Heading Blast Fume Clearance Time and Air Changes

Dilution decays exponentially, so each air change removes the same FRACTION rather than the same amount, and intuition gets it wrong. One change takes a heading to 37% of the starting concentration, two to 14%, three to 5%, and 4.6 to 1%. The first minute does most of the work and the last decade of concentration takes as long as everything before it. A 48,000 cu ft heading with 17,500 cfm delivered turns over in 2.74 minutes, so clearing to 1% takes 4.61 changes, about 12.6 minutes as a minimum. A CREW RE-ENTERING AT TEN MINUTES on the belief that most of it clears fast is walking into 2.6% of the blast fume load. And the airflow lever is real: repair the tubing to deliver 22,500 cfm instead and clearance falls to 9.8 minutes -- three minutes saved on every round, every day. TWO FIELD CAUTIONS BELONG WITH THE NUMBER. Real headings do not mix perfectly. Dead corners, the muck pile, and a tubing end set too far back all leave pockets that clear far more slowly than the average, which is why the required practice is to TEST the atmosphere with a calibrated instrument before re-entry rather than trust a clock. And fumes continue to be released from the muck pile and from any misfire long after the shot, so a heading that tests clean at the portal can still be unsafe at the face. The calculation sets the minimum wait; the gas detector sets the actual one. A perfect-mixing dilution estimate. Continued fume generation from the muck and from any misfire is not modeled and is a real hazard after the calculated time has elapsed. It does not determine the applicable re-entry criterion, which is set by regulation and by the mine's own ventilation plan, and it does not substitute for atmospheric testing, which is the actual requirement and the only thing that establishes a heading is safe. It does not address misfire procedures or the separate waiting periods those require. The mine ventilation plan, the blaster in charge, and MSHA govern.

concentration decays as exp(-Q t / V); one air change = volume / airflow; the air changes to a target fraction = ln(100 / target percent); clearance time = the two multiplied; the airflow for a target time = volume x changes / that time.

The perfect-mixing dilution relation by name, with atmospheric testing by calibrated instrument named as the actual re-entry requirement rather than an alternative to it. The mine ventilation plan, the blaster in charge, and MSHA govern.

Exponential-decay arithmetic on the user's own heading and airflow figures; no regulatory re-entry criterion is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: heading_volume_cuft, delivered_cfm, target_fraction_pct, wait_time_min, target_time_min, air_change_min, air_changes_required, clearance_min, remaining_pct_at_wait, airflow_for_target_time_cfm

• Each change removes a FRACTION 37% remains after one, 14% after two, 1% after 4.6exponential dilution
• Real headings mix imperfectly dead corners and the muck pile hold fumes far longer than the averagemine ventilation practice
• The instrument sets the wait, not the clock and continued fume generation is not modelled at allMSHA re-entry requirements