how accurate are our CO2 detectors?

A recent scientific study has found that the CO2 sensors found in many of the detectors supplied by Edaphic Scientific are highly accurate. Reseachers found that low cost, CO2 sensors have the same accuracy and performance as highly expensive laboratory equipment.

Our range of Non-Dispersive Infra-Red (NDIR) CO2 sensors were found to have a 1:1 relationship when compared with an independent measure of carbon dioxide.

The CO2 sensor inside of our CO2 detectors is based on Non-Dispersive Infra-Red (NDIR) technology. NDIR is a low cost approach to measuring carbon dioxide for indoor air quality (IAQ) and HVAC applications. Since it is low cost, some users have questions the accuracy of the sensors.

Undoubtedly, there are some NDIR CO2 sensors that are poor quality. However scientists in Sweden recently compared the CO2 sensors in our detectors against a standard and found them to be highly accurate.

The CO2 sensors supplied by Edaphic Scientific cost in the order of hundreds of dollars, yet the standard the Swedish scientists compared them against is a high precision, scientific laboratory instrument that costs in the hundreds of thousands of dollars.

The proof can be easily seen in the following graph. The x-axis is our CO2 sensor and the y-axis is the expensive laboratory instrument standard. The line through the graph is the 1:1 line – meaning that if the data (circles in the graph) fall on this line then there is an exact relationship between the two different measurement techniques.

CO2 Sensor Comparison

A comparison of an NDIR CO2 sensor versus an expensive, laboratory CO2 instrument. Source: Bastviken et al 2015, Figure 2.

 

As can be seen in the above graph, the data fall exactly on the 1:1 line. Therefore, you can have high confidence that the CO2 measurements you are recording with many of our sensors matches exactly with some of the most expensive scientific measurements available.

 

reference

Bastviken et al 2015. Biogeosciences, 12, 3849 – 3859

 


7 reasons why you need to measure CO2 inside buildings

CO2 has been a hot topic for a number of years mainly in relation to climate change and government policy. Leaving all of that aside, monitoring and controlling indoor levels of CO2 is important for everyone to consider for health, safety and even energy efficiency.

CO2 is a known indoor pollutant affecting performance in the workplace, at school, and even at the gym. Extreme levels of CO2 can lead to death, particularly in enclosed spaces such as laboratories, some hospital rooms, and breweries. CO2 can have a number of effects on home and workplace health and safety.

Controlling CO2 can also improve building energy efficiency, saving costs by up to 80%, and is even considered in the scoring of Green Star Ratings in building design.

Here, we outline 7 reasons why you should be measuring carbon dioxide levels inside buildings.

 

1.      CO2 can kill you

Outside air has a CO2 concentration around 400ppm and each human breath contains around 30,000ppm. CO2 concentrations greater than 20,000ppm cause panting; above 100,000ppm (10%) CO2 can cause tremors and loss of consciousness; and values above 250,000ppm (25%) can lead to death (Satish et al 2012). CO2 can be hazardous in one of two ways: by displacing oxygen in the blood or as acting as a toxin.

The Canadian Centre for Occupational Health and Safety has provided detailed information on how CO2 can be a hazardous gas under many circumstances.

 

2.      CO2 can decrease productivity

In the office and classroom, elevated levels of CO2, in the range between 1,000ppm and 2,500ppm, have been found to decrease information utilisation, increase headaches, decrease performance, and increase rates of absenteeism (Satish et al 2012). Generally, CO2 concentrations at 1,000ppm can lead to a statistically significant decrement in decision making performance. CO2 concentrations at 2,500ppm, on the other hand, lead to large and highly significant decrements in decision making performance.

Although CO2 is not the only factor, elevated levels can lead to that feeling of lethargy and tiredness often associated with office workers. Studies have shown that performance, associated with lethargy induced by elevated CO2, can decrease by up to 10% for adults and over 20% for schoolchildren (Wyon and Wargocki 2013).

 

 

3.      CO2 can increase rapidly in poorly ventilated rooms

Figure 1 shows how rapidly levels of CO2 can increase in a poorly ventilated office. For example, in a 3.5-by-4-metre sized office with a single occupant, CO2 increased from 500 ppm to over 1,000 ppm within 45 minutes of ventilation cessation.

 

Edaphic Scientific CO2 Levels for HVAC and IAQ

Figure 1. An example of increasing CO2 concentration in a poorly ventilated office with a single occupant.

 

In surveys of school classrooms in California and Texas, average CO2 concentrations were above 1,000 ppm, many exceeded 2,000 ppm, and in 21% of Texas classrooms peak CO2 concentration exceeded 3,000 ppm (Satish et al 2012). Such high levels of CO2 could have a particularly adverse effect on concentration during exam periods.

Generally, where large numbers of people gather then CO2 will increase rapidly and lead to poor indoor air quality and pollution. In offices, this could be meeting rooms where a number of staff gather for extended periods in confined spaces.

Other places, such as gyms, shopping centres, cafes with soft drink vending machines, or libraries, are increasingly being recognised as indoor environments with elevated CO2 leading to poorer performance. For example, a newly built, state-of-the-art, library in Geelong, Victoria, Australia, installed CO2 detectors.

 

4.      Some locations have naturally high CO2 levels and need to be monitored

There are certain locations where indoor CO2 in an enclosed room or area can potentially reach extreme and life threatening levels.

Any enclosed or poorly ventilated location where CO2 cylinders are stored or used will potentially have harmful levels of atmospheric CO2. Examples of such locations include laboratories and hospitals.

Other spaces where CO2 is regularly used in the manufacturing or work process are also potential areas of harmful levels of CO2. Breweries are potentially extremely hazardous. Pockets of high CO2 can form in tanks and cellars and can quickly lead to death. Even bars, clubs and pubs, where CO2 cylinders are stored in a room, are increasingly required to monitor CO2 levels for workplace safety.

Using CO2 sensors for ventilation control can assist in these cases. However, other systems with audible and visual alarms, such as the ESRAD-102 can be installed (Figure 2) warning workers and occupants of dangerous levels of CO2. The sensor is installed in the space with the CO2 source and an alarm indicator is placed outside.

As an aside, there are those extreme and unlikely locations, especially in the outdoors, where CO2 asphyxiation has led to the death of many people. Volcanoes, for example, not only pose the hazards we are all familiar with but can also eject large amounts of CO2. There have been reported cases around volcanoes of death via CO2. Volcanic eruptions increase atmospheric CO2 to dangerously high levels in confined spaces or areas of poor ventilation. This includes topographic depressions such as valleys. But it could also include building basements, car parks or lower floors.

 

CO2Meter Alarms

Figure 2. The ESRAD-102 CO2 Storage Safety Alarm can save lives in locations where extreme levels of CO2 occur. 

 

5.      Monitoring CO2 for energy efficiency

Many facility managers are increasingly turning towards monitoring CO2 for Demand Controlled Ventilation (DVC). Ventilation units can automatically set air intake on the assumption of maximum occupancy rate of a room, office or classroom. However, occupancy is often intermittent and unpredictable therefore leading to over-ventilation and energy inefficiencies. Monitoring CO2 levels and automating ventilation to intake air at pre-defined CO2 levels, such as 800ppm, will lead to ventilation when it is actually needed.

One study found that monitoring CO2 for DVC saved between 5 and 80% on energy costs compared with a fixed ventilation strategy (Emmerich and Persily 1997).

Other technologies to monitor occupancy level may not be as efficient as monitoring CO2 levels. For example, some building controllers use relative humidity set points. However, humidity set points can vary widely, change slowly and not directly reflect occupancy. Another method is to use a PIR, or presence detector, sensor. This method is used widely to automatically turn on lights when a person enters a room. It uses infra-red sensor to detect motion and, therefore, whether there is a person occupying a room. However, it is difficult for this method to detect how many occupants there are in a room. Measuring CO2, on the other hand, can determine the presence of an occupant (CO2 levels will increase) and the number of occupants (the rate of change in CO2 levels will be higher with more occupants).

 

6.      Improving your green building score

The Green Building Council of Australia scores 1 to 2 points if CO2 levels are maintained below 800ppm or 700ppm respectively. This move recognises the relevance of optimal CO2 level for occupancy comfort and productivity.

 

7.      The novelty factor

Measure indoor levels of CO2 is new to most people. In fact, most people would not have a clue what the CO2 levels in their room are, what they should be, and how they change throughout the day with various factors.

Monitoring CO2 levels with a data logger is interesting, or having a wall-mounted or desktop LCD read-out, showing real-time CO2 levels, is also interesting. Informing your guests that you are controlling the ventilation in your building with a CO2 detector will certainly raise a few eyebrows!

 

CO2 technology, installation and maintenance

Indoor CO2 is measured with a relatively inexpensive technology called NDIR, or Non-Dispersive Infra-Red technology. Figure 3 is a diagram of a simplified NDIR based CO2 sensor. At one end is a LED source emitting infra-red light and at the other end is a detector with a specific light filter to detect only CO2. Gas diffuses into a chamber between the LED source and detector. The amount of CO2 in the chamber is directly proportional to the amount of absorbed light (read more detailed explanation on NDIR measurement principle).

 

NDIR Simple Schematic (Small)

Figure 3. A simplified diagram of the NDIR principle of CO2 measurement.

 

CO2 sensors for facility managers can be one of two types. The first is a simple detector, such as an ESSE-12, that has either voltage or 4..20mA output that can run back to a BMS. These detectors are ideal where multiple units need to be installed and operated by a single BMS.

These detectors can connect directly into a HVAC unit to control ventilation. This type is ideal where only one sensor is needed.

Other types can additional measure temperature and humidity, such as the ESSE-34, providing a complete monitoring solution.

Installation of sensors should be in an occupancy space rather than ducts. Maintenance depends on the type of model. Most CO2 detectors are delivered pre-calibrated yet require periodic calibration (you will need to check the manufacturer’s specs as this can vary from months to years). One set of models has Automatic Background Calibration (ABC) and provides a maintenance free option. Around 4am each night, the sensor detectors CO2 levels in the office and assumes it should be at 400ppm as there are no occupants. If the value drifts from 400ppm over successive nights, the sensor automatically recalibrates to provide accurate measurements.

 

conclusion

A work or learning place designed for optimal productivity leads to better performance and, ultimately, is better for the bottom line. Monitoring and controlling CO2 levels is one approach to a healthier workplace environment.

 

references

Emmerich SJ. & AK Persily (1997). ASHRAE Transactions 103(2):229–243

Satish et al (2012). Environ Health Perspect. doi:10.1289/ehp.1104789

Wyon DP. & P Wargocki (2013). REHVA Journal, August, 6-10