Overview of APD Vision Electronics Features:

All features described refer to Vision firmware version (2.1.3), the current release as of Fall 2007.

• A Single Wrist Display showing information for both controllers C1 & C2, eliminating the need for a second handset. Note, Either Controller can assume the duty of Master Controller for operating the solenoid.
• Heads Up Display (HUD) showing information for both controllers C1 & C2 using fiber optics
• Scrubber Monitor via patented Temp Stik, which is displayed on the Wrist Display with a Bar graph indicating usable percentage left. With 20% left, both an HUD alarm and Audible Buzzer is activated.
• Audible Alarm Buzzer providing critical warnings along with the HUD.
• Real time Nitrox/Trimix Decompression computer with adjustable Gradient Factors
• Automatic or Manual Setpoint switching during descent and Ascent
• Adjustable Low Setpoint from .5 to .9 PO2 and High SP from .9 to 1.5 PO2. Default Low SP is .7 and High SP is 1.3 PO2
• Download Dive information to PC, including PO2 and CO2 monitor during dive
• Upload Software upgrades from the factory via your PC.
• Maintain your Decompression obligation when switching to Open Circuit Bailout with 5 selectable gas mixes, which can be adjusted during the dive.
• Intelligent Battery Power Management algorithm allowing always providing maximum power for operating the solenoid

APD Vision Electronics component summary

The APD Vision Electronics consist of three controllers, three displays, two batteries, an intelligent bus, a buzzer and an TempStik scrubber monitor:

• One computer in the wrist display unit, referred to as the WDC (Wrist Display Controller).
• Two redundant, but independent controllers in the lid of the scrubber with as main function oxygen level (PO2) control, referred to as SPC (SetPoint Controllers). These three computers are fully self-supporting: they have their own program code 'on board'. You can easily see that when you upload new firmware to the Vision: you see the Communicator software uploading and verifying three sets of code.
• A serial databus that connects the WDC with the SPC's and also houses a real-time clock (RTC) to supply the logbook functions of the Vision with the correct date & time.
• It also contains a 'hub' to provide electrical and functional isolation in the event of a failure of any one component hooked up to the databus;
• Physically, the databus bus is represented by the electric cables running through the rubber medium-pressure hose between scrubber lid and WDC;
• One LCD wrist display, built into the WDC that shows the detailed status of the system;
• Two redundant, but independent HUD displays, comprising a green and red light each, placed on top of the mouthpiece. It is important to note that the actual LED's themselves are located in the scrubber lid, near the SPC's, and the light is transported to the end of the HUD through a fiber-optic cable. This was done to avoid having to use a thin, vulnerable electric cable all the way to the mouthpiece and having to use relatively big LED's there, as in most other HUD designs.
• Two batteries (non-rechargeable CRP2 type Lithium), placed next to the SPC's in the scrubber lid, in a water-resistant housing.
• A 'buzzer on a stalk', located to the left ear, and driven by the SPC's.
• A TempStik, that measure the progressing warmth front that travels through the scrubber bed as the Sofnolime progressively gets spent, causing an exothermic reaction in the process. The measurement is done by a number of serially placed temperature sensors in the central pole of the scrubber container.

Functions by Component for APD Vision Electronics

Let's look at which component does what. This is VERY important in order to determine later on which functionality you may lose if a given component fails on you!

Contrary to popular opinion, in my view the WDC is NOT the most important component of Vision. Yes, it gives you a detailed view of it's status, and it lets you control all settings of the system, but it also does NOT do some important things:

• WDC does NOT control the PO2 (set point maintenance),
• WDC does NOT control the solenoid,
• WDC does NOT control the two HUD's (that is done by the SPC's: each SPC runs its own HUD),
• WDC does NOT control the buzzer, and
• WDC does NOT control power management (switching between batteries, and combining them, in case of an empty battery)

The WDC does some other important things though:

• WDC contains the pressure transducer (digital pressure gauge) for depth measurement. It is located in the bottom of the WDC, shielded by the bottom plate that holds the straps;
• WDC executes all CCR and OC decompression calculation functions;
• WDC does control the value of the two set points, and (auto-)switching between them;
• WDC contains the round-robin (FIFO) log memory for alter readout of dive profile and warnings.

So, losing the WDC does NOT mean losing a reliable set point maintenance, or even the ability to verify that, as the HUD's are driven directly by the SPC's (each SPC has its own HUD). However, you would lose your decompression information, but more on that later. Realize also that for proper CCR deco calculations, you only need three external inputs:

• depth (coming from the built-in pressure transducer in the WDC),
• time (coming from its internal oscillator in the WDC; see below) and
• PO2 (supplied to the WDC by the SPC's, its data stream transported to the WDC over the databus).

For OC calculations you even only need depth and time and the gas mix, not more; the rest is algorithm, executed by the program code in the WDC.

The RTC is located near the SPC's on the databus; it is only used for time-stamping logbook entries (dive profile, warnings and errors). However, each controller (WDC and the 2 SPC's) maintains its own internal tick count, based on its own built-in oscillators.

The solenoid is controlled by both SPC's. The Master SPC controls the set point and thus the solenoid if everything is normal and o.k., but the solenoid is linked through a so-called 'wired-or' configuration to both SPC's. The secondary SPC (slave is actually not a good word anymore in Vision terms) also independently monitors the PO2 (through its own cell readings), and starts acting intervening when the PO2 falls below 80% of the current target set point. This happens without the need to switch from master to slave, and acts as a 'catch all' safety-net function.

A similar thing happens for power management. Normally the primary ('master') SPC will perform battery management (switching to the secondary battery in case of battery-low, and later combining the two parallel when both are low). But also here, the secondary SPC can intervene if the active 'master' SPC is not able to perform this function.

The SPC's also contain the calibration correction factors, established during calibration of the unit. Each SPC contains and retains its own set of correction factors: 3, one for each cell.

Now, let's look closer at the WDC: As mentioned before, it contains the log memory, on non-volatile, round-robin, FIFO memory. This means a 'dead' battery will not cause a loss of logbook info, as unfortunately many diving computers do. The WDC receives the following data from the SPC's over the databus:

• Real-time PO2 info, supplied by the SPC's;
• Battery status info, also supplied by the SPC's;
• TempStik data, received directly from the TempStik (no SPC 'interference' here);
• PO2- and cell-related warnings and errors, as generated by the SPC's.

The WDC on its turn can send commands to the SPC's:

• Power up/down;
• Do calibration;
• Change set point. The SPC however does a validation of the new set point request. It will e.g. refuse a new set point of 2.0 if for some error reason the WDC would send that.
• Switch set point. This is effectively a set point change, initiated by the WDC, either on instruction of the user (long press of center button), or by the auto-switch function, driven by passing a certain depth. Remember that only the WDC 'knows' the current depth!
• Warnings and errors that are not PO2-related, e.g. decompression errors and warnings. They are sent from the WDC to the SPC with the sole purpose of showing them on the HUD's and the buzzer, as both of these are controlled by the SPC's.

The WDC cannot trigger the buzzer by itself: it needs a SPC to do that. The same applies to displaying errors and warnings on the HUD's.

'What if component x fails?'

I will not go into what bail-out or other actions are appropriate if a component fails. That is up to the instruction agencies or your smart self to decide. E.g.: do you stay on the loop or not in case of a WDC or databus failure? In my PERSONAL view it is totally justified to do that, flying it on the HUD's, but at the same time for sake of 'peace of mind' I could very well defend going OC, as you have less info to draw on.

So by now it should be clear that e.g. auto-switching, or in fact any change in set point, will not occur anymore if the WDC malfunctions, or the databus link is lost. The latter was the most likely reason of the failure reported by the diver in the introduction, by the way. If the databus link is lost (e.g. broken cable in the wiring loom between WDC and SPC's, the following will occur:

• The WDC will not show the proper PO2 numbers anymore, the WDC will show asterisks (*.**) instead of the last received value when it does not receive proper PO2 data anymore. This will make it much easier to spot, even though static numbers should be a clear tell-tale of trouble for a CCR diver. I stress the wording 'frozen PO2 display' instead of 'frozen computer', as the other display functions (e.g. decompression, time, depth) continue showing valid data if only the databus fails.
• The TempStik data will not show anymore, its black and white temperature progression bar replaced by dashes. This was also reported by the same diver;
• Depth and time continues to run, and is logged correctly in memory, as can be clearly seen in the logbook download file supplied by the affected diver on Internet. This includes things like ascent alarms till the very end of the dive, but at the same time will not contain proper PO2 data and/or PO2 related warnings and errors anymore.
• Switching off is not possible anymore: the WDC cannot instruct the SPC's to do this anymore.
• Setpoint changing (either auto or manual) cannot be performed anymore. The most recent set point is maintained;
• Setpoint maintenance however can be verified: a quick squirt on the manual O2 button, followed by a diluent flush, will run the HUD's and buzzer through all main functions: rapid blinking and red due to set point higher that 1.6, followed by green, followed by slow blinking and red due to set point low, followed by green again when the SPC instructs the solenoid to open till the set point is o.k. again. This is a quick set of actions that can easily be performed.
• Your CCR deco-calculations are however not linked to the actual PO2 value in the loop anymore: they will feed on the current static set point value. You could elect to switch the computer to OC-mode, even if you decide to stay on the loop. As this is in most cases safe, because OC deco stop-times are typically longer, this would be a justified action. However, the unit will default to the current set point value and so will act as if it is an unlinked deco computer and as such can safely be used. Very much like a Buddy Nexus or a VR3 without 4th cell!
• Similarly, if the frozen PO2 values are close to the set point then the value used is again sufficiently close to act as an unlinked deco computer, and is again safe to use. Proper dive planning should however ensure you always have tables as backup.

Simple Error Scenarios

• if a SPC fails, its HUD will go off (its lights will go off), and its values will not show anymore on the WDC.
• If you suspect a SPC of being frozen, do the 'squirt with O2 + dil flush' test as described above. It that case the WDC can instruct the secondary SPC to take over. This SPC already has the above described 'catch all' function that kicks in if the PO2 drops to 80% of the current set point.
• If the WDC or databus fails, deco and ascent related errors will not pop up on the HUD's, or trigger the buzzer.
• If the WDC or databus fails, and you do not feel comfortable flying it solely on the HUD's (which give less info than the WDC screen), you can still elect to do the ascent on OC, taking extra care when using hypoxic mixes, but go back to CC at 6 meters, effectively running it as an oxygen rebreather. You can then use the HUD as indication you should inject O2 again, as you likely strive to maintain a PO2 of 1.5 or 1.6, which will cause the HUD the blink fast. If the PO2 drops to 1.3 (assuming the last selected set point was 1.3), the HUD's will go steady green, indication you to inject O2 again. Nice feature!
• If only the databus link fails, that will show up as frozen or asterisk PO2 data and dashes in the TempStik data (if fitted), but depth and time will still run. Check it, to isolate the error. It also means the WDC is still usable as an unlinked multi-gas constant PO2 and OC computer, as described above. Or at the very least as depth gauge + timer.
• If the last selected set point was 1.3, and the WDC or databus fails, the unit will NOT switch back to low set point at 3 meters of depth. So: stay deeper than 3 meters to avoid wasting O2. The Vision will keep the solenoid continuously open if the set point is too low.
• Once you get to the surface, do NOT close the O2 valves: in the stress of the situation (after all, you experienced an annoying failure), you might keep the CC mouthpiece in and get hypoxic. Better to lose all of your O2 slowly than go hypoxic! Better still: go to OC once on the surface. Once on the boat, unit taken off, close the O2 supply. You have to, as you will also not be able to switch the system off, as this is done by the WDC instructing the SPC's. you'll have to remove the batteries.

I hope this contributes to a better understanding of the APD Vision Electronics, and effects of possible failure modes.