How gradient factors affect NDL?

[Ghost Diver #]

I have a good understanding of how gradient factors change my decompression profile. I am alittle fuzzy on how they impact my NDL? Once I begin my accent I understand how addjusting these settings effects my first stop depth and length. It’s the effects these settings have on the time before I (the computer) switches in to deco that I’m unsure off. I have played with the GFhi to adjust the NDL to be more or less conservative, but I don’t have a good handle on what the computer is actually doing to make this happen! I hope I explained my question effectively haha
I dive Shearwater perdix

 

[tbone1004]

basically the GF-Hi determines how long your decompression stops last to get you to the surface. If you are diving within NDL's, that stop length has to be 0 minutes at 0 ft. The lower the GF-Hi is, the lower the saturation percentage you are allowed to take on before it hits 0. 70%, 80%, 90% saturation, whatever it is.

A very simplified way to think about it is to think about it like a bucket. A 5 gallon bucket is GF-Hi=100, and water is flowing in at 1qt/minute so you have 4*5=20 minutes before the bucket is full. If you change the GF-Hi to 80, you are only allowing the bucket to fill to 4 gallons, but still at the same rate so you have 16 minutes. GF-Hi=60 and 3 gallon bucket=12 minutes.

The algorithm in your Perdix uses 16 buckets that fill at different rates from each other, and obviously fill and drain at different rates depending on depth. When you choose your GF-Hi, in a simple term, you are only allowing those buckets to fill to whatever percentage you chose.

 

[Ghost Diver #]

That’s a amazing explanation! Thanks, makes sense!!

 

[Ayisha]

That is an amazing explanation, but realize that manipulating GF's should only be done if you have a really solid understanding of current research. Do lots of reading of reputable sources, and then decide what you want your GF's set to.

 

[scubadada]

Your GF High dictates your NDL On a NDL dive, which computers' NDLs are not affected by GFLo?

 

[DogDiver]

Using my perdix for cave diving I use a 70/30 GF. For tropical single 80 nitrox 32% I use 80/45 GF. Seems to work well for me.

 

[scubadada]

A GF High of 75 would be considered conservative, 85 moderate, 95 liberal. Corresponds to Shearwater rec presets.

 

[NAM001]

Using my perdix for cave diving I use a 70/30 GF. For tropical single 80 nitrox 32% I use 80/45 GF. Seems to work well for me.

Im assuming your gf's are written backwqards 70/30 instead of 30/70.

 

[NAM001]

in a 30/70 gf the 70 says the max allowable supersaturation allowed. That max condition in the rec world happens at the surface. That assumes you do ascents at the prescribed rates. The problem is similar to the psi on board setting. What psi at depth will result in ascending and getting on the boat with 500#. form 100 ft it may be 800 psi. 800# at 100' use 300 getting to the surfae leaving 500 psi. The gf hi is used to calculate how long you can be exposed to ongassing at say 100', and in the rec world, proceed at the prescribed rates to the surface and not exceed in the 30/70 example 70% of allowed super saturation . 100% causes big bubbles. 100% is what is called the M value. the M value is a ratio 2:1 between the tissue pressure and the ambient pressure of Nitrogen. The greater the ratio the greater the off gassing rate,,,, the greater the off gassing rate the more bubbles are caused. Just like the coke bottle and taking the cap off fast.. Since your ascent rate is assumed to be proper for calculating the off gassing. the only other factor is how long you on gas as say 100'. If you use a say 30/95 your ndl time at 100 ft may be 18 min. if you want less gas to off load and you use a 30/85 then your 100' NDL would be perhaps 15 min and if you want to be super conservative and use a 30/70 then your 100 ' ndl might be 12 minutes of allowed on gassing. By theory the first number gf lo is not used until you exceed NDL and have to make a deco stop.