Three moments
- Before entry — after the space is opened and ventilated.
- At entry — at different depths; heavy gases gather at the bottom, light ones at the top.
- Throughout the work — continuously or at defined intervals.
Measured thresholds
| Parameter | In the site procedure we measured |
|---|---|
| Oxygen | 19.5% – 23.5% |
| Hydrogen sulphide | Below 5 ppm |
| Carbon monoxide | Below 25 ppm |
| Ammonia | Below 25 ppm |
| Instrument calibration | Within the last 6 months |
These values come from that site's own procedure; they vary by country, substance and task. Your own thresholds follow from your own legislation and exposure limits.
Oxygen binds at both ends
The oxygen limit binds not only at the low end but at the high end. An enriched atmosphere accelerates combustion; materials that normally would not ignite begin to. Knowing the lower limit and not the upper one is a common gap.
Calibration is an escalation factor
The instrument is a barrier, and an overdue calibration quietly takes that barrier down. On paper this is a tick box; in software it can mean the permit cannot open until the device record has been read.
Who tests
The person carrying out the test is a distinct role and is recorded. "Testing was done" is an unverifiable sentence unless who did it and with which instrument is written down.