BSAC

1. Rebreather Diving

Why dive with rebreathers?

Why must you have nitrox knowledge to dive rebreathers?
The basic gas mixture breathed in any rebreather to depths of 40 metres will be nitrox and therefore the diver must understand the considerations of using nitrox for recreational diving.

Explain the basic differences between closed and semi closed circuits
SCR units such as the Dräger Dolphin and the San-o-Sub Azimuth use a pre-selected, constant flow, nitrox mix that is chosen to match the Maximum Operating Depth (MOD) of the planned dive. Venting of excess gas occurs periodically during the dive and ascent.

Closed Circuit Rebreather (CCR) units dynamically mix 100% oxygen with a diluent to form a breathable gas mix. For entry-level rebreather diving, the diluent used is air and this combines with 100% oxygen to form a Nitrox mix. For deeper dives the diluent gas may be heliox or trimix, depending on the rebreather manufacturers recommendation. This mix varies continuously with depth to maintain a pre-selected PO2 set point. Venting of gas only occurs on ascent due to expansion.

What are the main differences of rebreathers compared to a normal diving set?
The diver no longer experiences a change in buoyancy when breathing in and out. Furthermore the diver's buoyancy can now effected by up to three independent gas volumes - namely the diver's lung/ rebreather counter-lung, the BCD or wing and the dry suit. These all have to be considered - particularly in an emergency.

What are the main considerations of rebreathers when compared to a normal diving set?
Considerations are the same as open circuit air, nitrox or mixed gas plus a few more. These additional factors include increased risk of hyperoxia, hypoxia, and hypercapnia.

The diver must take a meticulous approach to both diving equipment and diving practises when diving with any form of rebreather. Consequently adequate consideration should be given to;

What units are suitable for use in branch diving?
All rebreather types are used in BSAC branch diving under the following conditions;

In order to use gas mixes other than nitrox, the rebreather should have been explicitly specified by the manufacturer as suitable for mixed gas use in recreational diving.

In addition, units used in branch diving need to comply with the following:

Is decompression diving allowed when using rebreathers within the BSAC?
Not all manufacturers have designed their units for decompression diving.

Where rebreathers are so designed and specified as such by the manufacturer, planned decompression is permitted.

The recommendations of the CCR rebreather diver's training agency should be followed. In all cases, the planned decompression should not exceed that permitted by the unit manufacturer and/or the training agency certification held by the rebreather diver. The most conservative case should be the one adopted.

In addition the BSAC recommend that;

What modifications to a rebreather are considered acceptable to BSAC?
With the growing range of independently produced modifications for rebreathers and the different combinations in which they can be applied to rebreathers, it is impossible for the BSAC to evaluate these possibilities. BSAC recognises however that some members will wish to adapt their rebreathers to suit the nature of their own particular diving. As with open circuit equipment the implications of any modifications should be carefully thought through, particularly, as in the case of rebreathers, where they may have an impact on the breathing loop.

Before applying any modifications to their rebreathers, divers should understand that any modification extends the equipment beyond the design parameters considered by the original manufacturer. Any subsequent modification is solely at the discretion and responsibility of the user who needs to satisfy them that the modification is not detrimental to the performance of the equipment.

BSAC strongly advises divers who modify their rebreathers from standard, other than by manufacturer approved modifications to take out independent third party liability insurance since the use of any modifications are entirely the responsibility of the individual diver.

What is BSAC's recommendation regarding the purchase or diving of rebreathers outside the UK?
If someone dives outside the European Community (EC) or buys a personal non-CE unit outside the EC and brings it into Britain, in principle they are approved to dive it provided these rebreathers comply with the criteria for use on BSAC branch dives as defined above.

It is not against European law to buy a personal non-CE rebreather outside the EC and bring it into Britain provided the units are not being sold or used for paid work. It should be noted that under EC directive EN14143, it is an offence to sell a rebreather that does not conform to CE marking in any EC member state. This directive is now incorporated as a British Standard as BS EN 14143. The offence is to sell it, not to use it. This applies equally to second hand sales as it does to the sale of new units. If you intend to sell your rebreather, for your own protection, you should ensure it is standard as 'CE Mark' before selling it on.

What diver qualification is necessary to dive rebreathers in BSAC branch diving?
Divers must have completed at least an entry level unit specific training course. For rebreather dives involving the use of gas mixtures other than Nitrox mixes, the diver must also have completed a unit specific mixed gas course. All courses should be provided by a recognised Training Agency which is also recognised by the unit manufacturer.

Why is there a 40 metre maximum depth limit for nitrox mixes in rebreather diving?

The entry level rebreather training for both SCR and CCR rebreathers provided by the recognised training agencies generally qualify rebreather users to dive nitrox mixes to 40 metres, with minimal decompression diving.

BSAC has also independently considered in detail the physiological implications of re-breather diving using nitrox mixtures. The 40 metre limit is based on the risk of carbon dioxide retention (hypercapnia) and the pre-disposition because of this to CNS oxygen toxicity.

What is mixed gas?

In terms of this document the term mixed gas is used to mean a helium based gas - either trimix (oxygen, helium and nitrogen) or heliox (oxygen & helium).

Can BSAC rebreather divers use mixed gases on BSAC branch dives?

Yes. Suitably qualified rebreather divers may use mixed gas on BSAC branch dives in accordance with the manufacturers published recommendations, their training agency qualification limitations and BSAC Safe Diving.
Why has BSAC enabled mixed gas use in rebreathers on branch dives?

It is clear that BSAC needed to give back to rebreather divers the depth range that is available to air and nitrox open circuit divers. With the training agencies generally agreeing on a maximum of 40 metres when breathing nitrox mixes in rebreathers, the only way to achieve this was to enable the use of mixed gases. This provided BSAC with an opportunity to give parity to BSAC rebreather divers and open circuit divers on branch dives.

What qualifications are required to be able to use mixed gas on BSAC rebreather dives?

A normoxic/intermediate or full mixed gas rebreather qualification from a recognised training agency. A list of such agencies and courses is contained in the document "Recognised Rebreather Training Agencies and Courses"

Can I dive with mixed gas in my rebreather if I have an open circuit mixed gas certification?

Not on BSAC branch dives. Whilst there are many common factors in open circuit and rebreather mixed gas diving there are also significant differences. The bailout and emergency strategies can vary considerably, partly due to the huge difference between gas efficiency when using the rebreather (on loop) and when having to bailout to open circuit. For this and other reasons the BSAC strongly recommends that a dedicated rebreather mixed gas certification be obtained.

To what depths can a BSAC mixed gas certified rebreather diver go to on BSAC branch dives?

To a maximum of 80 metres or to the maximum depth allowed by his/her qualification, which ever is the shallower depth. With the National Diving Officer's permission, divers can dive below the 80 metre threshold.

Typically divers with rebreather full mixed gas qualifications will therefore be able to go to 80 metres. Divers holding rebreather Normoxic/Intermediate mixed gas qualifications are normally limited to approximately 60 metres by their qualification.

Can I dive to depths greater than 35 metres if I have a mixed gas certification but I hold the BSAC certification of Sport Diver?

Yes. You can dive to the depth and under the conditions that you experienced during your mixed gas rebreather training. Experience gained since completing training can also be recognised at your DO's discretion (for instance, experience in a thin wet suit in tropical waters would not, necessarily, read directly across to temperate water diving in a dry suit).

The ultimate maximum depth that can be dived will be the shallower of 80 metres or your rebreather certification unless the NDO's permission has been obtained.

Why is there a 80 metre depth limit on mixed gas diving with a rebreather?

It is generally accepted that for dives deeper than 80 metres the logistics, planning, management, gas management and back-up considerations become much more complex. 80 metres is generally accepted as the maximum depth at which the divers can be self sufficient and bailout directly to the surface. Diving below 80 metres requires the discipline of team diving.

80 metres also conforms with BSAC's recommended maximum depth for open circuit mixed gas and allows suitably qualified rebreather divers to dive with open circuit mixed gas members on branch dives.

What is the minimum oxygen content in the gas that BSAC mixed gas rebreather divers can use on branch dives?

Rebreather divers should use gas mixes no leaner than their certification allows, and should also ensure that the oxygen content in the gas mix is adequate to sustain life from the maximum depth through to the surface.

What is the maximum Equivalent Narcotic Depth (END) that a mixed gas rebreather diver may use on BSAC branch dives?

BSAC recommends an END of 30 metres for rebreather diving. This is based upon the higher risk of carbon dioxide retention when using a rebreather and the consequential effects upon the susceptibility to nitrogen narcosis.

What is the maximum planned decompression time and depths a mixed gas rebreather diver may use on branch dives?

The maximum decompression time should be within that allowed by the divers certification or manufacturer recommendations.

Why is the maximum set point limited to 1.3 bar for the target operating depth?

A maximum set-point of 1.3 bar is recommended for closed circuit rebreathers with electronically selectable set points to ensure a safety margin against over-shoot of the PO2 - particularly critical if the descent is too fast. However, a set-point of 1.4 is allowed for decompression to facilitate the efficient off- gassing of inert gas providing this does not exceed 80% CNS toxicity.

In addition, breathing inspired PO2 above this level for extended periods significantly increases the risks of CNS oxygen toxicity. It should be remembered that relatively high percentages of CNS can be achieved when mixed gas is used for relatively longer dives with extended decompression schedules on high PO2. Consequently, it may be wise to choose a lower set-point than 1.3 when conducting such dives.

What is the maximum PO2 that can be used with rebreathers on BSAC dives?

Can 100% oxygen be used for open circuit bailout from a CCR rebreather?

CCR diving to 40 metres with minimal decompression diving, as per entry level rebreather qualifications, does not necessarily warrant the use of open circuit oxygen for emergency purposes.

However, if the mixed gas certified rebreather diver chooses to have access to any or all gases carried, whether on-board or off-board, appropriate safeguards should be utilised to avoid inadvertent gas loss and incorrect selection of gas for maximum operating depth.

What is the maximum PO2 that can be used by BSAC rebreather divers for open circuit bailout?

1.6 bar for mixed gas qualified rebreather divers. 1.4 bar for entry level rebreather divers.

The bailout plan of a mixed gas rebreather diver will possibly involve the use of multiple bailout gases. Choice of the bailout gases should be carefully planned in line with the diver's qualifications and not exceed the PO2 of 1.6 bar.

Is there a maximum decompression for the open circuit diving buddy?

The maximum decompression limit of the dive plan should be based on the most conservative case taking into consideration the qualifications and experience of both open circuit and rebreather divers.

I've seen a lot of articles about rebreather divers being solo, can you explain?

In accordance with BSAC procedures solo diving is considered to be outside normal recreation sport diving parameters and not permitted. Owing to the possible increased risk of hypercapnia, hypoxia and hyperoxia, it is particularly important to ensure good buddy monitoring. Unfortunately, many of the early rebreather fatalities occurred while diving solo - it is impossible to say whether these events could have been avoided.

It is essential that the buddy be appropriately qualified, experienced and properly briefed prior to the dive.

Why should a rebreather diver always pre-breathe the rebreather before diving?

Pre-breathing the rebreather warms up the absorbent material towards its optimum working temperature before the dive begins. Pre-breathing also allows the diver to confirm that the rebreather is functioning correctly whilst you are safely out of the water. Very importantly, pre-breathing is your only check for CO2 contamination before your dive. If you get a CO2 hit whilst pre-breathing on the boat or land you will survive. Underwater might result in a different outcome!


Why should the diluent cylinder of a CCR be full at the start of a dive?

The diluent may be used for suit inflation and be performing the function of bailout gas. It is therefore good practice to have the maximum bailout available to the diver in case a bailout situation and/or if the need for the buddy to perform a controlled buoyant lift arises. In these cases a typical 2 or 3 litre, 232bar cylinder will be barely sufficient. Hence, BSAC recommends the use of an additional (off-board) bailout cylinder when diving deeper than 15 metres.

In the case of mixed gas, the CCR diver may choose to use an independent off board source so that the mixed gas is not used for suit or BC inflation, since the poor thermal qualities of helium make it less suitable for suit inflation.

The situation is more flexible in relation to the O2 cylinder since normally very little O2 is used during a dive. Occasionally, 232 bar fills are not always available. However, since there is typically only a 2L or 3L oxygen cylinder, it is prudent to maximise the O2 available to the diver - in particular in the case of a leaking mask or diluent flushes at depth or even leaks in the loop then the cylinder can be depleted quickly.

Many rebreather divers consider it good practice to carry a small cylinder of oxygen or >70 % nitrox that can be plugged into the CCR rebreather as back up should the main O2 supply be lost for any reason.

Can a mixed gas rebreather diver safely dive with a trimix open circuit diver on BSAC branch dives?

Yes. Care should be taken to ensure that decompression schedules and bailout plans are similar to facilitate efficient buddy diving but with good planning it is perfectly possible.
Obviously, it is preferable for two mixed gas rebreather divers to dive together using similar diluent mixes and similar decompression tools. In this case, the decompression and bailout run time schedules will be similar and dive planning is simplified.


Can a mixed gas rebreather diver dive with a nitrox rebreather diver on BSAC branch dives?

This is also possible, but it does have its limitations. Decompression and bailout schedules may vary slightly. Care should be taken in the planning to ensure that they are similar and do not impair buddy diving. This is also simplified if both divers are using the same decompression tool.

Can a mixed gas rebreather diver dive with an open circuit air diver on BSAC branch dives?

Again, this is quite possible, but as the depth and duration of the dive increases the planning considerations increase. However, as the pair will be limited by the maximum depth of either 35 metres or 50 metres for the open circuit air diver the problem is minimised and can be overcome by careful planning.

It should be remembered that the lower molecular mass of helium means that the characteristics of on and off gassing of the rebreather diver may vary from that of the air diver. As a consequence the decompression schedule for similar bottom times will differ. The main differences are likely to be that the mixed gas rebreather diluent diver will have stops at deeper depths and a longer decompressions schedule for a similar bottom time than the air diver. This will become more pronounced as the depths and bottom times become more adventurous. The mixed gas diver may accrue deeper decompression stops than the air diver. On staying together the air divers shall consequently accrue a larger decompression penalty over and above their own dive plan. The key to this is as always to be meticulous in dive planning and to adjust bottom times accordingly.

Why should I register my qualifications with BSAC HQ?

When users register with BSAC HQ, a log book endorsement sticker will be sent to the rebreather users and their qualifications will be entered onto the technical database at HQ. This database will have several benefits;

Registration requires a photocopy of your rebreather qualification to be sent to HQ.

What are the implications if I do not comply with the BSAC recommendations?

As far as the insurers are concerned BSAC's expertise is as the Governing Body for Sport Diving and thus the BSAC insurers follow BSAC Safe Diving. Hence, if these recommendations are not adhered to, the BSAC 3rd party liability insurance may be invalidated for the individual(s) involved.

Why should SCR diving be a progressive step towards CCR diving?

It is clearly not a requirement to learn SCR diving prior to CCR diving, however, certain aspects of rebreather diving are generic to all rebreathers:

Clearly each unit needs specific training and experience, but the learning curve for a SCR diver to become a CCR diver is less steep than from an open circuit diver to a CCR diver - as many experienced CCR divers and instructors will confirm.

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2. Dive Managing a Rebreather Diver:


What proof should a dive manager request to ascertain that a rebreather diver is qualified?

The qualified rebreather user is required to register with BSAC Headquarters, recognition will be sent in the form of a Qualification Record Book sticker.

How many dives can a rebreather diver perform each day?

The entry level rebreather diver will be governed by the normal considerations of nitrox diving such as CNS loading, UPTDs and decompression considerations. This is a practice that BSAC strongly recommends.

Mixed gas use in sport diving is still in its infancy. Mixed gas computers will allow the diver to conduct more than one mixed gas dive per day. However, it is good practice to limit deep diving to one excursion per day. This is a practice that BSAC strongly recommends.

Are rebreather divers subject to similar surface interval requirements as ordinary divers?

Yes. No different to the principles of planning nitrox or mixed gas diving.

Can I use PC programmes for rebreather dive planning?

Yes - "commercially" recognised PC based dive planning tools can be used . Appropriate instruments (computers or depth-timers) should be used to ensure compliance with the dive plan in the normal manner. When carrying out mixed gas diving it is strongly recommended that multiple decompression tools are carried - for example at least one computer could be carried in addition to a dive timer/depth gauge with (cut) hard tables showing run times for back-up.

The dive planning must also take into consideration the experience, interests, equipment and ability of the buddy as in normal branch dive planning.

How do tables and computer fit together?

In each case the dive plan should be discussed and the most conservative dive plan agreed between the divers in accordance with the dive pairing guidelines provided. It is worth noting that some computers and PC based decompression tools will have differing decompression stop depths. An example of this is that many computers have stops based at 12, 9, 6 and 3 metres, Some programmes and computers will have much deeper but short stops within their schedules. This must be carefully considered when planning a dive. It is much better to have both divers of a buddy pair using the same decompression tool.

Can an Ocean Diver dive with a rebreather diver?

Yes. A qualified Ocean Diver can dive with a rebreather diver with the explicit approval of the DO. However, both divers will be governed by the Ocean Divers depth limitation of 20 metres. In addition, consideration and empathy must be shown to the Ocean Diver when confronted by a complex piece of equipment.

How does the buddy check differ?

Review/modify the buddy check procedure to accommodate the rebreather layout especially the shutdown operation of the mouthpiece.

The buddy check should ensure the buddy is aware of the risks and knows the appropriate emergency procedures. For example; the rebreather user should state the possible additional problems with a rebreather (hypoxia, hyperoxia, hypercapnia) and brief/demonstrate the appropriate action for each.

Please send any further questions you may have to BSAC Technical Chief Instructor

Questions can then be included in updates to this list of FAQs



V13 28.12.06