How to choose the right CO2 sensors

Indoor air quality is a hot topic right now as organisations begin to reopen their doors. Studies show that a good general measure of indoor air quality can come from looking at the amount of carbon dioxide (CO₂) present. 

Unlike outdoor air, indoor air tends to be continuously recycled, causing it to trap pollutants which can then build up within these confined spaces. The higher the concentration of occupants, the higher the levels of CO_2. Without the right ventilation, this can not only lead to virus transmission but also leads to dizziness, tiredness, nausea and even vomiting.

There’s a delicate balance to find in achieving the optimum air quality, with optimum conditions. CO₂, temperature and humidity all play their part. Getting the balance right can help employees operate at their most productive. It can also reduce illness absence and boost employees’ health and wellbeing.

Which is where CO₂ sensors can play a vital role. Monitoring indoor CO₂ forms an integral part of smart workspace solutions. Using the data, building managers can implement data-driven ventilation to allow the flow of fresh air to be automatically optimised. As organisations seek to transform their buildings into ‘healthy’ buildings, CO₂ sensors are a simple, yet powerful tool to optimise indoor air quality.

What are CO₂ sensors?

CO₂ sensors measure the concentration of CO₂ particles in the air – parts per million (ppm for short).

There are three main types of CO₂ sensor:

1. Non-dispersive infrared sensors
2. Electrochemical sensors
3. Metal oxide semiconductor sensors

Each has its own strengths and weaknesses, which we’ll explore below.

1. Non-dispersive infrared (NDIR) sensors

Simply put, NDIR sensors use specific wavelengths of light to measure the amount of CO₂ in the air.

Every single element absorbs certain types of light. When things are broken down into atoms and molecules, we can pinpoint what kind of light each absorbs, even light we cannot see.

This means we can use sensors to test if certain elements are present, and if so, in what quantity.

How they work:

As air enters the sensor, a light set at one of the specific wavelengths for CO₂ — usually around four microns — is activated at one end of the sensor. The other end holds a receptacle to measure how much light makes it to the other side. Once the light is activated, CO₂ gas molecules absorb the specific band of infrared light. The amount of light it absorbs depends on how much carbon dioxide is present. The more CO₂ present, the more light the sensor will absorb.

Pros:

• Low cost compared with other technology
• Very long-lasting, with some lasting more than 10 years
• Other substances will not interfere with readings
• Work well at common CO₂ ranges (0-2550ppm)

Cons:

• Can be affected by extreme temperature and humidity levels

2. Electrochemical sensors

Electrochemical sensors measure electrical current or conductivity to determine how much CO₂ is present in the air.

How they work:

When CO₂ enters this sensor, it creates a chemical reaction. As this reaction occurs, the sensor experiences an electrical change. Depending on the specific type of sensor, the reaction can make the sensor pick up an electrical current, change an existing current, or change how well the sensor would carry a current.

The sensor will then use the type and amount of electrical change to determine how much CO₂ is present.

Pros:

• Less susceptible to humidity and temperature changes than standard NDIR or MOS sensors

Cons:

• Other substances can throw off readings by cross-responding to other gas species, such as water vapour
• Not as long-lasting as NDIR sensors
• The sensor can ‘drift’ or lose accuracy over time

3. Metal oxide semiconductor (MOS) sensors

MOS carbon dioxide sensors use the resistivity of metal compounds to test the amounts of gas in the air.

‘Resistivity’ describes how easily electricity flows through something. For example, copper, which is used a lot in wiring, would be less resistant than rubber, which is used to stop electric currents.

How they work:

A MOS sensor has a metal strip or film that is exposed to the air being tested. This strip has a constant electric current running through it. As the air comes into contact with the piece, it will interact with the metal and change the chemical composition, either through a reduction or oxidation reaction.

When this happens, the resistivity, or conductivity, of the metal will be altered. The kind of resistance change, whether increasing or decreasing, and its magnitude, determines the concentration of the target gas. Based on what kind of metal it is, different gases will react to the strip.

Pros:

• Very simple design makes them easy to use

Cons:

• Can be affected by temperature and humidity
• Usually used at higher, less common CO₂ concentrations (>2000ppm)
• Other substances in the air can throw off readings by cross-responding to other gas species, such as water vapour

How to choose the right sensor

The case for using a CO₂ sensor is clearly strong. But when it comes to choosing the right one, there are a few things to consider:

1. Measurement range

While MOS sensors do very well in CO₂ levels above 2000ppm, this level is already high enough to affect you. NDIR sensors can provide information at lower CO₂ levels. Pressac CO₂ sensor can measure from 0-2550ppm with a very high degree of accuracy, which is much more helpful in a CO₂ monitor.

2. Longevity

NDIR sensors are the longest-lasting monitor currently on the market. While both electrochemical and MOS sensors have made considerable strides in this department, they are still behind the NDIR sensor type, which usually lasts 10-15 years or more.

3. Cross-sensitivity

Both MOS and electrochemical sensors can have something called cross-sensitivity bias. When a substance other than CO₂ reacts in the sensor, such as water vapour, it changes the electrical properties of the sensor and the readings it gives you. NDIR sensors do not have this problem, as only CO₂ can absorb the light emitted in the sensor.

4. Reading ‘drift’

Electrochemical and MOS sensors may eventually lose electrons, and the readings will ‘drift’. This means the readings displayed on your CO₂ monitor can be significantly higher or lower than the true value.

 5. Cost

NDIR sensors are a very cost-effective solution. They are often much cheaper than other technology and can be operated using a battery or solar-powered. This makes them easy to install and very low maintenance. It’s also worth considering whether you want to just measure CO₂ or multiple air quality parameters. Multi-sensors may be more expensive, but they can include CO₂, temperature and humidity sensors all in one device, saving you money overall.

6. Ease of data integration

Look for sensors that offer easy data integration and secure data transmission. Using Pressac’s Smart gateway, our sensors send data to a local network or cloud platform so it can be easily integrated with your ventilation system.

Need advice?

As technology advances, you can access multiple types of Indoor environment monitoring sensors for collecting data. Each has its advantages and disadvantages, so it’s important to work with a smart sensor expert when making your decisions. Contact our IoT business development team to see how we can support you to keep your internal air quality at optimum levels.