Calculating volume of a loop

dantheman

Too many smilies, dude...

caveseeker7

Witty, Mike, I got a good laugh out of it.

__________________
Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

Sutty

Fill loop with air, remove a known volume and replace with O2, seal the loop and pump round by pressing on the counterlungs until fully mixed, then check the resulting FO2. Work out the volume by how much dilution of O2 there has been.

schford

Now that sounds like a plan - have a blob!

Stuart

AD_ward9

No answer to my question on why you want to do this.

The normal reason is that when calibrating O2 sensors, you inject O2. You need to know the %O2: it is not 100% but is the original gas diluted by the O2 that is added. To know the O2 % you need to know the minimum volume of the loop, and the max volume, so the level of O2 you are calibrating against can be known.

For example, if min volume is 1 litre after adjusting for the fact that the pressure is less than ambient (about 0.7 of ambient) because there is negative loop pressure w.r.t. ambient at the start of O2 cal, max volume is 6 litres, min volume contains 20.9% O2 after injecting O2 until the OPV releases gas, then the total O2 % is (5.21/6)*100. Assume the underpressure at the start is 700mbar and overpressure at the end is 1300mbar, and I have factored both into the litre sizes.

These numbers I plucked out of the air for min volume are fairly minimal: most RBs are a lot more because of the scrubber housing. A figure of 5 litres is not unknown, in which case the error in the above calculation from assuming O2 cal at 100% O2, becomes gross.

I think the above error is a common one in design and would result in there being more DCS issues on rebreathers than on O.C. per hour of use. With RBs, many DCS hits go unrecorded, because the casualty self-treats using the pure O2 they have available.

I notice many Inspiration users think there is 100% O2, or 98%, on cal. The figure is a fair bit lower. The result is that throughout their dive, their PPO2 is less than what they think. This is a minor failure mode, that should be removed by the rebreather calibrating the O2 sensors in air when the scrubber is open then measuring what the loop pressure is at the start of the test, then filling with 100% O2 and measuring again.

I do not know any other reason to know the min volume of the loop. What dive computer or electronics are you using to do this?

Cheers

Alex

NB 1: I use percentages above to keep things simple. Using PPO2 one has to include compensation for atmospheric pressure, which should be obvious to all. I do not see why the user should enter atmospheric pressure either: most decent sensors can just measure it. Asking the user to do this just invites another error.

NB 2: How come we get no blobs for giving you the right answer on how to measure your min volume, and no blob even for exposing to all the reason why people want to do this? What an ungrateful lot on this thread!