To err is human.
Fundamentally people make mistakes. That is a given.
There are procedures to minimise (eliminate) mistakes.
Point 1 at Blending
The blender knows what he is attempting to blend. The proposed mix should be marked on the cylinder
The blender should make the pressure required (O2 and air for Nitrox, Helium, O2 and air for Helium) on the cylinder.
As each gas is added the required pressure should be marked through once it is confirmed.
If settling time is required, after adding the gas, the cylinders should be set aside, the pressure circled. On returning to the cylinder the pressure can be double checked, if required, additional gas added. Then the pressure is marked through, and the next gas added.
When I used to do trimix, after adding helium and O2, prior to adding air. I always used to confirm the pressure and O2 percentage at that stage, if it didn't match the calculation, it was vented down and we started again.
In the early days, Helium analysers where very expensive, and seldom available. So checking each stage of the mix was a fundamental principle.
Many years ago, although working part time for a shop, I was one of the principle blenders. Because we rigidly used the system of marking the cylinders target mix prior to blending, marking the target pressure for each gas. We could do a stage of the filling, and leave cylinders over night or the weekend for the other blender to finish.
At the end of the blending process the blender double checks the mix. Remarking the cylinders with the final mix (which should be the target mix).
Point 2. At pickup
The diver CHECKS the cylinder(s) confirming the correct mix, and the cylinder is appropriately labeled.
(They sign for it in the log, next to the blenders signature and mix analysis, adding there measurement, and signature).
Point 3 Directly prior to diving
On the boat, prior to fitting the regulators the cylinders are rechecked, confirming the cylinder labelling matches the ACTUAL mix.
At point 3 you have the final check, not only does this confirm the blender, got it right, it confirms the diver checked the cylinder at the pickup (shop) correctly. Most importantly, it confirms that the cylinder is the CORRECT cylinder, and that it has not been used and remixed, incorrectly labelled or swapped for a similar cylinder.
The more cylinders that are available, in storage, in the car, on the boat, the higher the risk that the cylinders can become confused. For those handling large numbers of cylinders, or storing them between dives, it significantly reduces mistakes in cylinder selection, accidental confusion, or even forgetting a cylinder has had a change of mix.
It is VERY VERY bad practice to fail to check the cylinder on the boat at assembly of your kit.
I have watched divers half way through checking and kit assembly be disturbed or distracted. The best of them stop, and start the procedure again, rechecking the mix, rechecking the labelling and fitting the reg's.
The procedures for multi cylinder diving - checking each and every cylinder, is as valid for a single cylinder dive.
Remember people make mistakes. Mitigation of this risk is achieved by proper procedures.
That means checking the cylinder just before you use it. You wouldn't get in the water not knowing if the cylinder had gas in it, not knowing if it was full, empty or half full. If you wouldn't gamble on a cylinder being empty or full, why would you gamble on the mix?
I would guess that we have at some point or other all seen a 'full' cylinder turn out to be empty when kitting up on the boat. If a simple error like that can occur, mixing errors can occur.
