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FAQs of Electrochemical Gas Sensors

1. Conventional operating life

Electrochemical gas sensors for popular gases, such as CO, H2S and O2, can operate normally for 2 to 3 years. The operating lifetime of exotics, such as H2, ETO, SO2, NO2, NO, NH3, HCL, PH3, CL2, CLO2, HCL, HCN, CH3SH, VCM, O3, HF, CH2O, THT, C2H2, C2H4, varies depending on the design of the sensor. In general, they all should have minimum of 1-year operating life. Some could work for several years. For example, HF sensor has an operating life of 12 to 18 months, and the standard operating life of SemeaTech 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 temperature, humidity, dust, etc. In an ideal environment where the humidity is maintained between 20% to 60% RH with no contaminant intrusion, some sensors can continuously function per specifications for more than 10 years.

Most of 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 well resistive to chemical reactions. Therefore, periodic exposure to target gases will not reduce the sensor operating life.

The datasheet of a SemeaTech sensor includes so-called storage life. Typically it is 6 months after production, and it is based on the assumption of storage temperature between 10℃ to 30℃. The sensor sensitivity may become unstable beyond this period. A small portion of this period is inevitably devoted to production, finished good storage and transportation. Therefore, it is very important for customers to make careful plans when placing purchase orders.

2. Factors affecting sensor lifetime

Humidity is the most influential factor. Due to the limitations of technology in electrochemical gas sensors, the humidity in the operating environment should not significantly exceed 95%RH. Above this point, the electrolyte inside the sensor can be diluted by absorbing the moisture from the environment. The volume of the diluted electrolyte can expand 2-3 times, and it eventually causes the electrolyte to leak out from the sensor plastic enclosure. When the humidity in the operating environment is much lower than 20%RH, the electrolyte can gradually dry out followed by 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 humidity, the dilution or drought of the electrolyte is reversible. Over a period of several days, the weight of the sensor and the electrolyte concentration can be restored to their original states, along with the sensor performance.

Extreme temperature is another major effect on the sensor lifetime. In general, the operating temperature that sensor manufacturers state in the datasheet varies from -30℃ to +50℃. However, high-quality sensors can withstand temperatures beyond this range for a short period of time. For example, a good H2S or CO sensor can be exposed up to 65℃ for 1~2 hours without a problem. But repeated exposure to such a high temperature may cause drought of electrolyte, baseline drifting and slower response. Some of the electrochemical gas sensors may function well at -40℃, but the tradeoff is a drastically reduced sensitivity to the target gas. In some cases, the sensitivity can decline as much as 80%. On the other hand, the response time can tremendously increase along with the frozen electrolyte in such a low-temperature environment.

The concentration of target gas also affects sensor lifetime. Generally speaking, the higher concentration is, the shorter lifetime of the sensor is if the sensor is not designed with high-quality catalyst that is able to withstand in the high concentration of target gas.

It is important to note that the sensitivity of the sensor may vary according to the surrounding environment. The change of the humidity may improve the insensitive and slow-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 is likely to change within 2 or 3 weeks in accordance with local temperature and humidity. This is very common if the sensor is stored in an extremely dry space, such as an air-conditioned office before installation in the field.

Under special circumstances, interfering gases may be absorbed by the catalyst or react with the catalyst to form by-products, thereby suppressing the catalyst and damaging the sensor electrodes.

Strong vibrations and mechanical overstress may also damage interconnections or solder joints that hold together the electrodes, platinum wire and interfaces, thus damaging the sensor.

3. Filtrable gas sensor

Chemical filters are installed on some sensors to minimize the possible effects of interfering gases, particularly sulfide gases. These filters have a limited operating life and are usually defined in terms of PPM hour as their tolerance level to interfering gases. Since the gas concentrations are different, the unit of PPM hour may not be accurate. Even a 1000ppm hour filter may not be able to double the operating time when the target gas exposure time is halved.

When the filter is saturated, the cross-reaction between the sensor and the interfering gas will increase (such as a SO2 sensor with a filter to H2S). When the filter is used up, it is hard for users to tell whether the sensor is reacting with SO2 or H2S.

The organic filter (carbon-based) is highly efficient but not renewable, and it will be saturated with blowholes clogging when ambient humidity exceeds 50%RH. As a result, the efficiency of the chemical filters decreases in high humidity.

4. Plan for gas sensor replacement

Instrument operators expect to plan sensor replacement in advance by predicting the operating life. In that case, service engineers can bring new sensors to the site for maintenance, avoiding equipment downtime or repetitive staffing problems. If users can be sure to extend the routine sensor replacement cycle, they can also reduce the cost of replacing sensors.

With regard to the prediction of operating life, it will be 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 on historical data (such as every 2 or 3 years), or if the sensor is not sufficiently responsive to the test gas. Only when the sensor shows a significant decrease in sensitivity (or excessive response time) will it be possible for the sensor to be replaced between service cycles.

5. How to find sensor breakdowns?

In the past decades, several patents and technologies have been applied to gas sensors. Although they all claim to be able to detect breakdowns in electrochemical sensors, most simply infer sensors working under some certain electrodes stimulation. The only reliable way to show that a sensor is working is to measure the sensor's response by using test gas – which is called bump test or full calibration.

In fact, electrochemical sensors do not automatically prevent breakdowns. They emit zero signal current both in clean air and even when exposed to the target gas before being invalid. Therefore, there is no guarantee that a gas detector will automatically identify the existed breakdowns. However, a gas detector can report events that may affect the sensor's performance: smart gas detectors and transmitters can detect the surrounding environment and emit an alarm when the temperature exceeds the sensor’s thresholds. The transmitter can also compare the gas concentration to the sensor's maximum allowable value, and issue a warning if exceeded. In these cases, the user should appropriately test the sensor by using test gas to verify if the sensor can respond correctly.

6. Meaning of calibration

Calibration is to calibrate the indicator value of the alarm with a certain concentration of standard gas, including zero calibration and span point calibration. Zero calibration generally refers to the calibration in high purity nitrogen or clean air environment. Span point calibration refers to the calibration of alarms at a certain concentration of target gas.

7. How long does the sensor need recalibration?

The duration of the recalibration after use depends on various factors, such as the operating temperature, humidity, pressure, gas and exposure time. In general, SemeaTech electrochemical sensors can provide a very stable signal over time.

Calibration intervals depend on application requirements, sensor technologies, industry requirements and legal requirements. It is better to check the calibration when you receive the sensor and recheck the accuracy one month after installation. When the indication stabilizes, the inspection period can be extended to 3, 6 or even 12 months depending on the application. Having said that, performing calibration in compliance with the regulations is mandated.

8. How fast can the sensor stabilize when first used?

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)

H2S

2

10

CO

2

10

O2-LF

12

720

SO2

2

10

NO

12

720

NO2

2

10

Cl2

2

10

HCl

12

240

ETO

12

720

HCN

12

10

PH3

2

10

NH3

12

240

O3

2

10

H2

2

10

9. Why do electrochemical sensors have to age before use?

● Electric charges will accumulate on the counter electrode, which can be neutralized by aging.
● Other gases will be adsorbed on the sensor when not in use, which can be reacted by aging for more stable operation.
● When aging, working electrode is controlled within the required bias voltage range relative to reference electrode, so that the sensor is ready-to-work.

10. Why is the working and reference electrode of electrochemical sensors(zero bias sensors) short-circuited during storage?

This is for zero bias electrochemical gas sensors. Charges will accumulate at both ends of the electrode when the element is not in use. A short circuit (shorting spring on the sensor) aims to release the charges, which is also called neutralization. The shorting spring must be removed before the sensor is installed on the instrument, and the sensor requires about 10 minutes for the baseline to become stable before the initial calibration.

The biased gas sensor does not need to be short circuited, but it requires a warm-up time 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 each use regardless 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.

11. What happens if the pressure range exceeds that defined in the data sheet?

All sensors use similar sealing systems that rely on hydrophobic PTFE to prevent fluid from flowing out, even if there are holes for gases. Leakage will occur if the pressure at the sensor inlet suddenly increases or decreases beyond the allowable limit. If the pressure changes slowly enough, sensors can be used over a wider pressure range.

12. Optimal storage conditions for sensors

Sensors stored in original packaging will not deteriorate significantly even beyond their storage life. This period will last longer when sensors are kept under mild conditions and avoiding extremely hot and direct sunlight.

If the sensor is removed from its original packaging, it must be stored in a clean area away from the various solvent gases. The solvent gas may be absorbed into the electrode and cause related problems.

13. Requirement of power

Two electrode sensors (e.g. O2 sensor and two electrode CO sensor) are self-powered with no consumption of their own. Three and four electrode sensors require power because they have to operate on special constant potential circuits. Essentially, sensors do not need power supplies, as they generate an output current directly from the oxidation or reduction of the target gas. However, the amplifier in the circuit is not so perfect that it will consume some current. The power can usually be reduced to really low levels if needed.

14. How long can the internal filter last?

Some sensors have an internal chemical filter to remove gases that will interfere the signal. The filter is placed behind the diffusion barrier, so the velocity of gas passing through the filter is much lower than that through the main gas channel. As a result, small amounts of active chemicals can last a long time.

In general, the filter is designed to achieve the expected operating life of sensors in the intended application. However, this may be difficult in some applications where the concentration of interfering gases is high, such as emission monitoring. For these applications, we recommend 3-series sensors with internal filters for longer lifetime.

For some pollutants, the filter works through adsorption rather than chemical reactions, in which cases high concentrations may easily overload the filter. This is usually the case with organic vapors. SemeaTech technical support team can provide more information about specific cases.

15. What happens if gas concentration exceeds the maximum overload?

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. The sensor will become progressively nonlinear with the increasing overload and takes longer to recover because the sensing electrode cannot consume all the diffused gas. As the overload increases, the gas will accumulate in the sensor and diffuses into the internal space. It may react with the reference electrode and then change its electric potential. In this case, it may also take a long time for the sensor to recover (days) though placed in clean air in time.

The design of the circuit has an important role in ensuring a rapid recovery from the high concentration, because the Op-Amp in the potentiostat will not reach current or voltage saturation when the high concentration gas is injected. If the Op-Amp limit the current entering the sensor, it will also limit the gas reaction at the working electrode, which will cause an immediate gas accumulation within the sensor.

Finally 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's at the highest gas concentration likely to be seen. If larger voltage drops are allowed to take place across the load resistor this will cause similar changes in potential of the sensing electrode, which will take time to recover from once the gas is removed.

16. How much oxygen needed for the sensor to work properly?

Electrochemical sensors that produce output by oxidizing the target gas, such as CO sensors, need to supply oxygen to the counter electrode to maintain the oxygen-consuming reaction there. Usually a maximum oxygen concentration of several thousand ppm is required, which can be provided by the sample gas. Even if there is no oxygen in the sample gas, there is enough oxygen inside the sensor to last for some time.

For most sensors, the reference electrode also requires a small amount of oxygen. As a result, problems will be caused if the sensor works continuously in an oxygen-free environment.

17. Why is my sensor indicator below the specification data?

There are many reasons for the question, mainly because the following:
● Different gas flow rate
● Additional diffusion barriers, such as explosion-proof mesh or PTFE dust-proof membrane, are placed in front of the sensor, especially if a large dead volume is formed between them and the sensor
● The adsorption error of pipeline and pressure reducing valve is easy to cause when the measured gas has strong adhesion, such as HCL, CL2, HF, etc.
● Accuracy of cylinder standard gases or so-called calibration gases.

18. Does temperature affect the sensor indicator?

Electrochemical gas sensors are sensitive to ambient temperature. Both sensitivity (nA/ PPM) and zero current (equivalent PPM or nA) vary with temperature. When setting up the software calibration, please keep in mind that the sensor's technical data sheet has given the temperature dependence from -20℃ to +50℃. SemeaTech is implementing an ongoing test plan, so if there is no data in the product data sheet, please feel free to contact us. In some cases we have the data but haven't updated the datasheet yet.

19. About cross interference data

SemeaTech sensors have high selectivity and little interference with other reactive gases. Results of cross interference tests for common gases are listed in the product data sheet of each sensor. SemeaTech is implementing an ongoing test plan so if the data you need is not found in the product data sheet, please feel free to contact us. In some cases we have done relevant tests, but have not posted or update the datasheet yet.

20. What is the recommended storage period?

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 0℃ and 20℃. Do not store in the area 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.

21. Uncertainty of cross-sensitivity data

The cross-sensitivity data in our datasheet are based on tests performed on a small number of sensors, which are designed to indicate the sensor’s response to other gases rather than the target gas. There may be different values under different environmental conditions, and different batches of sensors may differ by 50% from the values listed in the datasheet.

22. What if the gas temperature is different from the sensor temperature?

Although zero output current of an electrochemical gas sensor varies based on the ambient temperature, the temperature of the measured gas has little influence on the sensor zero output current.

The signal output range of a sensor depends on the rate at which the gas molecule diffuses through the capillary hole to the sensing electrode. The temperature of the gas diffusing through the capillary hole is different from that of the gas inside the sensor, which may influence little on the sensitivity of the sensor. It may also cause tiny shifts or transient currents before equilibrium is fully established.

23. What substances may cause damages?

SemeaTech sensors can work under various environmental conditions. However, it is important to avoid exposure to high concentrations of solvent vapers during storage, installation and operation process.

Formaldehyde may temporarily inhibit the operation of NO sensors. Some organic solvents may produce high baselines that cause sensor errors. The PCB should be cleaned before installing the sensor. Do not glue directly onto or near the sensor, as solvents may cause cracks of the plastic.

24. How to prepare and use SemeaTech electrochemical sensors?

Zero Bias Sensor: The sensor is guaranteed to be operational before shipment by connecting the reference electrode and the sensing electrode with a wire (shorting spring), which is the same as the recommended J-FET running circuit. (A SemeaTech product using an auxiliary electrode also has a melt connecting the auxiliary electrode and the reference terminal). Remove the wire and place the sensor in the working circuit, it is all ready.

Bias Sensor: Do not connect the reference electrode to the sensing electrode with a wire for bias sensors because they will be permanently damaged. Some bias sensors, such as O3, C2H4O, NO, HCL, C2H3CL, THT and C2H4 sensors, can be connected to a special PCB to maintain the bias potential. This keeps the sensor in ready-to-work condition. If the sensor is not in the bias state, it will take longer to stabilize the baseline. We recommend to maintain the bias potential even if the instrument is turned off. If not, a long start-up time will be required when the instrument resets.

25. How long does the sensor need to be replaced?

The expected lifespan of SemeaTech electrochemical gas sensors can be found in the product data sheets. The sensitivity of the sensor will gradually decrease over time, which determines the operating life and replacement of the sensor. When the sensor in the instrument cannot be successfully calibrated, it needs to be replaced.

26. About interfering gases

Every gas sensor shall be calibrated periodically with its target gas to ensure the maximum accuracy. It is best to calibrate with a mixture of gases close to the test concentration. Do not use gases beyond the sensor's measurement range to avoid the inaccuracy during the detection.

Before shipment, SemeaTech sensors have been tested in detail using the target gases to ensure conformities with the specs. The cross-sensitivity data in the data sheet are based on a few batches of sensors so it may vary widely from batch to batch in manufacturing. Therefore, it is strongly recommended to avoid the use of alternative gases (surrogate gasses) for calibration.

27. Can sensors be exposed to target gas for a long time?

SemeaTech lead-free oxygen sensors can be used for continuous detection of oxygen at a concentration of 0-30%. SemeaTech air quality monitoring (AQM) CO, NO2, SO2, O3, and H2S sensors can be used for continuous detection of the target gases causing environmental pollutions.

SemeaTech toxic gas sensors are designed for intermittent detection of target gases. They are generally not suitable for continuous detection applications, especially for those involving high gas concentrations with extreme humidity and temperature. Continuous detection can sometimes be achieved by circulating two (even three) sensors in and out of the airflow, so that each sensor is only exposed up to half the time, and recover for the other half in fresh air.

28. What is the difference between 3 electrode and 4 electrode gas sensors?

SemeaTech 4 electrode gas sensors are mainly used for air quality monitoring with PPB level resolutions while measuring the gases causing the environmental pollution. In addition to the working, reference and counter electrode, the 4 electrode gas sensors have a fourth electrode named Auxiliary, that maintains the stability of the sensor zero current and reduces the background noises.  

29. What is the sensor housing made of?

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.

30. Are SemeaTech sensors intrinsically safe?

Although there is no intrinsic safety certification required, SemeaTech electrochemical gas sensors and the instruments used such sensors can still easily meet intrinsic safety requirements. Please contact SemeaTech for further assistance in obtaining intrinsic safe approvals.

31. How to test circuits?

SemeaTech electrochemical sensor is designed to operate in a special circuit called a potentiostat. The purpose of the circuit is to control the potential of the working electrode relative to the reference electrode and to amplify the current flowing in or out of the reference electrode. The potentiostat circuit can be easily checked by:
1. Remove the sensor;
2. Short-circuit the reference electrode to the counter electrode;
3. Measure the electric potential of the working electrode when the reference electrode is short-circuited to the counter electrode. For zero bias sensors, the measured value should be zero (±1mV). For bias sensors, the measured value should be the recommended bias voltage;
4. Connect a current source between the working electrode, reference electrode and opposite electrode to verify if the voltage output can meet expectation.
In most cases, the above steps are enough to confirm if the circuit is working properly. Now you can also reinstall the sensor and stabilize it. Measure the voltage between the working electrode and the reference electrode. For zero bias sensors, the voltage should be zero again. For bias sensors, it should be the recommended bias voltage.

32. Will the sensor response faster with pump-suction air supply?

The use of pump-suction air supply will not increase the response rate of the sensor itself, but it does enable the measured gas to pass through the sensor quickly and efficiently. As a result, the pump affects the overall response time of the entire instrument.

33. Can I place a membrane or filter in front of the sensor?

A membrane or filter can be placed in front of the sensor to provide additional protection, but it should be noticed that no dead corner occurs that will increase the sensor response time.

34. What factors should be considered when designing a sampling system?

In the design of sampling system, it is very important to use materials that can prevent adsorption of the measured gas. The best materials are fluoropolymers such as PTFE, TFE and FEP. Condensation of water in the airflow can lead to clogging and overflow, so it is also important to use a proper water collector to remove the condensed water. Alternatively, water can be removed in a gas phase by using a Nafion tube.

For high temperature gases, the airflow should be cooled to the sensor's temperature operating range, and appropriate filters should be used to remove any particles from the gas sample. Additional on-line chemical filters can also be integrated into any sampling system design to eliminate any effects of cross-interfering gases.

35. The traceability of SemeaTech sensors

SemeaTech has passed ISO 9001 certification. Each sensor has its unique serial number that can be fully traced with production batch, manufacturing date and other information.