NITROX FILLS COST MORE THAN straight air fills. With a membrane continuous-blending system, this reflects the cost of the equipment involved. With partial-pressure blending, this reflects the cost of buying in pure oxygen, and the additional time taken
to blend it.
Some liveaboards and dive centres with a membrane system, especially those overseas, offer free nitrox or a flat rate for as much as we can breathe in a week, so we dont need to think any further about it.
Where we pay by the fill, however, some divers pay for nitrox without worrying too much about the cost, and have the perfect nitrox fill for every dive. The rest of us are typically more careful with our money, using nitrox only some of the time.
So if you are one of the divers who likes the advantages of nitrox, but doesnt want to pay for it on every single fill, what can you do to make the most of it

FILLS AND TOP-UPS
The first trick is always to think two dives ahead: about the dive for which you are getting the fill, and the dive after that.
With a nitrox fill for the current dive, which fill would we have for the next dive if we topped up with air Get this right, and we can benefit from nitrox for every dive, while paying for it for only half of those dives.
The top-up mix depends on the mix with which we begin, the pressure to which we breathe it down, and the pressure to which we top it up. We know that an air top-up will always result in a weaker mix. This is a bit awkward, as the first dive is usually the deeper in a day, so would benefit from a weaker mix, while the shallower second dive would benefit from a richer mix.
So to make the most of a top-up fill, we need to get our nitrox fill for a second dive, then use an air top-up for the first dive of the next day.
On a typical day of charter boat diving, where there is no on-board compressor, many divers will bring a cylinder, or even a twin-set, for each dive. If these are of similar size, the set from todays second dive can, with an air top-up, become the set used for tomorrows first dive.
The set used for todays first dive is blown down, then filled with nitrox ready for tomorrows second dive (and then, with an air top-up, used for the first dive of the day after).
It is much easier with an on-board compressor and oxygen bank for nitrox-filling. Now the time to buy nitrox is mid-day after a deeper first dive, and ready for a shallower second dive.
After the second dive, an air top-up will give a weaker mix for tomorrows first dive.
While a scattering of charter boats about the country are equipped for this, virtually all the boats in Scapa Flow in the Orkney Islands are equipped for on-board nitrox. Little surprise then that this trick for getting the most out of nitrox has become known as the Scapa Fill.
With this in mind, we will use Scapa Flow for a couple of examples. For a nice easy second
dive on the cruiser Karlsruhe at a maximum depth of 26m, we get a fresh fill of 39%, the richest mix compatible with a maximum PO2 of 1.4 bar, giving an equivalent air depth of 18m.
Our first dive tomorrow will be the battleship Kronprinz Wilhelm, with the seabed at 38m at the stern. If we breathe the 39% down only to 100 bar on the Karlsruhe, an air top-up to 232 bar will give us a 29% mix for the Kronprinz, with a Maximum Operating Depth (MOD) of 39m and an Equivalent Air Depth (EAD) of 33m.
Its an almost perfect top-up plan.
Our next example begins deeper, with a second dive on the cruiser Dresden. The seabed is at 36m, but a good dive can be had without venturing below 32m, so we plan a fill of 33%.
Tomorrow morning, were off to the steam trawler James Barrie, with a maximum depth of 43m. Again breathing the nitrox fill down to 100 bar, an air top-up to 232 bar gives us a 26% mix with an MOD of 44m and EAD of 40m.
If we used more gas on the Dresden, an air top-up from 70 bar would give us 25% for the James Barrie.

IS NITROX NEEDED
Whether a dive is worth a nitrox fill depends on how you value your diving. Ignoring the safety factor of nitrox, what we really need to know is how much additional bottom time we will get from a nitrox fill - and can we carry enough gas to make it worthwhile
Firstly, we estimate a surface air consumption of 18 litres per minute. This is fairly typical for an experienced diver who is relaxed and settled. Most training manuals recommend calculating at 25 lpm, so beware that personal consumption rates could be higher than our estimates.
Suppose we are diving on a single 12-litre cylinder. Allowing a one-third reserve, we have a gas duration of 30min at 24m. The air No-Stop Time (NST) is 31min, so there is little point paying for a nitrox fill.
On the other hand, at 27m, the single 12 will give us 28min gas duration. The air NST is 23min and the nitrox 38 NST is 56min, so we gain five minutes extra dive time with nitrox. Wear twin-12s, and we can make use of nitrox for a dive of 56min, with the gas duration matching the NST.
This trend continues as we go deeper. At 30m, a single 12 gives us 26min duration, but the nitrox 35 NST is 25min and the air NST 18min.
A no-stop dive is now limited by the nitrox NST to 25min, a 7min gain over air.
To gain more, we need both to carry more gas and be prepared to decompress. With twin 12s, allowing for 20min decompression, we could have 44min at 30m, again limited by the volume of gas we are carrying.
Going deeper on a single 12, we continue to have more gas than we need for an air or nitrox no-stop dive, but the time gain for nitrox is less worthwhile. At 39m, we gain only 3min additional NST with nitrox.
Past 39m, the absolute gains of only a minute or two seem hardly worthwhile for the cost, but perhaps nitrox becomes more important for reducing narcosis.
Here we come across an issue on which diving physiologists are divided. Does nitrox decrease narcosis Some believe yes, that narcosis is governed by the EAD. Others argue that oxygen is also narcotic, so there is no difference in narcosis for nitrox.
The graph above shows the increased dive time available by using nitrox compared to air, taking into account any limitation from the volume of gas carried.
With a single 12-litre cylinder, there are no gains for nitrox shallower than 24m. We cant carry enough gas to make it worthwhile.
With twin-12s, considerable gains can be made for shallower no-stop dives, but once we go deeper or get into longer decompression, these gains diminish in significance.
For deeper decompression diving, perhaps the twin-set would be better filled with air, and the cash spent on a rich decompression mix.

Air Top-Up Formula
To calculate the mix from an air top-up, add O2 partial pressures of the starting mix (Start P x Start Mix) and the amount of air added (Fill P - Start P) at 20.9%, the proportion of O2 in air, then divide by the final fill pressure:
Start P x Start Mix + (Fill P - Start P) x 20.9
                  Fill P