This is a brand new pressure relief valve The thread is standard 1/4" BSP These are known as "Pop Off Valves" or "Pressure Relief Valves" When the pressure in your tank reaches 200 psi this will automatically allow air to escape and relieve the system of too much pressure. Or you can manually pull the ring to let air out. In theory your pressure switch should turn the compressor off before these dizzy heights are reached but people like to know it's their for safe measure and some governing authorities insist upon it.
