Dear Shout,
I use a Nonin fingertip pulse oxymeter. There are some other threads on accuracy and use of pulse oxymeters on DB is you are interested. Because it senses the blood that is in the fingertip there is about a 20 sec delay from the low oxygen blood to get from your brain to the sensor. Also dive reflex shunting can throw sensor off as blood flow to arms can be restricted. That all being said I love my Nonin sensor. It seems to correlate very well to me percieved O2 state, ie when it is in th 60's I feel need to breathe alot, when in the 50's it feels a bit sketchy and when it the 40's it feels a bit "scary" or close to the edge. My lowest reading was at the pool after a dynamic, it read 47%. Here is link, they are about $300. I have the model 9500, I would probably buy the newer model 9550. The model is not sold as water proof or water resistant so I am very careful to never get it wet so pool use is only for "special" occasions. Another option is to haunt e-bay for used pulse oxymeters sold from hospitals, there are various threads on this as well.
Pulseoximeter- oximeter