Shearwater Perdix AI

[scubadada]

Just to confuse things a bit, as I recently learned from the boys at Subsurface, the calculation above will only give an approximation for RMV. It's a good enough approximation for comparing dives but is not accurate.

First, 1 bar = 0.986923 atm, so that should be applied in the conversion.

Second, air does not act like an ideal gas under compression so the compression factor would need to be determine depending on the pressure of the air. The compression factor changes with nitrox, helium, etc.

As I said, ignoring compression factor and bar to atm conversion will still give a RMV which is useful for comparing one dive to another...

This seems like a really trivial correction, could you provide an example of the calculation vs the "real" RMV

 

[ATJ]

This seems like a really trivial correction, could you provide an example of the calculation vs the "real" RMV

Certainly, but I'll have to do it tonight when I get home.

 

[scubadada]

Certainly, but I'll have to do it tonight when I get home.

Thanks, I can wait

 

[ATJ]

Let's assume a simple dive...
A 50 minute dive with an average depth of 10 metres where 100 bar was consumed.
That's a SAC of 1 bar/min.

If a 12.2L tank was used, the RMV would be 12.2 L/min using the simple calculation.


If the tank started at 200 bar:
12.2L x 200 bar x 0.986923 atm/bar / 1.0326 = 2,332 L @ 1 atm.

and the tank ended at 100 bar:
12.2 x 100 x 0.986923 / 0.9930 = 1,212.5 L @ 1 atm.

So 1,119.5 L were consumed.
That's 22.39L/min on the surface.
The RMV is 11.195 L/min

That's quite a significant difference from the simple calculation.

It gets interesting when you look at where that 100 bar was used.

If the tank started at 150 bar:
12.2 x 150 x 0.986923 / 1.0074 = 1,792.8L @ 1 atm

and ended at 50 bar:
12.2 x 50 x 0.986923 / 0.9909 = 607.6L @ 1 atm

so 1,185.2 L were consumed.
That's 23.7 L/min at the surface and an RMV of 11.852 L/min @ 10 m,

Not as big a difference from the simple calculation but big enough and also different from the "more accurate" calculation at different starting pressure.

Another example, an 80 minutes dive with an average depth of 10 metres where 160 bar was consumed.
That's still a SAC of 1 bar/min and RMV of 12.2 L/min using the simple calculation.

If the starting pressure was 200 bar and the ending pressure was 40 bar:
12.2 x 200 x 0.986923 / 1.0326 = 2,332 L @ 1 atm
12.2 x 40 x 0.986923 / 0.9901 = 486.4 L @ 1 atm
Consumption: 1,845.6 L in 80 minutes @ 1 atm and RMV of 11.535 L/min.

Of course, these calculations assume a constant temperature of 300K (~27ºC). It is almost impossible to factor the temperature in as we don't know the temperature of the tank. On a hot day, the tank could be quite warm at the start and even add up to 5 bar to the measured pressure (let alone any other affects of temperature on the calculations).

 

[dmaziuk]

Let's assume a simple dive...
a SAC of 1 bar/min.

If a 12.2L tank was used, the RMV would be 12.2 L/min using the simple calculation.
...
The RMV is 11.195 L/min
...
Another example, an 80 minutes dive with an average depth of 10 metres where 160 bar was consumed.
That's still a SAC of 1 bar/min and RMV of 12.2 L/min using the simple calculation.
...
Consumption: 1,845.6 L in 80 minutes @ 1 atm and RMV of 11.535 L/min.

As long as the actual is less than calculated by the simple formula, I'll take the simple formula for my gas planning any day.

 

[oceancurrent]

I was wondering myself about the lack of RMV calculations some time back. Oceanic has had SAC/RMV functionality in OceanLog for decades, so hard to believe that Shearwater can't get it implemented properly. I wrote to Shearwater and received the following response:

The Perdix can calculate SAC based solely on the readings from the transmitter. It is true that RMV calculations depend on having the cylinder volume, and we are considering how we can add this information. However, it's a bit more complicated than it first seems... for example, sidemount divers consume air differently and it can throw off the calculations. Because RMV can be used to calculate information that a diver might depend on for their next dive, we need to make sure that we implementing it correctly. All that being said, it's definitely on the list of future improvements.

 

[stuartv]

I was wondering myself about the lack of RMV calculations some time back. Oceanic has had SAC/RMV functionality in OceanLog for decades, so hard to believe that Shearwater can't get it implemented properly. I wrote to Shearwater and received the following response:

Right. RMV is not in the Oceanic computer itself (at least, not the one I have). It's in the desktop computer software.

Exactly like Shearwater...

 

[Kevrumbo]

. . .
That's 23.7 L/min at the surface and an RMV of 11.852 L/min @ 10 m,

Should be: "That's 23.7 L/min Depth Consumption Rate (DCR) at 10 m (2ata), and an RMV of 11.852 L/min @ surface".

. . .Not as big a difference from the simple calculation but big enough and also different from the "more accurate" calculation at different starting pressure.

Another example, an 80 minutes dive with an average depth of 10 metres where 160 bar was consumed.
That's still a SAC of 1 bar/min and RMV of 12.2 L/min using the simple calculation.

If the starting pressure was 200 bar and the ending pressure was 40 bar:
12.2 x 200 x 0.986923 / 1.0326 = 2,332 L @ 1 atm
12.2 x 40 x 0.986923 / 0.9901 = 486.4 L @ 1 atm
Consumption: 1,845.6 L in 80 minutes @ 1 atm and RMV of 11.535 L/min.

Of course, these calculations assume a constant temperature of 300K (~27ºC). It is almost impossible to factor the temperature in as we don't know the temperature of the tank. On a hot day, the tank could be quite warm at the start and even add up to 5 bar to the measured pressure (let alone any other affects of temperature on the calculations).

Ultimately, it's whatever the SPG or AI indicates real time at a planned turn or minimum reserve pressure, and other practical vital macroscopic factors such as -for example- physical exertion, breathing consumption rate of teammates, and environmental conditions (i.g. Current; Flow; Visibility etc), which are more the more important margins significantly overwhelming and "swamping-out" any of the esoteric FYI minutiae above regarding the "precise" or "accurate" volumetric free gas capacities of various cylinders vis-à-vis ideal versus real gas thermodynamics. A potential maximum 11% variance short in an initial twinset fill of larger volume HP cylinders is at most an annoyance and an issue to correct on subsequent fills; but a 30% or greater depth consumption rate than expected at my first time interval SPG check is objectively a more "genuine", meaningful and critical red flag indicator to abort the dive.

 

[ATJ]

I had an interesting and somewhat surprising issue with my Perdix on Sunday.

I always calibrate the compass whenever I change the battery, as per the manual. It was less than a month since I last changed the battery and calibrated the compass so I assumed it was all good.

On Sunday morning daylight saving ended and I updated the time on my Perdix (and other devices).

A couple of hours later I was diving at one of my usual sites. We normally jump in from the rocks, descended and head at between 30 and 60º to the sand line. I use the compass on my Perdix for guidance. I was following my buddy and thinking he'd heading almost due north but the compass was saying around 60º. The spot we hit the sand line was around due north from the entry so the compass was wrong.

I had dived the same site the day before and the compass was working fine.

When I got home I checked the compass as I know which way north is and it was out around 60º. I calibrated it again and it seems to be fine even 3 days later.

Why would changing the time affect the compass?

 

[Mike1967]

Why would changing the time affect the compass?

FYI I just changed my clock and it didn't affect the compass.