What should be done before connecting a regulator to a gas cylinder valve?

Before connecting a regulator to a gas cylinder valve, it is crucial to open the valve slightly and then close it immediately. This practice is important because it helps to clear any dust or debris that may be resting on the valve or inside the fitting. By briefly allowing gas to flow, you can ensure that the pathway for gas is unobstructed and that no contaminants are present, which could potentially cause issues when establishing a secure connection. This action aids in ensuring safety by verifying that the valve is working correctly and that there is no buildup of materials that could pose a hazard when the regulator is attached.

The other options would not be appropriate actions to take in this scenario. Warming the cylinder, pre-hydrating the regulator, or lubricating the valve are not common practices associated with safely preparing to connect a regulator to a cylinder valve, and they could lead to unsafe handling or improper functioning of the gas system.