Spirometry results and scuba diving

[tank29]

I was wondering if you can use the results of a spirometry test to determine how much your bouancy may change based on the amount of air you inhale and exhale while diving?

If I understand things correctly, for scuba diving, the important number is tidal volume, which isn't necessarily measured during a spirometry test (I don't think it's on my results). I am hoping there is a way to infer tidal volume from the measurements that I have.

The reason I am interested in this, is that I have pretty big lungs and breathing affects my trim and bouancy quite a bit while I'm diving. I'm likely going to buy some tank weights to help with the trim, but sizing the weights is a bit tricky when I don't know how much my lungs are changing things. I'm still new to the sport, so I'm sure that my respiration rate is higher than it will be when I'm more comfortable, and perhaps that is playing a roll in this as well.

Any thoughts or advice? Thanks in advance!

 

[spectrum]

While I'm as much of a numbers guy as anyone I think you are over thinking this. Move some weight up on your tank or into trim pockets if you are foot heavy to get into the ballpark.

Your tidal volume is counteracted somewhat by your breathing rhythm as you take advantage in the lag between a buoyancy change and the physical movement it causes in the water column. Just as you are about to actually begin to sink you begin to inhale and just as you are about to rise you exhale. If you are down in to a nice very slow and deep pattern some amount of porpoising is likely and it's not the end of the world. Since what you set for trim weight is based more on the average the variation does not matter.

 

[TSandM]

You weight yourself for mid-lung volume. Then you use your respiratory excursion to adjust your buoyancy to go over obstacles, or to initiate descents or ascents. But mid-lung volume is still the place where you want to be neutral.

Knowing your resting tidal volume on land would be marginally useful in predicting how powerfully you can adjust your buoyancy underwater, because your tidal volume when diving is almost certainly larger than your resting tidal volume on land (or it should be).

Rick Murchison has an excellent essay on "Why Neutral Buoyancy Isn't", which may give you a better grasp on these ideas.