Electrochemical gas sensors for popular gases, such as CO, H2S, and O2, can operate normally for 2 to 3 years. The operating lifespan of exotics, such as H2, ETO, SO2, NO2, NO, NH3, HCL, PH3, CL2, CLO2, HCL, HCN, CH3SH, VCM, O3, HF, CH2O, THT, C2H2, and C2H4, varies depending on the design of the sensor. In general, they all should have a minimum of 1-year operating lives. Some could work for several years. For example, HF sensors have an operating life of 12 to 18 months, and the standard operating life of SemeaTech's Lead-Free O2 sensor and Long-Life NH3 sensor is about 5 years. Having said that, the operating life of an electrochemical gas sensor changes relative to working environmental conditions such as temperatures, humidity, dust, etc. In an ideal environment where the humidity is maintained between 20% and 60% RH with no contaminant intrusion, some sensors can continuously function per specifications for more than 10 years.
Most electrochemical gas sensors are non-consumptive. They should not deplete while exposed to target gases. Good sensors are equipped with sufficient catalysts and durable conductors that are resistant to chemical reactions. Therefore, periodic exposure to target gases will not reduce the sensor's operating life.
The datasheet of a SemeaTech sensor includes the storage life. Typically, it is 6 months after production, and it is based on the assumption of storage temperatures between 10°C and 30°C. The sensor sensitivity may become unstable beyond this period. A small portion of this period is inevitably devoted to production, finished goods inventory, and transportation. Therefore, it is very important for customers to make careful plans when placing purchase orders.
For more details, please refer to Application Note 220607.
Humidity is the most influential factor. Due to the limitations of technology in electrochemical gas sensors, the humidity in the sensors' operating environment should not exceed 95% RH. Above this point, the electrolyte inside the sensor can be diluted by absorbing moisture from the environment. The volume of the diluted electrolyte can expand 2-3 times, and it eventually causes the electrolyte to leak out of the sensor's plastic enclosure. In contrast, when the humidity in the operating environment is much lower than 20% RH, the electrolyte can gradually dry out, resulting in a significantly prolonged response time. The dilution or drought of the electrolyte can be determined quickly and easily by weighing the sensor. Compared with the manufacturing specifications, a change of more than ±250mg indicates that the performance of the sensor is likely to be affected. In most cases, by exposing the sensor to the opposite extreme of humidity over a period of several days, the dilution or drought of the electrolyte in the sensor can be restored, as can the sensor's performance.
Extreme temperature is another major factor in the sensor's lifespan. In general, the operating temperature that sensor manufacturers state in the datasheet varies from -30°C to +50°C. However, high-quality sensors can withstand temperatures beyond this range for a period of time. For example, a good H2S or CO sensor can be exposed to temperatures up to 65 °C without a problem. But for most of the electrochemical sensors, repeated exposure to such a high temperature may cause a drought of electrolytes, baseline drifting, and a slower response. Some of the electrochemical gas sensors may function well at -40 °C, but the tradeoffs are in sensitivity and response time to the target gas. They may lose 80% of their sensitivity and increase the response time significantly due to the frozen electrolyte.
The concentration of the target gas also affects the sensor's life. Generally speaking, the higher the concentration, the shorter the sensor's life. But the design of a sensor with good catalysts will increase the sensor's lifespan at high concentrations of the target gas.
It is important to note that the sensitivity of the sensor may vary according to the surrounding environment. The change in humidity may improve the sensitivity and slow the response of a sensor. This is especially the case in areas with seasonal climate changes. For example, the performance of hydrogen sulfide sensors is particularly related to the surrounding environment. The sensitivity and response time of the sensor in a fixed gas monitor are likely to change within 2 or 3 weeks, depending on local temperatures and humidity. This is a very common phenomenon for an electrochemical gas sensor that has been kept in a dry place for some time before installation in the field.
Under certain circumstances, cross-interfering gases could be absorbed by the sensor catalyst or react with the catalyst to produce by-products that inhibit the catalyst and damage the sensor electrode.
Strong vibrations and mechanical overstress may also damage the sensor enclosure and interconnections or solder joints that hold together the electrodes and platinum wires of the sensor.
For more details, please refer to Application Note 220607.
In general, a sensor's operating life is affected by various factors, and each specific situation is different. In practice, the user will replace the sensor either on a fixed cycle recommended by the manufacturer or based on its historical service data. If the sensor's response time T90 is much longer than the specification or shows a significant decrease in sensitivity during calibration or a pump test, it needs to be replaced.
For more details, please refer to Application Note 220607.
In the past decades, several patents and technologies related to this subject have been publicized. They all claim to be able to detect electrochemical sensor failures, but most of them simply infer whether a sensor is working under certain electrode stimulation. The only reliable way to find whether a sensor still functions normally is to measure the sensor's response by conducting a "bump test" or "calibration."
In clean air, the output signal of a functional electromechanical sensor is always zero current. A failed sensor exposed in the target gas also has zero current output. Therefore, there is no guarantee that a gas detector can automatically identify sensor failures. Users should appropriately test the sensor by using the target gas to verify if the sensor is still working correctly.
Every gas sensor needs to be calibrated after it is installed on a gas monitoring instrument to ensure the accuracy of gas measurements. Calibration is the process of matching the sensor's output to the known concentration of the target gas. In general, the calibration includes zero calibration and span point calibration. Zero calibration refers to the calibration in a high-purity nitrogen or clean air environment, and span point calibration refers to the calibration at a certain concentration of target gas. The gas monitoring instrument's user manual should include the details of calibration procedures and other information to support the proper maintenance and timely calibration of these instruments.
The recalibration after sensors have been used for a period of time depends on various factors, such as the operating temperatures, humidity, pressure, gas, and exposure time. It also depends in many cases on applications, sensor quality, industry standards, and government regulations.
Although SemeaTech electrochemical gas sensors have excellent consistency, stability, and environmental dependency over time, it is required to calibrate the sensor after it is installed on the gas monitoring instrument and then recheck (bump test) the accuracy one month later. When the sensor stabilizes, the calibration interval can be extended to 3, 6, or even 12 months, depending on the operating environmental conditions. The gas monitoring instrument's user manual usually includes the calibration requirements that should be strictly followed.
Different sensors require different stabilization times when first used. The following table lists some of the requirements:
Gas Type |
New or long-unused sensors(hrs) |
Temporarily-unused sensors(mins) |
CO |
2 |
10 |
H2S |
2 |
10 |
O2-LF |
2 |
15 |
H2 |
2 |
10 |
ETO |
12 |
12 Hours |
SO2 |
2 |
10 |
NO2 |
2 |
10 |
NO |
12 |
12 Hours |
NH3 |
2 |
10 |
PH3 |
2 |
10 |
CLO2 |
2 |
10 |
Cl2 |
2 |
10 |
HCl |
12 |
240 |
HCN |
2 |
10 |
CH3SH |
2 |
10 |
THT |
12 |
12 Hours |
C2H3CL |
12 |
12 Hours |
O3 |
2 |
10 |
HF |
2 |
10 |
CH2O |
2 |
10 |
C2H2 |
2 |
10 |
C2H4 |
12 |
12 Hours |
If the sensor is not in use for a period of time, its counter electrode may build up electrical charges that affect the sensor's accuracy. The warm-up process eliminates such electrical charges. On the other hand, the sensor electrodes may absorb other gases from the environment. Warming it up before use can clean up the electrodes and make the sensor function more steadily. In addition, the warm-up process ensures the bias voltage between the working and reference electrodes is within specs to prepare the sensor for use.
Only unbiased gas sensors are shipped with a shorting spring connecting two of the three pins (the working electrode and reference electrode). If an unbiased sensor is not in use, electrical charges will gradually build up on these electrodes. Shorting them will release the electrical charges (called neutralization) and keep the sensor stabilized during storage. The shorting spring must be removed before the sensor is installed on a gas monitoring instrument. The sensor then needs about a 10-minute warm-up for the baseline to become stable before the initial calibration and test.
Biased gas sensors do not need to be shorted electrically between any electrodes, but they require a warm-up time of about 6 hours or longer after they are installed on a gas monitoring instrument for the baseline to become stable enough before the initial calibration and test. It is recommended that the instrument be designed with the correct bias voltage for such sensors to avoid the warm-up time prior to each use, regardless of whether the instrument is on or off. To avoid the warm-up time before the sensor installation, it is recommended to use a simple electronic device that supplies the correct bias voltage to keep such sensors warmed up.
For more details, please refer to "8. How fast can the sensor stabilize when first used?"
Electrochemical gas sensors are sealed with a hydrophobic PTFE membrane to prevent fluid from flowing in and out, even if there are holes for gases to flow through. Leakage will occur if the pressure at the sensor inlet suddenly increases or decreases beyond the specified limits in the sensor datasheet. If the pressure variation is slow enough, the sensor might have a chance to sustain a wider pressure range over the spec.
SemeaTech sensors are recommended for a storage period of six months. During this period, the sensors shall be stored in a dry area, and the ambient temperature should be between 10°C and 30°C. Do not store in areas containing organic solvents or flammable liquids. Under these conditions, the sensors can be kept for up to six months without shortening their expected operating lives.
Electrochemical gas sensors stored in their original packaging will not deteriorate significantly, even beyond their storage life. They will last longer if they are kept under mild conditions and avoid high temperatures and humidity. After a sensor is removed from its original packaging, it must be kept away from any organic solvents or flammable liquids that may be absorbed by the sensor and subsequently affect its performance.
Two-electrode EC sensors (e.g., lead O2 sensors and two-electrode CO sensors) are self-powered and have no internal consumption.
Theoretically, three- and four-electrode sensors also don't consume power because they generate and output current from either oxidation or reduction processes with target gases. The reason why these sensors require power to function is because they need to operate on a potentiostatic circuit. So the power consumption is to optimize the performance of the amplifier in the circuit. The power applied to the sensor can be adjusted to a very low level.
Some electrochemical gas sensors have an internal chemical filter to reduce the potential impact of interfering gases. These filters have a limited operating life, and their tolerance threshold for interfering gases is typically expressed in terms of PPM-hours. When the filter is saturated, the sensor output is not only from the target gas concentration but also from interfering gases.
SemeaTech attempts to design the filter's operating life to match the sensors'. However, it may not be achievable for certain applications in which the concentrations of interfering gases are too high, such as stack emission monitoring and car emission testing. For these applications, we recommend that you choose the 3-series gas sensors, which have a longer-life internal filter.
The carbon-based organic filter is highly efficient but not renewable, and its blowholes will become gradually clogged when ambient humidity exceeds 50% RH. In general, the efficiency of the chemical filters is inversely proportional to the ambient humidity.
For some pollutants, the filter works by absorption instead of chemical reactions, and the pollutants can easily overload the filter. This is very typical for organic vapors. Please contact SemeaTech if you require additional information for your applications.
Maximum overload refers specifically to whether the sensor can maintain a linear response and recover quickly after exposure to the target gas for over 10 minutes. As the overload increases, the gas will accumulate inside the sensor, and the sensor electrodes cannot consume all the diffused gas in a short period of time. The sensor will become progressively nonlinear with the increasing overload and take longer to recover. The overload reacts with the reference electrode and then changes its electric potential. In a worst-case scenario, it may take several days for the sensor to recover in a clean air environment with forced air flow.
The circuit design with the sensor plays an important role in a rapid recovery from the overload. The op-amp in the potentiostat will not reach current or voltage saturation when the high-concentration target gas flows into the sensor. If the op-amp limits the current entering the sensor, it also limits the reaction at the working electrode. Eventually, an immediate gas accumulation inside the sensor occurs.
On the other hand, the load resistor connected to the sensing electrode should be chosen to ensure the voltage drop across it is never more than a few mV at the highest gas concentration likely to be seen. If larger voltage drops are allowed to take place across the load resistor, it will cause similar changes in the potential of the sensing electrode, and then the sensor will take a long time to recover after the gas is removed.
The answer is no. First of all, the measurement of reducing gases generally requires the consumption of oxygen on the counter electrode. This part of the oxygen can be either dissolved oxygen in the electrolyte or oxygen in the measurement environment. On the other hand, most electrochemical gas sensors are designed for the measurement of toxic and harmful gases in the air, so oxygen is required to maintain the sensor's stability during operation. Working for a long time in an environment with large variations in oxygen concentration will lead to sensor baseline drift and sensitivity changes.
There are many reasons for the sensor's sensitivity not meeting the specs, mainly because:
1. There are variations in the calibration gas flow rate.
2. Additional filters, such as explosion-proof mesh or PTFE dust-proof membrane, are placed in front of the sensor.
3. The components of the calibration system (tubing, valves, regulators, etc.) may absorb the gas from the cylinder. This is very common for reactive or sticky gases like CL2, CLO2, NH3, O3, NO2, HCN, HF, and HCL. Running a higher concentration of the target gas to "soak" the system for 30 minutes or longer before the test usually solves the issue. It is highly recommended to use only Teflon tubing and corrosion-resistant regulators for the calibration system and make the tubing as short as possible.
4. Standard gases, so-called calibration gases, are not certified or accurate enough.
For more details, please refer to the application note, "EC Sensor Calibration with Reactive or Sticky Gases."
Electrochemical gas sensors are temperature-sensitive. The sensitivity and baseline of a sensor vary along with ambient temperatures. The product data sheet posted on the SemeaTech website contains the temperature data that must be taken into consideration for compensation while developing the gas monitor or system software. The datasheet can be viewed or downloaded in pdf format directly from the website.
Cross sensitivities, also known as interfering gases (off gases), are gases that can cause the electrode inside the sensor to react even if the target gas is not present. In other words, cross sensitivities are a gas sensor's reaction to interfering gases rather than the target gas. Cross-sensitivities can result in either positively or negatively skewed sensor outputs. A positively skewed output can lead to the belief that there is too much of the target gas present, which will result in false alarms on the gas monitoring instrument. Negative effects produced by cross-sensitivities result in a lower reading on the instrument than the actual target gas concentration in the environment, which creates a dangerous and life-threatening situation. In order to fully accept the reading as accurate, it's important to take into account the hazards in the environment that may produce "off gases" potentially impacting the performance of the instrument. SemeaTech attempts to design each gas sensor only for a specific target gas or vapor, however, quite often the target gas is not the only gas detected by such a gas sensor as a result of cross-sensitivity.
Please be aware that the cross-sensitivities in SemeaTech's product datasheet only represent several batches of the sensor manufactured during product development or an upgrade. In SemeaTech production, each batch of sensors is fully tested using the certified calibration target gases to ensure conformity with the specs, but SemeaTech does not validate every batch to guarantee the consistency of the cross-sensitivities from those interfering gases in the datasheet. Therefore, it is not recommended to use any interfering gases (also called surrogate gases) for sensor calibration.
The ambient temperature affects the output sensitivity of an electrochemical gas sensor but not the measured gas temperature. The output sensitivity of a sensor is related to the rate at which the gas molecule diffuses through the capillary hole to the sensing electrode. If the temperature of the measured gas diffusing through the capillary hole is different from that of the gas inside the sensor, it will have very little impact on the sensitivity before the temperature equilibrium inside the sensor gets fully established.
SemeaTech gas sensors can work under various environmental conditions. However, it is important to avoid exposure to high concentrations of solvent vapors during the storage, installation, and operation processes. For example, formaldehyde may temporarily inhibit the function of NO sensors. Some organic solvents may raise the sensor baseline and cause errors on the gas monitoring instrument and inaccurate readouts. To ensure the sensor works per its specifications inside the instrument, the PCB must be thoroughly cleaned up before the sensor is installed.
Mechanical overstress may cause deformation or cracks in the plastic enclosure of the sensor. Extremely high temperatures and low humidity can cause the electrolytes in the sensor to dry out, while high humidity may cause leakage. If the target gas concentration is too high, the performance of the EC gas sensor will be degraded.
For more details, please refer to Application Note 220607, "Lifespan and Shelf Life of Electrochemical Gas Sensors."
The unbiased gas sensor is shipped with a shorting spring connecting the reference and the sensing electrodes to keep the sensor stable before use. The shorting spring must be removed when the sensor is installed on a gas monitor or system. Then it takes about 10 minutes for the sensor baseline to become stable prior to the initial calibration and test.
The biased gas sensor requires a warm-up time of about 6 hours or more after it is installed on the instrument for the baseline to become stable enough before the initial calibration and test. It is recommended that the instrument be designed with the correct bias voltage for such sensors to avoid the warm-up time prior to each use, regardless of whether the instrument is on or off. To avoid the biased sensor warm-up time before the initial installation, a simple electronic device supplying the correct bias voltage to warm up such sensors is recommended.
Please refer to "8. How fast can the sensor stabilize when first used?" for the warm-up time for each type of sensor.
Caution: Do not connect the reference and the sensing electrodes of a biased sensor with a shorting spring. The sensor will be permanently damaged as a result.
Some of SemeaTech's gas sensors can detect the target gas concentration continuously, such as the lead-free oxygen sensor and the air quality monitoring (AQM) sensors.
Some of the toxic gas sensors are designed to detect gas leakage for industrial safety. The sensitivity of these sensors is stable for a short period of time and may decline over a long period of time. So they are generally not suitable for continuous detection applications, especially those in high gas concentrations with extreme humidity and temperatures. Continuous detection can sometimes be achieved by using two (or more) sensors, so each of them takes turns spending half the time in detection and recovering for the other half in fresh air.
SemeaTech's 4-electrode gas sensors are designed specifically for Air Quality Monitoring (AQM). These sensors are very stable in the long run and sensitive to target gases with high resolution in ppb. In comparison with the 3-electrode gas sensors, which have the sensing, reference, and counter electrodes, the 4-electrode gas sensors have an additional electrode called "auxiliary" that maintains the sensor's zero current stability and reduces background noise.
Please see Application Note: AN 190921, AQM Sensors, for 4-electrode gas sensor design references.
It is highly recommended to solder sensor receptacles on PCBs and then plug sensors into the receptacles. If it is needed to solder gas sensors directly on PCBs in some cases, only SemeaTech 4-series gas sensors can take the heat from solder irons, but the soldering duration must be minimized to less than 5 seconds to avoid too much thermal shock causing damage to the sensors. Having said that, using electrically conductive epoxy instead of soldering is always a better approach to making the interconnections between gas sensors and PCBs.
A number of different plastics are selected for the sensor housing. Their compatibility with both the internal electrolyte system and durability in expected applications has been rigorously tested. ABS, polypropylene, or polyphenyl ether are commonly used. For more details, please refer to the data sheet of each sensor.
There are no intrinsic safety certificates required for SemeaTech's electrochemical gas sensors due to the small currents and voltages they produce. When used in compatible intrinsically safe gas monitors and systems, they are all suitable for meeting intrinsic safety requirements such as Class 1, Division 1.
SemeaTech electrochemical gas sensors are designed to be operated by a special circuit called a potentiostat. This design controls the potential of the working (sensing) electrode in relation to the reference electrode for guaranteed bias voltage. At the same time, it converts the signal current flowing into or out of the working electrode into a voltage output. Here are steps to verify if your circuit is properly designed or not:
1. Remove the sensor from the circuit board.
2. Shorten the reference electrode and the counter electrode on the circuit board.
3. Measure the electric potential of the working (sensing) electrode. For unbiased sensors, the measured value should be 0 (±10 mV), and for biased sensors, it should be the bias voltage (±10 mV).
4. Connect a current source between the working electrode/reference electrode and the counter electrode of the circuit board to verify if the voltage output meets expectations. Note: This current source is used as a simulation of the sensor, and the current range should be less than the full-scale output current of the simulated sensor.
The use of a pump in front of gas sensors to increase the gas flow rate does not make the sensor respond faster, but it allows the measured gas to pass through the sensor more quickly and efficiently. As a result, the pump makes gas monitors or systems respond to the measured gases faster.
Electrochemical sensors are designed to have an embedded membrane as a filter to keep dust and moisture outside the sensor. For additional protection, adding more filtration in front of the sensor is fine as long as the sensor's response time to the target gas still meets the application requirements.
Because of the reactive properties of some gases, such as CL2, CLO2, NH3, O3, NO2, HCN, HF, and HCL, the calibration system must be built with non-reactive materials. Regulators must be corrosion-resistant, and tubing and tubing joints should be made of PTFE (aka FEP or Teflon) or PTFE-lined soft tubing such as Norprene. Furthermore, the tubing should be as short as possible to avoid condensed moisture inside the system. All of these are meant to reduce the absorption of the gas in the gas delivery system.
For high-temperature gases, the gas flow should be cooled down to the sensor's temperature operating range before it reaches the gas sensor. Removing particles from the gas flow with appropriate filters is another factor that should be taken into consideration. If possible, add chemical filters to eliminate cross-sensitivities from interfering gases.
For more details, please refer to the application note, "EC Sensor Calibration with Reactive or Sticky Gases."
SemeaTech is an ISO 9001-certified manufacturer. We are committed to providing the most reliable and best-performing products to our customers. Our products are fully traceable to ensure that defective or unsafe products can be quickly located and removed to protect the safety of the end user and also help prevent avoidable product recalls.
1.Some type of gases (such as: ETO, THT, C2H4, etc.) are required to apply bias voltage between electrodes to change the electrochemical reaction rate and electrode surface state in order to monitor and analysis target measurement parameters.
2.The biased electrochemical sensors (operating on the principle of an electrolytic cell) only limit potential difference between working electrode and reference electrode and the potential on the counter electrode can drift due to the electrolyte solution, external temperature and humidity, stored gas environment, etc. This drift results in a significant increase or decrease in the sensor baseline. The counter electrode needs several hours to be stable (mainly because the electrolyte ions involved in the counter electrode reaction equilibrium need time to move to reach the potential required for the counter electrode reaction.). The higher the bias voltage, the longer it takes for the sensors to stabilize. We recommend that users maintain the bias voltage in the circuit even when the devices are turned off. Otherwise, it will take much longer time for devices to start up.
Pressure is one of the influencing factors that affects the performance of the sensor. Generally speaking, sensor’s sensitivity will change linearly when pressure change slowly. The higher atmospheric pressure is, the higher sensor’s circuit output it is. The relationship between them is linear and positively correlated.
When pressure change quickly, sensors may be damaged at the same time. When the environment changes from atmospheric pressure to high pressure, the sensor current will quickly become high and then drop back to normal. In this case, it depends on whether sensors have pressure resistant design.
On the contrast, sensor’s sensitivity will decline when pressure change from high to low. However, if the ambient air pressure at the sensor drops rapidly, the tiny air bubbles inside the sensor will expand and burst, "pushing" the electrolyte out of the sensor's air inlet and causing a leak. When air inlet has been blocked, the sensor is also permanently damaged.
Designs that can be considered for resistance to sudden pressure changes include:
1. Install sensors at the end of sample gas flow to avoid pressure fluctuations.
2. Alternatively, an airflow restrictor can be fitted to the front of the sensor to achieve the same effect.
3. The other effective way is to ensure that sensor’s back pressure is zero, allowing unrestricted venting of the airflow to the atmosphere. Please notice that prevent back-diffusion in the surrounding atmosphere is important, which may dilute the sample gas and result in lower concentration. Attaching a small piece of exhaust pipe can prevent back-diffusion.
This is the common phenomenon for Oxygen gas sensors, but it will disappear after 20 seconds. This phenomenon is especially noticeable on elevators that rise and fall at high speeds.
When humidity and pressure around electrochemical gas sensor change rapidly, the output signal also undergoes a sudden change in signal. However, the peak of this signal declines quickly and then stabilizes. This stable output value is solely related to the concentration of the detected gas and the ambient temperature, so there is no need to compensate for pressure and humidity variations. Please note that sensors must be used within the specified pressure and humidity ranges.
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