BP60 Modification Treatise


Disclaimer (Please read):

Taking a device engineered for one purpose and then modifying it beyond it's intended purpose and outside the proper manufacturers training may kill you.  It's as simple as this, you may end up dead.  I survived doing the modifications discussed below but maybe I was lucky, you may not be so lucky.  If you use a BP60 without the proper training from an authorized training facility, or god forbid you modify it and use it underwater and then end up dead or a vegetable, don't say I didn't warn you or your loved ones.  Think about it. 

What I present below is not the end all way to modify a BP60 for diving,  it's just the way I did it.  You should look at what I present here with a critical and objective eye but leave your mind open to the viewpoints and suggestions.  In the end you'll hopefully come up with some good information that'll be helpful in making some creation that will keep you alive underwater.

Going forward:

I went about the process  of converting a BP60 for use underwater at a very very slow pace.  I thought about it a lot and discussed it at length with people who knew much more about it then I did.  I proceeded with great caution and fear for my life.  Bench testing, pool testing, bench testing, more pool testing, and then shallow easy dives.  Don't jump in and make drastic changes and deep long hard dives before you really know what's going on.  Rich Pyle once said that after diving a rebreather for a month he knew all there was to know, after diving it for a year he realized how little he really knew (or something like that).  Do yourself a favor, go slow and learn all you can.

I really pushed the BP60 to it's limits (140 fsw) with the stock scrubber, given what I know now I wont do that again.  I still think the BP60 is a fun little O2 unit and I might even take it to moderate depth of 60 fsw.  But with the restriction in the centersection that comes into the plenum under the scrubber from the breathing hose, seen very well here, if you had to work really hard the WOB might be a bit of a problem with CO2 build up, plus the scrubber is so damn thin.

The following is my response to discussions that took place on a mail list long ago when we first started doing this conversion, maybe 1997 thru 2001.


I currently use the Teledyne K1-d in my BP60.  Since there isn't a lot of room between the stainless cover and the top of the scrubber I  chose the K1-d for it's size.     

3/10/2001- I was playing around with my new R22D's and found that they do fit under the SS centersection cover on the BP60, there wasn't a lot of room but they did fit.   I could press in on the top and feel it deflect in a tad and hit the sensor.  Some tests of your own are warranted.  I may in the future try to install one on in my O2 rig for testing and get away from the K1D's since the R22D's should last much longer.

Loop flow direction (part 1):

All this talk about OTS CL's (over the shoulder counterlungs) and Back mounted CL's brings up a question I had regarding the loop flow direction in the back mounted CL designs. Typically I understand the conventional loop flow to be Diver=>Scrubber=>CL=>Diver. And one of the reasons I hear for this is that your exhaled gas is warm and by going directly to the scrubber it helps to keep the bed temperature up. Also it is easier to exhale and push the gas through the scrubber than it would be to pull it across the scrubber if the loop direction was reversed because we can exhale stronger than inhale. Well I'm wondering what's wrong with having the loop flow Diver=>CL=>Scrubber=>Diver ? 

(1) Kevin mentioned the other day, during the hypercapnia discussion, about having to push the water through the scrubber bed to get the water out that was introduced into the loop from the DSV. Well this wouldn't be a problem in this Diver=>CL=>Scrubber=>Diver direction because the CL would act as a water trap. 

(2) All the gas, at least on my BP60, is injected into the CL. So in the Diver=>CL=>Scrubber=>Diver loop flow direction the gas has some time to homogenize before it gets to the diver. Where as if flow was conventional Scrubber=>CL we inject gas and immediately breath the gas we just injected which might have a high concentration of O2 (a concern when diving as a mixed gas rig).

(3) I'm not sure where the sensors are placed for other units but the best place I could find for mine are directly on top of the scrubber. This allows the sensors to get heated up by the scrubber and stay at an elevated temperature. But, if loop flow is conventional Scrubber=>CL the gas coming out of the scrubber and across the sensors is going to be at a higher temperature albeit maybe not much. So condensation is more likely to form than if the loop flow was CL=>Scrubber. The exhaled gas is cooled a little in the CL and passes across the sensors which are warmed by the scrubber and are more likely to be at a higher temperature than the gas. 

(4) With sensor placement on top of the scrubber if loop flow is conventional Scrubber=>CL the gas the sensors are reading is exhaled gas not breathed gas. With CL=>Scrubber loop flow the sensors are reading the gas your gonna be breathing. 

Loop flow direction (part 2):

I've been running my O2 unit by exhaling into the CL and inhaling from under the scrubber. My thought here is that: 

1) If I exhale into the CL and am ascending, the gas that is vented off hasn't gone through the scrubber and removed CO2 that didn't need to be removed. 

2) A lot of the water vapor that would be condensed out will be condensing in the CL and not in an area where it has a higher probability of getting into the scrubber material. Like if loop flow was from the scrubber to the CL then as gas comes out of the scrubber it will condense quickly on the SS cover. I'll also counter my own thought by saying that the condensation would most likely roll into the CL from the vents around the scrubber. 

3) The Porex filter is on the bottom of the scrubber canister and with loop flow from the CL to the scrubber the Porex filter is filtering the gas before it goes into your lungs. This helps reduce the Sofnolime dust ya get. I don't see that foam filter on the top of the scrubber doing much filtering if the loop flow is from the scrubber to the CL. 

4) The gas is warmer and has more water in it when it gets to you if the loop flow is from CL to Scrubber. 

5) I planned on attaching my sensors to the top of the scrubber canister with some type of a fixture. As the canister and metal top heats up the sensors also heat up. With loop flow from CL to scrubber the sensors will be at a higher temperature than the gas flowing across them plus the gas will be less saturated with water and condensation on the sensors will be averted. I don't like the idea of mounting the sensors on a surface that is not as warm as it could be. but I haven't yet played with this so what do I know, just thoughts. 

6) If I add gas/O2 into the CL and have loop flow from CL to scrubber by the time the gas gets to me it has been homogenized If loop flow was the other way, from scrubber to CL, I'd be getting a quick O2 spike. 

7) I agree that by breathing into the bottom of the scrubber first the higher gas temperature will help to get the scrubber going, but with a good pre-breath and the fact that after you get it started the scrubber is surrounded with gas and not in contact with the surrounding water, I don't think it adds much value considering the other things. 

Loop flow direction (part 3):

My logic, or illogic, was that we're trying to make the loop as efficient as possible. I agree that venting gas that hasn't been scrubbed yet will not produce a big savings, but all the little savings add up. As John stated "The scrubbers on these rigs have a 'not so good' reputation for breakthrough", so why not optimize? My point was that all the small savings add up. This isn't to say I'm not going to try the loop direction the other way, I will. How about this as another reason for CL=>Scrubber=>Diver loop flow. When your purging the loop at depth with diluent to verify PPO2 meter readings or purging with O2 on the hang gas is injected into the CL. If you use the CL=>Scrubber=>Diver loop flow you inhale the gas you want to get rid of and purge it through your nose. Do two of these then breath a little bit, purge again. If loop flow was scrubber=>CL=>Diver the gas you breath in from the CL, where the gas was just injected , is the gas you want and now your going to purge it from the loop. Anyone else have a different purge method that gets around this? Of course with the CL=>Scrubber=>Diver loop flow and a purge your now purging off scrubbed gas so I guess savings is not as much of an issue as efficient purging. As Patrick said "the unit is not designed to work that way", but I would also have to add that it wasn't designed to work underwater either (3/2005 - Not exactly true). We're trying to make the best of a system that was designed to keep people alive while fighting fires not to keep someone alive at 130 feet. 

Yes I have a foam pad on top of the scrubber but I ass-u-med that it was mostly used to retain the scrubber material. I can't see the foam absorbing moisture from the loop since its probably at the same temperature as the gas going through it in the scrubber=>CL=>Diver direction and at a higher temperature of the gas in a CL=>Scrubber=>Diver direction. I find most of my condensation/moisture in the CL. 

Cleaning Porex filter:

I soaked my Porex filter in water for awhile and then blew it out with air.

Trimming mold part line:

I was looking at my center section last night as I was installing the wiring for my sensors and noticed something. The "vent" area where the gas moves between the CL and the scrubber on my unit had an excess amount of material left over on the mold separation line which is inside these passages. I used a razor knife to trim away this excess material and it looks to have opened up these passages considerably. Seems like trimming this up will possibly help to decrease the WOB. 

Center Section Ports: 

Yup, I used an O-ring to seal them. I just bought a 5/16-24 stainless hex bolt and cut a shoulder into it on the lathe added an O-ring and I was done. The pressure gradient there is zero so no big deal. Bummer on the old unit not having the material needed to put in new ports. I always wondered why I had seen photos of BP60's with the center section rotated and the plumbing attached to the old ports. I just figured it was easier for who ever did the work. 

Okay, now some new stuff, 3/2005.

Useless parts:

The stock parts I removed were the two tank pressure gauge's, the End of Service Alarm (low tank pressure warning whistle), the flow restrictor (O2 bleed "orifice"), one section of the Over Pressure Valve (OPV), the compression spring that pushes on the counterlung,  cooling rings in the lid, the six plugs in the backplate with the screen mesh, the harness, and the Anti-Anoxia valve.


Note in this picture of my backplate that I've added about ten holes to the backplate to allow the water to move in and out more easily.  With the standard six holes air flows in and out just fine but drop the rig in water and the higher viscosity of the water makes for hard breathing.  The holes I added were slightly larger then the originals and the total area of the holes equaled what's on the MK15 series Biomarine rebreathers.

In this picture you can see I made a ring to allow me to rotate the centersection so the breathing hose connections were at the top of the rig and to increase the counterlung volume a bit.  I found the CL volume to be slightly smaller then I liked and this fixed that problem.  On the bench I sucked all the gas out of the loop until the CL bottomed out, then with the loop closed I took the biggest breath I could, I then opened the loop and breathed out into the loop as much as I could.  I made the CL volume so it would just start to vent at the very end of my exhale.  The ring worked good but I'd make sure I radiused the inside top edge otherwise it has a tendency to adversely affect the counterlung material.  A to scale AutoCad  drawing of the ring can be found here or a PDF file here.

I've also used standoffs to raise the centersection, not shown well here (standoff in the middle right of the picture). This also worked well.  If you used standoffs on the original backplate you wouldn't really have to drill the extra holes in the backplate.


Two modifications I made to the cover were to cut out a slot for the hoses to come out, now that they were at the top of the rig.  I also had to open up the center of the cover where the scrubber lid comes through.  After raising the centersection a bit off the backplate, as mentioned above, it made for a tight fit.  A little plastic removal and things worked well.


You can see in this picture  that I've rotated the centersection 135 degrees clockwise so the breathing hoses are coming out of the top of the rig and that the gas connections are still in the same orientation as the original.  One of the things you have to look out for in a BP60 is the extra material some molds have in them to allow you to do this and others that don't.  Here's how to check it out: disconnect the centersection from the backplate, remove the CL, now while looking at the back of the centersection check the area inside 135 degrees clockwise from where the current gas connection fittings are. There may or may not be some extra meat in the mold here to drill new gas connections holes.  If you have the extra material you can add new gas connections but if you don't have the extra material all is not lost.  You could always make a banjo fitting like here

Look here to see a BP60 used with a banjo fitting, from www.divenet.com

Look here to see a shot of a BP60 without the extra material.

Look here to see  a shot of a BP60 with the extra material.

If you rotate the centersection, block off the old ports, and drill some new ports, what else is there to do. Well here you can see I've done that and can see the stainless tube that feeds the shrader valve is installed in the new port.  Unfortunately when you rotate the shrader valve assembly the old holes don't line up with the new position so you have to plug the old holes and drill new holes.

While we're on the shrader valve assembly there were a few things that I needed to do to it to make it work for me.  I plugged the connection where the Anti-Anoxia valve was connected and I removed the flow restrictor (O2 bleed "orifice").  To plug the Anti-Anoxia valve connection I inserted a machine screw and then silver soldered it in place.  I installed a standard first stage port plug where the flow restrictor was. Check the shrader valve assembly out here.

Over pressure valve (OPV):

The stock OPV has a dual stage vent and I thought it just added extra height and lessened the effective CL volume.  I removed the top section of the OPV and removed the threaded portion of the lower section on a lathe.  

Mouthpiece/DSV (Dive surface valve):

Currently I use a mouthpiece with DSV and hoses from a Draeger Ray.  The hose ends fit right onto the connections on the centersection and although the hoses are a tad shorter then I would like it works quite well.  You can see the progression on my homebuilt  mouthpiece/DSV here. After switching to the Ray parts I went back to the homebuilt one for a comparison, there is none.  The WOB on the homebuilt one is very noticeable compared to the Ray's. 

Just thinking, it might be an option to use the original short sections of breathing hose with the screw on connections on them and the Ray hose.

Copper tubing:

First Stage:



Tank pressure gauge on tank valve:

I'll be working on this ........ 3/27/2005

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