Silly Math Tricks

[FLDVR]

Heres a safe topic

I had trouble with calculating Psi from an Al40 using 1.25 as my tank factor.
It seems I just cant multiply with 1.25 without using a calculator. Heres what
someone suggested to me.

Divide by 5 then multiply by 4.
5/40 =8 and 8 X 4=3200 PSI.

I'd be willing to bet that a lot of people out there have some cool tricks to help in
on the fly dive planning.
We can write them all up when we are done and have a nice addition to our wetnotes.
What say Ye?

Henry

 

[SparticleBrane]

Don't you have that backwards?

Shouldn't you multiply by 5, then divide by 4...?


Example:
You have 2000psi in an Al40. How many cf is that?
(2000/3000) * 40 = ~26.66cf (exact answer)
(1.25*20) = 25cf (using brute force multiplication)

Multiple by 5, then divide by 4:
20*5 = 100
100/4 = 25cf


YOUR way:
20/5 = 4
4 * 5 = 20.

By dividing by 5 and then multiplying by 4, you're effectively using a tank factor of 0.8 rather than 1.25.

Remember that 5/4 = 1.25 and 4/5 = 0.8
:no

 

[SparticleBrane]

I will also point out that in my GUE-F text it uses 1 as the tank factor for Al40s rather than 1.25.

 

[Kevrumbo]

Use the Half, and then Half again Rule of Thumb to get 25%.
In the above example, half of 20 is 10; and then half again of 10 is 5;
Add 5 to what you started out with (20 plus 5) equals 25cf (or 1.25 * 20 = 25).

For those of you down here in Los Angeles taking AG's Ratio Deco Class, apply the same trick above to figure out your First Deep Stop at 75% of your Max Depth:
Example: max depth is 160'.
Half of 160 is 80; Half again of 80 is 40;
Subtract 40 from your max depth of 160, and your first Deep Stop is at 120'.
(Check: 75% of 160; 0.75 * 160 = 120).

 

[FLDVR]

Spartcle Brane. I think you are calculating from PSI to CF. I was going from CF to PSI thus the algebraic rearrange of the formula. I'll play with both.

However I see KevRumbo's contribution here feels a lot better to me, I'll be giving that one a try also.

Keep em coming, thanks for the input.
Henry

 

[SparticleBrane]

Spartcle Brane. I think you are calculating from PSI to CF. I was going from CF to PSI thus the algebraic rearrange of the formula. I'll play with both.

I see now what you are doing, but the fact that you said this really threw me off:

Divide by 5 then multiply by 4.
5/40 =8 and 8 X 4=3200 PSI.

First you say to divide by 5, so you immediately take 5 and divide it by 40 to get 8...
I would have understood immediately if you had labeled things out a bit more. A number without units is meaningless...

 

[Rainer]

NO! That's not approved so it wont work.
You must multiply by 4 and divide by 4. Since 4 is even, and its much easier to work with an even number, there's less chance to make a mistake!

WRONG! There are LOTS of even numbers. You need to work in primes. Ideally even primes (since you *are* correct that it IS easier to work with even numbers). Multiply by 2 and then divide by 2. It's really the only safe way to work this out. God you guys are a bunch of strokes.

 

[Kevrumbo]

Rainer, I think you mean easy integer arithmetic --planning using cardinal numbers (e.g. 10', 20', 30', 40'. . .in imperial, progressive/regressive units of 10'; and 3m, 6m, 9m, 12m . . .in metric in units of 3m); rounding down psi/bar values to divisibility by 3 (Rule of Thirds Calculations); using 10/3 and 3/10 conversion factors when converting between meters & feet and vice versa etc.

For those of you taking AG's Ratio Deco Class here in LA, Andrew will help you recall the old arithmetic tricks learned in grade school, and show you how to utilize them again. . .

 

[limeyx]

WRONG! There are LOTS of even numbers. You need to work in primes. Ideally even primes (since you *are* correct that it IS easier to work with even numbers). Multiply by 2 and then divide by 2. It's really the only safe way to work this out. God you guys are a bunch of strokes.

OK, I concede to your higher levels of mathematical skills.

 

[Doc Harry]

More Math Tricks

Confuscious said:

"He who can use math tricks is probably good enough with math that he doesn't need the tricks."

2.5 x 30 = ? (The answer is not obvious)

Change it to:

25 x 3 = ? (The answer becomes obvious)