Storage conditions
The conditions under which an oxygen sensor is located before or when it is not in use directly affect the sensor's displacement. Manufacturers typically list recommended storage conditions in the sensor's datasheet.
• The electrochemical oxygen sensor should be placed in a refrigerator at room temperature (or preferably), but not below -20°C. At these low temperatures, the electrolyte inside the electrochemical sensor will freeze and damage the casing.
• The maximum storage temperature for the gas sensor is 60°C. Above this temperature, the ABS resin housing of the sensor will melt and damage the sensor.
• Condensation on the sensor's gas diffusion membrane should be avoided. Fortunately, once the condensation disappears, the sensor should recover over time.
Conversely, storage in extremely dry environments will alter the lifespan of electrochemical sensors due to excessive evaporation of the liquid electrolyte.
• Atmospheric pressure can also affect the lifespan of electrochemical oxygen sensors due to excessive evaporation of the liquid electrolyte and rupture of seals.
• If the sensor is refrigerated, it should be normalized to ambient temperature before use to obtain an accurate reading.
Generally, the best storage for any gas sensor is in a dark, cool container away from light. If an oxygen sensor arrives in a sealed bag, it should remain sealed until used. If the oxygen sensor is not in a sealed bag, it must be sealed. Some oxygen sensors require a small amount of oxygen to maintain their internal calibration.
Shelf life
Storage life, or "shelf life," is particularly important for electrochemical oxygen sensors. These sensors operate through chemical reactions within the sensor itself. When the sensor is exposed to a target gas, the chemicals release an electrical output (voltage or amperes) proportional to the gas level. This means that even when the sensor is not in use, exposure to air will deplete the internal chemicals. At high temperatures and low humidity, the absorption dries out. Eventually, the chemical reactions slow down to the point where the sensor no longer provides accurate readings.
Therefore, it is crucial to order only "fresh" electrochemical sensors. Most electrochemical sensors have a specified shelf life of 6 months. This is why they must be ordered from suppliers who store sensors in appropriate environments and rotate their inventory regularly.
To maximize sensor lifespan, sensors should only be ordered when you are ready to use them. If there are multiple gas detection devices using the same sensor in the field, keep extras on hand and indicate the delivery date on the packaging. The initial replacement sensor used by the gate should be the next one you intend to use (FIFO).
Operating conditions
Oxygen sensors perform best when used within the temperature, pressure, and humidity parameters recommended by the manufacturer.
While all oxygen sensors can operate over a wide range of temperature, pressure, and humidity, any sudden change in these conditions can affect the sensor's accuracy. For example, if a sensor is moved from a refrigerator to normal air temperature, it should be acclimatized to the new temperature several hours before use.
• When calibrating an oxygen sensor, both the sensor and the calibration gas (usually nitrogen) should be at ambient air temperature and pressure. Because the stored gas cools as it is released from the cylinder, a long tube should be used to allow the calibration gas temperature to rise, and a flow regulator should be used to limit the pressure from the tank.
• All gas sensors should be protected from condensation. If the breathable membrane becomes wet during use, it should be dried for several hours or days.
• During use, the oxygen sensor should be protected from shocks and vibrations. Excessive shock or vibration may cause temporary instability in the sensor's output. In most cases, allowing the sensor to remain stationary for several hours under normal indoor air and temperature conditions will allow it to return to normal performance. However, excessive shock may damage the sensor beyond repair.
• Avoid contaminating the oxygen sensor with molecules or gases not detected in fresh air. For example, some sensors may be damaged by high concentrations of alkali metals, brine spray, or ammonia. Additionally, adhesives or silicone rubber vapors generated during factory manufacturing can fluorinate the gas diffusion membrane.
Life expectancy
The expected lifespan of gas sensors ranges from 12-18 months for electrochemical gas sensors to over 15 years for optical or NDIR sensors. For example, the electrochemical AlphaSense O2-A2 oxygen sensor has a rated lifespan of 2 years, while the O2-A3 oxygen sensor has a rated lifespan of 3 years. The LuminOX LOX-02 25% oxygen sensor, which uses oxygen for fluorescence quenching, is rated for over 5 years.
