21ft must be a new school number and conservative. One atmosphere sea water is 33 ft or 34ft fresh water. So technically 33ft is the no decompression limit. You can stay at 33ft or less for any length of time and ascend with no decompression issues however, if I were to embark on such an experiment i would 1: ascend very slow, 2: make at least 2 or 3 stops for 10 minutes or more and 3: use oxygen at 20ft thats my opinion anyways. When Im doing deco dives I use oxygen above 20 ft for deco.