Scientists have long had an idea about the effect of CO2 gas (carbon dioxide, carbon dioxide) on the human body. According to the information specified in the classifier of hazardous substances GOST 12.1.007-76, carbon dioxide is considered a low-hazard substance (class 4), has a low concentration in the atmospheric air. By itself, CO2 has a low degree of harmful effects on the environment, but an increase in the concentration of gas in the air to 7% can harm the human body: breathing becomes difficult, suffocation occurs. A feature of carbon dioxide is that it does not have the ability to heat up the body, with a decrease in the concentration of CO2 in the air, breathing is completely restored.
ASHRAE: standardization of HVAC equipment
High levels of CO2 concentration in the atmospheric air (from 0.1 to 0.7%) have a negative impact on a person, sharply reducing his performance. Unlike carbon dioxide, oxygen can change its concentration over a wide range without causing harm to he alth. The ASHRAE HVAC Standards Committee has establishedthe allowable rate of carbon dioxide in rooms with people at the level of 0.1% of the total air volume. It is the allowable CO2 indicator, indicated by ASHRAE, that is considered the baseline when calculating air exchange.
Purpose of measuring CO2 concentration
In a general sense, 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 for indoor air quality, therefore, focusing only on the concentration of carbon dioxide, and with a ventilation system containing CO2 sensors, indoor air quality can be effectively regulated.
When breathing, the average person, inhaling oxygen, is able to exhale from 0.35 to 0.5% of carbon dioxide. In other words, the mixture of gases exhaled by a person exceeds the concentration of CO2 compared to the outside air by 100 times. If a person is indoors, within a few hours the concentration of carbon dioxide in the air increases many times over, and the air quality drops sharply.
Inhaled CO2 limits
Despite the fact that carbon dioxide has neither color nor smell, its increased concentration is easily felt by a person. When inhaling air with a high content of CO2, fatigue is felt, absent-mindedness occurs, and a person becomes inattentive. The problem of air with an excessive content of carbon dioxide is especially acute in closed public and educational institutions, medicalestablishments.
Experts in the laboratory found that the gas concentration above 0, 1% is already able to have a negative effect on humans. 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 overcrowded rooms and public transport, a similar proportion of gas in the air is not capable of causing much harm and is considered acceptable for breathing.
Increased concentration of carbon dioxide (from 0.05 or more) contributes to low activity of the human body, drowsiness, slow reactions and a low indicator of the thought process, there is a feeling of suffocation.
Monitoring room air quality: wall-mounted CO2 sensor
Wall-mounted CO2 sensors continuously measure the CO2 concentration and send a control signal to the ventilation unit to remove excess carbon dioxide. Sophisticated climate systems may have built-in sensors, but it is possible to use an external CO2 sensor and then connect through separate outputs to the fan.
There are various options for wall sensors on the market, there are devices with relay or analog outputs, as well as outputs for the monitor screen. Since manufacturers can supply control sensors with only one output, some owners modify the devices themselves. CO2 sensor, ownmanually improved and containing all the listed options for transmitting the output signal, it is most effective because it is compatible with any ventilation system. Modern CO2 sensors should implement a self-calibration system to improve 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 indicator and ventilation system mode control buttons; a sensor that does not contain LED indicators and individual control buttons.
The sensors are powered by low-voltage AC networks. Some manufacturers provide an additional option to connect a power supply to the CO2 sensor.
Functionality of CO2 sensors
Almost all sensors are able to measure the concentration of carbon dioxide in the air stream, controlling the limit values. CO2 sensors are capable of measuring gas concentrations in the following ranges:
- 0 to 2000 ppm (0.02%);
- 0 to 3000 ppm (0.03%);
- 0 to 5000 ppm (0.05%);
- 0 to 10000 ppm (0.1%).
The data received by the device is converted into active 0-10V output signals. The sensors for calculating the CO2 concentration absorb non-scattered infrared radiation (NDIR). The devices are equipped with a protective shell of the maximum protection classes IP65-IP68.
In the absence of integrated devices for visual display of resultsmeasurement uses a CO2 sensor with analog output. Carbon dioxide meters have an automatic and manual zero calibration function. Before calibration begins, uninterruptible power must be supplied to the instrument for 10 minutes. The room where the sensor is installed must be ventilated. The corresponding zero point carbon dioxide concentration level is 300 ppm (0.003%). Most carbon dioxide sensors are calibrated once, with subsequent periodic calibrations performed automatically. After the CO2 sensors are first powered up and started up, the instrument performs its own testing and setup procedures. During the first five minutes after launch, the output may not correspond to the actual values.
Adaptive residential ventilation
Adaptive ventilation differs from traditional ventilation only by operating modes. Traditional fans operate in one mode, energy consumption does not depend on the number of people in the room and the quality of the air in it.
The adaptive ventilation mode is controlled automatically, for which a CO2 sensor for ventilation is used, which controls the carbon dioxide content in the air. Thanks to an intelligent control system, the fan will supply the amount of air that is needed and sufficient.
The need to control ventilation by sensorCO2
The admissibility of the level of CO2 concentration is regulated by state standards, one of which is GOST 2.1.005-88 (sanitary and hygienic requirements for the air of the working area). According to GOST, when considering the allowable values of carbon dioxide in the air, the minimum performance indicators of ventilation equipment are also taken into account (30 m3/h per person). Based on the requirements of GOST, each person present in the room should receive 30 m33 of running air in 1 hour.
CO2 controlled ventilation systems
HVAC specialists often use the concept of air distribution efficiency. The air distribution efficiency index is understood as the speed with which the fresh air flow reaches the recreation area or workplace (breathing zone). The quality of the supply air entering the breathing zone should not decrease as you move around the room, in other words, the fresh air flow should not come into contact with the one that contains a high concentration of CO2.
Modern climate systems and technologies quite effectively and economically perform the functions of ventilating the premises. Built-in carbon dioxide sensors and meters are able to control the ventilation system to ensure proper indoor air quality while minimizing energy consumption.
Climatic systems in operation are guided by indicators of CO2 concentration in the air, electronicscompares the received value with the given one. CO2 sensors provide control of the ventilation system, maintaining the air quality at optimal parameters. Such systems are successfully used in rooms with a variable number of people. A high energy saving class is achieved by optimizing the ventilation power.
Where to install a CO2 sensor or monitor
The choice of the location of the carbon dioxide sensor must be carried out based on the restrictions:
- the device must be at least 1 m away from the permanent location of people;
- household CO2 sensor is not placed closer than 1 meter to the supply ventilation;
- organization of optimal power supply of the device implies its close location to the energy source.
