Al and all, Which is why two-thirds of the sub operators in the world were and are quite content to have a dependable pair of 02 meters and a good barometer. That pressure change from C02 buildup is pretty dependable. Set the barometer to zero after closing the hatch then turn the scrubber on for 15 minutes or so out of the hour to when you get a couple of inches of pressure. Simple as pie. Vance