Oxygen gas sensors are vital to consumers and businesses across many different industries today and can incorporate a variety of sensing technologies like electrochemical, optochemical, oxygen quenching, and zirconia components at their core.
Dependent upon the specific type of sensor that you purchase and your use case or environment, the industry agrees that there are important factors in keeping these sensors operating at optimal performance over their operational life.
Today, we are partnering with long-time sensor supplier SST Sensing, to provide 5 easy steps towards extending the lifetime of an oxygen sensor.
Now let's get started.
What is the life expectancy of an Oxygen Sensor?
- Clean, dry air (aircraft OBIGGS) applications: 10+ year
- Good quality natural gas (low Sulphur): 5+ years
- Biomass (wood chip, pellet, etc.): 2+ years
- Coal (low sulphur): 2+ years
- Composting: 1+ years
These lifetimes are typical and are not guaranteed. The lifetime of an oxygen sensor can be dramatically reduced if they are physically damaged (high shock or vibration), contaminated with chemicals, or of the heater supply is too low or too high for the chosen sensor and the environment in which it is used in.
Step 1. Ensure Your Gas Sensor or Interface is Set Up Properly
- Verify the oxygen sensor unit is mounted securely and sealed correctly if appropriate or applicable.
- If fitted, ensure any baffles are installed in the correct position
- Verify the oxygen sensor and wiring are all undamaged
- Ensure the cables are strain-free and not twisted
- Ensure the oxygen sensor is connected properly, with all its inputs and outputs complete. If appropriate, all screw terminals are properly tightened.
- Test the power supply to ensure it is delivering the correct voltage before wiring to the device.
- Failure to test the suitability of the power supply BEFORE first power on could result in irreversible product damage.
Step 2. Assess the Environment where the Gas Sensor will be Used
For example, the application in which the zirconium dioxide oxygen sensor is operating (industrial processes) influences the oxygen sensor lifespan.
Fail Safe Operation and Sensor Asymmetry
One of the main benefits of the dynamic and active cell employed within most oxygen gas sensors that incorporate zirconium, is that they are inherently fail safe. The continual cycling and measurement of the generated Nernst voltage is effectively the heartbeat of the sensor, if this stops, something fatal has occurred within the cell. This can be very quickly detected by the interface electronics.
Operating in Aggressive Humid Environments – What Causes An Oxygen Sensor to Fail?
When operating the oxygen sensor in warm, humid environments it is important the sensor remains at a higher temperature than its surroundings, especially if there are corrosive components in the measurement gas. During operation this is less of an issue as the heater operates at 700°C, however this means when the oxygen sensor or application is being powered down the sensor heater must be the last thing to be turned off after the temperature of the surroundings have suitably cooled. Ideally the sensor should be left powered or at a lower standby voltage (2V typically) at all times in very humid environments.
Failure to adhere to these rules will seriously effect the lifetime of an oxygen sensor and result in condensation forming on the heater and sensing element. When the sensor is re-powered the condensation will evaporate, leaving behind corrosive salts which very quickly destroy the heater and sensing element as illustrated. Note how the sensor’s external metalwork looks completely normal.
Protecting the Gas Sensor from Excessive Moisture or Humidity
In environments where excessive moisture or falling water droplets are likely, the sensor should be protected from water reaching or falling directly onto the very hot sensor cap as this can cause massive temperature shocks to the cell and heater. Popular methods include a hood over the sensor cap or for the sensor to be mounted in a larger diameter cylinder.
At a very minimum the sensor cap should be angled downwards in the application as this will deflect any falling moisture and prevent the sensor cap from filling with water.
Step 3. Avoid using the Gas Sensor with Silicones
Typically, Zirconium dioxide oxygen sensors are damaged by the presence of silicone in the measurement gas. Vapors (organic silicone compounds) of RTV rubbers and sealants are the main culprits and are widely used in many applications.
These materials are often made of cheaper silicones, that when heated still outgas silicone vapors into the surrounding atmosphere. When these vapors reach the sensor, the organic part of the compound will be burned at hot sensor parts, leaving behind a very fine divided Silicon Dioxide (SiO2).
This SiO2 completely blocks the pores and active parts of the electrodes. If RTV rubbers are used we advise using high quality, well cured materials. Guidance can be provided on request.
Step 4. Protect the Gas Sensor from Harmful Chemicals
Small amounts of combustible gases will be burned at the hot Pt-electrode surfaces or AI2O3 filters of the sensor. In general, combustion will be stoichiometric as long as enough oxygen is available, the sensor will measure the residual oxygen pressure which leads to a measurement error.
Oxygen sensors are not recommended for use in applications where there are large amounts of combustible gases present and an accurate O2 measurement is required as these gases will dramatically affect the lifetime on an oxygen sensor.
- H2 (Hydrogen) up to 2%; stoichiometric combustion
- CO (Carbon Monoxide) up to 2%; stoichiometric combustion
- CH4 (Methane) up to 2.5%; stoichiometric combustion
- NH3 (Ammonia) up to 1500 ppm; stoichiometric combustion
Vapors from metals like:
- Zn (Zinc)
- Cd (Cadmium)
- Pb (Lead)
- Bi (Bismuth)
These will have an effect on the catalytic properties of the Pt– electrodes. Exposures to these metal vapors must be avoided as they can influence the lifetime of an oxygen sensor.
Halogen and Sulphur Compounds
Small amounts (< 100ppm) of Halogens and/or Sulphur compounds have no effect on the performance of the oxygen sensor. Higher amounts of these gases will, in time, cause readout problems or, especially in condensing environments, corrosion of sensor parts and affect the lifetime of an oxygen sensor. Gases investigated:
- Halogens, F2 (Fluorine), Cl2 (Chlorine)
- HCL (Hydrogen Chloride), HF (Hydrogen Fluoride)
- SO2 (Sulphur Dioxide)
- H2S (Hydrogen Sulphide)
- Freon gases
- CS2 (Carbon Disulfide)
Step 5. Avoid Reducing Atmospheres, Fine Dust or Vibration Environments
Long time exposure to reducing atmospheres may in time impair the catalytic effect of the Pt-electrodes and must be avoided. Reducing atmospheres are defined as an atmosphere with very little free oxygen and where combustible gases are present. In this type of atmosphere oxygen is consumed as the combustible gases are burned.
Fine Dust and Heavy Shock or Vibrations
- Fine dust (carbon parts/soot) may cause clogging of the porous stainless steel filter and could have an effect on the response speed of the sensor.
- Heavy shocks or vibrations may alter sensor properties resulting in the need for recalibration.
If you would like more information on oxygen sensing solutions and integration within your application or industry, please contact us at Sales@CO2Meter.com