Ask how long ventilation must run before confined space entry and most sites give a fixed figure. GB 46768-2025 gives none, because the answer depends on the volume of the space, the gases present, the ventilation method and the material left inside. The same half hour that clears a small sewer manhole falls well short in a large storage tank. Three criteria settle it — not a clock.
Criterion one: readings sit inside the acceptable range
Ventilation exists to bring hazardous gas into a safe band, and the standard is written as an outcome rather than a duration. Entry is permitted when testing passes, not when the fan has run for a set time, so a rule of the form “ventilate for thirty minutes, then enter” does not hold on its own. GB 46768-2025 requires oxygen between 19.5% and 23.5% by volume, flammable gas below 10% of the lower explosive limit, and toxic gas within the exposure limits set by GBZ 2.1. All three must pass.
The trap is concluding from a single reading. A value taken while airflow is still disturbed, right after the fan stops, can differ from one taken after the atmosphere settles. The compliant routine is to test again after ventilation ends and immediately before entry.
Criterion two: test points cover the worst-case positions
Testing the entrance is not enough. Hydrogen sulfide and flammable gas are heavier than air and tend to collect at the bottom of a space, while methane is lighter and gathers at the top. A vertical scan is what catches them.
GB 46768-2025 calls for at least three test points along the vertical axis, with the top and bottom points no more than 1 m from the tank roof and floor and no more than 8 m between points; where the vertical distance is under 2 m, two points are used instead. Ventilation has run long enough only when the worst-case position passes — not when the entrance reads clear.
Criterion three: the ventilation method matches the space
How the air moves matters as much as how much of it moves. In long linear spaces — pipelines, cable trenches, pipe racks — airflow tends to short-circuit: in at one end and straight out at the nearest opening, leaving the far section unchanged. These runs need staged ventilation rather than a single fan at the access point.
Large-volume spaces behave the other way round. In a storage tank, material left at the bottom keeps releasing vapour throughout the job, so ventilation has to run during the work rather than being switched off after a single pass before entry.
Why this needs data rather than a stopwatch
None of the three criteria can be demonstrated with a stopwatch and a clipboard. Each needs a reading tied to a point and a time. Our confined space monitoring unit (LSM3000 series) separates an indoor and an outdoor module, with the gas detection unit positioned close to the working face and running continuously, so pre-entry results and post-entry gas trends both carry a location and a timestamp. At inspection it is possible to show when each reading was taken, at which point, and at what value — which is what turns a ventilation claim into a record.
The criteria are not a substitute for the permit process. Passing all three tells you the atmosphere is safe to enter at that moment. It does not replace entry control, the attendant, the rescue plan or continuous monitoring during the job, all of which remain requirements in their own right.