The R2P™

complete unit & owner (17.9 kB)

The finished unit.



mouthpiece (10.7 kB)

An RGU-FM mouthpiece (courtesy of Harbour Supplies), cut down to accept standard mouthgrip and bung to stop water ingress. Two AP Valves AP2 BC inflators are fitted iether side of the mouthpiece - new end caps were kindly machined out of nylon by Jopo. Diluent addition is on the inhale side, O2 addition is on the exhale side.


counterlung (14.9 kB)

Yet another Buddy BC has been pressed into service!

end stop (4.06 kB)

An aluminium plate has been bolted through the pocket of the BC to hold the scrubber cannister using a cam strap and bit of aluminium channel to locate the tube.


scrubber (38.5 kB)

Our old friend, 6" OD polyolefin gas pipe (5" ID), holds about 2 lbs of sodalime.

scrubber base (22.9 kB)

Base of the scrubber open with the end of the dip tube, diffuser and filter pads.

scrubber top (10.7 kB)

Looking down on the top of the scrubber. Fitted with three 22 mm tank connectors - one for the exhaust from the mouthpiece which flows down through the stack to the base of the scrubber, another for the hose to the inhale side of the mouthpiece and the third port leads to a T-piece to the counterlung and handset for O2 display.

sensor array (17.6 kB)

The top section contains two O2 cells on a plate attached to the lid of the scrubber.

scrubber top without sensors (11.2 kB)

With the cells removed it is possible to see the dip tube passing though the sensor plate and the elastic cord retainers for the cells. The hole for the dip tube is oversized so that the gas comes back up through the scrubber past the ends of the cells.

end stop (4.06 kB)

More parts from AP Valves secure the breathing hoses to the short length of rigid pipe required to mate with the male fittings on the scrubber. Very neat!


sensors (19.1 kB)

The O2 sensors are a pair of PSR-11-39-MD cells made by Analytical Industries and distributed in the UK by Gotec. These are equivalent to the R22D cells (R22-BUD) used in the KISS and Inspiration rebreathers. They have 3-pin Molex pin outs with the central pin unused.

connector (8.32 kB)

Suitable connectors for these can be made by taking the 4-pin power connectors used for PC disc drives and slicing one pin off.

Velleman lit (25.8 kB)

The readout for the O2 level is made from a modified LCD Velleman Kit K2651 digital voltmeter from Maplin Electronics (part number VE61R). Tom Rose explains how to use a similar meter to make an O2 display and I've basically copied this.

PCB (24.7 kB)

For this particular meter, it was assembled as a conventional DVM 200 mV except that the gain resistor R4 was replaced with a 2M2 resistor and one of the trim pots (RV2) with a more sensitive 22 turn 10K trimmer.

O2 meter (19.1 kB)

For this particular meter, it was assembled as a conventional DVM 200 mV except that the gain resistor R4 was replaced with a 2M2 resistor and one of the trim pots (RV2) with a more sensitive 22 turn 10K trimmer. A notch was cut in the LCD readout PCB so that the trim pot can be adjusted more easily. DP1 is shorted to get it to read ppO2 in atm. Finally, the sensor requires a 10 K ballast across the meter which is soldered across IN- and IN+ (the leads from this are sticking out the bottom).

calibration in air (12.1 kB)

Calibrating the display in air (admittedly using a different cell from the one's above).

calibration in oxygen (12.1 kB)

Checking it with oxygen - not bad!

box (16.5 kB)

The case for the displays is made from an Otterbox case, as recommended by Tom. The port for the cable is another tank connector from B&Q (15 mm this time) which conveniently accepts a hose pipe coupling which has a gland in it to make it water tight.

back and front view of the O2 readout (12.5 kB)

Back and front of the box holding the dual O2 displays. The cable connecting the sensors to the readouts is routed through a length of PVC pipe connected to the scrubber. Everthing is at ambient pressure so there should be no leaks. Both displays are driven from a single 9 V battery. The battery life should be several months so there is no need for an on/off swich - just connect the 9 V battery when calibrating the sensors before use.

handset (10.1 kB)

The finished handset connected to the 15x22x22 mm unequal T connector which sits on the counterlung port of the scrubber. Ideally, a 15 mm port directly to the srubber lid would avoid the T piece but there isn't space. A tampon (!) in the end of the tube leading to the displays acts as a moisture trap but allows equalisation of pressure with the scrubber.


finished device (65.5 kB)

All assembled without off-board diluent and O2 supplies.

Last updated 28 March 2010 by Duncan Priceback