I need a very small 200 mV meter to build an UW PO2 meter

Thread Starter

NetDoc

Joined Jan 6, 2014
22
Hey guys,

This might be my first post here. I Scuba dive for a living. I live it, I love it and I like learning all about it. Recently, one of my oxygen analyzers bit the dust. It wasn't apparent until I bought the $80 Oxygen cell for it. Needless to say, it's not being made any longer. Consequently, I took it apart and finally reverse engineered it enough to fix it as well as to create another one for when this one fails again. Here is the story on that: http://2sb.us/473123

A couple of years ago, I became certified in Rebreather diving. This is where you scrub your breath of C02, add a bit of oxygen and breathe it over again. Some rebreathers use quite a bit of electronics, but in the end I want something that I fully understand to act as a back up. A fully redundant method of measuring the PPO2. I've seen a number of home brewed devices and a couple that are being sold for what seems a sizable profit. Here's a diagram of the circuit I sussed out earlier:


The meter is a Lascar SP200. I want something about half the size. I want to fit the circuit, including batteries, in a small tube. I don't know what diameter tube I'll need until I come up with the final circuit. The battery package needs to last 10+ hours of constant use. I'll need to calibrate this just once before I dive and then it will be sealed. It can be off by 3 or 4 percent, maybe even more. It's a backup to make sure I don't go below 0.2 PPO2 or above 1.4 PPO2.
 

Thread Starter

NetDoc

Joined Jan 6, 2014
22
That's pretty small. The Lascar is 22.4x35.0mm. So you need roughly 1x2 cm.
I was just sent a GEM semi-closed Rebreather and since it's not a CCR, it doesn't need electronics to add oxygen. It uses a very small readout that''s half the size of the Lascar unit. That's where I got the idea from. Since this is a loaner, I don't feel comfortable breaking the seals to check out any numbers on it.
 

GopherT

Joined Nov 23, 2012
8,012
Do these rebreathers use compressed oxygen as the make-up? Miners use a chemical oxygen generator and requires no electronics. Each molecule of CO2 you exhale releases a molecule of oxygen. Very simple, no batteries, no electronics. Two Pittsburgh area companies have been making chemical oxygen generators for years (CSE Corp and MSA Corp).
 

paulktreg

Joined Jun 2, 2008
784
If you've seen smaller meters then they may have been custom made?

Just an idea but have you considered building something based on a window comparator? A green LED for your stated range and red if it strays outside.
 

Thread Starter

NetDoc

Joined Jan 6, 2014
22
Do these rebreathers use compressed oxygen as the make-up? Miners use a chemical oxygen generator and requires no electronics. Each molecule of CO2 you exhale releases a molecule of oxygen. Very simple, no batteries, no electronics. Two Pittsburgh area companies have been making chemical oxygen generators for years (CSE Corp and MSA Corp).
We're too wet for that to work. Way too wet. Also, the miners are breathing one atmosphere while we are breathing multiples of that. The problem is that oxygen becomes toxic at depth leading a diver to do the chicken dance (convulsions) if the PPO2 exceeds a certain point. Go the other way and and hypoxia will surely kill you. It's important to know the PPO2 (Partial Pressure Oxygen) as you dive. Important enough to have redundant systems monitoring it.

If you've seen smaller meters then they may have been custom made?
Probably not but anything is possible.

Just an idea but have you considered building something based on a window comparator? A green LED for your stated range and red if it strays outside.
I would love to do that as well. Nothing like idiot lights in addition to the actual gauge. Red on top set to >1.0, green set from .2 to 1.4 and blue on the bottom set <.4. That way, you would start to see red or blue while you were still in the safe zone but getting close.
 
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