Simon Campbell

Posted at 2049 hours on 02/02/09 | Posted in Diving

Sentinel maintenance workshop

It’s not that I don’t trust re-breathers, it is more the fact that I like to understand as much as possible about the kit I am diving; especially if it may kill me at any second…

When I first bought by excellent Sentinel LSS rebreather, I took it to bits, quickly realising that there are very few ‘user serviceable’ parts on board. This is good and bad… Bad, if you are stuck in the middle of nowhere with a unit that isn’t happy. Good, if you a manufacturer and you don’t want some ding-bat pulling the unit to pieces, breaking it and then trying to claim the fault under warranty.

Not really happy about my lack of knowledge, I called up Kevin (Gurr: the VR Technology big cheese) and asked him if they ran a ‘service course’ for the unit. He said no, but as he knew the sort of diving & expeditions we were planning, he said he would put something together specifically for the Triton team.

The date was set and it happened to run the Monday following a BSAC Instructor Foundation Course in Blackpool. I was ‘bossing’ the event and finished at 1830 on the Sunday.

Snow

I set out with heavy snow forecast to VR Technology (based near Poole, Dorset) picking up my colleagues, Tony Bridge and the mighty O-Ring King at the Tickled Trout (yes, this is a real place) on the way past. We arrived at the Premier Inn after an uneventful and ‘snow free’ drive at 0005. I was totally buggered and went to bed.

Next morning I was starving; you don’t have much time to eat properly at these BSAC events, so I really stuffed my face with a huge, unhealthy breakfast.

We arrive

The course started at 1000 and we arrived a few minutes early. It was bitterly cold, even for us stout Northern chaps. We entered the building to be met by Kevin who quickly passed us over to Al (Wright). Some of you may know Al as he, and his delightful wife Freda (who also works at VR Technology), used to run the MV Salutay.

Part one: Electronics

Al brewed up and gave us an agenda for the day. It looked very professional and highly detailed. First up was the electronics department where we were introduced to Steve. He took us through replacing the LCD/PCD in the secondary handset and some stuff on the primary. I have to say its not the kind of thing you want to be doing on a boat, or not at all if you are not totally comfortable with a soldering iron.

Delicate bits!!!

One of the most interesting observations was how delicate the HUD unit is. Basically, its just a PCD in a waterproof tube sealed by o-rings. The ‘vibrator unit’ is soldered on to the board with double sided sticky foam holding it in place. I will be very careful next time I pack the unit for travel.

The handsets are also interesting. They never tell you this but the ‘reed’ switches (that are activated by the magnetic push switches) are made of glass and mounted on the handsets’ PDB. These are quite delicate and handsets need to be treated with great care!

The final couple of really delicate bits are the high pressure sensors attached to the Poseidon Extreme first stages that provide data on cylinder pressures to the primary handset. These are ‘potted’ but still can go wrong if mistreated. The expedition workaround is by turning off the faulty sensor on the handset and screwing in a traditional Submersible Pressure Gauge!

Looking at replacing the O2 solenoid was also fascinating; something that can readily be achieved in the field with a steady hand, a soldering iron and of course, a spare… Once the full unit was fully dismantled, the consensus was access to the batteries and the Central Processing Unit is really a return to factory job.

Following a few connector cleaning tips (white vinegar and a toothbrush), we moved into the main workshop to look at the more mechanical side of the operation.

Part two: Putting it all together

We were introduced to Jan (very nice bloke) who builds up and tests the units before dispatch. It was very cold in the unit where assembly took place, so we donned hats and coats readying ourselves for part two.

There was a customer unit in for service that was about to be reassembled; indeed convenient! Jan firstly showed us breaking down and reassembly of the mouthpiece. This is one of the most essential operations as many leaks (causing failure of the negative pressure test) can be attributed here. The crucial mushroom valves, which ensure the flow of gas is in the correct direction, also sit on either side. Extreme care and patience required!

Every time you mess with the unit you can see the quality of build; everything is carefully machined out of stainless or ‘Delrin’.

He showed us how to replace every o-ring on the unit and then went on to describe the use of the ‘service port’ on the head unit. This is used to increase the pressure of the inside of the unit by one bar following maintenance. When they submerge the head for testing they see the bubbles coming out rather than damaging water going in! Good thinking batman.

Even before I attended this course, I was told quite forcibly that you don’t tighten anything with a spanner, even the hoses: finger tight is right! This was again reinforced by Jan. Cells are replaced very easily but its a good idea to clean of the jack plugs with white vinegar and a toothbrush before replacement.

Next up was the flow cone and CO2 seal replacement. Get this wrong and you are a goner for sure, but again relatively easy. This is the only seal on the unit that should be very lightly greased with Dow Corning Molykote 111 release agent (the grease of choice); all the others can be smeared liberally.

Take a dump

The over pressure valve (OPV) at the bottom of the unit is a standard APEX shoulder dump which assembles and reassembles readily. The ‘Canadian’ made Automatic Diluent Valve (ADV) is a simple piece of KISS – sorry kit :-) and easily maintained.

Next up was the re-tensioning and fitting of the ‘swagelock’ pipe-work. I have to say this is difficult to deal with as it is so tough. Again the quality is fantastic.

Jan showed us the installation of the fitting on the single counter-lung and subsequent testing. The O-Ring King looked at this with interest, as I had asked him to make some of the test fittings for us: anything in the breathing loop must be perfect! We did note that only ‘Virkon’ tablets are recommended for disinfecting the loop components.

This was followed by a demonstration of the checking of the first stage inter-stage pressure check / pressure relief valve and nice cup of tea.

Restrictive blocks

The two gas blocks are great bits of kit, one for manual oxygen addition and one for the manual addition of diluent.

The unfortunate thing is that that the design restricts the flow of gas. This makes the high quality Poseidon Extreme Deep Bail Out Valve (BOV – supplied with the unit), almost useless at depth in a high stress situation, where high volumes of gas may need to be delivered.

The blocks can be retrofitted (and the Triton spec ones are) with ‘Swagelock’ connectors that allow the use of off-board oxygen and ‘dil’ to be connected to the system. Many users incorrectly blame the flow capacity of these fittings for the inability of the BOV to deliver at depth and fit them with larger ones. This may make them feel better but its a waste of cash; the block itself is the culprit.

Moving on, they are easy to service albeit they are flitted with peculiar set of o-rings. 8×2 (two off), 7×2 (three off). More spares required. Finally the isolate on the block was stripped down and reassembled: easy peasy.

The canister

This was the last thing checked. The tabs here are vital, as their proper operation lock the canister into the main body when fired up into the CO2 seal. Replacement, piece of cake, but ‘Loctite 222’ required to ensure they screws don’t come adrift.. Bit of a spring length check and there it was, done.

The rebuild

The final stage was putting the unit back together which, when done methodically, is easy. Jan, who had been building these units almost from the beginning, expertly showed us the best way to do it and route all the hoses correctly.

And that was it. Tony picked up some more 2L cylinders (he had left his originals in the Philippines for when he returns in April), Freda divested us of some cash and we made our way back. Tony was off to Cardiff so we deposited him at Salisbury Station following a couple of pints of excellent ‘Doom Bar’ bitter, being his birthday and all!

O-Ring and I drove home content, feeling considerably more ‘illuminated’ on the subject of VR Technology and the mighty Sentinel LSS Rebreather!

Conclusion

They can be repaired in the field BUT its much better to take a spare head with you if possible. Saves a whole heap of hassle.

For those Sentinel users there is a:

• brand new firmware update for the unit which removes some of the software bugs. Specifically the TPM reading faulty in very warm water (I don’t have a problem here :-)
• modified end (one way valve) to the over pressure pipe which allows the OPV to be screwed down more to prevent constant dumping
• Kent Tooling 5mm stainless backplate which replaces the factory unit and almost removed the need for additional lead in the UK (heaven)
• new manual (which is much better) available to download from the Technology at Depth website

The see more photos of this course, take a look at my flickr photostream or if you want a chat, just contact me!

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