by Ray Hicks, President, GasLab.com
Read the specifications of any oxygen sensor and you will see one or more of these specifications:
- Storage Conditions
- Storage Life
- Operating Conditions
- Operating Lifetime
Each of these are important when selecting an oxygen sensor. Here’s what you should know.
The conditions oxygen sensors are stored in before they are used or when not in use can have a direct impact on the life of the sensors. Manufacturers will typically list the recommended storage conditions on the data sheet of the sensor.
- Electrochemical oxygen sensors should be stored at room temperature or (preferably) in a refrigerator, but no colder than -20°C. At these low temperatures the electrolyte inside electrochemical sensors can freeze and break the casing.
- The maximum storage temperature for gas sensors is 60°C. Above that temperature the ABS resin casing of the sensor can melt and damage the sensor.
- Condensation on the sensor's gas diffusion membrane should be avoided. Fortunately, once the condensation dissipates, over time the sensor should recover.
- Conversely, storage in an extremely dry environment will shorten an electrochemical sensor’s life due to excessive evaporation of the liquid electrolyte.
- Atmospheric pressure can also impact the life of an electrochemical oxygen sensor due to excessive evaporation of the liquid electrolyte and breaking of the seals.
- If a sensor is in cold storage, it should be allowed to normalize to the ambient temperature before use to obtain accurate readings.
In general, the best storage for any gas sensor is a dark, cool, non-humid container. If the oxygen sensor arrives in a sealed bag, it should be left sealed until used. If the oxygen sensor is not shipped in a sealed bag, it should not be sealed. Some oxygen sensors require a small amount of oxygen to maintain their internal calibration.
Storage life or “shelf life” is especially important for electrochemical oxygen sensors. These sensors work by a chemical reaction inside the sensor. As the sensor is exposed to the target gas, the chemistry gives off an electrical output (voltage or amperage) that is proportional to the gas level. This means that even if the sensor isn’t being used, if it is exposed to air it is still using up the chemicals inside. Under high temperature and low humidity, the electrolytes will dry out. Eventually the chemical reaction slows down to the point where the sensor no longer gives an accurate reading.
As a result, it is important that you only order “fresh” electrochemical sensors. Most electrochemical sensors have a stated shelf life of 6 months. This is why it is important to order them from suppliers who store the sensors in a controlled environment and regularly rotate their stock.
For maximum sensor life you should only order replacement sensors when you’re ready to use them. If you have several gas detection devices in the field using the same sensor, keep a few extra on hand, and mark the delivery date on the packages so the oldest replacement sensor is the next one you use (FIFO).
Oxygen sensors work best when they are used within the temperature, pressure and humidity parameters recommended by the manufacturer.
- While all oxygen sensors are rated to work within a wide range of temperature, pressure and humidity levels, sudden changes in any of these conditions can have an impact on the sensor’s accuracy. For example, if you move a sensor from a refrigerator to normal air temperature you should let it become acclimated to the new temperature for several hours before using it.
- When calibrating an oxygen sensor, the sensor and the calibration gas (typically nitrogen) should both be at ambient air temperature and pressure. Because stored gases cool when released from the cylinder you should use a very long tube to allow the calibration gas temperature to rise and a flow regulator to limit the pressure from the tank.
- Condensation is to be avoided for all gas sensors. If the gas permeable membrane becomes wet during use, it should be dried for several hours or days.
- Oxygen sensors should be protected against shock and vibration during use. Temporary instability in the sensor’s output may result from excess shock or vibration. In most cases, allowing the sensor to sit motionless in normal room air and temperature for several hours will allow the sensor to return to its normal performance. However, excessive shock may damage a sensor beyond repair.
- Avoid contaminating oxygen sensors with molecules or gases not found in fresh air. For example, some sensors can be damaged by high concentrations of alkaline metals, salt water spray or ammonia. In addition, adhesives or silicone rubber vapors from factory manufacturing processes can clog the gas diffusion membrane.
The life expectancy of gas sensors range from as little as 12-18 months for exotic gas electrochemical sensor to greater than 15 years for optical or NDIR sensors. For example, the electrochemical AlphaSense O2-A2 Oxygen Sensor is rated for 2 years, while the O2-A3 Oxygen sensor is rated for 3 years. The LuminOX LOX-02 25% Oxygen Sensor that uses the principle of Fluorescence quenching by oxygen is rated at 5+ years.
While there is no guaranteed lifetime beyond the manufacturer’s warranty, all sensors are engineered to operate far beyond their rated lifetime when used within the storage and operating parameters listed for the sensor.