Fourth or Fifth cell
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David Harrison made a new designed cell holder for the Inspiration. He now offers the possibility to add a fourth or even Fifth cell. I asked David to write a small article about the development of his idea.

APD Inspiration

4th Cell Holder

After buying & using a commercially available 4th cell holder for my Inspiration, I realised that it had several limitations which affected its performance to the point where it was worse than useless.

a, Due to the placement of the 4th cell, i.e. outside of the scrubber head & out of the main gas flow, it did not track the ppo2 as accurately as the 3 main cells.

b, Calibration was nigh on impossible, despite following the manufacturers instructions to the letter.

c, It leaked!

So out of frustration I set out to devise a better alternative, the MK1 version was cobbled together in true Heath Robinson style in the shed. This at least proved that the concept was sound. I.e by placing the 4th cell inside the head, adjacent to the 3 main cells an accurate consistent reading could be achieved.

I dived the home made version throughout the 2004 season with great results. Dives were in the 10 - 80M range using air & trimix diluents. During the autumn / winter of 2004 I was approached by a friend who had taken the concept & had a CAD design drawn up. After slight modifications to the drawing we had a prototype (MK2) made up out of white Nylon. This was checked dimensionally in a scrubber & test dives were carried out early 2005. The design was "tweaked" & a production run was ordered from a contact that I have in the precision engineering industry. The production version is made from black Delrin (pom h) & is of an extremely high quality (the same fella makes parts for nuclear subs, but we can't mention that!)

One of the main design criteria was that no permanent modification should be necessary to the rebreather to enable fitting of the 4th cell, this was achieved by placing the cable entries in an extended portion of the cell centre tube, see photo below

The version shown is the "double" option. This is to allow 2 VR3s to be connected to 2 separate cells, in effect a 4th & 5th cell. All the centre tubes are drilled to take upto 5 R22 bud or similar cells. Due to the fact that no drilling of the unit is involved, fitting is pretty straightforward, anybody can do it, honest!

If preferred, Fischer connectors can be fitted to the cell holder in lieu of cable glands, a double ended cable would then be used to connect the cell holder to the VR3. Personally I would avoid this option as it introduces additional electrical & mechanical joints into the system, it is also more expensive, considerably.

In addition to being used as a 4th / 5th cell holder for dive computers, the cell holder is ideal for use where a cable needs to enter the head for a HUD application. In fact it is perfectly feasible to use one cable entry for a VR3 4th cell & the other entry for a HUD as in Uri Barans unit

This image shows a cell holder fitted with a VR3 4th cell cable & a Fischer port for a Rebreather Guardian HUD


Full details and ordering instructions can be found here


David, thanks for your contribution to my website!

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