sensors are rated for some service life in the presence of leaded gasoline, but sensor life will be shortened to as little as , depending on the lead concentration. Lead-damaged sensors typically have their tips discolored light rusty. Another common cause of premature failure of lambda probes is contamination of fuel with silicones (used in some sealings and greases) or silicates (used as corrosion inhibitors in some antifreezes). In this case, the deposits on the sensor are colored between shiny white and grainy light gray. Leaks of oil into the engine may cover the probe tip with an oily black deposit, with associated loss of response. An overly rich mixture causes buildup of black powdery deposit on the probe. This may be caused by failure of the probe itself, or by a problem elsewhere in the fuel-rationing system. Applying an external voltage to the zirconia sensors, e.g. by checking them with some types of ohmmeter, may damage them. Some sensors have an air inlet to the sensor in the lead, so contamination from the lead caused by water or oil leaks can be sucked into the sensor and cause failure. Symptoms of a failing oxygen sensor includes: sensor light on dash indicates problem, increased tailpipe emissions, increased fuel consumption, hesitation on acceleration, stalling, rough idling. Diving applications The type of oxygen sensor used in most underwater diving applications is the electro-galvanic oxygen sensor, a type of fuel cell, which is sometimes called an oxygen analyser or ppO2 meter. They are used to measure the oxygen concentration of breathing gas mixes such as nitrox and trimix. They are also used within the oxygen control mechanisms of closed-circuit rebreathers to keep the partial pressure of oxygen within safe limits. and to monitor the oxygen content of the breathing gas in saturation diving systems and of surface supplied mixed gas. This type of sensor operates by measuring the voltage generated by a small electro-galvanic fuel cell. Scientific and production applications Soil respiration In soil respiration studies oxygen sensors can be used in conjunction with carbon dioxide sensors to help improve the characterization of soil respiration. Typically, soil oxygen sensors use a galvanic cell to produce a current flow that is proportional to the oxygen concentration being measured. These sensors are buried at various depths to monitor oxygen depletion over time, which is then used to predict soil respiration rates. Generally, these soil sensors