ACR is one of the cornerstones of indoor air quality (IAQ) since the best way to keep the air inside a room healthy is to replace all of it at sufficiently regular intervals. Changing the air is the simplest and most effective way to stop pollutants, gases, particles, bacteria and viruses from building up inside a confined space. ACR, as its name suggests, represents the number of times that all of the air in a room is replaced in an hour.

CO2 as an indicator of stuffiness

Previously, ACR was measured against a table set by regulation, which recommended generic values for different types of venues and activities. But a recent paradigm shift has seen professionals acknowledge CO2 concentration as a top priority – and ACR values are now treated as targets rather than guidelines. That’s because CO2 concentration, expressed in parts per million (ppm), is the most reliable and effective measure of IAQ.

France’s air stuffiness index

In France, for instance, the government has developed ICONE, an air stuffiness index that demonstrates the importance of CO2, since it’s based on average concentrations of the gas over time. The equipment used to measure CO2 concentrations has to meet certain requirements:

- Measurement range of 0 to 5,000 ppm
- Uncertainty of less than 50 ppm + 5% of the measured value
- Measurement resolution of less than 2 ppm
- Measurement frequency of 10 minutes or less
- Measurement storage capacity of at least 8 days

In addition, when calculating the ICONE value, readings must be taken on weekdays at the normal room occupancy level.The index operates on a scale from 0 to 5, with these two values being the extremes:

- 0 : no stuffiness = CO2 concentration of less than 1,000 ppm 100% of the time
- 5 : extreme stuffiness = CO2 concentration of more than 1,700 ppm 100% of the time

The underlying formula was developed based on extensive research by the French Scientific and Technical Centre for Building (CSTB):

Checking ACR: a four-step process

Checking that the air in a room is being replaced as it should involves taking several readings using different instruments.

Important: These recommendations only apply to air-conditioned commercial and industrial buildings, and to controlled-atmosphere environments in particular. They don’t apply to residential buildings, offices without air conditioning systems or rooms equipped with a simple split system (a conventional stand-alone air conditioning unit).

• Measuring the incoming air flow
  This is the first and most important measurement in the process. The aim is to check that the ventilation system is blowing enough air into the room – in other words, that the active part of the system is doing its job. The most reliable instrument for taking this kind of measurement is an air flow meter such as our DBM 620 model, which is compatible with all types of air vents.
  
  With its accompanying SmartKap mobile app, the DBM 620 can automatically calculate the ACR in a room based on the vent air flow readings and the overall volume of the space in question. Another option is to use a vane or hotwire thermo-anemometer to scan or zone the air vents, although this is a more laborious process. It also doesn’t work with diffusers, a type of vent that only air flow meters can measure.

• Measuring the outgoing air flow
  In order to make sure the air is circulating properly, it’s also important to measure the air flows through the outlet vents, which work typically via passive ventilation: the air isn’t blown out of the room, but is instead forced through the vents by overpressure. The best approach for these vents is to use a vane or hotwire thermo-anemometer, coupled with a cone of the correct size. For large outlet vents (600 x 600 mm), the DBM 620 is an ideal option because it automatically detects the air flow direction, meaning it can also measure outgoing air flows.

To learn more, read our blog article and watch our webinar (in French) on air vent measurement techniques..

• Measuring overpressure inside the room
  In controlled-atmosphere environments, checking that the ventilation system is functioning properly involves additional measurements. One option is to use a differential pressure sensor to compare the pressure inside and outside the room. Ideally, there should be a slight overpressure inside the room, although the difference shouldn't be too excessive. Anything greater than 20 Pa indicates that the outlet vents are too small or excessively clogged, or simply that the blown air flow rate is too high (and the system is therefore using too much energy).

• Installing or checking a CO2 monitor
  At the start or end of an inspection, it’s always a good idea to use a CO2 monitor – such as our AQ 110, CO2 Stats or KCC 320 models – to check whether the ACR value suggested by the previous readings is correct. Ideally, these instruments should be used to keep this parameter under constant observation. If the previous measurements are satisfactory but the CO2 concentration is still high, this could point to other problems:
  
  The most likely cause is: air of poor quality entering the room (too much air recycled from the air treatment unit, a filtration issue or an external inlet vent located in a polluted environment).
  Another possible cause is: that some parts of the room could be in an air change “blind spot”, perhaps because an outlet vent is missing or not positioned correctly, leading to poor air circulation. This is why it can sometimes be necessary to map CO2 concentrations at different points in the same room, using multiple instruments.