CO2 Sensors: Overview, Design, Manufacturers
Despite the fact that in use the word "air" is often replaced by "oxygen", in fact, it is not its only component. In only 21% of oxygen and almost 79% of nitrogen. And only 0.03% is the level of CO2 in the air. However, this gas is essential for living organisms. Related to this is the relevance of using CO2 sensors to measure its concentration.
The need for sensors
In many cases, the concentration of carbon dioxide — CO2 is measured not in percent, but in ppm.
If the level of this gas in the air increases, the person begins to feel drowsiness, fatigue, stuffiness, loss of concentration function, attention is lost, excess irritability appears, and efficiency decreases.
A strong increase in the concentration of this gas leads to asthma attacks or even loss of consciousness.
Gas exchange in cells
In the closed unventilated rooms in the air there is a lack of oxygen and an excess of carbon dioxide.At the same time, a person begins to consume the remaining oxygen in a large volume, more and more saturating the air with carbon dioxide. The diffusion capacity of the latter is 25–30 times higher than that of oxygen, which makes a person more sensitive to his excess.
Normal gas exchange in the human body occurs only at the optimum value of the partial pressure of carbon dioxide in the blood.
The change of this value in any direction contributes to the fact that the transport of oxygen to the cells deteriorates. When holding the breath, the transfer of carbon dioxide does not stop; deep breath after this is due to the increase in the partial pressure of CO2.
Thus, the presence of a person for a long period of time in a room with a high concentration of carbon dioxide contributes to the occurrence of negative consequences. Therefore, the emphasis has always been placed on the need to ventilate the room for ventilation. A CO2 sensor allows control of the air exchange.
By power source, CO2 sensors are classified into:
- stationary, which operate from the usual electrical network, installation of which may be carried out on the wallstables, floors and air ducts;
- autonomous, which work at the expense of battery energy.
In the ducts, the CO2 concentration is determined using channel sensors, in office and residential premises - room sensors.
Advanced sensors are capable of managing the ventilation system, adjusting the flow of fresh air and calibrate in open areas.
In modern devices of this type, it is possible to control not only the level of gas pollution with carbon dioxide, but also the temperature and humidity conditions, some models may inform about the appearance of smoke in the room.
Carbon Dioxide Gas Analyzer Device
Non-dispersive devices are equipped with light filters or receivers, the principle of operation of which is similar to the first. Also in this device there is a working chamber, from which air flows into the next receiver.
The radiation source can be an infrared laser, an LED, a heated coil.
Selective receivers, semiconductor elements, bolometers and thermopiles are used in infrared sensors.Non-monochromatic radiation is used in the selective receiver, in other cases the sensors incorporate interference or gas filters. In some models, a combined filter is used, consisting of two, one of which passes the IR band of the specified range, and the other does not.
The signal mismatch is accomplished by using a computing device and a precast detector located behind the filter.
Those sensors that are designed to work in open areas are made of PC polycarbonate. Acrylonitrile-butadiene-styrene devices are used for indoor work, as they have a lower impact resistance and a lower temperature range.
Carbon dioxide sensors are capable of selectively absorbing a CO2 molecule. Dispersion sensors use single-wave radiation received by a monochromatograph.
They are mounted in the area of direct service or exhaust duct.
To prevent gas exchange between the oxygen of the air duct and the analyzer, the body of the latter must be isolated.
In order to make information available from the sensors, they are placed at the optimum height in the illuminated space.
If the CO2 level is determined in the open air, then after the removal of the device according to the instructions, it is purged.
In case the device is subject to wind, dust, precipitation, vibration, steam blow, a mud guard should be installed directly in front of it.
If water gets into the device, it can fail. In most cases, the effect of water extends to the ends of the wires, since the device has a fairly tight enclosure. Then there is the risk of corrosion, weakening of the terminal connections. In order to prevent this, it is necessary to treat with anti-corrosive liquids and block ports that are not currently used.
Ventilation of several rooms with one sensor
For this purpose, supply and exhaust sensors ZENIT and ZENIT HECO, as well as consumer equipment I-VENT and CAPSULE can be used.
A proportional valve is installed on the inlet channel in each room. In the presence of exhaust in each room there is also located a similar valve.For each exhaust channel or even for a room, a separate CO2 sensor is installed on which the VAV system is mounted by the manufacturer.
When a person enters the covered premises, the sensor detects an increase in the concentration of carbon dioxide. The proportional valve has an electric drive, on the basis of which the air exchange is adjusted.
In this case, the air quality in the room is maintained at the highest level. The concentration of CO2 in the air is at optimum values.
As people enter and exit the premises with the air exchange controlled by the sensors, it increases or decreases to the optimum values.
It is possible to use one sensor instead of several in the exhaust duct, however one device records the total concentration for all rooms with a single exhaust fan. Air exchange will be regulated equally, which is not always justified.
Ventilation of one room with one sensor
For this purpose, only a CO2 sensor in the exhaust duct is installed (installation is carried out by the manufacturer). Air exchange is regulated automatically. The same brands can be used as sensors as for ventilation of several rooms.
This creates a ventilation system with optimal performance, fully automated, characterized by low operating costs.
Calculation of the ventilation system for carbon dioxide
For the calculation, it is necessary to have information about the level of the gas being studied outside the window, the human flow in the studied rooms, what physical activity is occupied by the people who are in these rooms, what concentration of carbon dioxide is needed.
In order to compensate for the release of CO2 by each person, air exchange is calculated using the formula L = (G × 550) / (X2-X1). We make the necessary explanations:
- L - air exchange, m3/ h;
- X1- carbon dioxide level in the supply air, ppm;
- X2- MAC of CO2 in the air of the room, ppm;
- G is the volume of carbon dioxide emitted by each person, l / h;
- 550 - coefficient used to convert X values1and X2in g / m3from ppm.
The release of carbon dioxide by each person averages 23 l / h, while in a larger volume the secretions are noted during heavy physical work (up to 63 l / h), light work (up to 36 l / h) and walking (up to 33 l / h). The smallest amount of this gas is released during sleep (12 l / h), sedentary and office work (15-18 l / h).
The concentration of this gas in rural areas is 332 ppm. Cities are more polluted. So, in small cities with a population of up to 300 thousand people, this value is 409 ppm, and in cities with a larger population - 511 ppm.
Air quality in residential and office premises according to DIN EN 13779
This regulatory document establishes a classification for the excess of the CO2 content in a room in relation to the outdoor air concentration.
Here it is established that high-quality air has a typical range of <400 ppm, medium-quality air - 400-600 ppm, air of satisfactory quality - 600-1000 ppm, low-quality air -> 1000 ppm.
The efficiency of the exhaust fan operation depends on how efficiently the measurements are made by the carbon dioxide sensor. At a time when there are a lot of people in the room, the ventilation system should work at its limits. As the number of people in the room decreases, the air supply should gradually decrease, until the air supply stops completely. This can be achieved using successive cycles of switching on and off of several fans or the possible use of fans with a two-speed electric drive,as well as with a dynamic decrease or increase in the number of revolutions of the electric drives of the fans with frequency regulation. The latter method is most preferred.
Consider some sensors of the concentration of carbon dioxide produced in Russia, as well as foreign ones.
SenceAir CO2 Level Sensors
This Swedish company is a world leader in the design and production of the considered types of gas analyzers. The devices produced by this company are distinguished by high performance properties. They can be connected to a PC, as a result of which long-term operation of these devices can be achieved. The measurement range is up to 5000 ppm with an accuracy of 30 ppm. Used sensor NDIR.
The detector of carbon dioxide from the company "Master Kit"
This device, according to the manufacturer, is able to determine the CO2 concentration in the range from 0 to 3000 ppm with an accuracy of measuring from 0 to 2000 ppm in the region of 10% (for large values, the accuracy is not indicated).
The device should be operated at a temperature of +25 ° C. If the temperature does not match, it is necessary to make appropriate corrections. The device cannot be used at t exceeding +50 ° C, and if it is below 0 ° C. This parameter is measured by the detector under consideration.Temperature can be expressed both in degrees Celsius and in degrees Fahrenheit.
The device has LED indicators that show the level of gas pollution with carbon dioxide: green at a concentration of up to 800 ppm, yellow - from 800 to 1200 ppm, red - at a level of more than 1200 ppm. This information is put on the box with the device.
The country of manufacture is China. The device is completed with a USB cable and the instruction which is given in Russian. A cable is required to provide power to the instrument. Operation of this device involves the use of "native" USB-cable, because not everyone will fit here.
The life span of the sensor is 5-10 years.
Calibration needs to be done every three years. To do this, as well as to plot graphs from the data obtained using the detector, a special program is available that is available for Windows OS, as well as source codes for the GNU / Linux OS family (graphics) created by an independent programmer and not supported by the company.
The sensor is not a chemical, but an NDIR-sensor, related to modern devices.
Designed CO2 sensor for home. This is due to the fact that it has a low accuracy of 100 ppm.
Kvazar LLC produces a variety of gas analyzers, including the CO2 sensor under the brand name OKA-T-CO2.
This sensor is mounted on a DIN rail. Indication indications - digital.
Like foreign analogs, it has an interface for communicating with a computer.
Unlike foreign analogues, this device is graded not in ppm, but in volume percent. The measurement range is from 0 to 5. At 0.5%, the device gives a signal that you can adjust. The relative error of the device, according to the manufacturer, is 25%, which is quite a lot compared to the previous models considered.
Verification should be done annually. The advantage is a wide temperature range of work - from -40 ° C to +50 ° C.
Carbon dioxide sensors display information in digital form. It is necessary for a person to always be in good shape, feel good. There are similar devices both foreign and domestic production.