CO2 Detection

I was in a casual conversation a few years ago when I heard that you could not measure CO2 in a rebreather loop. That's silly I thought as the moisture problem is easily solved. Instead of opening my fat mouth, I went home and filed a patent application.

The following is a basic explanation of the system ripped from the US and foreign patent applications.

The Problem

Scrubbers can fail due to a number of reasons. They may be over breathed by a user under stress, poor packing, poor design, failure to start the scrubber reaction, excess moisture, exhausted absorbents or others. Excess CO2 has lead to numerous deaths of rebreather users in civilian and military applications.

Gases from the scrubber are at an elevated temperature and saturated with water vapour. Water comes from the user as well as the exothermic reaction in the scrubber where a molecule of water is generated for each molecule of CO2 is scrubbed. When the gas leaves the scrubber, it begins to cool down and water begins to condense on the cooler surfaces of the hoses, counterlung, in the gas steam, and optical surfaces of sampling cells that are below the dew point of the gas in the breathing loop.

The problem as I saw it was to extract and dry the gas changing the dew point and preventing condensation.

The Solution

At the exit of the scrubber, a sample of the gas stream (1) is extracted and flows through a moisture reduction tube (4). Within the moisture reduction chamber is a tube (3) with a special membrane that passes water vapour but not CO2. Flowing inside of the tube is the dry replacement oxygen (2) we love so much. In other systems such as semi closed circuit rebreathers the dry gas flow may be Enriched Air.

Water vapour travels through the tube walls from saturated to unsaturated gas streams through permeation distillation. The dew point of the sample gas is greatly reduced and flows through a sample chamber (5) where the carbon dioxide concentration is analyzed using infrared.

Exiting the sample chamber the sample gas flow the sample is mixed with the now moisturized oxygen at an aspirator (6) that also assists in drawing the sample, back into the breathing loop. The location of the sample extraction allows for a number of options in how the measurement system can be used.


Note that the system is entirely passive in that no pumps are necessary, no heaters are required, and the only power is the minimal power to run the monitor. Other embodiments of the system place the sample extraction within the scrubber be it axial or radial. These placements allow the Carbon Dioxide Sampler to determine the amount of remaining scrubber life.

To say the least, there was considerable interest in the system at DEMA.

Additional patent applications are pending for control of temperature drop, signal conditioning for pressure and temperature effects on the gas volume in the sample cell.


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