According to the UHMS (undersea hyperbaric medical society) a nurse must work 1 year/480 hours in a hyperbaric unit either mono or multiplace. This is after attending a Primary training either in Texas or South Carolina. Look on the UHMS website and you will find the info. Then after the year you have an exam that is fairly intense. If you read the hyperbaric medicine practice book, this will help you through the nursing exam for CHRN