iMixer Tutorial
Blending
Trimix 26/17 (Helitrox) with EAN 32:
Suppose you
are trying to mix a cylinder of 26/17
using EAN 32 as your top off gas. You
have heard others say that you can make
26/17 by topping about 500 PSI of Helium
to 3000 PSI with EAN 32; but what is the
exact mixing plan?
You start by
entering your starting mix (in this case
an empty cylinder of Air 15 PSI of
21/0), target mix (3000 PSI of 26/17),
and your top off gas of EAN 32 (FO2 =
32, FHe = 0). When you press the
Calculate button you get the
message:
Warning
Unable to
create blending plan with
current top
off gas.
You get this
message when it is mathematically not
possible to create an exact blending
plan. In this case you can get very
close to 3000 PSI 26/17 but it is not
exactly 26/17. What can you do?
In this
case, switch Top Off Mix on
and enter the mix parameters manually.
Start with 500 PSI of Helium (FO2 is 0%,
FHe is 100%), enter your target pressure
of 3000 PSI (ignoring the FO2 and FHe
values), and set your top off gas to EAN
32. When you press Calculate,
iMixer adds EAN 32 to your cylinder to
3000 PSI and displays the resulting mix.
In this case, you get a cylinder of gas
that is 26.7% Oxygen and 16.7% Helium:
pretty close to 26/17.
If this is
not close enough we can try again. To
get the Helium exact, we know that 17% x
3000 PSI is 510 PSI. Change the
Starting Mix to 510 PSI of 100%
Helium and recalculate. The result of
this top off is a mix that is 26.6%
Oxygen and 17% Helium.
Is this mix
close enough to 26/17 for you? You
decide. Keep in mind the MOD of the gas
has been reduced to 141 FSW (from
145FSW) because of the slightly
increased fraction of Oxygen in the mix.
Correcting a mistake
You are
blending a cylinder of 16/40 for a
customer and you find the final analysis
is off by a significant amount. In this
example, suppose you end up with a
cylinder of 17/45 at 3100 PSI. How do we
correct this blending error?
Set your
Starting Mix to 3100 PSI of 17/45,
your Target Mix to 3000 PSI of
16/40, Top Off Gas to the
appropriate value (in this case Air),
and press Calculate.
To correct
your blending error, you must bleed the
tank down to 1871 PSI, add another 358
PSI of Helium, and top off with Air to
3000. Since you are adding Helium to a
tank already at a high pressure, you may
require a booster to complete this mix.
What did it
cost to fix this mistake? Using the
default set of cost values supplied with
iMixer, if you are filling an Aluminum
80 cylinder, your shop cost is $7.23 to
make this correction ($14.47 if you are
filling doubles).
Using a Bank Mix  Blending Trimix 26/17
(Helitrox) with EAN 50 and Air:
Suppose you are trying to mix a cylinder
of 26/17 using Air as your top off gas
with EAN 50 in your Nitrox bank. As
before you start by entering your
starting mix (in this case an empty
cylinder of Air 15 PSI of 21/0),
target mix (3000 PSI of 26/17), set your
top off gas to Air (FO2 = 21, FHe = 0),
and your bank mix to EAN 50 (FO2 = 50,
FHe = 0).
With the Use Bank Mix switch set
to off pressing the Calculate
button gives you the following plan:
 Add O2 to PSI: 340
 Analyze O2/He: 96.5 / 0.0
 Add He to PSI: 850
 Analyze O2/He: 38.6 / 60.0
 Add 21/0 to PSI: 3000
 Analyze O2/He: 26.0 / 17.0
When you set Use Bank Mix switch
to on the plan recalculates:
 Add 50/0 to PSI: 902
 Analyze O2/He: 49.5 / 0.0
 Add He to PSI: 1412
 Analyze O2/He: 31.6 / 36.1
 Add 21/0 to PSI: 3000
 Analyze O2/He: 26.0 / 17.0
In this case you can blend 26/17 without
having to use 100% O2.
