Oxygen deficiency and oxygen enrichment testing
Oxygen deficiency and oxygen enrichment testing
This domain tests the thresholds and the reasons oxygen readings change.
Core rules
- Normal air is 20.9% v/v oxygen.
- Deficiency: below 19.5% is the usual confined space entry limit. Effects worsen quickly: at 15–19% coordination is reduced; at 10–12% judgement is impaired; at 6–10% nausea and loss of consciousness; below 6% convulsions and death can be rapid. Causes include displacement by nitrogen, argon, CO2 or methane, consumption by rusting, combustion or bacteria, and purging.
- Enrichment: above 23.5% v/v. Oxygen-enriched atmospheres make clothing and hair burn violently, and oil or grease can ignite. Causes include leaking oxygen lines, oxygen used for ventilation (never permitted), and welding or cutting operations.
- Measurement: electrochemical sensors are common for portable meters; check in fresh air before use and calibrate with zero gas and span gas. Pressure and humidity affect readings.
- Test oxygen first, because a pellistor's flammable gas reading depends on oxygen and because oxygen deficiency or enrichment changes the whole risk picture.
- Entry range: typically 19.5–23.5% v/v. Some permits use a narrower band; follow the permit.
Worked example
A fermentation pit reads 20.9% O2 at the top and 14% O2 at the bottom. Reasoning: CO2 produced by fermentation is heavier than air and has displaced oxygen at the bottom. The top reading alone would have passed, but entry would be lethal. Ventilate the pit, re-test at all levels, and monitor continuously.
Traps
Using oxygen to improve the atmosphere; assuming a normal reading at the entrance applies throughout; ignoring oxygen enrichment as a fire risk; forgetting that oxygen deficiency also invalidates a pellistor's %LEL reading.