Been mulling something over this evening. I've been looking into minimum gas and running some examples. I have a question I fear may be silly and embarrasing.

If you have a dive planned to say 35m for 40 mins and next gas source is at 21m, you have a 2 min travel time to 35m to 21m, is it possible to work out beforehand what the average depth would be so you can work out your gas consumption rate?

In terms of minimum gas, the average depth fo the dive is irrelevant. You need to know the max depth, as sod's law this is where you would have a problem. Additionally, the bottom time is also irrelevant - at the level we are talking about.

A la dir....

so, let's say we have a 33 metre dive ( Is saturday 33 metres?)

1 minute for the problem

1 minute to get to 80% of the ATAs (24 metres)

1 Minute to get to 21 M

1 Minute for the gas switch

So it's a 4 minute ascent time.

We assume 30 litres per minute for both divers. so thats 30X2X4 = 240 litres. Now we multiply that by the average depth. Given our ascent, I would use probably 25 metres for this. That's 3.5 bar. 240 X3.5= 840 litres, or 35 bar. We can't really tell 35 bar on a guage so call it 40 bar. So minimum gas is 40 bar. I'd probably say 50 bar tbh. However, bear in mind that this is the DIR approach, so it's assuming you have a deco gas to swap to at 21 Metres.

for the dive on Saturday, the numbers also work out that the 50 bar would still allow you to deco out on your backgas if you had a deco gas failure. However, this is not always the case, especially if you move away from standard gases, so you need to consider this as well.