Diving and Hyperbaric Medicine Volume 41 No. 2 June 2011 90 Technical report Solid-state electrolyte sensors for rebreather applications: a preliminary investigation Arne Sieber, Rainer Baumann, Stefanos Fasoulas and Anatol Krozer Key words Rebreathers/closed circuit, oxygen, carbon dioxide, transducer, capnography, helium, trimix Abstract (Sieber A, Baumann R, Fasoulas S, Krozer A. Solid-state electrolyte sensors for rebreather applications: a preliminary investigation. Diving and Hyperbaric Medicine. 2011;41(2):90-6.) Introduction: Recently developed prototypes of zirconium dioxide and NASICON-based micro solid-state electrolyte oxygen (O 2 ) and carbon dioxide (CO 2 ) sensors were tested for their potential suitability in rebreathers. The O 2 sensor has a quasi-indefnite lifetime, whilst that of the CO 2 sensor is approximately 700 h. This is a preliminary report of a new technological application. Methods: The O 2 sensor was tested in a small pressure chamber to a partial pressure of oxygen (PO 2 ) of 405 kPa (4 bar). The CO 2 sensor was tested up to 10 kPa CO 2 . The response times to a step change of pressure were measured, and cross- sensitivity for helium tested using trimix. A rebreather mouthpiece was modifed so that breath-by-breath gas recordings could be observed. Power consumption to heat the sensors was measured. Results: The O 2 sensor demonstrated non-linearity, particularly above 101.3 kPa (1 bar) PO 2 , whereas the output of the CO 2 sensor showed an inverse logarithmic relationship. Cross-sensitivity to helium was observed. The mean t90 response times were 90 (SD 10) ms for the O 2 sensor, and 100 (SD 10) ms for the CO 2 sensor. Breath-by-breath recordings showed slight damping of the CO 2 trace due to electronic fltering. Power consumption was 1.5–2 W per sensor. Conclusions: The fast response times would allow accurate breath-by-breath measurement. Even though the O 2 sensor has a non-linear response, measurement is possible using multi-point calibration. Further design is necessary to allow trimix to be used as the diluent. A major disadvantage is the high power consumption needed to heat the sensors to high temperatures. Introduction GAS MONITORING IN CLOSED-CIRCUIT REBREATHERS While in open-circuit scuba diving, exhaled gas is usually vented into the water, in a rebreather it is returned to a counterlung, carbon dioxide (CO 2 ) is removed in a ‘scrubber’ and metabolised oxygen (O 2 ) is added from the supply tank. Information about the various types of rebreathers available can be found elsewhere. 1,2 In a closed-circuit rebreather (CCR), the partial pressure of O 2 (PO 2 ) is usually kept at a constant level and only metabolised O 2 is replaced with fresh O 2 from the supply tank. 3,4 Oxygen is diluted in the breathing circuit with nitrogen or helium, or a combination of these gases. To maintain the PO 2 at a constant level, a control loop is needed. Therefore, electrochemical oxygen transducers, whose output signals are proportional to the PO 2 , are used as sensing elements. In a manually controlled rebreather, the diver reads the PO 2 from a display, then, if necessary, adds O 2 manually and/or, as in one model, adjusts the O 2 injection needle valve. In an electronically controlled rebreather, this control task is usually performed by a micro-controller and a solenoid valve. CCRs have many advantages, such as: • high gas effciency, close to 100%; • stealth (silent, bubble-free diving); • warm, humid breathing gas; • extended diving time; • reduced decompression obligations due to optimised gas mixtures and decompression mixtures. Risks and accidents associated with rebreathers have been discussed in detail elsewhere. 5 The most commonly identifed systems failures that cause fatalities are: • PO 2 outside of life-sustaining limits; • high CO 2 levels. In current rebreathers, O 2 is measured using galvanic sensors. The core element of this transducer is an electrochemical cell (fuel cell) consisting of two electrodes of dissimilar metals (cathode − a noble metal behind a diffusion barrier, made usually of Tefon; anode − lead) in contact with a liquid or semi-solid basic electrolyte, usually potassium hydroxide. The transducer output is linear in the range up to 203 kPa (2 bar) PO 2 with a slope of 40–70 mV bar -1 . The most common and, for the diver, most dangerous failure mode of a PO 2 transducer is an incorrect electrical output for a given PO 2 . http://archive.rubicon-foundation.org