Gas Sensing Solutions (GSS) and AP Diving have collaborated to develop the CozIR™ Wide Range CO2 sensor that increases diver safety.
Recreational diving is enjoyed by people around the world and diving with rebreather equipment is booming. Rebreather technology recirculates the air, removes the CO2 and tops up the oxygen level. As a result, a small tank of air and a small tank of oxygen is all that is needed, rather than big tanks of compressed air.
One limitation is the CO2 removal. This is done chemically by passing the exhaled gas through a scrubber containing soda lime but, of course, this has a finite absorption capability. The capability of the scrubber depends on the user refilling with the correct soda lime at the correct time and unfortunately it has been known for users to get it fatally wrong.
The trouble with increased levels of CO2 is that the diver cannot detect it by smell or any other sense. The next stage of increasing CO2 levels is a narcotic effect – so the diver stops thinking about the details of the dive time and can run into problems including losing consciousness.
The GSS new ultra-low power, custom CO2 sensor gives a vital – and potentially life-saving – early warning of a rise in CO2 levels.
The GSS sensor has many technical advantages: It is the only sensor that can be run off batteries, which makes this solution for diving rebreathers possible; It uses very little power and it turns on almost instantly so that it can be incorporated in the normal start up sequence of battery and component tests. In addition, it also measures the partial pressure of CO2 to give warnings as low as 3 mbar – which is only 0.025% at 100m.
Calum MacGregor, GSS’ CEO, commented, “Diving is a very challenging environment for a CO2 sensor as it has to cope with large changes in temperature. “Fortunately, our LED-based designs have self-correction for temperature changes built in. However, the real challenge was preventing water ingress to the device and coping with humidity in the breath so that it did not condense on the optics and form droplets that would interfere with readings. We often produce custom versions of our sensors to meet challenging applications and indeed worked with AP Diving to produce a special version of our CozIR Wide Range CO2 sensor. The sensor is part potted to prevent water ingress to the electronics and includes a desiccant to absorb water in the breath before it gets to the sensor’s optical cavity. It also has a special hydrophobic coating on the optics that we developed to further negate the effects of any humidity.”
Martin Parker, AP Diving’s Managing Director, explained, “We have been trialling this new design for over a year. It is working perfectly, and we are doing a major push to educate people about how important and life-saving it is to have a CO2 sensor in a rebreather. We have designed the sensor into a ‘Plug and Play’ unit so that it can be retrofitted to any of our InspirationTM rebreathers with VisionTM electronics that were made as far back as 2005. Not only is it a safety feature, but it also has a major benefit for divers. Now they can use virtually any brand of ‘diving’ soda lime in their rebreathers, as opposed to just the brand that is specified, which can be challenging to obtain when travelling to different parts of the world. In addition, if they should push past the recommended usage times, they have the added safety of an appropriate CO2 warning system.”
CO2 sensors work by measuring how much light is absorbed by CO2 molecules in the 4.2 and 4.4 microns range as it passes through the sample gases, which is called Non-Dispersive Infra-Red (NDIR) absorption. The amount absorption indicates how much CO2 is present. GSS developed LEDs that are specifically tuned to emit at these wavelengths. These LEDs use very little power and turn on almost instantly, enabling sensor readings to be made in less than a second. As a result, GSS CO2 sensors are the only NDIR CO2 sensors that can be powered by batteries. Competitor sensors use IR sources that require significantly more power per measurement and also take much longer to reach a stable condition for a measurement so that they need to be mains powered.