It depends on an individual trip by trip, and individual basis.
I am the tank bee-otch as a general rule for the large group of people that I dive with.
This is because I have a large truck and I work very near our local fill station of choice.
There are as many as 20 tanks at a time in my garage.
Essentially when we come back in from a boat trip,
everybody throws their tanks into the back of my truck,
and I get them filled during the work week.
I generally fill the tanks myself.
And generally we are just diving premixed gas that comes out of the bank.
And generally this mix ranges between 36 and 38%.
So when I analyze the tanks, I of course label them with a piece of tape.
And then I usually find myself transporting the tanks to whatever boat we are going out on.
Once in a while somebody will come in from out of town and use the tanks that we toss out there from my group, and they will very often analyze the tanks themselves.
Which I think is a very solid practice.
I would say if you are using RENTAL tanks it is a really really good idea (even if you tested your tank at the shop yourself, but somehow lost chain of custody even if it means for 15 seconds, you handed your tank to a dive master on the back of a boat and he/she stowed stowed it) to re-check that tank at your seat.
Rental tanks can look remarkably alike, with nearly identical scuff marks, peeling patches of paint, initials, dive shop decals, and every other dangerous similarity in the world, is a VERY REAL DANGER or other possible situation.
The other wild card to this is that if you are having your tanks filled at a partial pressure fill station,
there can be a 2% to as much as 4% range in the true percentage of oxygen that the sensor will register if you do not roll of the tanks around for a little bit immediately after a partial pressure fill.
There have been several conversations on Scubaboard in the past, about this process but here is the quick rundown on this potential variance in an accurate O2 reading.
If you put in 300 or 500 psi of oxygen, and then put air on top, and then test the tank without rolling around on the floor, there can be up to a 4% variance in the accurate reading of the gas that is in the tank.
4% would be the most extreme differential that I have ever personally heard of.
Some people are of the opinion that this only really happens if you test the tank immediately or very shortly after the fill being performed.
There is a body of thought that says if you leave the tank sitting there overnight that the gas will evenly distribute and you will get an accurate reading.
I am of the opinion that if a tank has been partial pressure filled, even if it has been sitting in the shop for a week waiting for pick up, you ought to at least roll it across the floor for 15 seconds before you put it on the analyzer.
My personal situation is this.
I very, very, very, rarely rent scuba tanks.
If I pick up my or my friends tanks at the shop that have been filled, I test them there.
They then go into my garage.
Then if I am going somewhere I throw them into my truck, and I drive to a boat, and I dive them without testing them.
But I have a certain laissez-faire attitude about this, because almost all of the tanks in my garage are within a point or two of one another.
Generally in the 35 to 38% range.
This is a whole different ballgame if we are doing some special deep wreck or reef or even decompression diving.
When that happens, I will test the tanks at the shop, and when we arrive at the boat,
not only are we testing the percentages again, but we are also checking to make sure that the tanks are full.
But the mistakes do happen.
As recently as three weeks ago, I ended up grabbing two empties that somebody slipped into my garage, and did not test them.
The rule with my group is upon depleting a tank you have to remove the percentage mix label/tape from the crown.
Somebody did not do that, and I did not know that the tanks were in the garage empty.
My buddy and I had to do two dives on one tank apiece.
Which was not the end of the world, because a low pressure 120 can really do nice things for you if you know how to fill them correctly.
We ended up diving one hour and 45 minutes instead of the two hours that was allowed for that particular trip.
So it was no big deal.
Chug
Often laissaiz fair but nosy at the same time.