Continuous trimix blending with 2 nitrox sticks (English)

I've been playing for a while now with a second nitrox stick in series with the first one.
The idea is to be able to pre-mix trimix in a cheap way and still be able the check the mix at all times.

Setup is simple; 2 nitrox sticks in series.
In the first one you introduce the Oxygen to blend nitrox, this nitrox percentage is measured after the first stick.
Now after the first measurement I introduce the helium to the nitrox and at the end of the second stick before the gas enters the compressor is another O2 sensor.

That's right, there is no helium analyzer.
You can easily measure the helium content by calculating the amount of nitrox is replaced by the inert helium.
So for example you want to blend 21/35.
you replace 35% of the air with helium, therefor the oxygen percentage will drop also with 35%.
So when introducing 35% helium to the airflow you'll get (1 - 0.35) * 0.21 = 13.7% oxygen left.

Now we add oxygen back with the first stick until the sensor reads 21%.
You'll see that the O2 sensor after the first stick will now read 32.3% which confirms that 21/35 can be made by topping off helium with 32% nitrox.
You can also calculate this if you don't thrust my numbers, or want to make another mix.

It's very simple:
the total amount of O2 to add in bar (end pressure * desired O2%) for example 21% => 0.21 * 200bar = 42bar O2
This number is then divided by (end pressure - starting pressure - the amount of helium to add in bar)
The amount of helium being ( end pressure * helium percentage ) => 200bar * 0.35% = 70bar
So that will be:
42bar O2 / ( 200 - 0 - 70) = 0.323 or 32.3%

Now you have 2 O2 values you need to keep at the set point to check your mix.

In my earlier test with just one stick I had no idea how much helium I was pumping when the compressor flow was drifting and adjustments were needed to the oxygen and helium flow since I only had one O2 meter for both oxygen and helium.

Some interesting points:
Since you introduce gas in the second stick that gets it's gas from the first stick; the flow in the first stick will drop since part of it is replaced by the introduced gas.
Since the flow of the gas introduced in the first stick is constant it's percentage will rise...
for example if you first adjusted the first stick with O2 to 32.3% and added the helium next, you'll see the 32.3% O2 in the first stick will rise to about 38% O2 and you need to adjust...
This is because the flow in the first stick is reduced by 35% (the helium content that's introduced in the second stick, so that's 35% less gas that needs to come from the first stick)

It is however quite simple to anticipate this.
You can calculate this by subtracting the fraction of O2 in the input of the stick (air => so 0.21) from the desired fraction (in this case 0.323 or 32.3%)
This number is then multiplied by:
(1 - (the amount of He to add in bar / (end pressure - starting pressure))
Now add the fraction of the input gas (0.21) back and you have the adjusted value.

for example: to fill an empty tank with 21/35 we know we have to add 32.3% nitrox to the helium.
For a 200bar fill, we have to add 70 bar of helium in there (0.35 * 200bar = 70bar)
To know the initial set point of the nitrox before you add the helium you can calculate it like this:

((0.323 - 0.21) * (1 - (70 / (200 - 0)) + 0.21 = 0.284 or 28.4%

When you adjust the first nitrox stick to 28.4% and introduce the helium later, the nitrox percentage will climb to 32.3% when the end O2% reaches 21%

The above is only valid when oxygen is introduced in the first stick and the helium is introduced second just before the gas enters the compressor.

air -->Oxygen---------->analyser1--->helium---------->analyser2-------->compressor

When you change the order the above calculations are not correct anymore.
For example, when you first introduce helium, you'll have to account for the drop in the flow of the first stick and have to actually mix a higher percentage of helium in the first stick to end up with the desired percentage at the compressor intake.

I have put all this into an excel sheet for various options for blending.
full continuous blending for both setup options of O2 or He first/last
combined partial blending and continuous blending (handy for deco mixes with high O2%)
re-mixing an existing mix

When I find out how, I will upload the blender sheet somewhere and post a link to it...
For now, here is already a picture of my setup.



Duikje Roche Fontaine

Afgelopen zondag nog een duikje te Roche Fontaine gemaakt.

Het zicht was weer niet geweldig zoals we dat de laatste jaren gewoon zijn.
Waterstand is nog steeds erg laag.
Temperatuur rond de 11 graden.

De duik zelf uitgevoerd met trimix 21/35 en ean50 decogas.
Afgedaald in de put en even de 50m aangetikt, maar vanwege het erg slechte zicht beneden na 3 minuten alweer opgestegen tot boven de 45m.
na 15min rondzwemmen aangekomen op het 35m plateau waar het wrakje ligt en Stephan wat video heeft proberen schieten.
Daar na een dikke 20min aan de opstijging begonnen.

Tijdens de opstijging nog een scubapro automaat die besloot te free-flowen :-)
En zelf gemerkt dat ik een snee in mijn long hose heb waar gas weglekt, maar niks ergs.
Waarschijnlijk van in het wrakje te kruipen.

Verdere opstijging gaat vlot en we zwemmen over de bodem in het ondiep naar de kant.
Totale duiktijd 65min.

Bodemgas dubbel 18l tx21/35
decogas 40cuft. ean50

Bodem profiel:
~15min op gem. 42m max. 50m
~20min op gem. 33m

21   3   ean50
18   2
15   2
12   2
9     3
6     5
3     8

Een filmpje van de duik kan je vinden op de koelakant web site onder de hoofding "Roche Fontaine".

12:08 Gepost door Davy in General | Permalink | Commentaren (2) |  Facebook |


France cave diving trip to the Lot Nov. 2005 (English)

Last year we went again on a trip to the Lot area in France for some cave diving.
We visited the classical caves like Ressel, Landenouse, Cabouy, St. George, Trou madame and Truffe.
Conditions were great above and below the water.

Since it was already a year ago, I'm not writing a full report, but Harald made a nice DVD of the trip, and below you can see a short teaser featuring Ressel and Landenouse.
(Size 11MB.)

09:46 Gepost door Davy in diving trip reports | Permalink | Commentaren (0) | Tags: cave trip lot 2005 |  Facebook |


How not to lose your Halcyon reel (English)

Many people have mysteriously lost their Halcyon reel.
Most of the time it's the way the supplied double-ender is clipped to the handle.
There is indeed a good and a bad way to clip it; and unfortunately, the most obvious way is the wrong way.
The picture below shows the wrong way, clipped "from the inside out"






The next series of pictures show how the clip gate can be accidently pushed open if the reel is twisted in the right way.








Below is the correct way to clip a double-ender to a Halcyon reel... (from the outside in)
the other pictures show that there is now far less chance to accidently push open the clip gate.








To close this, a small video that demonstrates the unclipping when clipped in the wrong way followed by the proper way to clip the reel.

12:26 Gepost door Davy in diving tips & tricks | Permalink | Commentaren (0) | Tags: reel |  Facebook |


Adding a protective cover over the P-valve (English)

After installing a P-valve to your dry suit, there is always some “knob” and hose that catches on stuff like your dry suit underwear while donning.
Especially the older styles balanced style P-valve with the balancing check valve outside of the actual valve.






What I always do to create a smooth surface again on the inside of my dry suits is to add a protective cover of neoprene over the P-valve.
That way there is no problem anymore with entering the suit.
Not that it is a “problem” without the cover… It’s just more comfortable and smooth.









You start with cutting a piece of thin (2mm) neoprene the size of the cover.
I always lay it on the inside of the suit on top of the valve and mark the neoprene so it covers the entire valve assembly.
Leave some extra room on the top and bottom to secure it to the suit. (5cm)

Glue the top of the cover to the suit so it covers the whole P-valve assembly like you initially cut the sheet of neoprene.
Next you cut a hole on the top to feed the hose through.
Make sure there is no stress on the hose at the hole.









Now you can mark where the bottom of the cover meets the suit.
Here you can glue a piece of Velcro as well at the lower end of the cover.
This way you can still easily access the P-valve assembly to perform maintenance.






And now your basically done.
The only downside I found with this cover is that in case of a dry suit flood, the cover dries slower than the rest of my (trilaminate) suit.

10:31 Gepost door Davy in dive gear projects | Permalink | Commentaren (0) | Tags: p-valve, dry suit |  Facebook |


Easy donning of dry gloves with a latex seal (English)

Everybody who ever tried those pesky dry gloves with a latex seal knows what a struggle it can be to put them on.
However, there is a little trick that makes this quite easy.
You should not try to fight your hand into the narrow seal opening, but instead you can point the seal into the glove so it’s facing the same direction as your dry suit’s wrist seals when you put your suit on.




Now it’s very easy to just point your hand and slide it into the glove…
A little talcum powder on the seal helps this a lot, but it’s very nice to have your seals in good condition so they don’t “stick”. I don’t use talcum powder or anything at all.
Your hand should ideally just slide through.




Once your hand is in the glove, you should pull the side of the seal/glove to pull the glove seal’s edge up onto the dry suit wrist seal.








Make sure you check the entire circumference of your wrist to see if the glove seal is over the dry suit wrist seal, there should be no skin, liner or under glove showing or it will leak.
This part can be tricky at first, but just needs some practise.
Don’t worry that the seal is facing the “wrong” way; if you did it right it won’t leak a drop.








12:18 Gepost door Davy in diving tips & tricks | Permalink | Commentaren (0) | Tags: dry suit, dry gloves |  Facebook |


DIR-BE dag 18-11-2006

Afgelopen zaterdag 18 November was weer eens een DIR-BE dag.
Gewoon een dagje duiken, bijpraten enz.

Afspraak was om 10h aan het Zilvermeer te Mol, waarna er eerst wat gegeten en gedronken werd in Atlantis.
Zo rond de middag werd er een eerste duik gemaakt.
Harald en ik zouden Robert en Cor een lift geven met de scooters tot aan de silo's aan de andere kant van de plas waar Cor foto's zou maken en van daaruit terug zou zwemmen richting steiger.
De rit naar de silo's gaat vrij vlot en nadat we onze lifters afgezet hebben maken Harald en ik nog enkele rondjes om de plas.

Terug aan de ingang laten we Ramon en Roel zoals beloofd een rondje doen met de scooters.
Uit het enthousiasme kan ik afleiden dat ik binnenkort wel enkele scooter buddies bijkrijg
Tegen de avond wordt er door enkelen nog een 2de duikje gemaakt, maar zelf blijf ik mooi in de cafetaria
's Avonds gaan we dan nog iets eten om dan toch op een fatsoenlijk uur thuis te zijn.
Leuk dagje met mooi weer.




15:21 Gepost door Davy in General | Permalink | Commentaren (0) |  Facebook |