notdeadyet
12-05-2013, 07:49 PM
I promised a few people I would do some pics and a write-up.
Since the demise of Teledyne cells in the rebreather market I've been toying with the idea of buying a cell checker. However I've got oddball cells in my unit which don't fit any of the connection options on commercially made units, they are fatter than regular cells so wasn't sure if they would fit the chamber and as they are high output then they'd be off the scale of a standard 0-200mv DPM LCD display. I also didn't fancy paying £300-500 for a pot either.
So after a year of apathy after my last Teledyne cells died I finally got round to building one. The design is nothing fancy, it's not a million miles different to many of the units shown on CCRX or RBW. The basic chamber is a 10" water filter houseing which is rated to 135psi, plenty for what I needed. It has a built in purge button and brass 3/4" NPT ports.
First step was to reduce the ports to a more manageable 1/4" NPT (I've got shitloads of 1/4" stuff in my spares box as a lot of KISS and Mk15 connections are based on it). Next I added a 0-2.5bar gauge to one side to monitor the pressure in the chamber. On the other side I added a t-piece with an oxygen add port and a gland for the cable to the cells. The oxygen port is just a push on hose barb with an old piece of LP hose cut down and connected to a spare first stage. I may swap that to a quick release in the future to save tying a first stage up permanently. I should really dial the IP of the reg down to below the rating of the chamber.
The cable gland is just a 1/4" Swagelok tube connector as used on Hammerhead handsets and other bits of kit. It's basically just a compression fitting with a piece of cable rather than a pipe. The cable itself is 4-core telecom cable from Homebase. Once in place the whole fitting was potted with a bit of hot melt glue.
Connection to the cells is via some mini crocodile clips on the cell connectors. As it's a 4 core cable they share a common negative (as they do in the rebreather anyway). I was having trouble with gas leaking through the sheath so I sealed it with some more hot melt, it's ugly but does the job.
Cell monitors are just some cheap multimeters on the 2000mv setting. The test leads are spliced on to the cell cable. It isn't as pretty as a NarkedAt90 but it is a 6th of the price:
http://i113.photobucket.com/albums/n233/lizardland/2013-05-12194650.jpg
Operation is dead simple. Connect the cells, let them sit in air and take the reading. With these they were in the region of 20-22mv. A bit of maths would say they should read 95-105mv in a pure O2 environment. Screw the housing on to the lid and pressurise to 1bar and then purge back to ambient. I thought about adding a needle valve but the cylinder valve gives fine enough control. Obviously the atmosphere in there is now half air and half oxygen so I repeated the flush and purge cycle until I started getting the predicted value on the multimeters. It took about seven flushes to get the readings right. The cells were showing 102-112mv with the pressure at ambient which suggested the atmosphere inside was now more or less pure oxygen.
A bit more maths and I should get 190-210mv when I up the pressure to 1bar (10m depth). I brought it up, let it settle and the cells were showing 195-216mv which I'm happy with. It isn't dead on but equally there is a bit of tolerance on the gauge so it probably isn't dead on 1bar.
It follows that 2bar (or 20m depth) should give 285-315mv. Bring the pressure up again, let it settle and the cells are showing 298-320mv. Again, fine by me.
Purge the chamber back down to ambient and let it settle just as a final check in oxygen at 0bar (surface pressure). Final final check is to make sure they settle back to their original readings in air with the chamber open (which they did).
So...
Filter housing: £13 from Ebay
2x3/4"NPT reducing adapters: £6 from Ebay
1/4" NPT Cable gland/compression fitting: £3 from Ebay
T-piece: I already had but I think is in the region of £7 from Swagelok
Cable: £1 from Homebase
Crocodile clips: £2, Ebay
3xMultimeters: £9, Ebay
Pressure gauge: £15 from a pneumatics supplier
BSP/NPT adapter for the gauge: £3 from same place as gauge
Hose barb: already had but about £3 from Swagelok
OK, I've gone over my 50 quid budget (and doesn't include the first stage) but not by much. It does the job surprisingly well and easily. I think my next step might be to knock up a spreadsheet to track the results and predict the values in oxygen at ambient, 1 & 2 bar. Might also include something for gases other than O2.
It is a very easy project, easy enough that everyone with a rebreather should have a cell checker.
And if anyone mentions the Little House on the Prairie table cloth then you can get fooked.
Since the demise of Teledyne cells in the rebreather market I've been toying with the idea of buying a cell checker. However I've got oddball cells in my unit which don't fit any of the connection options on commercially made units, they are fatter than regular cells so wasn't sure if they would fit the chamber and as they are high output then they'd be off the scale of a standard 0-200mv DPM LCD display. I also didn't fancy paying £300-500 for a pot either.
So after a year of apathy after my last Teledyne cells died I finally got round to building one. The design is nothing fancy, it's not a million miles different to many of the units shown on CCRX or RBW. The basic chamber is a 10" water filter houseing which is rated to 135psi, plenty for what I needed. It has a built in purge button and brass 3/4" NPT ports.
First step was to reduce the ports to a more manageable 1/4" NPT (I've got shitloads of 1/4" stuff in my spares box as a lot of KISS and Mk15 connections are based on it). Next I added a 0-2.5bar gauge to one side to monitor the pressure in the chamber. On the other side I added a t-piece with an oxygen add port and a gland for the cable to the cells. The oxygen port is just a push on hose barb with an old piece of LP hose cut down and connected to a spare first stage. I may swap that to a quick release in the future to save tying a first stage up permanently. I should really dial the IP of the reg down to below the rating of the chamber.
The cable gland is just a 1/4" Swagelok tube connector as used on Hammerhead handsets and other bits of kit. It's basically just a compression fitting with a piece of cable rather than a pipe. The cable itself is 4-core telecom cable from Homebase. Once in place the whole fitting was potted with a bit of hot melt glue.
Connection to the cells is via some mini crocodile clips on the cell connectors. As it's a 4 core cable they share a common negative (as they do in the rebreather anyway). I was having trouble with gas leaking through the sheath so I sealed it with some more hot melt, it's ugly but does the job.
Cell monitors are just some cheap multimeters on the 2000mv setting. The test leads are spliced on to the cell cable. It isn't as pretty as a NarkedAt90 but it is a 6th of the price:
http://i113.photobucket.com/albums/n233/lizardland/2013-05-12194650.jpg
Operation is dead simple. Connect the cells, let them sit in air and take the reading. With these they were in the region of 20-22mv. A bit of maths would say they should read 95-105mv in a pure O2 environment. Screw the housing on to the lid and pressurise to 1bar and then purge back to ambient. I thought about adding a needle valve but the cylinder valve gives fine enough control. Obviously the atmosphere in there is now half air and half oxygen so I repeated the flush and purge cycle until I started getting the predicted value on the multimeters. It took about seven flushes to get the readings right. The cells were showing 102-112mv with the pressure at ambient which suggested the atmosphere inside was now more or less pure oxygen.
A bit more maths and I should get 190-210mv when I up the pressure to 1bar (10m depth). I brought it up, let it settle and the cells were showing 195-216mv which I'm happy with. It isn't dead on but equally there is a bit of tolerance on the gauge so it probably isn't dead on 1bar.
It follows that 2bar (or 20m depth) should give 285-315mv. Bring the pressure up again, let it settle and the cells are showing 298-320mv. Again, fine by me.
Purge the chamber back down to ambient and let it settle just as a final check in oxygen at 0bar (surface pressure). Final final check is to make sure they settle back to their original readings in air with the chamber open (which they did).
So...
Filter housing: £13 from Ebay
2x3/4"NPT reducing adapters: £6 from Ebay
1/4" NPT Cable gland/compression fitting: £3 from Ebay
T-piece: I already had but I think is in the region of £7 from Swagelok
Cable: £1 from Homebase
Crocodile clips: £2, Ebay
3xMultimeters: £9, Ebay
Pressure gauge: £15 from a pneumatics supplier
BSP/NPT adapter for the gauge: £3 from same place as gauge
Hose barb: already had but about £3 from Swagelok
OK, I've gone over my 50 quid budget (and doesn't include the first stage) but not by much. It does the job surprisingly well and easily. I think my next step might be to knock up a spreadsheet to track the results and predict the values in oxygen at ambient, 1 & 2 bar. Might also include something for gases other than O2.
It is a very easy project, easy enough that everyone with a rebreather should have a cell checker.
And if anyone mentions the Little House on the Prairie table cloth then you can get fooked.