Co2 (Carbon Dioxide) Concentration in Cabins is just as critical- if not more so than the removal of Airborne Particulate and Fibre from the Cabin.
Some known facts about Co2 Concentration in Cabins:
- Primarily, results from the natural breathing of the Occupant(s) via consuming of the contained oxygen in the Cabin.
- Excess will contribute to Operator Reduced Concentration, Fatigue, Micro- sleeps and Acidosis.
- Is normally off- set by the Quality Cabin Environmental Air System's Fresh Air Supply Unit- and not the Recirculation Air Supply.
- Can exceed allowable limits of 700- 1000 ppm (WHO) with a single occupant in a Vehicle Cabin- within 8 minutes ion a relaxed state / ambient conditions (eg no Operator activity such as driving) and extremes of ambient conditions (enlarge the Image in the LHSide Column to see our Test Cabin results).
The AIRAH (Australian Institute of Refrigeration, Air- conditioning and Heating) states that to avoid harmful Co2 Concentration then Ventilation / "Exhaust" Air Flow or "Leakage" from a Cabin, with a single occupant should between 7.5- 10 Litres / second. If you have 4 people in a Light Vehicle then the ventilation / Exhaust / leakage Air form the External Supply Unit of your Quality Cabin Environmental Air System should be between 30- 40 Litres / second.
Important Note: Exhaust / Ventilation Air Flow can only be provided by the External Air Supply Unit. So one needs to consider the Air Flow rate of the External Air Supply Unit only as the Recirculation Air supply Unit just "recirculates" air inside the Cabin. See the video of our Test Cabin for Co2 Testing at this link.
Other aspects of Co2 Concentration to consider when it comes to your Quality Cabin Environmental Air System design are:
- Cabin Sealing / Air Loss / Cabin leakage: Whilst a well sealed Cabin is required (correct Cabin Pressurisation), it should not be sealed ("air tight") to the extent that Co2 will not be displaced / expelled from the Cabin. Subsequently, part of the design of the Quality Cabin Environmental Air System is to ensure optimum Positive Cabin Pressure, Fresh Air supply and "controlled leakage" (air loss) from the Cabin.
- Full Recirculation Air Supply Mode: So as to increase Operator / Occupant Comfort or where travelling in Dust Conditions, most Operators will switch the HVAC System to Full Recirculation Mode. This a serious error, as not only will this Depressurise the Cabin ("sucking contamination into the Cabin) but also cause rapid Co2 Concentration.
- External Filter Blockage / Loading: A In- Cabin Pressurisation & Monitoring device must also be installed so as to monitor and warn the Operator if Cabin Pressure and respective Fresh Air Flow falls below the safe set- point, otherwise rapid Co2 concentration will occur (and depressurising of the Cabin- ingress of Contamination).
Co2 Testing Results:
See the Table below of our Co2 Concentration monitoring in our Quality Cabin Environmental Air System Test Cabin with a single occupant:
- Cabin External Air supply was turned Off- Cabin depressurised ("blue" graph line) and Recirculation Air supply was left On (that is cabin Air was Recirculation Supply only).
- Co2 is measured and displayed as the "brown" graph line and it climbed to nearly 6 x times the allowable Co2 limits within 15 minutes- the allowable limit (700- 1000 ppm) was exceeded only after >4 minutes.
- Once the Co2 level of 4000 ppm was reached, the External Air Supply was turned back on and within 5 minutes the C02 Concentration levels returned to external / normal levels of 500-600 ppm.levels quickly dissipated to allowable limits.4000 ppm
See the video of the Co2 Testing at this link