Questions answered about building a BP60


This is an old repost from Rebreather World.

Why build up a BP30 BP60 BP240 and how much?

This post is in response to a question that was PMd, to me, that I thought would answer some other lurkers questions as well, as this is not the first time that I have been asked. I believe I reflected a less detailed answer in another thread as well.


 Originally Posted by WartedEmperor

How much time and money would I have to put into it if I did it all right the first time?


This is a difficult question to answer, and depending on who you ask, you might get a couple of answers. That I know of, there are only two people still diving their BP60 conversions, myself and Mark Munro, and of us, Mark has severely slowed working on his due to a near fatality, and I have been going forward with mine, using it as a platform for remote cave exploration. Right off, that should tell you something about the unit. It is not the ideal combination of utility, cost and usefulness, nor is it particularly simple to convert into a safe, diveable unit. I have a particular reason that I stick with mine, and that has mostly to do with small size and portability. However, I do still consider it a good way to start, and with a couple of molding differences, it could really be a very good rebreather. unfortunately, new molds are not going to happen, so some shortfalls must be expected.

With that disclaimer aside, let me try to answer your question in as verbose and round about manner as possible.

First, some other divers gave me an idea of your background, so let me start with how I think would be most prudent to continue with a BP-60. I stand by my initial assessment of starting with an oxygen only system and progressing from there. If you manage to procure a complete unit (30,60,240), they all have the same basic structure and vary only at the scrubber depth and stainless steel cover size) you will have perhaps half the parts required to start a diveable unit. I will give you the rough outline in steps that you will need, and try to guesstimate how much you might spend in making the conversion.

BP-60 to O2 rig. $745 - $1590

1. Rotate the counterlung housing so that the hoses point towards your head. $0
2. Space the counterlung housing off of the plastic baseplate and/or increase the number of holes to allow better water flow to the diapharagm. $0-5 depending on what hardware you have around the house.
3. Scavenging the MAV (Manual Addition Valve) and DAV (Diluent Addition Valve) fittings, reconfigure the piping so that you have straight oxygen addition with no continous injection. $40-50 in stainless swagelock fittings and brass or stainless piping, assuming you have a small pipebender. If you jack the fittings up removing them , you can triple that price to $150, those fittings are NOT cheap.
4. If you choose to use the original tank (not the best idea) you will find it is not enough gas for the duration of your scrubber at depth (60 max feet for short durations, 30 max normal, check navy O2 tables) and you will want to replace it with a 13cf tank, which will definitely be sufficient for a 30, probably for a 60, and maybe not for a 240, and first stage regulator. $250
5. Plug the extra port in the DAV. $0
6. Purchase appropriate hoses- $95 (depending on source, may be much more)
7. Purchase DSV (Dive Surface Valve) or BOV(Bail out Valve). Here is a major argument- I have both, and I will never again dive on a simple DSV. There is just no reason to, as if you have a CO2 event, you WILL NOT be able to switch back to an open circuit regulator in time. Trust me. I learned the hard way. Golem BOV $900. If you are an idiot, the cheapest DSV I found that worked was about $350. You might be able to build a serviceable piece of crap from plumbing parts- $25
8. Plumb your BOV into your O2 bottle- $10
9. Add a constant addition orifice- $130 for the orifice and filters, $55 for the volumetric tester to check it before every dive. This is not really necessary, as the DAV will add every time you need gas, but can make breathing easier, as you are basically breathing through a tire valve otherwise.

That's probably it- you don't have to worry about decompression, you cant go below 30 feet really, but you can cruise around in the shallows for a significant period of time, and for some that is a really good start. The nice part, is you have bought one of the really expensive parts already (the BOV) which you can move around to other units later.

BP60- Oxygen to Mixed Gas (In addition to the high number above)
$608, if you are a machinist, and you hate dive computers.
$1410, if you are not a machinist, and you hate dive computers.
$3410, if you like to know what your decompression is based on what you really are diving, and you are not a machinist.

Now you have to fix some of the critical issues with the system, as you will be going deeper and with gasses that would not be suitable for a direct ascent to the surface. For your benefit, I have tried to include two prices- that if you happen to be a machinist, or are good friends with one (free work), and if you might have to be paying for one-off production (cause nobody else is doing this.)

1. First, the breathing path of the BP-60 is not optimal. depending on how you plumb it, there is an obstruction in either the inlet or exit tube from the diapharagm housing that severely limits gas flow. Mark Munro found this out the hard way. The BP-series will NOT flow enough gas through the loop to sustain a heavy breathing diver during high exertion events. This is particularly true at depth, where the density of the gas increases, and worsens the effect. You have two options in this case- control your breathing, (not always possible), or go to open circuit on the BOV. Mark and I looked at making a piece to add into the counterlung housing mold that would alleviate this problem, but he had lost interest in diving the unit, and I found it easier to go to OC on my BOV instead of chopping up hard to find BP-60 counterlung housings. The best you can do to reduce breathing effort is to make the path of water flow to the counterlung as unrestricted as possible. The original housing was designed to move air, not water through the ports, and as such provides insufficient flow. There were a number of answers to this problem in the early days, one of the best of which was to space the counterlung housing off of the original plastic backplate using machined spacers- leaving a 1/4-3/8 inch gap all the way around the bottom, But resulted in having to replace the counterlung spring to provide the same amount of inhalation assistance. This provided enough flow, while retaining the original backplate. I went a different direction, losing the backplate all together, and developing a steel bracket which retains the original spring length, but completely opens up the counterlung, preventing wetsuit and drysuit from blocking the inlet ports. New spring and machined spacer $250, Steel retention plate (of my design) $200, If you can cut and bend your own $18

2. You are now going to have to figure out a manual addition O2 valve, in addition to the orifice that you should have installed in step one. There are a couple of ways to go about this- You can buy an all in one unit/swagelock nightmare from a number of sources ($??) but I'm not even sure most places will sell such a thing without some proof of certification in their specific unit, you can cobble one together off the internet from swagelock parts ($200) and I think I saw one that some guy in europe was making, very smooth unit, around $500? Can't remember. Or you can build one similar to mine ($500 if you don't tell the machinist what it is for) or $150 if you can cut it yourself (some parts and special tools.)

3. Now, you will still have a fiesta of cabling and hoses, which you can probably just shove under the housing, or you can do an arrangement like mine, with the bulkhead bracket and special fittings. ($600 in machining, $80 if you can cut and machine it all yourself.)

4. From there you are left with a way to secure bottles. For now, everything I do is offboard, so no special bracketry. but you will need the quick release fittings from swagelock. Not cheap at all. $200.

5. You will also need a way to monitor your gas mixture. You can buy and assemble a munro board and build a housing $60, and then machine a holder for the sensors, as well as run the cabling and such (another $100). Or you can have someone else do it for you ($250) or you can install an X1 and X-link or a Shearwater. (2000-2400 when all done) Without the computer, you are flying blind decompression wise, and will eventually bend yourself, or spend an inordinate amount of time watching your PO2 display.

So just off hand, that is what i came up with.

You can see that obviously, (though it didn't seem so at the time) money is not the reason that I dive my BP-60.
I could easily have saved all this money (Nearly $4600 and I am an amateur machinist) and bought something nice like a Kiss Classic, or something else. Anything else.
As it is, I can't step on a dive boat with my machine (no sane boat captain would let something like that on their boat without knowing me) and every time I come out of a popular cave (Jackson Blue, for instance) people react like they've seen the Creature from the Black Lagoon. I invariable get one or two side comments (not always directly) about how I am going to kill myself on that piece of shit, thus my naming it the Deathbox.
On the upside- It is, by far, the smallest 4 hour duration rebreather available. It is also one of the cleanest, in the sense of configuration on the body. There are no hoses or counterlungs or any of that stuff on the chest.
I also get the satisfaction of diving something I have built with my own hands, with parts that I designed with my own flawed brain. I enjoy the modeling in 3D cad, and the machining, almost as much as actually diving it.

If this does not sound like the types of motivations that are making you want to get a BP-series machine, I would seriously consider saving up the bucks, or crashing the Visa, and getting a production rebreather. You will get a phone number to someone who cares when you have problems. I will give you my email, but I can't guarantee that I will care if you have problems with your BP series machine- and good luck finding anyone else who will.

Hope this answers your (and anyone else's) questions. If you wish to go forward with your project, send me some pictures of where you are at on it, and I will be happy to converse with you via email, about the best way to go forward, and as I switch from Inventor to Solidworks, I will be happy to share some of my drawings.