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Minimum remaining cylinder pressure

Aviation Mechanic Airframe (AMA) practice question 9 of 20 on Metallic Structures. ACS element AM.II.A.R3 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONAs a recommended safety practice, welding gas cylinders should not be depleted in content below what pressure?

  1. A5 psi
  2. B20 psi
  3. C15 psi, the acetylene limit
Show the answer and explanation

Correct answer: B. 20 psi

  • A — incorrect Drawing a cylinder this low leaves too little pressure to prevent reverse flow from the other cylinder.
  • B — correct The handbook recommends that cylinders not be depleted below 20 psi, which prevents possible reverse flow of gas from the opposite tank.
  • C — incorrect 15 psi is the maximum working pressure for acetylene, not the minimum remaining cylinder content.

BACKGROUND

Setting up oxy-acetylene equipment follows a definite order intended to protect both the equipment and the people using it. The oxygen cylinder shutoff valve has a double seat and is opened fully against its stop to seat the valve and prevent a leak, while the acetylene cylinder valve is opened slowly just far enough to read cylinder pressure and then one half turn more, so it can be shut quickly if needed. As a recommended safety practice, cylinders should not be depleted in content below 20 psi, because that residual pressure prevents possible reverse flow of gas from the opposite tank. Check valves and flashback arrestors guard against the same hazard from another direction, stopping reverse gas flow and keeping a high pressure flame or oxygen-fuel mixture from being pushed back into a cylinder. Leaks are checked with soapy water on the connections and never with a flame.

STUDY TIP

Do not confuse the two pressure figures in the setup procedure: 15 psi is the acetylene working maximum, while 20 psi is the minimum content to leave in a cylinder.

KEY POINTS

  • Do not draw a welding gas cylinder below 20 psi of content.
  • The residual pressure prevents reverse flow of gas from the opposite tank.
  • Check for leaks with soapy water on the connections, never with a flame.

SOURCES

  • sources/ama-airframe/ch05.txt "Equipment Setup — Hoses"
  • sources/ama-airframe/ch05.txt "Check Valves & Flashback Arrestors"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.