No compensation for ideal gas law deviations

[scubadada]

Deviations from the ideal gas law are not discussed in the Oceanic owners manuals.

The Shearwater Teric owner's manual clearly states no compensation is made:

 

[jborg]

Yeah I was thinking specifically of the "algorithm" to determine gas time remaining, not whatever is going on with deco. For example, the Suunto manual states that:

"The change in your air consumption is based on constant one second interval pressure measurements over periods of 30–60 seconds. An increase in air consumption decreases the remaining air time rapidly, while a drop in air consumption increases the air time slowly. In this way, an overly optimistic air time estimate, caused by a temporary drop in air consumption, is avoided."​

This is just to point out that the displayed gas time remaining on any computer may be more involved than simply the remaining gas divided by the instantaneous consumption rate, and it could be that Shearwater or someone else decided to "play it safe" in the beginning until a good baseline has been established.

 

[stuartv]

My Oceanic checks gas pressure once per second and updates the ATR based on the last 90 seconds. My Shearwater calculates SAC based on the last 2 minutes and uses that with current tank pressure to make the GTR calculation. The first 30 seconds of each dive is discarded in the calculation, see pp 52-3 Teric owner's manual

A very minor point, probably irrelevant to the topic at hand, but...

I believe the grey PPS transmitters only transmit once every 5 seconds. The green and yellow transmitters are slightly less often (7 and 8 second periods?).

 

[scubadada]

Hi @stuartv

So...my Teric seems to always start off with a lower GTR than my Oceanic. It seems to be closer at the end of the dive, I haven't paid enough attention to whether it is still lower, but gestalt that it is. Seems neither manufacturer compensates for deviations from the ideal gas law. Oceanic uses a 90 sec sample for the calculation, SW uses a 120 sec sample and discards the 1st 30 sec of each dive. Oceanic includes the SS, SW does not. I have my Oceanic reserve set at 500 psi and my SW reserve set at 560 psi, adding in my usual SS. I will gather some data and report back.

Hi @jborg

So is that 30 sec for for a increase in gas consumption and a 60 sec sample for a decrease in gas consumption? Both more rapid than used by Oceanic or SW.

 

[scubadada]

A very minor point, probably irrelevant to the topic at hand, but...

I believe the grey PPS transmitters only transmit once every 5 seconds. The green and yellow transmitters are slightly less often (7 and 8 second periods?).

So maybe there are less data points per unit time?

From my VT3 manual:

 

[stuartv]

Hi @stuartv

So...my Teric seems to always start off with a lower GTR than my Oceanic. It seems to be closer at the end of the dive, I haven't paid enough attention to whether it is still lower, but gestalt that it is. Seems neither manufacturer compensates for deviations from the ideal gas law. Oceanic uses a 90 sec sample for the calculation, SW uses a 120 sec sample and discards the 1st 30 sec of each dive. Oceanic includes the SS, SW does not. I have my Oceanic reserve set at 500 psi and my SW reserve set at 560 psi, adding in my usual SS. I will gather some data and report back.

How did you determine that Oceanic does not compensate for non-ideal gas behavior?

All it needs to know is the gas pressure, which is exactly what it does know, to determine that, at a starting pressure of 3000 psi, how much you breathed in 10 seconds is really (for example) 3% less than if you breathed down the same # of psi from 1000 psi in 10 seconds.

So maybe there are less data points per unit time?

From my VT3 manual:
View attachment 620619

There is certainly nothing to invalidate the idea that the computer looks at the cylinder pressure every one second in maintaining its GTR display. But, the pressure value it is using isn't going to change more often than once every 5 seconds. So, I guess if it does work that way, it prevents having a 4 second latency in updating the GTR...

Like I said earlier, a minor point, not really of any consequence to this conversation.

 

[ATJ]

A very minor point, probably irrelevant to the topic at hand, but...

I believe the grey PPS transmitters only transmit once every 5 seconds. The green and yellow transmitters are slightly less often (7 and 8 second periods?).

I'm pretty sure it is more like 5.1 or 5.2 seconds for the green and yellow transmitters. It is just a little bit off 5 seconds to make the chance of a clash less likely.

 

[stuartv]

I'm pretty sure it is more like 5.1 or 5.2 seconds for the green and yellow transmitters. It is just a little bit off 5 seconds to make the chance of a clash less likely.

Thanks for the correction. I looked around and found this:

This was from Shearwater, in a 2017 thread on SB:

We currently only sell one transmitter interval, but there are two others available (both still have FCC ID MH8A):
* Green housing - 4.80 seconds
* Grey/'platinum' housing - 5.00 seconds (standard OEM issue for other brands as well)
* Yellow housing - 5.25 seconds
Note the green housing does not necessarily designate 'O2 safe'. All units are capable of being O2 clean, although we don't currently sell them that way either.

Normally, using transmitters with the same interval does not pose a problem. However, we have on occasion seen (and had reports of) signal interference when using two identical transmitters. Turning one transmitter off for 2 minutes will normally resolve the problem, but of course this is not ideal.

Mitch Burton
Shearwater Research​

So a 5 sec interval is standard. It is not a random interval. If one turned on one grey transmitter and then exactly one minute later turned on a second grey transmitter, unless one of the clocks has a LOT of drift, you won't get any interference. They will never be much closer than 1 second apart. I try and turn mine on 2.5 sec apart.....and don't have any problems.

 

[scubadada]

How did you determine that Oceanic does not compensate for non-ideal gas behavior?...

I do not know that, I assume that. We know that SW does not compensate for deviations in the ideal gas law. If Oceanic did, the GTR would be lower than SW, the opposite is what I have consistently observed. I will collect GTR values from both computers when I have the opportunity to dive again.

 

[stuartv]

I do not know that, I assume that. We know that SW does not compensate for deviations in the ideal gas law. If Oceanic did, the GTR would be lower than SW, the opposite is what I have consistently observed. I will collect GTR values from both computers when I have the opportunity to dive again.

I think you have that backwards. Yes, SW does not compensate. If Oceanic does, then Shearwater would be consistently lower (in the early part of the dive), not Oceanic.

That is why the SW documentation that you quoted earlier says in the early part of a dive the SW computer will overestimate SAC and underestimate GTR.

Your observation is that your SW is consistently underestimating GTR (compared to the Oceanic). That is consistent with the theory that the Oceanic IS compensating for non-ideal gas behavior.