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    Updated: 10 May 2020


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Startpage > Products > Gas Detection > Nitrogen Dioxide

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Nitrogen monoxide, NO is a colorless, toxic gas. In air it is oxides into nitrogen dioxide NO2.

Nitrogen dioxide is a brown, toxic gas with a pungent odor.

NO and NO2, are named NOx.

These emissions along with sulfur dioxide causes acid rain. Because of these emissions, many new rules have been adopted in recent years.

Alarm limits for NO2 or NO for monitoring of the ambient air are set according to the occupational exposure limits:

  • Alarm 1 = 4 ppm 30 ppm, threshold lvl.

  • Alarm 2 = 5ppm 50ppm, Max exp. lvl.

If the source is exhausts set the following limits:

  • Alarm 1 = 1ppm 20ppm, threshold lvl.

  • Alarm 2 = 5ppm 25ppm, Max exp. lvl.

Alarms are preferably set according to these exposure levels with the threshold limit value as the first alarm limit and the top limit value as the second other alarm limit.

Using aspiratory systems can be probematic since the NO2 is easily absorbed by the material in gas pipelines.

Detector location
Both nitrogen monoxide and nitrogen dioxide are heavier than air, which theoretically means that the gas will fall to the floor in air, but these gases are also sensitive to drafts, etc.

Fire Hazard
Both NO and NO2 are combustible gases.

Nitrogen oxides NOx
These oxides cause acid rain, which gives both acidified lakes and algal blooms in eutrophic waters.

High concentrations may cause respiratory diseases.

Gas monitoring and demand controlled ventilation in garages and parking garages

Nitrogen dioxide NO2 is formed in bus- and truck garages, road tunnels, mountain cavities, etc.

Gas monitoring in garages meet two main needs:

  1. To warn when the quantity of harmful gases reaches an unhealthy level.

  2. To ensure that the management of ventilation is done the best and most profitable way, ie. according to fresh air requirements.

When the concentration becomes too high, the fans starts automatically and stops again when the NO2 concentration has down to the predetermined level.

Harmful gases
Harmful amounts of nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO) can be obtained from petrol and diesel exhaust fumes.

As a rule only carbon monoxide is monitored in the garage since it is often wrongly believed that the other gases do not reach harmful levels.

Carbon monoxide is an extremely dangerous toxic gas.

Earlier, people often commited suicide by inhaling exhaust fumes from cars.

Today, the concentrations are much lower, thanks to catalytic converters in cars and carbon monoxide is no longer an environmental problem, except perhaps at cold starts.

Regarding carbon monoxide and nitrogen dioxide from a gas monitoring point of view, it is appropriate to have two alarm levels, one of which occurs at 20 ppm (CO / resp 1-2 ppm (NO2), and the second at about 25 ppm or 5 ppm.

Hygienic threshold limit values for NO2 (gaser from diesel vehicles, propane driven ice makers).

  • NGV = 1 ppm if source from exhaust fumes: Warning, fan starts at half or full speed

  • TGV = 5 ppm: Alarm, upper threshold limit
    With NGV is understood a concentration that is acceptable for a full day (8 hours).

TGV is the concentration of the average value during 15 minutes of carbon monoxide level / nitrogen dioxide level.

The alarm level is exceeded in two gas sensors simultaneously for the warning lights to be activated.

At the higher alert level, for example, on a juice blender warns of unhealthy levels of carbon monoxide / nitrogen dioxide levels, it is advisable to have exceeded the higher of two gas sensors simultaneously for juice blender to be activated.

The reason is to avoid the "distorting load" it can give rise to if a car happens to be in direct proximity to a sensor resulting in of an overdose of carbon monoxide or nitrogen dioxide.

A sensor density of at least 1 sensor / 400-500m2 is appropriate.

If there is heavy traffic in the garage, it is important to take account of other harmful gases Nox (collective term for various nitrogen oxide compounds) and hydrocarbons.

In similar cases specific CO and NO2 sensors cannot cover the need for detection. Other sensors are required that can detect these gases.

Ventilation control
The minimum requirement to for ventilation control is that the gas monitoring facility affects ventilation in such a way that if there is harmful gas the fresh air intake should increase enough for the gas concentration to be reduced to harmless levels.

A well-regulated demand controlled ventilation in a garage not only improves the air quality but also minimizes energy consumption by avoiding unnecessary ventilation.

If you optimize ventilation according to the gas concentration it is enough to ventilate when the air is bad.

With a modern gas monitoring plant you can obtain not only alarm functions (two levels) but also control functions for air evacuation.

NO2 alarm in ice rinks
In the case of ice rinks where the nitrogen dioxide may occur at the incomplete combustion of LPG-powered ice-makers and thus cause health-damaging concentrations, it is recommended that a detector be placed at either short ends of the rink diagonally from each other to get the best coverage.

  • 1 ppm: Warning

  • 5 ppm: Alarm, Emergency services is alerted and the facility is evacuated

See also Projecting Guide

Choice of Equipment

The Stand-alone detector is a detector where power supply, alarm and control functions are integrated in the detector housing. This detector is used where very simple systems are sought and only few monitoring points are desired.


SNO2, Stand Alone detector - Wall
ModBus, Analog, Relay Output



Underst. SNO2  

deklaration RS485 RS485 Datasheet  


SNO2, Combi detector - Duct
 ModBus, Analog, Relay Output














Analog detector, ANO2 - Wall













Analog detector, ANO2 - Duct













Warning lights for nitrogen dioxide detectors

Gas alarm plate for nitrogen dioxide detector

















Warning Siren for nitrogen dioxide sensors


















Combined warning siren and flash light for
nitrogen dioxide sensors










BNO2, Field Bus detector - Wall
Modbus, BacNet, etc.



to do   BNO2  







BNO2, Fieldbus detector - Duct
Modbus, BacNet, etc.




Underst. Art &  




RS485 Science  
    Environm. Duct  
    declaration Kit  

The exchangeable sensor X-CHANGE turns the recalibration into a home game:

  • Delivery of recalibrated sensor cartridges

  • Easy exchange, no specialized company needs to come to you

  • Available for every gas type

  • At guaranteed exchange prices

  • Increased reliability (SIL2-software)

  • Important time and cost savings

The X-Change support guarantees a price based on the consumption of the sensor capacitance.

For each uncalibrated sensor returned to AP, you will receive a credit for the unused capacity of the sensor.

More >





Legal and safety requirements
The devices are manufactured according to the regulations and various directives such as EN 50545, etc.

Products delivered by AP meet and even exceed the new European Standard EN 50545.

The safety features check the connected warning devices on functionality and open circuit day and night (Level SIL2 according to EN 50271).


Quick and easy replacement of sensor

  1. Open the housing with a screwdriver.

  2. Disconnect the sensor cable and unscrew the disused sensor head.

  3. Exchange the sensor cartridge (3) and connect the cable of the new sensor head to the PCB.

  4. Close cover of the housing with the screws (1). The recalibrated sensor is ready for use at once.


Electrical products in an explosive environment
A small amount of energy - a flame, spark, high surface temperature or similar is required to ignite an explosive gas mixture.

Products installed in an area with a potential explosion hazard must of course in themselves  not be a potential explosion hazard.

For this reason, the products are manufactured in such a way that this cannot occur.
The equipment must also be labeled and tested - ATEX classified - by the approved testing authority.

The Ex-class information is available in "Swedish Standard"
The choice of which Ex-classification is required is determined by factors such as type of gas, permissible maximum surface temperature and potential explosive hazards in the area where the products are to be installed.

The potential explosion hazard in the different areas is determined by the zone classification according to "Swedish Standard".

More about ATEX-products can be found in the following link

ATEX explosionproof detectors

CNO2, Combi detector - Wall
ModBus, Analog, Relay Output




Underst. CNO2  

declaration RS485 RS485 Datasheet  




CNO2, Combi detector - Duct
 ModBus, Analog, Relay Output




CE missing  











E-mail: lars@fjellcom.se     

Phone: +46(0)31-287202

Mobile: +46(0)706-755561



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