Name the gases found in enclosed spaces, the instruments used to detect them, the minimum oxygen level for entry, and explain with a diagram how an oxygen content meter works?

flowchart TD
    A[Sample air] --> B[Diffusion membrane]
    B --> C[Electrochemical cell]
    C --> D[Current ∝ O₂ concentration]
    D --> E[Display % O₂]

• Gases in enclosed spaces: oxygen (deficient), flammable hydrocarbons, hydrogen sulphide, carbon monoxide, carbon dioxide
• Instruments: portable multi-gas detector, catalytic explosimeter, electrochemical oxygen analyser, toxic-gas tubes/sensors
• Minimum oxygen for entry: 20 % by volume
• Oxygen analyser principle: electrochemical cell — oxygen diffuses through membrane, chemical reaction generates current proportional to O₂ concentration
• Regulation: SOLAS XI-1/7 requires portable atmosphere-testing instruments for oxygen, flammable gases/vapours, H₂S and CO
• Procedure: test at different levels, use calibrated equipment, ventilate continuously