forum.heinrichsweikamp.com

I ran a search but did not really find the information I looked for. I have few question about active gases and how you set them up affect the deco calculation. Here's a few scenarii.

1)
1st gas air
2nd gas NX80 (not active)

If I understand it right,deco is calculated as if I was only diving with air. When I change to the 2nd gas (activating it in the process) it recalculates the deco with my new gas (in this example NX80), thus lowering my total deco time.

2)
1st gas air
2nd gas Nx80 (active = yes, change 10m)

This time the deco is calculated as if I'm diving with air AND Nx80. So my deco time should be less than the dive above (for the same dive parameters). Well it should be less than the deco time I had on the previous dive before I changed to NX80.

At 10m the gas will blink to tell me I have a better mix.

If I change gas and do the manipulation, my TTS remains unchanged.
What if I don't change gas ? My OSTC will recalculate my deco but when will it do that ?

3)
1st gas air
2nd gas Nx40 (active = yes, change 0m)

That's the part I'm not really sure about. How is my deco calculated ? as if I have a deco bottle or no deco bottle ? Will my unit blank at 30m to tell me I have a better gas ? (I think it shouldn't since I've purposely set 0m and for me it means that I will change it at any time I feel necessary but not obviously at 30m)

Thank you all for your insight.

2) If you don't switch the gas, the OSTC will recalculate deco after a few minutes.
3) The ostc will calculate a dive with air only because you did not tell the unit when you are going to switch to NX. I am not sure whether the unit will blink "better gas" in this case, but you can quickly simulate a dive with these gases using the simulator to check this.

tiefunten Wrote:
-------------------------------------------------------
> 2) If you don't switch the gas, the OSTC will
> recalculate deco after a few minutes.

I should have been more accurate in my question : after how many minutes ?

> 3) The ostc will calculate a dive with air only
> because you did not tell the unit when you are
> going to switch to NX. I am not sure whether the
> unit will blink "better gas" in this case, but you
> can quickly simulate a dive with these gases using
> the simulator to check this.

Ok. Thanks for the info. And I didn't think to simulate the dive to check all this.

Thanks a lot for your input. I really appreciate it.

> 3) The ostc will calculate a dive with air only
> because you did not tell the unit when you are
> going to switch to NX. I am not sure whether the
> unit will blink "better gas" in this case,

For those interested, I've just run a simulated dive and 1) the gas didn't blink and 2) the said gas was in blue as if it wasn't active (even though I activated it via the appropriate menu). This makes me believe that it wouldn't be counted when calculating the deco time until you switch the gas as in example 1 above.

Hi,

The OSTC strictly requires the gases to be set accurately when using it in a Multigas configuration:

- All gases that are going to be used must be active
- One Gas (Usually the "First Gas" ) should have change depth=0m
- Only one gaschange on the same depth (That includes the 0m gas)
- change depths are only taken into account for the decompression phase
- you'll blinking gas change recommendations only in a window of 3m above the configured gas change depth

regards,
Matthias

I guess what you posted stands true from a safety point of view. And it makes sense.

The question came from the procedure a friend of mine uses. He dives with his twin 10L and takes a extra 7L bottle filled with Nx40.
The idea is not really to improve deco time but more to have extra gas.

Why Nx40 ? simply because 1) Nx40 is free in our club but if you take a higher % of O2 you have to pay for it (and for some dives, it's not really necessary) 2) my friends feels better/less tired after a dive when he used a bit of Nitrox (he always does all his deco as if he was diving with air even when he uses his Nx40)

So the idea behind this is to have more gas for a single dive but also to have a little "better" deco. That's why we talked about how to setup the gases to reflect the fact he might not take the NX40 at 30m depending on the deco time. That was example 3 by the way.

Again, I agree with what you posted. I shall do it that way.

I just wanted to understand a little better how my OSTC is working if in case I make a setup mistake (which can happen I must admit)

Then you the choice:
1/ you want security. You program just air. Never change it. And do have extra margins (this correct assuming you don't have to bother about CNS).

2/ You want the shortest TTS. Program air + Nx40, with you switch depth. The OSTC will assume you will make the change, and predict the shorter stops.

3/ you not sure what you will do. You prefer to see the TTS computed on air, just in case you can't or finally don't want to make the switch. Program air + inactive Nx40. At anytime, you can decide to switch to any gas. TTS and decoplan is computed from all active gas PLUS the current one. First has is just the gas current when you start the dive. And gas with a Switching depth of 0m will be used at surface, ie. never. So Switching is enough for the OSTC to take your Nx40 into account.

Awesome.

Thanks JeanDo.

heinrichsweikamp Wrote:
-------------------------------------------------------
> - One Gas (Usually the "First Gas" ) should have
> change depth=0m
Hmmm, wasn't that supposed to be MOD of the gas ? So in case of air, takin PPO2 as 1.4, that would be around 57 m ?

Not for the first gas as you breath it from surface (which is 0m).

Stephane Wrote:
-------------------------------------------------------
> Not for the first gas as you breath it from
> surface (which is 0m).

Really ? I suppose the same applies than to any gas that You start the dive with ... any nitrox, any normoxic (+-) trimix ?
What if I am going deeper than normoxic range using TX21/35, switching to hypoxic afterwards, and coming back ? It looks fishy to me ...

By the way ... simulation with AIR, change depth 0m, and change depth 57m, on bottom time 27 minutes and depth 45meters, GF30/80, gives 97 minutes deco at 6 meters ??? Not speaking about other stops ...

The MOD of the gas is only valid on the way up to the surface, it is used to tell you that you can change deco (ou bailout) gas.

Let's assume you start with you Tx21/35,at 70m you change to a lower fiO2 gas and on the way up you go back to your Tx21/35. You'll have to set the MOD to 70m on the Tx21/35 if you want to have the blinking gas function. Deco will be calculated with the gas you are breathing at the moment (don't forget to change it on the MK2!).

I don't understand you simulation;: which (other) gases are you using? Only air?

@gorcio: Did you set last decostop to 6m? Nevertheless, this is much deco... MV-Planner (nothing else here at the moment) shows me an overall deco of approx. 60min for that dive and approx. 80min with last stop set to 6m. My OSTC Simulator shows me 72mins overall deco for that dive with last stop at 3m and GF30/80 using only air.

@stephane : MOD - seems to be calculated automatically now, in 2.0. However, the depth of gas switch is configurable by the user. So no way for me to setting MOD of TX21/35. But I can change it's gas switch depth ... since it is my FIRST gas, should it be 0 ?
My simulation is done on AIR only and the figures are way too high ...

@tiefunten : last stop is 6m, and I find 97minutes of deco a big deviation from what I would have planned. So, Yours is showing 17 minutes shorter ... which firmware version ? I am using 2.0

@Gorcio: V2.0. But using last stop at 6m, I have also got >100mins total deco. So there is +~20mins deco only because of the deeper and ineffective last stop when u are not using O2.
But I would never do this dive with these settings. Some questions: Why last stop at 6m when you are not using oxygen for decompression? Why those very conservative GF-values when using only air? When you are using ZHL16OC you have 46mins of deco for this dive with last top on 3m. I use GF30/90 or 40/90 when diving air or nitrox...

[quote=Tiefunten]I use GF30/90 or 40/90 when diving air or nitrox...[/quote]

GF low at 30 or 40 will give you deep stops which are not very necessary when diving air or nitrox. You'd better use 80/80 or 80/90 which will give you conservatism against ZHL16OC.

Hi,

I think he is using two gases set to air.
One as first gas with -0- mtr. MOD and
one as deco-gas set to 57 mtr. MOD.
That doesn't make sense as it is only one gas actually.
Try using just one gas set to air as first gas, MOD 0 mtr.
What has been mentioned about GF and 6 mtr. last stop also makes sense to me.
For air no need to use GF and no need for 6 mtr. last stop.

My half cent, Reiner

Sorry guys, but I have last stop at 6m, and won't be changing it when switching from TMX to AIR or nitrox and back. Call me lazy . And that was just a question, as I am normally not using neither the simulator, nor air alone at this depth .

@sailor - no, I do not have two air gases defined. I have o2, nx50, 21/35, 18/45, air. Sometimes I change 21/35 or 18/45 to whatever's there after topping up ...

@Stephane: I know that GF is not necessary using Air/Nitrox, but I want the security and it results in a nice smooth deco curve. When I would use GF 80/90 that wouldn't make much difference to plain ZHL16OC.

I didn't say that GF are not necessary with air or nitrox, I said that a too low GFlo will lead to a not so good decompression curve.
If you want security just increase the GFhi, you will stay longer in shallower depth but you will more efficiently offgasing.

gorcio Wrote:
-------------------------------------------------------
> @sailor - no, I do not have two air gases defined.
> I have o2, nx50, 21/35, 18/45, air. Sometimes I
> change 21/35 or 18/45 to whatever's there after
> topping up ...


That's what you wrote:

simulation with AIR, change depth 0m, and change depth 57m, on bottom time 27 minutes and depth 45meters

I can't see your O2 or any other gas then air in this statement.

sailor Wrote:

> I can't see your O2 or any other gas then air in
> this statement.

Then how did You come to conclusion that I had two air gases defined ? I simply gave You mu settings ... and on this simulation, the only air was the only active gas. The rest was inactive, however, with proper change depths already set.

I just had that impression when reading your statement and it could have made sense regarding your question in my opinion, sorry for that, Reiner

sailor Wrote:
-------------------------------------------------------
> I just had that impression when reading your
> statement and it could have made sense regarding
> your question in my opinion, sorry for that,
> Reiner

No problem mate. Honestly, You made me run to my unit to check it ... and uff, I only had what I had . No duplicated gases .