How it works
Applied Sensing Technologies offers galvanic-type microfuel cells for measuring the oxygen concentration in a gas mixture. These sensors are sensitive to oxygen and are an excellent choice for measuring oxygen in various gas mixtures without significant interference from other gases. These sensors are:
— Oxygen specific
— Linear response to oxygen concentration
— Relatively small size
— Disposable at end of life
— Absolute zero
— Single point calibration
— Low drift for long life
— No maintenance
The sensing element of the sensor, usually perforated silver- or gold-coated metal coated with a gas-limiting oxygen barrier, senses the oxygen concentration in the gas mixture and outputs a current signal proportional to the oxygen partial pressure. To facilitate the oxygen reduction reaction on the sensing element, an easily oxidizable metal, such as lead, serves as an anode (electron source). The current flows between the anode and the sensing element through an external circuit. The current strength is measured and expressed in understandable units of oxygen. Reduction and oxidation reactions are presented as
Reduction of O 2 + 2H 2 O + 4e ----> 4OH -
Oxidation 2Pb + 4OH - ----> 2PbO + 2 H2 O + 4e
Overall reaction 2Pb + O 2 ----> 2PbO
Applied Sensing Technologies offers galvanic-type microfuel cells for measuring the oxygen concentration in a gas mixture. These sensors are sensitive to oxygen and are an excellent choice for measuring oxygen in various gas mixtures without significant interference from other gases. These sensors are:
— Oxygen specific
— Linear response to oxygen concentration
— Relatively small size
— Disposable at end of life
— Absolute zero
— Single point calibration
— Low drift for long life
— No maintenance
The sensing element of the sensor, usually perforated silver- or gold-coated metal coated with a gas-limiting oxygen barrier, senses the oxygen concentration in the gas mixture and outputs a current signal proportional to the oxygen partial pressure. To facilitate the oxygen reduction reaction on the sensing element, an easily oxidizable metal, such as lead, serves as an anode (electron source). The current flows between the anode and the sensing element through an external circuit. The current strength is measured and expressed in understandable units of oxygen. Reduction and oxidation reactions are presented as
Reduction of O 2 + 2H 2 O + 4e ----> 4OH -
Oxidation 2Pb + 4OH - ----> 2PbO + 2 H2 O + 4e
Overall reaction 2Pb + O 2 ----> 2PbO
Main Features of Oxygen Sensors
Oxygen sensing elements are usually small in size, and the life of the sensor is strictly dependent on the rate of anode consumption, which in turn depends on the rate of oxygen supply to the sensing element. The design of the sensor is a compromise between the measured oxygen level and the amount of lead (Pb) needed to reach the sensor’s life to a reasonable time, typically 12−60 months. Depending on the gas barrier used, oxygen sensors generate a low to high µA current signal per unit of oxygen concentration. Sub-PPM oxygen measurements require a higher current signal to provide a good signal-to-noise ratio. On the other hand, for percent level oxygen measurements, a much lower current provides a good signal-to-noise ratio.
Oxygen sensing elements are usually small in size, and the life of the sensor is strictly dependent on the rate of anode consumption, which in turn depends on the rate of oxygen supply to the sensing element. The design of the sensor is a compromise between the measured oxygen level and the amount of lead (Pb) needed to reach the sensor’s life to a reasonable time, typically 12−60 months. Depending on the gas barrier used, oxygen sensors generate a low to high µA current signal per unit of oxygen concentration. Sub-PPM oxygen measurements require a higher current signal to provide a good signal-to-noise ratio. On the other hand, for percent level oxygen measurements, a much lower current provides a good signal-to-noise ratio.
Color coded ring
Applied sensing Technologies sensors are color coded for easy identification of the sensor application. SRX series sensors are designed to measure O2 in inert gases containing hydrocarbon and hydrogen; blue for PPM O2 and green for % O2. In the SRZ series sensors, brown is for measuring O2 in gases containing any level of CO2 and trace levels of H2S.
Applied sensing Technologies sensors are color coded for easy identification of the sensor application. SRX series sensors are designed to measure O2 in inert gases containing hydrocarbon and hydrogen; blue for PPM O2 and green for % O2. In the SRZ series sensors, brown is for measuring O2 in gases containing any level of CO2 and trace levels of H2S.
Sensor Calibration
The sensor signal is usually constant per unit of oxygen concentration. However, environmental conditions can affect the sensor signal, which requires periodic calibration. As already mentioned, the sensor signal is always proportional to the partial pressure of oxygen in the gas mixture, doubling the partial pressure — doubling the signal, the absence of oxygen — the absence of a signal. This feature of the sensor allows one-point calibration. Percent oxygen sensors can be calibrated in ambient air (20.9% oxygen at atmospheric pressure). PPM Oxygen sensors can be calibrated in ambient air, but it takes a significant amount of time for the sensor to recover to a low PPM level (up to 60 minutes), so it is recommended to use a gas containing 8−80 PPM of oxygen for calibration.
The sensor signal is usually constant per unit of oxygen concentration. However, environmental conditions can affect the sensor signal, which requires periodic calibration. As already mentioned, the sensor signal is always proportional to the partial pressure of oxygen in the gas mixture, doubling the partial pressure — doubling the signal, the absence of oxygen — the absence of a signal. This feature of the sensor allows one-point calibration. Percent oxygen sensors can be calibrated in ambient air (20.9% oxygen at atmospheric pressure). PPM Oxygen sensors can be calibrated in ambient air, but it takes a significant amount of time for the sensor to recover to a low PPM level (up to 60 minutes), so it is recommended to use a gas containing 8−80 PPM of oxygen for calibration.
Temperature Effect
The sensor signal is also dependent on the ambient temperature, the sensor signal increases exponentially with increasing temperature, typically 2.0% to 2.4% per C. When processing the sensor signal, a network of resistors/thermistors can be used to compensate for temperature effects.
The sensor signal is also dependent on the ambient temperature, the sensor signal increases exponentially with increasing temperature, typically 2.0% to 2.4% per C. When processing the sensor signal, a network of resistors/thermistors can be used to compensate for temperature effects.
The 48-hour graph data shows the stability of three SRX series PPM oxygen sensors, full scale range 0−10 PPM, nitrogen gas containing 0.8 PPM oxygen, sample flow rate 0.5 SCFH. Change in ambient temperature +/-10−15 F over 48 hours in a day and night cycle.
Measurement accuracy and stability
The most requested characteristics of a galvanic type oxygen sensor are measurement accuracy and long-term stability. Applied Sensing Technolgies has included a number of innovations in sensor design. The PPM sensor anode is coiled, allowing the anode to be completely immersed in the electrolyte, preventing anode dry patches that typically occur with sintered grained lead anode, thereby eliminating unwanted signal spikes. The electro-etched sensing element provides an extremely smooth sensing surface, low signal noise and high measurement stability. The design of the sensor provides fast response and fast recovery from a fault condition, excellent linearity, repeatability and stability even under varying environmental conditions.
The most requested characteristics of a galvanic type oxygen sensor are measurement accuracy and long-term stability. Applied Sensing Technolgies has included a number of innovations in sensor design. The PPM sensor anode is coiled, allowing the anode to be completely immersed in the electrolyte, preventing anode dry patches that typically occur with sintered grained lead anode, thereby eliminating unwanted signal spikes. The electro-etched sensing element provides an extremely smooth sensing surface, low signal noise and high measurement stability. The design of the sensor provides fast response and fast recovery from a fault condition, excellent linearity, repeatability and stability even under varying environmental conditions.
Recovery of SRZ Series PPM oxygen sensors after 2 minutes exposure to air below 10 PPM on pure nitrogen
Air to 100 PPM 7.5 minutes
Air to 10 PPM 32 minutes
Air to 5 PPM 55 minutes
Air to 100 PPM 7.5 minutes
Air to 10 PPM 32 minutes
Air to 5 PPM 55 minutes
Acid Gas Sensor Compatibility
The SRZ series sensors use a special electrolyte formulation that makes the sensor resistant to any level of CO2 in the sample gas and trace levels of H2S. The impregnation formula of the electrolyte increases the stability of the sensor at temperatures close to freezing.
The SRZ series sensors use a special electrolyte formulation that makes the sensor resistant to any level of CO2 in the sample gas and trace levels of H2S. The impregnation formula of the electrolyte increases the stability of the sensor at temperatures close to freezing.
Sensitivity to flow, position and motion
Since the sensor signal is limited by the diffusion rate of oxygen through the barrier membrane, the sensor is insensitive to changes in sample flow unless significant back pressure is applied. For optimal performance, a sample flow of 0.5−5 SCFH is recommended. The sensor is not position sensitive and can be mounted in any position, but the preferred position is sensor down. Minor mechanical vibrations have no effect, but sudden movements of the probe should be avoided.
Since the sensor signal is limited by the diffusion rate of oxygen through the barrier membrane, the sensor is insensitive to changes in sample flow unless significant back pressure is applied. For optimal performance, a sample flow of 0.5−5 SCFH is recommended. The sensor is not position sensitive and can be mounted in any position, but the preferred position is sensor down. Minor mechanical vibrations have no effect, but sudden movements of the probe should be avoided.
Recommended storage
Sensors are sealed in metallized bags under nitrogen. Sealed sensors can be stored on the shelf for as long as needed, but the ideal recommended shelf life for SRX Series sensors is six months and for SRZ Series sensors is three months. The recommended storage temperature is up to 35 C, but the preferred storage temperature is up to 25 C. The electrolyte used in the SRZ series sensors is volatile, so it is recommended to store them up to 10 C, preferably in a refrigerator.
Sensors are sealed in metallized bags under nitrogen. Sealed sensors can be stored on the shelf for as long as needed, but the ideal recommended shelf life for SRX Series sensors is six months and for SRZ Series sensors is three months. The recommended storage temperature is up to 35 C, but the preferred storage temperature is up to 25 C. The electrolyte used in the SRZ series sensors is volatile, so it is recommended to store them up to 10 C, preferably in a refrigerator.
Galvanic-type electrochemical oxygen sensors, also known as microfuel cells, are extremely versatile, highly oxygen-specific, easy to use, and require no maintenance other than periodic calibration. At the end of their service life, they can be disposed of like a battery. These sensors are widely used in a variety of applications, in particular to monitor sub ppm oxygen levels in inert gases, hydrogen and gaseous hydrocarbon streams. The cross-references are only for reference to the most commonly used PPM oxygen sensors. Contact us for clarification of the design and application of the sensor.
The SRX-CT series sensors are specifically designed for breathing equipment such as ventilators. The user must check the compatibility of the sensors with the intended equipment.
NOTE: For optimal accuracy, ventilator sensors should be calibrated before each use and 24 hours after continuous use in an oxygen environment greater than 90%.
NOTE: For optimal accuracy, ventilator sensors should be calibrated before each use and 24 hours after continuous use in an oxygen environment greater than 90%.
The sensors of the SRX-CF series are specially designed for applications where CO2 is present in the sample gas, such as oxygen measurement in food warehouses. Contact us for clarification of the design and application of the sensor.
The SRX-CT and AST series sensors are specifically designed for breathing air analysis and diving applications. The user must first check their compatibility with the intended equipment/breathing apparatus. Sensors must be calibrated before use and must not be used after the expiration date. Sensors for breathing equipment show a linear response up to 1.6 atm O2 (sensors with mV output in air only up to 16 mV).
Galvanic-type electrochemical oxygen sensors, also known as microfuel cells, are extremely versatile, highly oxygen-specific, easy to use, and require no maintenance other than periodic calibration. At the end of their service life, they can be disposed of like a battery. These sensors are widely used in various fields, such as oxygen control in air separation, food processing and packaging, personal safety area monitoring, inert gas welding and glove box purging. The cross-references are only for reference to the most commonly used % O2 sensors. Contact us for sensor design, output signal, electrical contacts and applications.
