Enclosed Space Entry with a Questionable Atmosphere Reading

Several red flags:

1. The gas meter's calibration is expired. An uncalibrated meter may read incorrectly - a false 'safe' reading is the most dangerous outcome in enclosed space entry. The crew's confidence in it is not evidence of accuracy.
2. No entry permit has been signed. The permit is the formal control that records the atmosphere test, the isolation, the rescue arrangements and the named entrants. Proceeding without it bypasses the whole safety system.
3. Pressure to meet a deadline. Commercial pressure is exactly the circumstance in which shortcuts are taken; the deadline is not a reason to accept risk.
4. The tank has been sealed for three weeks - organic cargo residues can deplete oxygen and generate toxic gases, and the atmosphere can change after the lid is opened.

My first action would be to stop the entry: tell the ratings not to enter, close or guard the lid, and inform the chief officer and master that entry cannot proceed until the meter is calibrated or a calibrated meter is obtained and a permit is properly completed. I would do this politely but unambiguously, and record the instruction. If the surveyor is present, I would explain the delay and the reason.

The atmosphere must be verified by a calibrated instrument, and the result must be recorded on the permit. Steps:

• Obtain a calibrated multi-gas meter - from the vessel's spare (if it has a current certificate), from the port, or from the surveyor/contractor. If none is available, entry does not take place; the deadline is secondary.
• Test from outside the tank at several levels and at several points, not just at the lid, because gases stratify and pockets can exist. Oxygen, flammable gas (%LEL), hydrogen sulphide and carbon monoxide are the standard four.
• Test again after forced ventilation, and continue ventilation during occupancy. Re-test at intervals and whenever anyone leaves and re-enters.
• Ensure the tank is isolated (valves, blanks, electrical isolation) and that the permit records the isolation.
• Confirm rescue arrangements: a standby person at the entrance, rescue harness and tripod/hoist, communications, and a means of raising the alarm. The entrants should be trained and medically fit.
• Only then sign the permit, with the test results, time, instrument identification and calibration date recorded.

If the meter cannot be calibrated in time, the correct answer is to miss the deadline, not to accept an unverified atmosphere.

I would acknowledge the point but explain why it does not change the decision. Double-bottom tanks are among the most dangerous spaces: they are small, poorly ventilated, and can contain rust, scale and residues that deplete oxygen; they are also difficult to escape from. 'It has always been fine' is survivorship reasoning - the fact that previous entries were uneventful does not mean the atmosphere was safe, only that the risk did not materialise.

I would also point out that the permit system exists precisely because the atmosphere can change without warning, and that an uncalibrated meter gives no reliable assurance. The chief officer's deadline is a commercial matter; an incapacitated entrant in a double bottom is a fatality and an investigation. I would offer a practical route: get a calibrated meter quickly, ventilate thoroughly, and if the survey can be done in a shorter window, do it - but not by skipping the controls. If the chief officer still presses, I would escalate to the master, because the master has the authority and responsibility for the safety of the vessel and persons on board.