How to measure?

Properly changing the air in your classroom reduces the risk of contracting the virus through the indoor air. But how do you know that enough air is changed?

The amount of CO₂ in the indoor air is a measure of the air exchange rate in the classroom and CO₂ can be easily measured with a CO₂-meter. The worse the air exchange rate, the greater the amount of CO₂ in the indoor air. If you keep the concentration of CO₂ in the classroom below 900 ppm, then you have good air exchange and there is less chance of virus particles accumulating in the indoor air.

A CO2 meter can help reduce the spread of the coronavirus COVID-19

Tips for buying a good CO2 meter

There is a large supply of CO₂-meters on the market. Before buying a meter or signing up for a group buy, check whether the meter offered is of high quality and suitable for use in the classroom.

When choosing a suitable and user-friendly CO₂-There are a number of things to watch out for.

Sensor type

Choose a NDIR (non-dispersive infrared) CO_₂-sensor.

Display

There are devices with a display that shows the measured CO₂-value. This has the advantage that you can quickly read the value, e.g. if your meter is in orange and you want to know how far you are above 900 ppm. With other devices, you can only view the values online or via an app.

Measurement error

Every measuring device, including a sensor, has a measuring error. This is the degree to which the result deviates from the actual value. For a CO2 meter (portable device, not used for controlling an HVAC system) a deviation of 10 % at 900 and 1500 ppm. In the case of devices with a measuring range up to 10 000 ppm, the deviation may certainly not be >10 % at 900, 1500 and 5000 ppm. The measurement error can be found in the technical data sheet of the meter.

Measuring range

In classes, CO₂-values can be high. So choose a broad measuring range of at least 5000 ppm.

Calibration method

Ageing of the sensor can cause "drift" (= a small, constant change in the measuring results of one and the same device in the same circumstances), which means that the sensor no longer measures correctly. It is therefore important that the sensor is calibrated regularly (=adjusting the sensor so that it measures correctly again).

There are two ways to calibrate sensors. Based on an external reference such as a calibration gas mixture or fresh outside air or based on an internal reference in the measuring device itself. Sometimes one speaks of single beam in the case of an external reference and dual beam in the case of a reference in the measuring device itself. In the long term, the measuring results of a device calibrated on the basis of an external reference are more reliable than the measuring results of a device calibrated on the basis of an internal reference.

In the group of sensors calibrated on the basis of an external reference, there are also self-calibrating devices. These devices use fresh outside air or air comparable to it (as is the case in an empty, well-ventilated classroom).

The easiest (no practical hassle of having the device calibrated by an external company) and cheapest (no lab costs for calibration) is to choose a sensor with a self-calibrating function, such as ABC (Automatic Background Calibration) Logic™.

LED indicator

At any time, you can see the measured CO₂-value, but with a colour LED indicator it is also quickly clear when there is too much CO₂-value is measured (the orange LED lights up) and therefore there is insufficient air exchange.

Warning levels LED indication

The CO₂-values whereby the LED on the device indicates a certain colour (green, red or orange) are often already set in the factory. Ask the distributor what values have been set and if necessary, ask them to adjust them. With some devices, you can also set the values yourself. The values that we recommend to set in order to reduce the risk of spreading SARS-CoV-2 are:

• From green to orange: > 800 ppm
• From orange to red: > 900 ppm

If you already have meters, but the set values do not match, see if they can be adjusted. If you cannot adjust the values, you can hide the lights to avoid confusion.

Sound

Some meters beep when they change colour. This can be annoying during the lesson. Make sure you can turn off the sound.

Model

Choose a table model that you can easily place on a table or low cupboard.

Datalogging

Some devices can also log, allowing you to read out the values later or view them online. The advantage of this is that you can easily follow the course of the CO₂-values over a certain period of time by means of, for instance, a ventilation diary and also the influence of certain interventions (e.g. opening extra windows or doors) to improve the air exchange rate. This option is not necessary but can be useful for the prevention advisor to map the ventilation in more detail.

Devices with data storage can be more expensive than devices without data storage. To view values online, you need a data platform. Manufacturers sometimes offer this, but then there is a chance that you are stuck with a system from that manufacturer. Don't forget to ask if the device can store the measurements if there is no internet connection. For some devices, the measurements can only be read with certain software (provided by the manufacturer or downloadable from the manufacturer's website). However, there is no guarantee that this software will work on your PC and that it will be available in the long term.

If you choose a system with logging via the Internet, make sure that the transmission of data takes place according to common standards (e.g. MQTT), otherwise you are stuck with the system of the supplier.

In devices that work with a data platform, an important aspect is the ownership of the data. Important questions here are: Where is the measurement data stored? Who owns the data? The simplest and safest devices are those that can store the values on a memory card. You can then import the values into, for example, Excel® for further processing.

We are pleased to say that our CO2 meter meets all the above requirements.

What you should know about your CO2 meter

Do you have a CO₂-There are a number of things you need to know about your CO₂-meter so that you can get started quickly.

Know how your device works 

Most devices are easy to operate, but it is best to read the manual. Check whether your device comes with a clear manual. If this is not the case, notify the person responsible for monitoring indoor air (management, prevention advisor). Perhaps an explanation of how the device works is organised at your school?

Check how to calibrate your device 

Without calibration, your device will measure incorrectly.

Does your device calibrate itself? Then make sure that the device can calibrate itself under the right conditions. For example, some meters need to be calibrated for a period of 4 to 8 hours per 24-hour period (e.g. during the evening and night) in an area with low CO₂-values (similar to the outside air concentration). Are you unable to provide the right conditions? If so, notify the person responsible for monitoring indoor air (management, prevention advisor).

Does your device not calibrate itself? If so, the device must be calibrated every 2 to 3 years unless the manual or technical sheet of the device states otherwise. Ask the person responsible for monitoring indoor air (management, prevention advisor) whether any agreements have been made on this, e.g. when you must return your meter for calibration.

Tip: Our CO2 meter calibrates itself automatically.

Check when the device was last calibrated  

When you have a new device, you should not check this. A new device is always calibrated in the factory before it is delivered.

Is your device not self-calibrating and has it been more than 3 years since your device was calibrated? Then you should know that there is a good chance that your device is no longer measuring correctly. Inform the person responsible for monitoring indoor air (management, prevention advisor).

Self-calibrating meters calibrate themselves only under the right conditions.

Check if the threshold values at which the LEDs change colour on your device are set correctly  

We recommend the following values:

• From green to orange: > 800 ppm
• From orange to red: > 900 ppm

Are the values on your device not set correctly? Inform the person responsible for monitoring indoor air (management, prevention advisor). Some meters allow you to change the values. How to do this can usually be found in the manual or the technical data sheet of the device. Can you not adjust the values? If so, you may have to cover up the lights.

Check if your meter is measuring correctly 

Place the device in the open air or near an open window. The CO₂-concentration in the outside air is between 400 and 500 ppm (higher in the city than in the countryside). Does your device measure a much lower value than 400 ppm or a much higher value than 500 ppm? Inform the person responsible for monitoring indoor air (management, prevention advisor). Your device may need to be calibrated.

Check if your device can log readings 

Some devices can also log, allowing you to read out the measurement values later or view and further edit them online. Check how your device logs the measurement values. You may need a memory card for this, cables to connect the measuring device directly to your PC or smartphone or you may need to install software.

Tips for getting started with a CO2 meter in the classroom

Everything checked? Then you can get started! We will gladly help you with our tips!

Where do you put the sensor? 

• Place the sensor on a table or cabinet, against the wall or in the middle of the room.
• Set the sensor not next to a door or window.
• Make sure the meter is in a safe place so that it does not fall over or be spilled on.
• Do not exhale near the sensor; this may affect the measurement.

How long do you have to measure? 

• Measure at least the whole school day. The CO₂-values in the classroom air fluctuate greatly during the school day. That is why it is best to measure throughout the school day. This way, you can be sure that enough fresh air enters the classroom throughout the day.
• Remember that self-calibrating meters should continue to measure after school hours. So leave them on.

Follow the measurement values 

• Check the device from time to time.
• If the meter reads green, there is sufficient air exchange.
• When the meter goes into orange/red, there is too little air exchange and the chance of bad substances accumulating and virus particles spreading through the air increases.

What if my meter goes into orange/red? 

If your meter goes into orange/red, this indicates too little air exchange. Try to limit these moments and look for solutions:

• Open windows and doors further or open more windows and doors.
• Take students from the classroom outside for an activity and leave windows and doors open.

If these moments occur frequently, signal this to the person responsible for monitoring indoor air (management, prevention advisor). Consider how to reduce the number of pupils in the classroom.

If your class is mechanically ventilated, there may be a problem with the ventilation system. The maintenance company may need to be contacted.

Don't get discouraged immediately if you exceed this limit. What is important is that you are already consciously working on ventilating and airing.

Involve the pupils 

• Explain to the students what the CO₂-meter, how it works and why fresh air is important.
• Make arrangements in the classroom about opening windows and doors.
• Designate an air boss to help you carry out these appointments. You can appoint a different air boss every week.
• Together with the pupils, make a CO₂-profile of your class.

High levels of CO2 are measured in my classroom. Is it more likely that I will be infected?

The worse the air exchange rate, the greater the chance that virus particles will accumulate in the indoor air. The likelihood of virus particles in the indoor air depends on:

• the presence of an infected person in the classroom
• that person is contagious. Research shows that especially very sick patients are contagious.
• that infected and contagious person emits micro droplets. These droplets are mainly released by (loud) talking, coughing and sneezing. We therefore advise against talking loudly or shouting in class. And sick children should not come to school.

A well-fitting mask limits the emission of microdroplets and also protects against inhalation of microdroplets. In primary school, children may not wear a mask, but they are less likely to transmit the virus.

High CO₂-values indicate poor air exchange. But how often and for how long these high values occur also play a role. Keep track of the number of violations (more than 900 ppm CO₂) and therefore the duration of the overrun as small as possible.