rongoodman:
There are various definitions of tech diving, but one common factor seems to be dives where you no longer have direct access to the surface in case of a problem, which then has to be dealt with where you are. The overhead could be a solid, physical barrier, such as the ceiling of a cave or wreck, or it could be virtual, such as a required deco obligation which you can't ignore without a serious risk of injury or death.
Well stated.
I was just wanting more info on heliums benefits and weakness really. . . . Sometimes for me diving it's not about the depth, its about the time spent down at the targeted depth. Stay at 100 ft for longer then 20 mins.
There is a recent thread that offers quite a bit of comment which you may find valuable:
http://www.scubaboard.com/forums/advanced-scuba-discussions/385336-when-go-trimix.html. With regard to helium and time at depth:
If you stay at 100 ft on 21% (air), according to the (PADI) tables, you have a NDL of 20 minutes.
If you stay at 100 ft 32% nitrox, according to the (PADI) tables, you have a NDL of 30 minutes.
If you stay at 100 ft 36% nitrox, according to the (PADI) tables, you have a NDL of 35 minutes.
HOWEVER:
If you stay at 100 ft for 30 minutes on 21% oxygen (air), you incur a deco obligation of approximately 8 min when using a combination of 50% and 100% O2 for decompression gases (and DPlan as the deco software).
If you stay at 100 ft for 30 minutes on 21% oxygen / 35% helium, you incur a deco obligation of approximately 10 min when using a combination of 50% and 100% O2 for decompression gases (and DPlan as the deco software).
If you stay at 100 ft for 30 minutes on 21% oxygen / 45% helium, you incur a deco obligation of approximately 11 min when using a combination of 50% and 100% O2 for decompression gases (and DPlan as the deco software).
IN CONTRAST:
If you stay at 100 ft for 30 minutes on 21% oxygen (air), you incur a deco obligation of approximately 19 min when using your backgas for decompression (and DPlan as the deco software).
If you stay at 100 ft for 30 minutes on 21% oxygen / 35% helium, you incur a deco obligation of approximately 32 min when using your backgas for decompression (and DPlan as the deco software).
If you stay at 100 ft for 30 minutes on 21% oxygen / 45% helium, you incur a deco obligation of approximately 46 min when using your backgas for decompression (and DPlan as the deco software).
You will get different absolute values, and relative benefit, for different software (and different algorithms) as noted.
ucfdiver:
It's only the ZHL-16b algorithms that have helium taking longer to decompress.
Therefore, the point of doing this computational exercise is to suggest that the immediate benefits of helium at that particular depth (100 ft) are more likely to be seen in clarity of thinking, and reduced effects of narcosis, not necessarily in times for decompression. As was said, helium may add to your deco schedule with some algorithms, not shorten it. But, you may find alternative value with helium.