Purchasing Strategy for dive computer(s)

[DiveLux]

Hi everyone,

My wife and me are relatively "fresh" divers with both around 19 dives with currently zero equipment and we are now thinking about our purchasing strategies for our dive computers.

• My wife is pretty sure that she wants to stay in "recreational diving" (she just interested in "seeing pretty fish, turtles and hopefuly whales" ),
• while I have become already fascinated with "decompression theory" and all the science and "skills" around diving
  (yeah, the fish are actually less important for me )
  so I have prety much decided that I want to venture into the path of "technical diving" within the next few years

So with that background in mind, I am currently thinking about which dive computers to purchases (as this is often recommended to be one of the first pieces of equipment). And if anyone would like to comment on my current status of thinking, I would be very gratefull - all comments are very welcome, even the more "robust" ones.

(1) What kind of dive computers for my wife and me
=============================================
My first thought is that since I know that I want to eventually go TechDiving, I will need two TechDive capable dive computers eventually. So the idea is to right away buy those two and let my wife use the second dive computer on during our joint "recreational dives".

In this case I have also often heard that it makes sense to buy two computers with (a) at least similar "algorithms" or better even with (b) simliar brands.

This strategy is to make sure that (a) during my future TechDives the deco calcualtions do not deviate too much between the two computers and (b) during our joint recreational diving, my wife and me can read and "understand" each others dive computers interfaces with ease (to reduce confusion under water).

So since we are in a hw forum, let's assume that I want to buy two "tech dive capable" hw computers.

(2) What model or features for the wife / second dive computer
=======================================================
So for the second dive computer I would then opt for an OSTC+ or an OSTC2 in the "sport" edition. This should give my wife all the features that she needs for recreational diving and if I reach the level of "Tech Diver" (hopefully in one or two years), then I can just upgrade the computer.

Since the second computer is planned as a "backup" computer for my future Technical Dives, there is no need for any kind of connectors, Fischer/S8/etc, on a "backup" computer correct?

However, in this scenario: What is the key difference between the OSTC2 and the OSTC+ ?
Or would maybe buying a "used" OSTC 4 be a better idea for such a "second" computer?


(3) What model or features for my / the first dive computer (Rebreather integration)
=========================================================================
Beeing a "nerd" i would opt for the OSTC 5 because it has all these nice gimmicks like "Air Intergration" that I would like to try out over the comming years.

However, one decision that I need to make here today already is about the "connectors" for a rebreather.

As just said, I am currently at 19 dives, so it will take some time until I will venture into "rebreather land" . However, the whole question about "rebreather integration" is anyway a bit unclear for me, so if someone can enlighten me, that would be very much apprechiated:

1. I understand that rebreathers are often offered with an "integrated" dive computer already (mostly a Shearwater), but I sometimes also see that they can be purchased without a computer. So if would own an OSTC5, so how likely is it that I could integrate the OSTC5 with my future rebreathers (e.g. a "KISS Sidewinder" or a "Dive Talk Go")? Am I then limited to certain rbreather models that are "OSTC5 compatible"? Or can basically every rebreather be connected to every dive computer?
   
   Because as far as I understand rebreather technology, you just need to connect and "calibrate" your dive computer with the "O2 sensors" that are inside of your rebreather.
   
   But is this connection between the O2 sensors in your rebreather and your dive computer just an ordinary "electric" cable? Or are there some special things in different rebreathers that restrict the dive computers that you could use?
2. Since I might switch between open circut dives (together with my wife) and closed circut dives dives during the same holiday trip in the future, I would prefer a dive computer that can be "detached" from the rebreather and can then be used in both scenarios (to not compromise my diving history during a trip)
   
   So the question is, do all rebreathers allow a "detachement" of the dive computer?
3. Concerning cable connections between rebreather and dive computer: I see that the OSTC5 is offered with S8/Fischer or optical.
   
   As far as I was able to read up:
   
   The "Fischer Conncection" is used with most rebreathers models, but actually not certified for underwater connections (?)
   
   The "S8 Conncetion" is less frequently used but actally more stable and certified for underwater usage
   
   And I have also read that there is a "converter" available for "Fischer to S8" or that you alternatively can ask your rebreather manufacturer to please produce your unit with an S8 connector.
   
   And on the hw page I have read some descriptions that the OSTC5 is able to incorporate (additional ?) CO2 sensors via the S8 connector. But I was never able to find anything solid about this yet.
   
   So as a conclusion, it looks like, if I do not know which rebreather I might use in the future, the smartest choice woudl be an S8 connector on the OSTC 5
   
   Sidenode: The OSTC is also offered with an "Optical" connections. However, I was not able to figure out in which use case or scenarios this would be a smart choice...

So, that is the status of my current thinking and any comments are very much welcome.
At miniumm your would be helping a "fresh" diver to enhance his understanding of the craft.

[Ralph]

Hi,

as you already wrote, a (set of two) OSTC+ or OSTC2 will serve you best. Start with the Sport version, and than upgrade to Tech later on. These computers will provide everything you need for sport as well as for tech diving. For sure the OSTC 4/5 is an option, it has a bigger screen, 3 instead of 2 buttons (makes adjusting it a little bit easier), but it is also more costly and bulkier, so check with your wife.

OSTC2: built-in, sealed battery (factory replacable), plastic housing -> scratches are almost invisible

OSTC+: user-replacable battery, aluminium housing -> different colors avail, but edges will soon all become natural alu color due to wear

Software is identical, internal hardware nearly also (the + can emergeny-operate on a 1.5V one-way-cell).

Regarding CCR diving: i know nowadays there is a direct path from REC to CCR, but i would recommend to go the "old path" via OC Tech before turning to CCR. Industry wants to sell CCRs, but to really make a benefit from a CCR and all its complexity, it is a machine for Tech / deco diving. To my perception, all SCR & CCR products that explicitly addressed rec diving (no deco, just extended bottom time) had a short life. Maybe because Nitrox (and if available carrying two tanks with different mixes) allows for the same profiles at much lower cost and logistics overhead. Last but not least, when heading for CCR, you will spend so much money on gear and courses - do not worry about the computer decision now...

As you asked, some info on rebreathers: every unit sold in the EU needs to have CE, and as such needs to be a self-contained, ready-to-go product, i.e. it needs to have its own setpoint controller with it. Depeding on brand, they may come with one computer with basic deco calculation functionality (e.g. the Inspi by AP), or two modified tech dive computers of which one controls the setpoit and both do independed dive calculation (e.g. SF2). Hooking an OSTC to a CCR either means to add a 4th cell, or to tap the OSTC to one or all 3 existing cells using Y-cables. Any way round, you will modify your unit, rendering the CE void, so in case something happens it will all be on your responsibility. Short: know what you (will) do! With the SF2 you could replace the 2nd OE-supplied dive computer with an OSTC via the Fischer connector, but then you will add dissimilarity to your kit.

BR
Ralph

[DiveLux]

Hi Ralph,

Thanks for answering so quickly! Your answers (a) really help but (b) open up some more questions:

As you asked, some info on rebreathers: every unit sold in the EU needs to have CE, and as such needs to be a self-contained, ready-to-go product, i.e. it needs to have its own setpoint controller with it.

Okay, so that means that if I buy a rebreather in the EU (and I would like to keep e.g. any "insurance" risks to a minimum) then I can only use the dive computer that comes with the original rebreather.

Quick question therefore: Does anyone know about a rebreather that comes with OSTC products as "primary" unit?

Hooking an OSTC to a CCR either means to add a 4th cell, or to tap the OSTC to one or all 3 existing cells using Y-cables. Any way round, you will modify your unit, rendering the CE void, so in case something happens it will all be on your responsibility. Short: know what you (will) do!

In my particular case: I am based outside of Europe anyway (so I am not sure if CE helps me) and I am currently thinking about something like the "Dive Talk Go" (https://divetalkgo.com/) or similar. What I like about such a unit like the DTG is that it is "light' enough to take onto a airplane and it should carry me during both recreational and technical scenarios in the foreseeable future.

So assuming that breaking the CE certification doesn't matter anyway in my case, I would need to check with the manufacurer (or an experienced dive shop) whether they can hook the OSTC to the O2 cells of the DTG as the "primary unit", which means that this would be some sort of "custom build".

Then the killer question is this: Is such a "custom primary unit" an absolute "no-go"? Is this just "unusual"? Or is it simply "not that frequent"?
How big are the risks of doing such a custom build?

And if this is all so unusal: Then my question becomes: Are OSTCs actually used anywhere in rebreather diving? If yes, in what role?


Also then the next (retorical) question to myself is about "why on earth"
===================================================================

Now, the question comes up: Why would I want to go down such a complicated road and custom build the rebreather to an OSTC instead of just using the "normal" primary unit that is already integrated with the rebreather anyway?

To be honest: I have tried out the Sheerwater Perdix and the OSTC+ and I have to say I somehow "like" the OSTC better. No very hard technical reasons, just personel preference.

Contributing here was that setting up the bluetooth connection for downloading the Sheerwater divelog was a pain in the ****. And I have made a principle of not "locking myself" into any manufacturers data formats for any data that I will rely on for many years in the future. Here OSTC buetooth connection to Subsurface was actually easy as a breeze. .

For the OSTC5 I also kind of like the idea of having Air integration in and thereby having "all relevant" data in one computer (O2, dilluent, bail-out, depth, time, etc.)

Interesting was also the small announcement in the OSTC5 description that there might be a CO2 sensor available in the (near) future?

So for deciding about the OSTC, the question again comes down to the question of "how unusual" is it have the OSTC as a "custom primary dive computer" with your rebreather? My understanding was that the O2 sensor connection in the rebreather is an analogue "electric cable" and the computer needs to be "calibrated" on its voltage anyway. So is this unusual?

Thanks again for all the comments.

Best!
Rasmus

[heinrichsweikamp]

My wife and me are relatively "fresh" divers with both around 19 dives with currently zero equipment and we are now thinking about our purchasing strategies for our dive computers.

Thank you for your enquiry and of course we share your enthusiasm for diving 100%. With 19 dives you are still at the beginning of a hopefully long career and you will get many questions answered on the way to the CCR. Even if you go full throttle in the direction of tec diving, that is still a few years and many, hopefully beautiful and educational, dives away. Ralph's tip was pretty good, an OSTC 2 or OSTC+ will accompany you now and later. Even if it is ‘only’ used as a backup - a backup can be the difference between ‘I'll just dive my plan properly to the end’ or ‘Now I have to think about a new strategy under stress’ on a demanding dive.
The OSTC 5 is just like an OSTC+ TECH or an OSTC 2 Tech: perhaps a little too many features for you at the moment and, quite honestly, we are not targeting the market for beginner divers. However, with a little familiarisation and practice, all OSTCs are very intuitive to use and everything comes from practical experience. Important functions, for example, are always accessible underwater with very few button presses. As we also use the ZH-L16-GF deco model without its own ‘flavour’ (See: https://www.heinrichsweikamp.net/downlo ... web_en.pdf ), the deco times are identical to other dive computers with this deco model. This is practical with an eCCR and necessary in a team with other dive computers from our market competitors.

Regards,
Matthias

[jb2cool]

I don’t really believe you need 2 computers, you’ll likely be diving with others so if your computer dies for any reason you can always complete the dive on theirs, if you get separated from them for any reason then you have yours.

OSTC mk2 served me well till I switched to an OSTC Plus, this is all you really need.

The difference between an OSTC Plus and OSTC2 is the battery, this is replacable on the OSTC Plus where it’s fixed on the OSTC2. I really like the replaceable battery as batteries don’t last forever and by replacing this you can refresh the computer and keep it going for years.

Also I think you are misunderstanding the ‘computers’ that are attached rebreathers. These look like computers but are really controllers where the computer handset actually controls what the rebreather does, this is different to a regular dive computer that’s just plumbed in to a rebreather to monitor O2 sensors.

[heinrichsweikamp]

So for deciding about the OSTC, the question again comes down to the question of "how unusual" is it have the OSTC as a "custom primary dive computer" with your rebreather? My understanding was that the O2 sensor connection in the rebreather is an analogue "electric cable" and the computer needs to be "calibrated" on its voltage anyway. So is this unusual?

While this is true for analoge O2 sensors, the OSTC 5 also supports Digital Solid-State (Factory calibrated) O2 and CO2 sensors.

With a current out-of-the-box eccr rebreather, the addition of another computer, e.g. an OSTC 5, is not intended by the manufacturer and is also not really necessary. This functionality is aimed more at exploration divers with an mCCR (like the the "Dive Talk GO" you mentioned) or a PSCR.

Matthias

[Ralph]

Even if you are homed outside of the EU, a product having a propper CE declaration, issed by a traceable "notified body", gives you some certaincy that the machine you are going to put your life on has gone through some propper tests. As it has been written before, what makes a dive computer a controller is, that it not only measureas the ppO2 in the loop and does some deco calculations with it, but also controls the solenoid for O2 injection. Depending on model and brand, the controller will also supervise some more electronics, like a srubber monitor, CO2 monitor, will provide power-up checks of those, checklists etc. So its software will be very geared to the specific CCR model it is gonna be used for, from interfacing with the electronics in the CCR to things like calculating O2 injection timing. HW OSTC computers do not support any of these functionalities, they can not be used as controllers.

Another point to consider: Maintaining a CCR requires quite a lot of spare parts over time, so with every entrepreneur and product entering the market, ask yourself: do i belive this company & product will still exist and be served in 5, 10 years? And as CCR training & certificates are bond to the specific CCR model: is there an instructor for the choosen model available?

[DiveLux]

Hello everyone,

I first would like to say thanks a lot for all the comments!
It was already extremly educational and I really learned a lot - so thanks for that and I am certain that others new divers (that will stumble through this forum) will also benefit from this thread.

So I am combining a few of the comments now:

Also I think you are misunderstanding the ‘computers’ that are attached rebreathers. These look like computers but are really controllers where the computer handset actually controls what the rebreather does, this is different to a regular dive computer that’s just plumbed in to a rebreather to monitor O2 sensors.

As it has been written before, what makes a dive computer a controller is, that it not only measureas the ppO2 in the loop and does some deco calculations with it, but also controls the solenoid for O2 injection. Depending on model and brand, the controller will also supervise some more electronics, like a srubber monitor, CO2 monitor, will provide power-up checks of those, checklists etc. So its software will be very geared to the specific CCR model it is gonna be used for, from interfacing with the electronics in the CCR to things like calculating O2 injection timing.

Oh okay, now this really helps to wrap my head around this:

Let's assume for a moment, that we are talking about an eCCR:

• In this case, I need a specialized control unit that manges the injections of O2 in the CC in order to keeping the correct ppO2 level (and maybe a bunch of other functionalities)
• This control unit may then also have additional dive computer functionalities but these functionalities are not "strictly" necessary to operate the rebreather
• So in these cases I always need a specialized control unit that comes with the rebreather
• An OSTC (or any other "dive computer" for that matter) that would connect to the rebreather's O2 sensors via an Y-cable would then just be an additional "monitoring device"
• My question is then: Why would I want to use an additional "monitoring device"? Just for "redundancy" of a "life critical system", right?

If we are however talking about an mCCR:

• Then I thought that the O2 injection is more like a "constant flow" that is controlled via a "mass flow orifice" and the diver just adds additional O2 manually every 7-8 minutes (depening on whether the ppO2 has drifted out of the target range)? ( https://www.kissrebreathers.com/mccr )
• So can we say that with these mCCRs there is "strictly speaking" actually no "control unit" because the O2 injection is mechanically controlled? The dive computer job is just to "monitors" the ppO2 and to alert the diver to remember to manually inject the O2? Or am I wrong?
• At least I see two (normal?) dive computers here in these role the "Shearwater Nerd" and the "Sheerwater Petrel". They seem to integrate for example with the:
  • the KISS sidewinder ( https://www.kissrebreathers.com/catalog ... p207591749)
  • or with the "Dive Talk Go" ( https://divetalkgo.com/shop/ccr/dive-ta ... ional-ccr/)
• My question is then: What role would an OSTC play in such an mCCR setup?

And to come back to the solid-state O2 sensors:

[...] the OSTC 5 also supports Digital Solid-State (Factory calibrated) O2 and CO2 sensors.
With a current out-of-the-box eccr rebreather, the addition of another computer, e.g. an OSTC 5, is not intended by the manufacturer and is also not really necessary. This functionality is aimed more at exploration divers with an mCCR (like the the "Dive Talk GO" you mentioned) or a PSCR.

This is the next thing I would like to understand?

• How would an OSTC5 be integrated into an CCR setup? And how would the "Solid State" sensors fit into this setup?
  For example, the KISS sidewinder or the DTG come with "analogue" O2 sensors, if I see this correctly.
• How would we then add solid state sensors into this setup? Would we "replace" the analogue ones with solid state ones? Or do we "add" additional sensors?

And the last angle on all this:

Another point to consider: Maintaining a CCR requires quite a lot of spare parts over time, so with every entrepreneur and product entering the market, ask yourself: do i belive this company & product will still exist and be served in 5, 10 years? And as CCR training & certificates are bond to the specific CCR model: is there an instructor for the choosen model available?

If e.g. the DTG still exists when I arrive at the "Tech Diving" stage, then I assume that it has survived for at least 2-3 years.
The KISS sidewinder (where I also have my eye on) seems to be around already for quite some time.

My bigger problem is currently more with the "instructor" and a "buddy". I am based in China (outside of the tier 1 cities) and finding an "English or German speaking" instructor and and "English or German speaking" buddy who will ideally dive the same rebreather, will be the next nightmare.

That's why I like the DTG, because it might be relatively easy to fly to the Philippines, where there is at least an instructor available.

[Ralph]

for eCCR: your summary is completely right. So why do people add a dive computer? I give an example, actually it is my story: the AP Inspiration has two setpoint controllers, each one acting independently, measuring the voltages from the 3 sensors, computing ppO2 and computing solenoid commands. Anyhow, one is the master actually operating the solenoid and the other one monitors the 1st and takes over in case it is unhappy with the performance of the 1st. Both controllers drive two lights (red & green) via a pastic fibre to the HUD, which in this case is not a fancy OLED display but only 4 light points in total. Anyhow, once this electronics is powered-up and in dive mode, that is all that is needed to keep the unit going (these controllers also read the scrubber monitor, the CO2 sensor and the levels of the 2 batteries). To do the setup, start the unit and make changes to the setpoint while the dive, there is one handset. If it fails during the dive, no problem, as said the two controllers carry on. The handset also implements a rudimentary dive computer, which only gives you the current ceiling and the total time to surface. So for serious diving, you need extra dive computer(s), to get more info on the deco stops and especially data for what-if scenarios. The OSTCs for example can calculate the bail-out deco schedule and cross-check its gas requirements against the gases & volumes carried.
To track the CCR part of the dive, the computer needs to know the ppO2 in the loop. Theoretically that is easy, say 1.3 bar from 10 meters of depth on. BUT: what if? You have problems with the loop and do a lot of flushing with diluent, your solenoid fails and you need to go into mCCR mode injecting O2 by hand, your oxygen supply fails completely and you need to go into SCR mode - the ppO2 is not steady 1.3. In all of these cases, it is nice to have the computer's calculations slaved to the sensors as in these (stressful) situations you can not steadily enter new ppO2 data into your computer.
How to hook a dive computer to the ppO2 of the loop? Usually, you need to drill a hole somewhere to route a cable out of the rebreather's head. Inside the head, where the gas flow passes over the sensors, you either place an extra sensor only serving the external dive computer, or you tap the already existing sensors. An extra sensor is the best solution (besides having to replace one more sensor about every year), as it does not interfere with the original electronics. In my case, that did not work, as there was no spare space in the head any more due to the CO2 sensor. So i needed to opt for the tapping solution. I tapped one sensor only, as the onboard controllers would already warn me in case there is a sensor disagreement. One sensor is enough to get a precise enough ppO2 value for the deco calculations, and in case there is a problem with the Y-cable the internal controllers only loose one sensor and that keeps them going.
Got it? eCCR instrumentation is a complex undertaking, you need to know what you are doing and also need to know what your original onboard controllers are doing to not make things worse, i.e. your whole unit less reliable than it originally was.

for mCCRs: as Matthias already wrote, they are some specific stuff that either survived from times where solenoids and controllers where not available yet, that are used because the religion of their divers forbit to use electronics, or that are very purpose-built self-made machines for extreme dives. I am personally not familiar with any of these, so to me mCCR is one of the emergency modes of my eCCR rebreather, but nothing i am going to start my dive on. Ah, i forgot one type of mCCRs: those devices that are made as-simple-as possible to be as-cheap-to-build-as-possible. They are a spin-off from the SCRs. In SCRs, a constant amount of O2-rich Nitrox gets constantly injected and by not violating a gas-dependent depth limit, the max ppO2 is naturally limited. The min ppO2 depends on your O2 consumption, the harder you work the lower the ppO2 in the loop will be. Again, safety margins at the lower ppO2 end are imposed by the rate of inflowing gas, which gives SCR their constant bubbling, and the add-valve that kicks in once the counter loung is completely sucked empty. The actual ppO2 in the loop during the dive? You don't know, anywhere between the min and max value. This is why many SCR divers added a O2 sensor to their loop and hooked either a pure ppO2 monitor to it, or a dive computer with sensor interface.
The step from SCR to mCCR is to remove the bubbling by switching from injecting a O2-rich Nitrox to pure O2. The pro: no constant bubbling, less overall gas consumption. The con: complete loss of the natural ppO2 min/max limiting. This now becomes your task! So for deco calculation a sensor and a sensor-enabled dive computer will do the math, and you will have to play the controller, monitoring the ppO2 (which usually constantly slowly drops) and adding O2 from time to time, or diluent in case you overdid with the O2. Wait, trust your life on one sensor only, or better use two, or three to be able to check the sensors for anomalies? Wanna do this during the whole dive (yes you are right: with mCCRs you become the setpoint controller! And the more stressful the dive becomes, the more O2 you will consume, the more frequent you have to check & adjust the ppO2!). What was the price difference between the mCCR and a proper eCCR? OK, all my opinion here...

Solid State vs. traditional sensors: traditional sensors generate a ppO2 proportional voltage in the range of 10..20 milli-volt. They can be tapped with a Y-cable, but if the cable to the external computer disconnects during the dive and the contacts get exposed to the water, these tiny voltage will break down and your original controller on the other end of the Y-cable will loose the sensor, too. Solid State sensors have a digital interface that can not be tapped, so an external computer will need to have its own sensor(s). Yes they (theoretically) last forever, but cost-wise they will have their break-even only after about 10 years compared to traditional throw-away sensors - if they do not break before due to severe flooding of the loop or whatever reason.

My last point: for 19 dives, you already know a lot about diving and i think you are on the right track. But please remain patient, do not rush things!
Richard Pyle once said: after 10 dives on a rebreather i thought i am a pro. After 100 dives, i believed being an advanced. After 1.000 dives i now know i'm a beginner...

keep diving!
Ralph