This calculator works out how long to ventilate a confined space before anyone goes in. It finds the air volume of the space, the air your blowers really deliver through their duct, and the time to bring your first reading down to a safe level, and it shows the purge in a 3D view of the space.
The purge time formula
Ventilation dilutes a contaminant exponentially: each minute removes a share of what is left. Rearranged for time, the dilution equation is:
Purge time = K × (V ÷ Q) × ln(C₀ ÷ C)
- V is the volume of air in the space, less any liquid or sludge left in it.
- Q is the airflow actually delivered into the space, not the blower's free-air rating.
- C₀ is your first reading and C the level you need. For low oxygen, the calculator dilutes the shortfall below 20.9%.
- K is the mixing factor: how far real mixing falls short of perfect. It is engineering judgement, from about 3 for an open tank to 8 or more for a pipe or tunnel.
The calculator also applies the blower makers' rule of seven complete air changes, half again for a toxic gas, and uses whichever time is longer. Where gas is still being released, it works out the level the space settles at and the airflow needed to hold it below your target.
Setting up the ventilation
- Put the blower intake upwind, away from engines, exhaust and the opening of the space.
- Run supply duct to the far end and the bottom of the space: supply air only reaches about 30 duct diameters.
- Exhaust where the gas collects: low for H₂S, propane and CO₂, high for methane and ammonia.
- Use an explosion-proof (intrinsically safe) blower and bonded duct in a flammable atmosphere.
- Keep ventilating while people are inside, and stop work if the blower stops.
Across Canada
Pick your jurisdiction in the calculator and it sets the targets to that jurisdiction's limits, warns when yours are higher, and lists its rules. In brief:
| Jurisdiction | Oxygen | Flammable gas for work | H₂S limit |
|---|---|---|---|
| Alberta | 19.5% to 23% | Below 20% LEL | 10 ppm 8-hour, 15 ppm ceiling |
| British Columbia | 19.5% to 23% | Below 20% LEL | 10 ppm ceiling |
| Saskatchewan | 19.5% to 23% | Below 20% LEL | 10 ppm 8-hour, 15 ppm 15-minute |
| Manitoba | 19.5% to 23% | Below 10% LEL | 1 ppm 8-hour, 5 ppm short-term |
| Ontario | 19.5% to 23% | Below 10% LEL | 10 ppm 8-hour, 15 ppm short-term (Reg. 833) |
| Quebec | 19.5% to 23% | Below 5% LEL | 10 ppm 8-hour, 15 ppm short-term |
| New Brunswick | 19.5% to 23% | Below 10% LEL | 10 ppm ceiling |
| Nova Scotia | 19.5% to 22.5% | Below 10% LEL | 1 ppm 8-hour, 5 ppm short-term |
| Prince Edward Island | 19.5% to 23% | Below 10% LEL | 1 ppm 8-hour, 5 ppm short-term |
| Newfoundland and Labrador | 20% to 22% | Below 10% LEL | 1 ppm 8-hour, 5 ppm short-term |
| Federal (Canada Labour Code) | 19.5% to 23% | Below 10% LEL | 1 ppm 8-hour, 5 ppm short-term |
| Yukon | 19.5% to 23.5% | Below 20% LEL | Check the current Yukon table |
| Northwest Territories | 19.5% to 23% | Below 20% LEL | 10 ppm 8-hour, 15 ppm 15-minute |
| Nunavut | 19.5% to 23% | Below 20% LEL | 10 ppm 8-hour, 15 ppm 15-minute |
In Alberta
Part 5 of Alberta's OHS Code governs confined spaces: a hazard assessment, a written code of practice, a permit, atmospheric testing before entry and while work goes on, a trained attendant and a rescue plan. Ventilation is one of the controls the assessment chooses. Our hazard assessment and program development services build confined space programs that meet it, and the COR readiness check shows where the rest of your program stands.
Sources: blower makers' purge charts (Air Systems International; Pelsue, citing Bell System Standard ISS10) for seven air changes, half again for toxic gases and 20 air changes an hour; Allegro's published bend ratings; NIOSH Pocket Guide for gas properties; Alberta OHS Code, Schedule 1, for exposure limits. Mixing factors are engineering judgement.