Oxygen gas sensors are an essential tool for many consumers and businesses across a wide range of different industries and sectors. These frequently encountered sensors may incorporate any number of sensing technologies such as optochemical, electrochemical, oxygen quenching and zirconia components.
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A number of factors must be considered to ensure that these sensors operate at their most optimal level throughout their service life. These factors depend on the specific type of sensor employed and the environment or use case in question.
The average working life of an optimized, well-managed oxygen sensor will depend on its use and application; for example:
- Clean, dry air, such as aircraft OBIGGS applications - 10+ year working life
- Good quality natural gas with low sulfur content - 5+ year working life
- Biomass applications including wood chip or pellets - 2+ year working life
- Coal with low sulfur content - 2+ year working life
- Composting applications - 1+ year working life
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While these lifetimes are typical, they are not guaranteed. An oxygen sensor’s viable working life may be significantly reduced in instances where these are physically damaged – for example, via vibration or shock, exposure to chemical contamination or in cases where the heater supply is too low or too high for the chosen sensor and operating environment.
There are five key steps that should be followed to maximize the working life of an oxygen sensor.
Step 1: Ensure The Gas Sensor or Interface is Set Up Correctly
The oxygen sensor unit should be mounted securely and, if necessary, sealed correctly. Baffles should also be installed in the correct position if these are fitted.
It is important to confirm that the oxygen sensor itself and any related wiring are undamaged and that cables are not twisted and remain strain-free.
The oxygen sensor must always be appropriately connected, complete with all necessary inputs and outputs.
All screw terminals should be properly tightened, and the power supply must be tested to guarantee that it is delivering the correct voltage prior to being wired to the device. This is especially important because the use of an unsuitable power supply can result in irreversible product damage when the sensor is first powered on.
Step 2: Assess the Gas Sensor’s Operating Environment
The working life of zirconium dioxide oxygen sensors is significantly impacted by their operating conditions, particularly when used in industrial processes.
Zirconia Oxygen Sensor System. Image Credit: CO2Meter, Inc.
Failsafe Operation and Sensor Asymmetry
A key benefit of the dynamic and active cell used within the majority of zirconium-based oxygen sensors is that they are fundamentally failsafe.
The constant cycling and measurement of generated Nernst voltage is essentially the sensor’s heartbeat, meaning that if this stops, it is clear that a fatal error has occurred. This error can be rapidly detected by the sensor’s interface electronics.
Sensor Failure When Operating in Aggressive Humid Environments
Oxygen sensors operating in warm, humid environments must be maintained at a higher temperature than their surroundings, particularly in instances where there are corrosive components in the measurement gas.
This is less of an issue during operation because the sensor heater is operating at 700 °C, but in practice, this does mean that the process of powering down the oxygen sensor or application must involve the sensor heater, and this must be the final component switched off )only after the temperature of the surroundings is suitably cool).
The sensor should consistently be at a lower standby voltage (2V typically) or even powered on in very humid environments.
Failure to adhere to this guidance can seriously affect the lifetime of an oxygen sensor as condensation forms on the sensing element and the heater. When this occurs, and the sensor is re-powered, this condensation evaporates to leave behind hard-to-detect yet highly corrosive salts, which rapidly damage both the heater and sensing element.
Protecting the Gas Sensor from Excessive Humidity or Moisture
The sensor should be protected when operating in environments where there is a high chance of excessive moisture or falling water droplets. It is important that water does not fall directly onto or otherwise reach the very hot sensor cap because this can trigger temperature shocks to the cell and heater.
To avoid this, it is common practice to include a hood over the sensor cap or to mount the sensor in a larger diameter cylinder.
The sensor cap should at least be angled downwards in the application because this will encourage the deflection of falling moisture and ensure that the sensor cap does not fill with water.
Step 3: Avoid Using the Gas Sensor with Silicones
Zirconium dioxide oxygen sensors can become damaged due to the presence of silicone in the measurement gas. Vapors (organic silicone compounds) of commonly used RTV rubbers and sealants are the primary cause of this issue.
As many of these materials are comprised of cheaper silicones, heating prompts the outgassing of silicone vapors into the surrounding atmosphere. As these vapors reach the sensor’s hot parts, the compound’s organic component is burned and leaves behind a very fine divided silicon dioxide (SiO2) which blocks the electrodes’ pores and active parts.
Where there is no option but to use RTV rubbers, it is important to use high-quality, well-cured materials.
Step 4: Protect the Gas Sensor from Harmful Chemicals
UV Flux 25% Oxygen Smart Flow Through Sensor. Image Credit: CO2Meter, Inc.
There is a risk that small amounts of combustible gases may be burned at a sensor’s hot Pt-electrode surfaces or AI2O3 filters. Since this combustion tends to be stoichiometric (as long as sufficient oxygen is available), this can cause the sensor to measure the residual oxygen pressure, resulting in a measurement error.
It is not advisable to utilize oxygen sensors in environments where there are large amounts of combustible gases present, and it is necessary to acquire an accurate O2 measurement.
Many of these gases can substantially limit the lifetime of an oxygen sensor via stoichiometric combustion; for example:
- Carbon Monoxide (CO) up to 2%
- Hydrogen (H2) up to 2%
- Methane (CH4) up to 2.5%
- Ammonia (NH3) up to 1500 ppm
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Vapors from metals such as zinc (Zn), cadmium (Cd), lead (Pb) and bismuth (Bi) can influence the catalytic properties of Pt-electrodes. It is important to avoid exposure to these metal vapors due to their significant impact on the working life of an oxygen sensor.
Halogen and Sulfur Compounds
While small amounts (less than 100 ppm) of halogens and/or sulfur compounds will not affect an oxygen sensor’s performance, exposure to higher amounts of these gases will eventually cause readout problems or the corrosion of sensor parts (particularly in condensing environments).
Gases to be aware of include halogens, fluorine (F2), chlorine (Cl2), hydrogen chloride (HCL) and hydrogen fluoride (HF). Other potentially damaging gases include sulfur dioxide (SO2), hydrogen sulfide (H2S), freon gases and carbon disulfide (CS2).
Step 5: Avoid Reducing Atmospheres, Fine Dust or Vibration-Prone Environments
Long-term exposure to reducing atmospheres should be avoided because this can impair the catalytic effect of the Pt-electrodes over time. A reducing atmosphere can be understood as an atmosphere that includes combustible gases and very little free oxygen. This leads to oxygen being consumed as the combustible gases are burned.
Fine Dust and Heavy Shock or Vibrations
Fine dust may stem from carbon parts or soot. This can lead to the clogging of an oxygen sensor’s porous stainless-steel filter, potentially impacting its response speed.
Heavy shocks or vibrations lead to changes in sensor properties, prompting the need for the sensor to be recalibrated.
This information has been sourced, reviewed and adapted from materials provided by CO2Meter, Inc.
For more information on this source, please visit CO2Meter, Inc.