the micro controller 35, where the light detector signal and the pressure value is received.

The method and means for measuring the pressure in a reaction chamber of a NOx gas analyser and using the pressure signal for compensating the pressure dependent varying light detector signal is of general nature and may be used as well as an improvement in gas analysers that the one of the invention. For the pressure to be measured in the reaction chamber 38, it is not necessary to have a pressure gauge inside the reaction chamber 38, as experiments have shown, but it is sufficient to measure it, for ex- ample, at the ejector pump 41 with the pressure sensor 43. In addition, the sample gas pressure may be measured before entering the reaction chamber 38 and be used alternatively or in addition for compensating the signal from the light detector. For example, the pressure of the sample gas may be measured before orifice 37, suitably by pressure sensor 59 at the zero velocity filter 36.

The flow through the zero velocity filter 36 may change due to the pressure from the engine 2 or in dependence of the contamination of the filter 36. hi a practical embodiment, the pressure as measured by the pressure sensor 59 in connection with an alarm function is used to indicate the case, where the pressure exceeds a certain limit, for example due to higher pressure from the engine 2 or due to contamination in the zero velocity filter 36.

The portable version of the gas analyser 1 according to the invention may be used for measuring combustion gas in diesel engine exhausts of ships. Typically, the combustion gas has an elevated temperature at more than 100⁰C or 200⁰C. When the gas sample enters the compartment 12 with the reaction chamber 38, there is a risk for condensation in the elements in which the gas sample propagates. In order to prevent such a condensation, the compartment with the reaction chamber 38 is heated by a heater 63 to a controlled elevated temperature of typically 200⁰C. This has a number of advantages. As experiments have shown, a better chemical reaction is obtained in the reaction chamber, which results in a better light signal for the detector. The controlled elevated temperature is chosen above 180⁰C in order to be above the condensation point for water and acids. On the other hand, it is chosen not to be above 200⁰C in order to prevent decomposition of the ozone. Thus, the region between 18O⁰C and 200⁰C is advantageous for the reaction and the light signal measurements.

As the complete compartment - largely corresponding to what is denoted probe in FIG. 2 - is held constant at the elevated temperature, temperature variation in the reaction chamber is minimised. This constant temperature has the consequence that the gas in the reaction chamber 38 has a constant mass during the chemiluminescence measurements so that light signal variations due to mass variations of the sample gas in the reaction chamber 38 is prevented.

The heater 63 is controlled by a control element 64 connected to the micro controller 35, which also receives temperature signals for the temperature of the heated compartment 12 from a temperature sensor 65.

However, this puts a stringent demand on the cooling of the light detector in the detector housing 49, as this is placed near to the reaction chamber 38 and inside the heated compartment. According to the invention, an efficient cooling is obtained by the vortex cooler 47. As instrument air is available for dilution, this cooling means can be established easily, and furthermore, no electric cooling, such as rather expensive Peltier cooling with high power consumption, is necessary. As prior experiments have shown, the large power consumption for Peltier cooling makes it difficult to remove the heat by easy means. Furthermore, cooling with a vortex tube is robust and reliable and therefore suited on ships, where repair of malfunctioning Peltier coolers usually is not possible. By using a Vortex cooler, the gas analyser according to the invention is more reliable than prior art gas analysers, which is important in off-shore applications.

hi order for the reaction chamber 38 to be calibrated, the reaction chamber 38 is con- nected through piping 60 to a gas inlet 15 for calibration gas, such as nitric oxide and oxygen certified gas.

The micro controller has suitable data exits, for example cabled databuses 66 such as RS 232 or RS 485 or a wireless databus 67, such as Bluetooth.

FIG. 4 shows a user interface that is presented in connection with the analysis program of the computers 20, 22, 24 connected to the gas analyser 1. It shows the calculated values of NOx and oxygen graphically in response to a time scan. The data as provided by the gas analyser 1 are stored and used in a further calculation, where fuel flow, ambient condition, fuel specifications, etc, are used in a further calculation which is presented as values on the user interface.

The gas analyser according to the invention may be stationary mounted to the exhaust system or, alternatively, the gas analyser may be portable and may be mounted when measurements are desired. During the mounting and demounting of the gas analyser, the operator may be exposed to combustion gas, which is undesired. In order to secure a safe mounting without exposure to combustion gas, the valve system as illustrated in FIG. 5a may be used. The system comprises a flange 70 with a conical tube 71, in which a ball 72 by a spring 73 is pressed against an opening 74 of the flange 70 in order to close the opening 74. The opening 74 has a width corresponding to the diameter of the sampling pipe 5 of the gas analyser 1 and may optionally comprise sealing means, such as an o-ring, to seal against the sampling pipe when introduced into the flange 70 through the opening 74. When the sampling pipe 5 is introduced into the opening 74, as illustrated in FIG. 5b, the opening sealingly surrounds the sampling pipe 5. The sampling pipe presses the spring loaded ball 72 into the flange compressing the spring 73. Combustion gas may thus propagate around the ball 72, which is indicated by arrows 75 and enter the inner channel 76 of the sampling pipe 5. As the front hole 78 at the front end 77 end of the sampling pipe 5 may be closed, when the front hole 78 rests against the ball 72, the front end 77 of the sampling pipe 5 may be equipped with side holes 79 for proper sampling of the combustion gas.

In order to analyse the sample gas in an external apparatus, the gas analyser may be equipped with a sample gas outlet. Advantageously, this sample gas is diluted with air in order to prevent condensation in the tubes from the gas analyser to the external apparatus. When properly diluted, sample gas may be transported through tubes with a length of 50- 100 meter without condensation problems.

Claims

1. A gas analyzer (1) for determining the concentration of an oxide of nitrogen in a sample gas comprising

- a reaction chamber (38) for chemiluminescent reaction of nitric oxide and ozone,

- a gas collector (5) for collecting a sample of gas with an oxide of nitrogen, the gas collector being connected to the reaction chamber,

- an ozone provider (54) connected to the reaction chamber for providing ozone to the reaction chamber,

- a detector housing (49) containing a light detector optically connected to the reaction chamber (38) for detecting light (59) from the chemiluminescent reaction and for providing a signal representative of the concentration of said oxide of nitrogen,

- a detector-temperature control unit with a cooling unit for controlling the tempera- ture of the light detector, characterised in that the detector-temperature cooling unit comprises a gas supply (14, 46) connected to a vortex cooler (47), where the cool gas exit of the vortex cooler is connected to a gas inlet (48) of the detector housing (49) for cooling of the light detector.

2. A gas analyzer according to claim 1, wherein the gas analyzer comprises an air intake (14) for dilution of the sample gas before provision of the sample gas to the reaction chamber (38), wherein the gas supply for the vortex cooler is branched off (46) the air intake.

3. A gas analyzer according to claim 1 or 2, further comprising an enclosed compartment (12) containing the reaction chamber (38) and the light detector and comprising a heating system for heating the reaction chamber to a temperature which is substantially higher than room temperature, preferably between 180°C and 200°C.

4. A gas analyser according to claim 3, further comprising a heating system (63) for heating the compartment with the reaction chamber to a temperature substantially higher than room temperature, preferably between 18O⁰C and 200°C.

5. A gas analyser according to claim 3 or 4, further comprising an electronics compartment (28) containing electronics for the light detector and optionally for other detectors, wherein the detector housing (49) also has a gas outlet (51) connected to a gas inlet of the electronics compartment (28) for cooling of the electronics in the electronics compartment with the same cool gas.

6. A gas analyzer according to claim 5, further comprising an ozone producing unit (54), wherein the electronics compartment (28) also has a gas outlet connected to a gas inlet of the ozone producing unit for cooling of the ozone producing unit with the same cool gas.

7. A gas analyzer according to claim 6, wherein a gas filter is placed between the electronics compartment (28) and the ozone producing unit (54) for filtering of the cool gas.

8. A gas analyzer according any preceding claim, wherein the gas supply (14) and the vortex cooler (47) are dimensioned to keep the temperature of the light detector at or below room temperature, preferably between -5⁰C and +2O⁰C.

9. A gas analyser according to any preceding claim, further comprising a pressure sensor (43) for measuring the gas pressure in the reaction chamber (38) and further comprising a compensator (35) for electronically compensating variations of the detector signal in dependence of variations in the gas pressure in the reaction chamber.

10. A gas analyser according to any preceding claim, wherein, the gas analyser com- prises a converter (13) for converting oxides of nitrogen, such as NO₂, into nitric oxide, NO, the converter comprising steel flakes or steel granules and a heater for heating the steel to an elevated temperature.

11. A gas analyser according to claim 10, wherein the heater is configured to heat the steel to a temperature of around 600⁰C.

12. A gas analyser according to any preceding claim, wherein, the gas analyser com- prises a linear power supply for minimizing electronic interference with the detector signal.

13. A gas analyzer according to any preceding claim with an air intake configured for intake of exhalated air from human beings.

14. A gas analyzer according to any preceding claim in combination with an exhaust system for exhaust gas analysis, where the gas analyzer with the gas collector (5), the ozone provider (54), the detector in the housing (49) and the cooling unit is directly mounted on the exhaust tube system of an engine.