SST Sensing Ltd.Coatbridge,

Studies show that heightened carbon dioxide (CO2) concentrations can have an adverse effect on the human metabolism, slowing down thought processes, hampering decision-making performance, and heightening the chance of human error. One environment where this is unavoidable is in spacecraft.

The CozIR sensor is contained in a 3D-printed housing that is clipped onto the clothing of the ISS crewmembers, with the sensors being worn on the back of their collars.

Within a self-contained system where humans are respiring, there will be a buildup of toxic CO2 levels over time. As a consequence, effective mechanisms for removing CO2 and resupplying oxygen need to be implemented; however, operational necessities still dictate that these will only be efficient enough to keep the CO2 levels down to a certain extent.

Air moves differently in such locations, without the influence of gravity involved. Heat does not rise, and this in turn leads to significantly less mixing of air. The occupants on the International Space Station (ISS) are exposed to greater levels of CO2 during their time onboard than the rest of us experience. On Earth, it represents just 0.03% of the air’s content (equating to a partial pressure of 0.23 mmHg). NASA has set a long-duration spacecraft maximum allowable CO2 concentration that is more than double that figure — at 0.7% (a partial pressure of 5.3 mmHg).

It is generally accepted that human adaptation to micro-gravity conditions will cause astronauts in space to become more sensitive to elevated CO2 levels. This can result not only in physical discomfort, but also impinge on their cognitive skills and reaction times, potentially leading to safety risks. It is not possible (either technically or financially) to remove enough CO2 to replicate normal conditions here on Earth, so the ISS has to function with relatively high ambient CO2 concentrations in the air.

As air does not mix well without the influence of gravity, wall-mounted sensors had proved themselves to be an unsuitable means by which to get an accurate reflection of a crewmember’s exposure to CO2. What was needed were wearable CO2 sensors. There were several key criteria that a wearable CO2 sensor had to meet. First, it would need to have prolonged battery life so that operation could be sustained for an extensive period without recharging. Second, for wearer comfort and convenience, the sensor would have to be compact and have a low mass. And finally, a simple data interface was important, to make the information acquired easy to access while at the same time keeping the system development process as short and uncomplicated as possible.

A solution from SST Sensing was developed with its engineering partner Gas Sensing Solutions (GSS). The two companies worked with NASA during the prototyping phase, providing detailed performance information on the CozIR CO2 sensor so it could be used with maximum effectiveness in NASA’s personal CO2 monitors.

The NASA team constructed a custom-designed baseboard around each CozIR sensor. This hardware provided all the necessary power regulation, processing, data storage, and wireless connectivity. The team also designed a housing that could be 3D-printed and clipped onto crewmembers’ clothing (with the sensors being worn on the back of their collars).

The low-power, high-speed, non-dispersive infrared (NDIR) devices can cover measurements down to ppm CO2 levels (with a range of 0 to 5000 ppm and a ±5% sensor accuracy). They comfortably support an operational lifespan of 10 years, and draw 3.5 mW at 2 samples/second acquisition rates. Connectivity is via a convenient serial connection facilitating data transfer.

The monitors provide simple and unobtrusive tracking of the astronauts’ individual CO2 exposure over an extended period of time. Astronauts will be able to view their own data via a custom iPad app. All data obtained will subsequently be compiled and utilized to get a better understanding of the long-term effects of heightened CO2 exposure during space travel. Not only could this prove highly beneficial to the ISS crew, but also for potential future missions to Mars.

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