Point of no return on-the-fly calculation

Faceless

It may sounds weirdo, but after playing with your simulation a little bit more, an interesting idea came to my mind.
As Bob has explained to me, deco obligation is being calculated on the fly ( i.e. there is no static dive tables), and given that O.R. rebreather has full gas sensing including gas pressure in both diluent and oxygen tanks, and in addition it can predict state of scrubber, is it possible with these data to derive a so called point of no return in real time as well ?

If you're using finite state machine then I guess, using just plain dull voting logic for O2 cells is silly, probably you have buit some mathematical model of O2 cell falure modes, then hardcoded it into set of states or trends, right ?
So, let say we have some dive/tank pressure/deco obligation/etc. trends and there are junctions of these trends which may lead to inability to perform a required deco obligation. If we can hit this junction i.e. when user is near that point or has reached it due to gas leak or smth else not so fatal but pretty dangerous, let's warn user about it.
Kinda a piece of common sense.
Sorry in advance if my idea is stupid


Offtopic: Alex, I wish you smooth and nice trip to DEMA and hope your demonstrations will go as you planned. Keep up the good work.

AD_ward9

Thanks for your good wishes. It was an excellent show.

As well as the photos I will put into the gallery next week, when I have some time, we will do a video of the unit underwater.

We predict the turning points based on the third rule, that is, when 3 * (1 - O2 in bar / starting O2 pressure in bar) >= 1. This might do what you want. Time to surface is displayed on the handset (top left), then ceiling time and ceiling depth.

On your question on O2 sensors, we eliminated all modes where the sensor fails high (by getting the sensor vendor to change the sensor design). We use 4 sensors, and we need only take the highest reading. This means 3 out of 4 can fail. When there are depth changes, we predict the O2 change, and if it is outside the expected range (which is tiny), then we check the O2 sensor and injector by widening the spread of PPO2 control. The PPO2 control can maitain PPO2 for just under 30 minutes even if all PPO2 sensors are out, including for depth changes.

Cheers,

Alex

AD_ward9

I just uploaded the pictures I promised into the Open Revolution gallery.

Alex

Genesis

Just out of curiosity.... how?

Unless I've missed something you have (1) a CO2 monitor in the INHALE side (not the exhale), (2) the equivalent of a mass-airflow sensor (e.g. you know how much gas is being moved around the loop), (3) O2 sensors and (4) the ability to differentiate between nitrogen and helium (percentages) in the loop.

But - mass airflow is only an indirect measurement of O2 consumption by the body. Unless you have oxygen sensors in both the inspired and expired sides of the loop, or some other means to sense true metabolic oxygen demand even with failed O2 sensors, how do you know what the diver's true metabolic demand is?

Without that, even with precision vernier injection, how does the system know how much oxygen is required at any given point in time?

Case in point - I can intentionally breathe deeper and harder than normal. This does not materially change my oxygen consumption, but it does change the mass-airflow through my lungs (and thus any loop connected to them.)

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
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AD_ward9

The same as a PSR, except electronically. This means for each breath, it gives you a certain amount of O2. On changing depth it compensates for lost O2, or extra dil. If you dil flush, ignores it and tries to keep steady state PPO2.

The sensor across the scrubber measures the tidal volume and respritory rate.

On the sensors, stopping them failing high is a long set of issues. We will try and get out that Five Year O2 sensor study we have into the public domain.

Alex

Genesis

Ok, that I understand, but the actual O2 consumed by the body (and thus needing to be replaced) is a function of workload and can vary by a factor of 1000% from "at rest" to maximum exertion. While tidal volume and breathing rate IS related to this, the correlation is not exact and is subject (to some degree) to voluntary override in the upward direction (that is, you can "overbreathe" compared to actual metabolic demand voluntarily - or involuntarily if due to fright, etc - even though that may not be correlated to actual exertion and thus metabolic requirements)

So if I have no working sensors and "breathe deep and frequently" compared to my workload, will the OR send the loop hyperoxic, thinking that I have a higher metabolic demand than I really do? Or, if I'm exerting myself, will it send it hypoxic, not knowing because the O2 sensors are offline?

I guess what I'm failing to understand is how the system can hold setpoint for up to 30 minutes without a direct reading when, from what I can discern, all the indirect means of trying to figure it out aren't going to be correct?

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket

AD_ward9

The correlation with tidal volume and rate is pretty good. That is how the Halcyon fridge and other PSR work. The error is typically 2%. When you work harder, you breathe deeper and faster. The amount of O2 you consume in each litre you inhale is remarkably constant.

The 30 min limit is due to an error of the injector cal, which causes an uncertainty which is cummulative, and that is why we have a 30 min limit for this mode: the PPO2 changes by 0.4 in that time, running at 40l/m to Table 4. It is fairly invariant on the O2 consumption. At 10l/m we get more error than for 75l/min.

Earlier we did this using a different principle, but the error was larger.

Alex

Genesis

Interesting.

I would think that if I intentionally breathed deeper/faster (e.g. intentionally hyperventilated) I would drive the system into hyperoxia..... not so?

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket

AD_ward9

You drive yourself into hyperoxia also.

There is a lot about these principles on variable volume SCRs, such as the Halycon Fridge and RB80. The Fridge uses it all the time: we only use to to check the O2 sensors throughout the dive, and then to manage things if they all fail. There is a complicated algorithm using the spare injector to work out if the injector has failured, or all the sensors.

Alex

Genesis

True, but I don't convulse doing that without some help from a PO2 that is climbing beyond safe limits!
This is true but an SCR uses add gas that can be breathed directly, and thus its safe even if you were to take a hit off the feed without any dilution.......

Its safe to say that if the OR thinks it has the PO2 under control and it does not, that this is an undocumented failure mode right?

Just seems to me that if you have all sensors down, given that the user CAN cause the unit to miscalculate a safe breathing mix (and there is no way to know if it has happened) you can't realistically advocate staying on-loop if other options are available.

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket