Untitled

Franken-Rig 02 Addition Module

Here is the key to the system. It's the 02 regulator from the East German MEDI RGU-FM escape rebreather. Essentially it adds a constant mass flow of oxygen to the loop at the rate of 0.9 liters per minute, and also has a manual 02 bypass button. The trickle of 02 is about right to maintain loop PP02 under conditions of normal swimming, and in case loop PP02 must be raised due to high workload or an orifice failure, the manual button is close at hand.

Looking at the regulator, we see the outlet port on the left, and an overpressure relief valve in the center. On the right is the cover for the diaphragm of the reducing regulator.

Peering into the outlet, we can see the orifice. The port for the manual add valve feeds this outlet as well.

Here is the manual bypass button. Pressing it in gives a freeflow of 02 for manual bypass or addition.

The diaphragm of the reducing regulator is not ported to the water pressure, thus the regulator is non-compensating. This is the basis for the constant mass flow.

The cylinder is a 200 BAR steel one with a 3/4 BSP outlet, and a one-way valve that was originally used for filling. I'll drill and tap this second port shortly for addition of a pressure gauge.

The on/off knob is a nice brass piece with a rubber grip on the periphery. It is flat, so in installation it hardly protrudes from the shell, yet offers a good surface for a gloved hand. There is one little flaw with this valve: It must be opened fully against an internal Teflon washer in order to keep salt water from the stem of the valve. The solution is simple: Disassemble the stem, lubricate it with Halocarbon grease, and when using it open it fully. Service it every now and then and it'll be fine. My final solution is to cut a little groove in the valve shaft with a lathe and to add an O-Ring for sealing. No big deal either way.

The valve was provided with a plate, allowing a thread to be run to act as a seal: Turning on the valve broke the seal, thus indicating that the unit had been used. It interfered with the installation, so I removed it.

This little connection was the original connector to the outlet port on the regulator. The MEDI rebreather used a plastic tube to take the 02 to the counterlung. It worked for them, and it now works for me too!

The original Russian system used two scrubbers internally. I decided to remove one scrubber and mount the 02 bottle and regulator in the vacated position. These brass adapters were machined to screw onto the fittings for the removed scrubber. I drilled one and tapped it and screwed in a 1/4 NPT to barb fitting, and it became the 02 injection port into the counterlung.

The completed 02 addition port, ready to install. This is the only complex part on the modification. It could just as easily be made from a rubber expanding plug, available at any hardware shop.

Here's the complete 02 addition system! It incorporates the constant mass flow system, the manual bypass valve, and the 02 injection port. Drop it in and let's dive!

As noted before, the low-tech system of aquarium tubing was used in the original MEDI system, and it works fine here too. Slipping off the hose from the barb on the injection port allows a quick predive flow-test as well, with a small flowmeter.

Here I've installed the one blanking port. The 02 add port will fit the other opening into the counterlung.

One bit of advice: Here's the big unknown: I have not yet measured the output pressure of the constant mass flow 02 regulator, so I'm not yet sure to what depth it will act as a constant mass flow 02 injection system. But since you are not relying on the automatic add feature of this rig any more than you would on any other CCR, you WILL always know the PP02 of the loop, right? And you WILL be checking to see if you need to manually add 02 every few minutes? In a few days I'll make up an adapter to measure the pressure produced by the regulator, and we'll have a better idea of the depth to which reliable CMF may be expected. BUT: Like ALL rigs using any sort of constant mass flow, you would be a DAMNED FOOL to rely on this system working correctly all of the time (or even part of the time!) But you knew that already, didn't you? Remember, the things that are most interesting in this world are things that can bite. Rebreathers can bite. Got it?

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	Here is the key to the system. It's the 02 regulator from the East German MEDI RGU-FM escape rebreather. Essentially it adds a constant mass flow of oxygen to the loop at the rate of 0.9 liters per minute, and also has a manual 02 bypass button. The trickle of 02 is about right to maintain loop PP02 under conditions of normal swimming, and in case loop PP02 must be raised due to high workload or an orifice failure, the manual button is close at hand.
	Looking at the regulator, we see the outlet port on the left, and an overpressure relief valve in the center. On the right is the cover for the diaphragm of the reducing regulator.
	Peering into the outlet, we can see the orifice. The port for the manual add valve feeds this outlet as well.
	Here is the manual bypass button. Pressing it in gives a freeflow of 02 for manual bypass or addition.
	The diaphragm of the reducing regulator is not ported to the water pressure, thus the regulator is non-compensating. This is the basis for the constant mass flow.
	The cylinder is a 200 BAR steel one with a 3/4 BSP outlet, and a one-way valve that was originally used for filling. I'll drill and tap this second port shortly for addition of a pressure gauge.
	The on/off knob is a nice brass piece with a rubber grip on the periphery. It is flat, so in installation it hardly protrudes from the shell, yet offers a good surface for a gloved hand. There is one little flaw with this valve: It must be opened fully against an internal Teflon washer in order to keep salt water from the stem of the valve. The solution is simple: Disassemble the stem, lubricate it with Halocarbon grease, and when using it open it fully. Service it every now and then and it'll be fine. My final solution is to cut a little groove in the valve shaft with a lathe and to add an O-Ring for sealing. No big deal either way.
	The valve was provided with a plate, allowing a thread to be run to act as a seal: Turning on the valve broke the seal, thus indicating that the unit had been used. It interfered with the installation, so I removed it.
	This little connection was the original connector to the outlet port on the regulator. The MEDI rebreather used a plastic tube to take the 02 to the counterlung. It worked for them, and it now works for me too!
	The original Russian system used two scrubbers internally. I decided to remove one scrubber and mount the 02 bottle and regulator in the vacated position. These brass adapters were machined to screw onto the fittings for the removed scrubber. I drilled one and tapped it and screwed in a 1/4 NPT to barb fitting, and it became the 02 injection port into the counterlung.
	The completed 02 addition port, ready to install. This is the only complex part on the modification. It could just as easily be made from a rubber expanding plug, available at any hardware shop.
	Here's the complete 02 addition system! It incorporates the constant mass flow system, the manual bypass valve, and the 02 injection port. Drop it in and let's dive!
	As noted before, the low-tech system of aquarium tubing was used in the original MEDI system, and it works fine here too. Slipping off the hose from the barb on the injection port allows a quick predive flow-test as well, with a small flowmeter.
	Here I've installed the one blanking port. The 02 add port will fit the other opening into the counterlung.