The interval between initial calibration and recalibration depends on multiple factors, including the sensor’s operating temperature, humidity, pressure conditions, the types of gases it is exposed to, and the duration of exposure.
The degree of cross-interference variation can be quite substantial. This is evaluated based on tests of a limited number of sensors, which measure the sensors’ responses to non-target gases rather than the target gases themselves. It is important to note that when environmental conditions change, the sensor’s performance may differ, and cross-interference values may vary by up to 50% between different batches of sensors. Therefore, in practical applications, these variables should be fully considered for the sensor’s accuracy and reliability.
Using a pump does not speed up the sensor’s own reaction rate, but it can rapidly and efficiently draw gas samples through the sensor from inaccessible locations. This allows the pump to influence the overall response time of the device.
A film or filter can be placed in front of the sensor for protection, but it must ensure that no "dead space" is created, which could prolong the sensor’s response time.
When designing a sample system, it is critical to use materials that prevent gas adsorption on the system’s surfaces. The best materials include polymers, PTFE, TFE, and FEP. Gas concentration may cause moisture condensation, which can block the sensor or lead to overflow, so appropriate dehydrators should be used—such as Nafion tubing to remove moisture at the condensation stage. For high-temperature gases, the sample gas should be cooled to meet the sensor’s temperature requirements, and appropriate filters should be used to remove particulate matter. Additionally, axial chemical filters can be installed in the sample system to eliminate cross-interference from gases.
The sensor's own temperature determines its minimum display current, and the temperature of the measured gas sample has a certain influence on this. The rate at which gas molecules enter the sensing electrode through the pores determines the sensor's signal. If the temperature of the gas diffusing through the pores differs from the temperature of the gas inside the sensor, it may affect the sensor's sensitivity to some extent. Slight drift or momentary current changes may occur before the device is fully set up.
Oxygen sensors can continuously monitor oxygen concentrations within a range of 0-30% by volume or partial pressures within a range of 0-100% by volume. Toxic gas sensors are typically used for intermittent monitoring of target gases and are not suitable for continuous monitoring, especially in environments with high concentrations, high humidity, or high temperatures. To achieve continuous monitoring, a method of cycling two (or even three) sensors is sometimes used, allowing each sensor to be exposed to the gas for a maximum of half the time and recover in fresh air for the other half.
We use different plastic materials considering compatibility with the internal electrode system and application durability requirements. Commonly used materials include ABS, polycarbonate fiber, or polypropylene. More detailed information can be found in the data sheet of each sensor.
Although there is no certificate proving its intrinsic safety, the product can stably meet the requirements for internal safety.
Three-electrode and four-electrode sensors are suitable for use in a special circuit called a Potentiostat. The purpose of this circuit is to control the potential of the sensing (and auxiliary) electrode relative to the counter electrode while amplifying the current flowing in or out. The circuit can be tested using the following simple method:
• Remove the sensor.
• Connect the counter terminal to its corresponding terminal with the circuit.
• Measure the potential of the sensing (and auxiliary) terminal. For a non-biased sensor, the test result should be 0 (±1mV), which is equivalent to the recommended offset voltage for a biased sensor.
• Connect the sensing (or auxiliary) terminal with the circuit to obtain the output voltage.
The above steps can confirm that the circuit is operating normally in most cases. After replacing and re-fixing the sensor, the voltage between the sensing and reference terminals of a non-biased sensor should still be zero, or equivalent to the recommended offset voltage of a biased sensor.
In most cases, the above steps can confirm that the circuit is operating normally. After replacing and re-fixing the sensor, the voltage between the sensing and reference electrodes of a non-biased sensor should be close to zero, or equivalent to the recommended offset voltage of a biased sensor.
Generally, Sensors cannot be cleaned in a typical cleaning system without causing irreversible damage or affecting their monitoring performance. High pressure and temperature will damage their sealing, and active chemicals such as ethylene oxide and hydrogen peroxide may destroy the electrocatalyst.
In terms of mechanism, low temperature generally is not a major issue. The liquid electrolyte in all sensors (except oxygen sensors) does not freeze until the temperature drops to around -70°C. However, long-term exposure to excessively low temperatures may affect the fixing of the plastic housing on the bracket.
For oxygen sensors, although high salt content means they may not be damaged immediately, the oxygen sensor electrolyte freezes at approximately -25 to -30°C, which may eventually lead to sensor failure.
Temperatures exceeding the upper limit will put pressure on the sensor's seal, eventually leading to electrolyte leakage. The plastics used to manufacture most sensor models become soft when the temperature exceeds 70°C, rapidly causing sensor failure.
All sensors use similar sealing systems, where the hydrophobic properties of PTFE materials prevent liquid from flowing out of the sensor (even with air holes). If the pressure applied to the sensor inlet suddenly increases or decreases beyond the allowable internal limits, the sensor's membrane and seal may deform, causing leakage. If the pressure changes slowly enough, the sensor may operate beyond the pressure tolerance, but consult technical support for advice.
Sensors stored in their original packaging typically do not deteriorate significantly even beyond the shelf life. For long-term storage, we recommend avoiding hot environments, such as windows exposed to direct sunlight.
If sensors are removed from their original packaging, keep them in a clean place and avoid contact with solvents or heavy smoke, as smoke may be absorbed into the electrodes, leading to operational issues. Oxygen sensors are an exception: once installed, they begin to be consumed. Therefore, they are transported or stored in sealed packages at reduced oxygen levels during unloading.
Two-electrode sensors, such as oxygen sensors and two-electrode carbon monoxide sensors, generate electrical signals through chemical reactions and do not require an external power source. Three-electrode and four-electrode sensors, however, must use a potentiostatic circuit and therefore require a power supply. In fact, the sensor itself still does not need power because it directly produces output current through oxidation or reduction of the target gas, but the circuit amplifier consumes some current—though this can be reduced to very low levels if necessary.
Some sensors have built-in chemical filters to remove specific gases and reduce cross-interference signals. Since the filter is placed behind the diffusion grid, and gas entry through the grid is much less likely than through the main gas channel, small amounts of chemical media can last a long time.
In general, the filter and sensor have a comparable expected lifespan for the required application, but in harsh conditions (e.g., emission monitoring), this may be challenging. For such applications, we recommend sensors with replaceable built-in filters, such as the Series 5 sensors.
For some pollutants, the filter does not remove them through chemical reactions but by adsorption, making it easy for the filter to be overwhelmed by high concentrations—organic vapors are a typical example.
The "maximum load" specifically refers to whether the sensor can maintain a linear response and recover quickly after being exposed to the target gas for more than 10 minutes. As the load increases, the sensor will gradually exhibit non-linear responses and require longer recovery times, as the sensing electrode cannot consume all diffused gas.
With increased load, gas accumulates inside the sensor and diffuses into internal spaces, potentially reacting with the counter electrode and altering the potential. In this case, the sensor may take a long time (days) to recover even when placed in clean air.
Another role of the circuit design is to ensure the sensor recovers as quickly as possible from high loads, as the amplifier in the circuit does not cause current or voltage saturation during signal generation. If the amplifier does limit current into the sensor, this will restrict the rate at which the sensing electrode consumes gas, immediately causing gas buildup inside the sensor and the potential changes described above.
Finally, select a resistor connected to the sensing electrode to ensure that even with sudden voltage drops at the foreseeable highest gas concentration, the change does not exceed a few millivolts. Allowing larger voltage drops across the resistor could cause similar changes in the sensing electrode, requiring recovery time after the gas is removed.
Sensors that generate output by oxidizing the target gas (e.g., carbon monoxide sensors) require oxygen at the counter electrode to balance the oxygen consumed by the oxidation reaction. Typically, a maximum of several thousand ppm of oxygen is needed, which is provided by the oxygen in the sample gas. Even if the sample gas is oxygen-free, the sensor has sufficient internal oxygen supply for short periods.
For most sensors, the counter electrode also requires a small amount of oxygen. If the sensor continuously operates in an oxygen-free environment, it will eventually produce erroneous readings.
There are many reasons for discrepancies in customer measurements, making it crucial to design equipment based on the sensor’s allowable calibration range and the natural decline in output capacity over its service life. Some causes we have identified include:
· Using different flow rates
· Placing additional diffusion grids (e.g., flame arrestors or PTFE membranes) in front of the sensor, especially if there is a large dead space between the grid and the sensor
· "Sticking" gases with absorbent tubing or brass calibrators (e.g., gas cylinders contaminated by chlorine; nitrogen cylinders degraded by oxygen ingress)
· Using cylinders outside the manufacturer-recommended minimum pressure
· Using "air" cylinders with diluted mixtures
· Failing to properly dampen pressure fluctuations in the sample system
· The design of the test device significantly affecting the measurement signal of combustible gas sensors
Sensors are typically connected to equipment via PCB connectors. Some sensors use alternative connections (e.g., data ports or specific connectors); refer to the relevant product data sheets for details.
For sensors connected via PCB connectors, do not directly solder the PCB connector to the equipment. Direct soldering may cause damage to the product housing and invisible internal damage.
Temperature data is available for most products and is specified in each product data sheet.
The maximum recommended shelf life for sensors is six months. During this period, sensors should be stored in a clean, dry container at 0°C to 20°C, not in environments with organic solvents or flammable liquids. Under these conditions, sensors may be stored for up to six months without reducing their expected service life.
The minimum flow rate requirement for sensors is comprehensively determined by design principles, medium characteristics, measurement accuracy, and practical application needs. When selecting and using sensors, users should choose appropriate sensor types and flow rate ranges based on specific application scenarios and measurement requirements.
Electrochemical sensors can be used in various environments, including some harsh conditions, but must be kept from exposure to high concentrations of solvent vapors during storage, installation, and operation.
Formaldehyde is known to disable nitric oxide sensors within a short period, while other solvents can cause erroneously high baselines. When using printed circuit board (PCB) sensors, install other components sparingly before mounting the sensor. Do not use glue or operate near electrochemical sensors, as such solvents may cause plastic cracking.
Catalytic bead sensors
Certain substances can poison catalytic bead sensors and should be kept away from the sensor. The failure mechanism may involve:
· Toxicity: Some compounds decompose on the catalyst and form a stable barrier on its surface. Prolonged exposure causes irreversible loss of sensor sensitivity. The most common substances include lead, sulfides, silicon, and phosphates.
Point 24. Reaction Inhibition
Other compounds, particularly hydrogen sulfide and halogenated hydrocarbons, can be absorbed by the catalyst or form new compounds upon absorption. This absorption is so strong that it blocks reaction sites, causing normal reactions to be inhibited. However, this loss of sensitivity is temporary—sensitivity will recover after the sensor operates in clean air for a period.
Most compounds fall more or less into one of the above categories. If any such compounds may be present in practical applications, the sensor should not be exposed to compounds it is not resistant to.
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