Final answer:
An acetylene cylinder valve is opened only 1/2 turn for safety reasons, enabling quick shut-off to prevent fire or explosion risks.
Step-by-step explanation:
It is recommended that an acetylene cylinder valve never be opened more than 1/2 turn because it allows for quick and easy shutoff in case of an emergency. Acetylene is a highly flammable gas, and in situations where a rapid escape of gas occurs, it is critical to shut off the flow as quickly as possible to minimize the risk of fire or explosion. Moreover, fully opening the valve can lead to a rapid release of gas that can create turbulence and static electricity, further increasing the potential for hazardous conditions. Therefore, for safety reasons, only opening the valve 1/2 turn ensures that it can be swiftly closed if necessary.