Using a TecMe Housing as a cell checker

4ster

I don't really know the best place to post this, I finally decided to post here in the Semi Closed forum since this rig is on a Dolphin.

I have been diving a Shearwater and a TecMe 3 cell housing for almost a year. I posted a write up on it here shortly after I set up the system. I am really happy with the setup, other than the fact that it makes the sleek Dolphin clunky looking. About that time I became aware of the current limiting problem of O2 cells at high PO2s and started reading about testing proceedures.

One challange initally was that the Shearwater requires calibration in O2 instead of air. At first I was placing the TecMe housing in a zip lock bag and feeding O2 from my mixing supply cylinder. This was pretty wasteful of O2.

But I didn't have a proceedure to test for current limiting at high PO2s. The Shearwater has a handy feature where you can read the mV on all 3 cells directly. The only thing needed to test the cells in place was some way to pressurize the housing for testing mV at various pressures.

Inspiration: TecMe offers a set of plugs for their housing, and Martin hints on his site that a housing with plugs can be used as a pressure pot to test cells. Ron Micjan has posted a description of his small test pot with a thumb operated valve. And last but not least, narked@90 has published the manual for their cell checker that has detailed instructions on how to test cells. John and Brent really did a good job developing test proceedures for the cell checker. I highly recommend reading the manual available for download at the bottom of the cell checker page: http://www.narkedat90.co.uk/37.html

Parts sources: The guage was purchased at my local welding supply shop, and all the plumbing bits were found at my (very good) local hardware store. All the parts were cleaned in Simple Green before assembly. I think I have no more than $25 in these parts. The O2 regulator is from my emergency O2 therapy kit.

Operation: After the plugs are attached to the housing O2 is supplied and purged a few times to make sure that the housing now has 100% O2. Once the purging is over I set the flow at the regulator at 1 liter per minute. The needle vave is adjusted to control the pressure in the housing and voltage measurements at various pressures can be taken to determine if any O2 cells are current limited at pressures up to 2 ATA.

Originally I planned to use my thumb to control pressure in the housing. This is Ron's great idea, it is a simple automatic overpressure valve. I found that it was difficult to take readings in my configuration using thumb control, so substituted the needle valve. To avoid overpressurising the housing I attach the supply hose by simply pushing it on the regulator hose barb. Surpisingly it stays on at the 2 ATA test point without leaking.

Data points are taken for all three cells in air, O2 at 1 ATA, and at points up to 2 ATA. The points are then plotted on a chart to check for linearity. Yea! On the first test - nearly a year after using these cells they passed!

Attached Images

Cell tester open.jpg (161.4 KB, 222 views)

__________________
Steve
"My problem is that I have trouble reconciling my gross habits with my net income." Errol Flynn

teomannaskali

Looks good. But may I suggest a procedture. AFAIK the cells do not like to be unpressurized too quickly...

Do not add a check valce to the sysdem.

1. Pressurize it to 2 atm.
2. Un plug the O2 tank.
3. Use the needle valve to de pressurize slowly the cell. Stopping at certain intervals to take the measurements.


Some more experienced people should be able to provide info on the speed.

Teoman

4ster

I'll check that, thanks.

With this setup it is probably better to keep the flow going, the volume of the pot and the lack of precision in the needle valve make it possible to hold a steady, dialed in pressure by having a "controled leak" in the system.

__________________
Steve
"My problem is that I have trouble reconciling my gross habits with my net income." Errol Flynn

teomannaskali

Thank you, that was appreciated.

I wonder if one could organise that to be in a more compact form to be able to use it in one of thoese new pda phones with excel

Teoman.

4ster

I thought about using the spreadsheet program on my PDA but didn't do it, mostly because of the nature of taking the readings. With my setup I am pretty busy checking ATA pressures, making sure the Shearwater is reading mV instead of the default partial pressure and all the time the O2 is flowing. Entering data correctly into a spreadsheet while doing the above is way too much multitasking for me. That is why I put the form with instructions on one page. I print out that page and use it to record the data, entering the numbers in the spread sheet later.

At some point I will probably merge this onto my PDA where I have a few other handy diving related programs.

__________________
Steve
"My problem is that I have trouble reconciling my gross habits with my net income." Errol Flynn