can anyone please tell me how to determine the CR for a trap that has a duration of 10 rounds? it is a repeated steam jet trap that emits 2 steam jets every round and each steam jet deals 2 damage + 1d4 subdual damage (fire damage?). damage can be avoided completely by making a succesful reflex save at a DC of 14 - x where x is the number of rounds that have elapsed since the trap went off.
some context for the trap: a spa / bath house on top of a rich mineral deposit is being terrorized by 2 thoqqua, when the players enter one of the baths for investigation the thoqqua have burrowed from an adjacent pool (situated slightly higher) to this one and attack the players. then water starts pouring through the 2 smouldering holes they left behind creating the 2 steam jet traps. I should probably also mention this will slowly fill the bath with very very hot water, so the floorspace available for melee decreases slowly..