In my experience, I have never seen a benefit to switching every 500 psi. or even every 1/6th.
It makes more sense to just use 1/3 rd of the gas from one tank then switch to the other and use 2/3rds from it, then switch back to the first tank for a final third you will normally use on the dive. (You turn after you use 1/3rd of the gas from the second tank.) Managaging the gas like this keeps each tank within 1/3rd of the other and ensures during the pentration that each tank has a reserve at least equal to the total you have used going in. The split in reserve also will ensure that you have enough gas in either tank to exit if you lose all the gas in the other.
But don't confuse it with diving "thirds" either as one does not require the other. If you have 3600 psi in each tank you could use 1200 psi thirds or you could, for example, back off a bit and use 1000 psi "thirds" to add 400 psi to the reserve in each tank. In that case you'd use 1000 psi from the first tank, then use 2000 psi from the second tank (turning the dive after 1000 psi is used) and switch back and use the second 1000 psi from the first tank (making the switch about half way back out of the cave). That would leave 1600 psi remaining in each tank for the reserve rather than 1200 psi. You can play with the numbers until you get the desired reserve and still only make 2 gas switches.
If you think it through and compare that to switching every 500 psi (or 500 psi then 1000 psi to maintain no more than a 500 psi differential), you'll find that you make two to four times as many gas switches but gain no real advantage in safety as you still have adequate reserve gas in either tank at any point in the dive with only 2 gas switches.
Plus switching to maintain only a 500 psi differential complicates the calculation for "thirds" as thirds are most often not divisible by 500 psi and making a math mistake at depth at max penetration (the point in the dive where it really matters) can create a problem with having an adequate reserve in both tanks.
In my opinon people badly over complicate gas management with independent doubles or sidemount to no useful effect or increase in safety. Simpler is better and having to remember only two gas switches and only two switch pressures (in the example above, 2600 on the first tank and 1600 on the second tank) will go a long way to preventing a potentially fatal mistake.
It makes more sense to just use 1/3 rd of the gas from one tank then switch to the other and use 2/3rds from it, then switch back to the first tank for a final third you will normally use on the dive. (You turn after you use 1/3rd of the gas from the second tank.) Managaging the gas like this keeps each tank within 1/3rd of the other and ensures during the pentration that each tank has a reserve at least equal to the total you have used going in. The split in reserve also will ensure that you have enough gas in either tank to exit if you lose all the gas in the other.
But don't confuse it with diving "thirds" either as one does not require the other. If you have 3600 psi in each tank you could use 1200 psi thirds or you could, for example, back off a bit and use 1000 psi "thirds" to add 400 psi to the reserve in each tank. In that case you'd use 1000 psi from the first tank, then use 2000 psi from the second tank (turning the dive after 1000 psi is used) and switch back and use the second 1000 psi from the first tank (making the switch about half way back out of the cave). That would leave 1600 psi remaining in each tank for the reserve rather than 1200 psi. You can play with the numbers until you get the desired reserve and still only make 2 gas switches.
If you think it through and compare that to switching every 500 psi (or 500 psi then 1000 psi to maintain no more than a 500 psi differential), you'll find that you make two to four times as many gas switches but gain no real advantage in safety as you still have adequate reserve gas in either tank at any point in the dive with only 2 gas switches.
Plus switching to maintain only a 500 psi differential complicates the calculation for "thirds" as thirds are most often not divisible by 500 psi and making a math mistake at depth at max penetration (the point in the dive where it really matters) can create a problem with having an adequate reserve in both tanks.
In my opinon people badly over complicate gas management with independent doubles or sidemount to no useful effect or increase in safety. Simpler is better and having to remember only two gas switches and only two switch pressures (in the example above, 2600 on the first tank and 1600 on the second tank) will go a long way to preventing a potentially fatal mistake.
