CO2 sensors: types, description

Scientists have long had an idea of the effect of CO2 gas (carbon dioxide, carbon dioxide) on the human body. According to the information specified in the classifier of harmful substances GOST 12.1.007-76, carbon dioxide is considered to be a low-hazard substance (4th class), has a low concentration in the atmospheric air. By itself, CO2 has a low degree of harmful effects on the environment, but increasing the gas concentration in the air to 7% can damage the human body: breathing is difficult, choking occurs. A feature of carbon dioxide is that it does not have the ability to be heated by the body, with a decrease in the concentration of CO2 in the air, respiration is completely restored.

ASHRAE: standardization of climatic equipment

High values of CO2 concentration in atmospheric air (from 0.1 to 0.7%) have a negative impact on a person, drastically reducing its efficiency. Unlike carbon dioxide, oxygen is able to change concentration in wide values, without causing damage to health. The ASHRAE Committee, dealing with standardization in the field of climate equipment, has established an allowable norm of carbon dioxide in rooms with people at the level of 0.1% of the total air volume. It is the permissible CO2 value, designated by ASHRAE, that is considered to be the basis for calculating air exchange.

The purpose of measuring the CO2 concentration

In general, the level of carbon dioxide in the air determines its stuffiness, which, in turn, depends on the number of people in the room. The amount of carbon dioxide is the main criterion of indoor air quality, therefore, focusing only on the concentration of carbon dioxide, and in the presence of a ventilation system containing CO2 sensors, it is possible to effectively regulate the air quality in the room.

When breathing, the average person, inhaling oxygen, is able to exhale from 0.35 to 0.5% of carbon dioxide. In other words, the exhaled gas mixture exceeds the concentration of CO2 in comparison with the outside air by 100 times. If a person is in an enclosed space, for several hours the concentration of carbon dioxide in the air increases manifold and the quality of air drops sharply.

Limit values of CO2 in the inspired air

Despite the fact that carbon dioxide has neither color nor smell, its increased concentration is easily felt by a person. When breathing in air with a high CO2 content, fatigue is felt, absent-mindedness occurs, a person becomes inattentive. The problem of air with an excessive content of carbon dioxide is especially acute in closed public and educational institutions, medical institutions.

Specialists in the laboratory established that the gas concentration index above 0.1% is already capable of having a negative effect on a person. The concentration of carbon dioxide in the range from 0.04 to 0.07% is optimal for human life. Carbon dioxide at a concentration of 0.07 to 0.1% is found in crowded rooms and public transport, a similar proportion of gas in the air is not capable of causing great harm and is considered acceptable for breathing.

The increased concentration of carbon dioxide (from 0.05 and more) promotes low activity of the human body, drowsiness, slowing of reactions and a low index of the thought process, a feeling of choking appears.

Room air quality control: wall mounted CO2 sensor

Wall sensors for carbon dioxide monitoring conduct a constant measurement of the CO2 concentration and provide a control signal to the vent to remove excess carbon dioxide. Complex climate systems can have built-in sensors, but it is possible to use an external CO 2 CO2 sensor and then connect through separate outputs to the fan.

In the market there are various variants of wall sensors, there are devices with relay or analog outputs, as well as outputs for the monitor screen. Since manufacturers can supply sensors only with one output, some owners independently modify the devices themselves. The CO2 sensor, with its own hands, improved and containing all of the listed options for transmitting the output signal, is most effective, since it is compatible with any ventilation system. In modern CO2 sensors, a self-calibration system should be implemented to increase the reliability and durability of the device.

Wall sensors have two most common modifications: a CO2 sensor with a relay output, containing a CO2 LED and buttons for controlling the modes of the ventilation system; A sensor that does not contain LED indicators and individual control buttons.

Power supply of the sensors is realized from low- current AC networks . Some manufacturers provide an additional opportunity to connect to the CO2 sensor power supply.

Functionality of CO2 sensors

Virtually all sensors are capable of measuring the concentration of carbon dioxide in the air stream, controlling the boundary values. CO2 sensors are capable of measuring gas concentrations in the following ranges:

• From 0 to 2000 ppm (0.02%);
• From 0 to 3000 ppm (0.03%);
• From 0 to 5000 ppm (0.05%);
• From 0 to 10,000 ppm (0.1%).

The data obtained by the device is converted into active output signals 0-10V. Sensors for calculating the CO2 concentration absorb unscattered infrared radiation (NDIR). The devices are equipped with a protective enclosure of the maximum protection classes IP65-IP68.

In the absence of integrated devices for visual display of measurement results, a CO2 sensor with an analog output is used. Instruments for determining the concentration of carbon dioxide have the function of automatic and manual calibration of zero value. Before calibration begins, ensure uninterrupted power supply to the instrument for 10 minutes. The room in which the sensor is installed must be ventilated. The corresponding zero point concentration of carbon dioxide is 300 ppm (0.003%). Most models of sensors that monitor the carbon dioxide content are calibrated once, further periodic calibration is performed automatically.

After the CO2 sensors first connect to the power and start up, the device performs its own testing and performs the installation procedures. During the first five minutes after launch, the output may not correspond to the actual values.

Adaptive ventilation of living quarters

Adaptive ventilation differs from traditional ventilation only by controlling the operating modes. Traditional fans work in the same mode, energy consumption does not depend on the number of people in the room and the quality of the air in it.

The operating mode of adaptive ventilation is controlled automatically, which is why the CO2 sensor for ventilation is used, by means of which the carbon dioxide content in the air is controlled. Thanks to the intelligent control system, the fan will supply the volume of air that is necessary and sufficient.

The need to regulate ventilation by the CO2 sensor

Admissibility of the level of CO2 concentration is regulated by state standards, one of which is GOST 2.1.005-88 (sanitary-hygienic requirements for the air of the working area). According to GOST, when considering the permissible values of carbon dioxide in air, the minimum performance of ventilation equipment (30 m ³ / h per person) is taken into account. Based on the requirements of GOST, every person present in the room must receive 30 m ^{3 of} running air per hour.

Ventilation systems with CO2 level control

Specialists in ventilation equipment often use the concept of air distribution efficiency. Under the indicator of the efficiency of air distribution is understood the speed with which fresh air flow reaches the rest zone or workplace (breathing zone). The quality of the fresh air entering the respiration zone should not decrease as you move around the room, in other words, the fresh air flow must not come into contact with one that contains a high concentration of CO2.

Modern climate systems and technologies perform efficiently and economically the functions of airing the premises. Built-in sensors and carbon dioxide concentration meters are able to control the ventilation system, ensuring proper air quality in the room, while minimizing power consumption.

Climatic systems in operation are guided by the CO2 concentration in air, the electronics compares the obtained value with the specified value. CO2 sensors provide control of the ventilation system, maintaining the air quality in the optimal parameters. Such systems are successfully used in premises with a variable number of people. A high energy-saving class is achieved by optimizing the ventilation capacity.

Where to install a CO2 sensor or instrument

The choice of the location of the carbon dioxide sensor should be based on the following limitations:

• The device should be removed from the place of permanent residence of people at least 1 m;
• The household CO2 sensor is not placed closer than 1 meter to the supply air;
• The organization of the optimal power supply of the device implies its close location to the source of energy.