Weight checks

[matt_yates]

Ok so I know ideally that a weight check should be carried out with nearly empty tanks, but its not always practical.

So will the following work.

1. Jump in the water with what ever is in your tanks and take a note of the pressure
2. Adjust your weighting till you are correct.

Then assuming that 1L of air is 0.0012Kg in weight

The following will tell you what you need to add to your weight belt in order to maintain a stop.

(Test pressure - Desired pressure) x Cylinder volume x 0.0012 = Weight of gas lost ( Bouyancy change)


So with easy numbers. At the end of the weight check I had 200 Bar in a 12L cylinder and 10Kg of Lead, And I want to maintain a stop with 30 Bar of gas in the cylinder

(200 - 170) x 12 x 0.0012 = 2.5Kg

To maintain that stop I would need 12.5 Kg of lead

Is that correct? or am i missing something?


Thanks

 

[Bubbletrubble]

Your example equation should read: (200 - 30) x 12 x 0.0012 = 2.5Kg
Other than that, you're not missing anything.

You just described one method of doing a weight check which involves compensating for the gas in a non-empty tank. Sounds fine to me. That's what I do.

 

[DevonDiver]

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[Charlie99]

Your method is sound.

For people in the USA using AL80 tanks (about 11 liter, 207 bar) the calculation is even easier.

The full tank at 3000psi has about 6 pounds of air. So every 500psi is about 1 pound of air.

So if, for example, you do the weight check at 2000psi, then you add 4 pounds of lead to compensate for 4 pound weight of the 4 x 500psi of air in the tank.

 

[Akimbo]

Your process is sound but I quibble a little with the actual value. There is a lot of variation of the weight of atmospheric air, due mostly to water vapor and particulate contaminates. Most of that is removed by Scuba air compressors so I find these numbers correspond a little closer:

Gas Weights
Nitrogen Density: Source: Nitrogen - Wikipedia, the free encyclopedia
1.251 g/L
35.42437509 g/Ft³
0.016101989 Lb/Ft³

Oxygen Density: Source: Argon - Wikipedia, the free encyclopedia
1.429 g/L
40.46477378 g/Ft³
0.018393079 Lb/Ft³

Air Density, Calculated 21% Oxygen and 79% Nitrogen
1.28838 g/L
0.016583118 Lb/Ft³

Argon Density Source: Argon - Wikipedia, the free encyclopedia
1.784 g/L
50.51725432 g/Ft³
0.022962388 Lb/Ft³

Helium Density Source: http://en.wikipedia.org/wiki/Helium
0.1786 g/L
5.057388801 g/Ft³
0.002298813 Lb/Ft³

The base value gets multiplied by some big numbers so precision makes a differance, especially on large doubles. Someday I am going to get a precision scale, thermometer, and pressure gauge to perform field checks… unless someone already has and is willing to share.

 

[Charlie99]

Your process is sound but I quibble a little with the actual value. There is a lot of variation of the weight of atmospheric air,

.....
Air Density, Calculated 21% Oxygen and 79% Nitrogen
1.28838 g/L
0.016583118 Lb/Ft³

And deviations from ideal gas law change the density of air by about 15% by the time you get to 350 bar, and even at 250 bar it's around 5%. With a very strong temperature dependence to confuse things even more.

For the purposes of determine the proper amount of lead, anything from 1.2kg/m^3 to 1.3kg/m^3 will result in the same answer for all practical purposes. I don't adjust my weighting with pellets of buckshot or adjust for what I had for breakfast that morning.

 

[1_T_Submariner]

Your approximations are close enough. Add a little for safety and your good. Does anyone weigh a 3lbs weight to make sure it is really 3lbs? Maybe your own wieghts or shop weights locally. But not on vacation.

 

[Akimbo]

And deviations from ideal gas law change the density of air by about 15% by the time you get to 350 bar, and even at 250 bar it's around 5%. With a very strong temperature dependence to confuse things even more...

Temperature and compressibility variations don’t make much difference in mass in the ranges divers deal with. I adjust to ± ½ Kg/1 Lb most of the time. Rounding errors can add up to 6-8 Lbs on a set of twin 120s plus decompression cylinders. It all depends what kind of diving you are doing and how close to neutral you prefer.

… Does anyone weigh a 3lbs weight to make sure it is really 3lbs? ...

I use a balance scale and log the weight belt and weights added or subtracted when fine-tuning in the water — especially for freediving. The log makes it easier to estimate changes for different working depth, wetsuit configurations, drysuit underwear, cylinders, and tools I carry.

 

[1_T_Submariner]

I use a balance scale and log the weight belt and weights added or subtracted when fine-tuning in the water — especially for freediving. The log makes it easier to estimate changes for different working depth, wetsuit configurations, drysuit underwear, cylinders, and tools I carry.

What increments do you add or subtract weight?

If you use a balance scale to weight the weights at a resort in Cozumel, Bonaire, Curacao or the Florida keys what are you going to do with the results? What is the tolerance on a weight +/- some %? That said I have taken a 4lbs soft weight back to the dive op that weighed less than a 3lbs weight, it must have lost some shot.

 

[daz88]

you guys have too much time on your hands.