Playing with numbers, waiting to dive

[coldwaterdufus]

Just bought 2 HP X7 100 steels, thanks to SB. It's still frozen up here, so I'm waitng to dive. First dive, I'll be doing weight checks but I'm just playing with some numbers. Comparisions are from AL 80 rentals to the X7. Hopefully you can make sense of the numbers. If you can't, thanks for trying.
AL 80 X7
tank weight 31.4 33
air 6.4 8.3
Lead 34 34

Total 71.8 75.3 Due to the buoyancy differences at dive's end, I know I can drop 7 # lead ( +4.4 vs -2.5 ), so
Total 71.8 75.3-7=68.3# Carried weight is lighter.
Now, buoyancy full = -1.4 and -10 resp . Therefore, going in with rentals, I'm 71.8 +1.4= 73.2# neg and 68.3+10=78.3 neg. Point is, a lot heavier at dives start.

Dive's end ( no air for calc ). Both have no air, so rentals weight 73.2-6.4= 66.8, X7 78.3-8.3=70 . It initially appears at dive's end I'm still heavier with the X7.

Now, this is where I get messed up. Buoyancy empty is +4.4 and -2 resp. I know I've already dropped the weight for buoyancy differences at the start, but empty rental weight = 66.8 from above. I know +4.4# positive, so I actually need 66.8-4.4=62.4 for safety stop if I didn't have the positive buoyancy. The X7 is still neg, so 70+2=72# call it negative or " downward force. The difference is approx 10#. Am I still very heavy ( and can loose more weight) or have I messed up the numbers, prob at the end. Thanks for the help.

 

[JohnB47]

First of all, you don't have the weight of air quite right. Air weighs about 7.5 pounds per 100 cu. ft. (You can see that from the fact that the buoyancy of the 100 goes from -10 to -2.5 between full and empty, so the air must weigh 7.5 pounds). Therefore, the weight of air in the AL80 is about 6.1 pounds versus 7.5 pounds for the X7-100.

Next, you forgot to subtract the 7 pounds of lead that you said you could drop (it is actually 6.9, but that is close enough) from the 34 pounds you were wearing with the AL80. With the X7-100, you should be wearing about 27 pounds.

So now, add it up:
AL80: 31.4 + 6.1 +34 = 71.5 pounds
X7-100: 33 + 7.5 + 27 = 67.5 pounds

So, with the X7-100, you are carrying 4 pounds less, and you have 20 cu. ft. more air! The only negative is that you probably paid a lot more for the X7-100, but IMHO it is worth it. (I bought one last year after doing this same calculation.)

I hope this helps.

JohnB47

 

[coldwaterdufus]

Thanks JohnB47. You are only half way along the calculations. I did subtract the 7#for dive's start. Please refer to my first underlines, they are very similar to your numbers, slight variation for weight of air.

Then, I added the negative buoyancies for each tank full at dives start to show I'm a lot heavier when i first enter the water.

What I'm wondering about is can I remove any more weight with my line of thinking towards the end of the dive. If you can, just follow along farther down the calculations.

Or do I just go in heavier ( using the numbers for the tank full neg buoyancies as above ) and the 7# is all I can remove.

Thanks again

And yes, I did pay more for the x7's, but I'm looking forward to them.

 

[JohnB47]

I tried to follow the rest of your calculations, but you are making things too complicated. I see where you subtracted the 7 pounds from the total weight of the AL80, but it would be more clear if you subtracted it from the lead you wear with the X7. After that, I have no idea why you added buoyancy numbers to the total weight.

When you are out of the water, the total weight you have to carry is what matters and buoyancy is irrelevant. In the water, you want to achieve neutral buoyancy, which means that the total weight (including your body) should equal the weight of the water displaced.

At the end of the dive, assuming you use up all the air, you are left with just the weight of the tank and the weight of the lead:
AL80: 31.4 + 34 = 65.4
X7-100: 33 + 27 = 60.0
Of course, you will have had a longer dive with the X7-100 to use up the extra 20 cu. ft.

Remember, what you are trying to accomplish is to make sure that you will be neutral or slightly negative at the end of the dive, when you have used up all your air. So you need enough weight to sink your body, wetsuit and all equipment except the tank PLUS the positive buoyancy (or MINUS the negative buoyancy) of the empty tank. So if you were weighted correctly with the AL80 (I assume that the 34 pounds was right for you) and you change tanks, you need to change the amount of lead by the difference in buoyancy between the two tanks. That is all you need to do!

You will be more negative at the beginning of the dive with the X7-100. At the beginning of the dive, you should always be negative by slightly more than the weight of the air you are carrying (or that you expect to use during the dive), so that you will be near neutral at the end. So, with the AL80, you should be about 6.1 pounds negative at the start, and with the X7 you should be about 7.5 pounds negative.

Did that make it more understandable?

 

[Puffer Fish]

Nice work John.

I would suggest that one can remove at least one more pound (perhaps two), as one should never be breathing their (expensive, non-rental) tank empty, and if you are doing a safety stop, you should still have some suit compression.

 

[coldwaterdufus]

Thanks JohnB47. I figured I was making it too complicated, especially at the end. I understood the removal of the 7#, but my LDS was still telling me I could take off more weight. I was trying to understand how. I know I'll be more negative at the start of the dive. When you said " That is all you need to do! " in reference to the different buoyancies, I totally get that. Thanks again