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The second factor affecting gas sensor readings is ambient temperature and humidity. Most gas sensors are sensitive to changes in ambient temperature. This is because performance parameters of chemical reactions, electronic components, and inorganic or organic materials change with temperature. This ultimately leads to changes in the current and voltage output by the sensor.
What Are the Operating Temperature Ranges of Different Sensors?
Non-dispersive infrared sensors (NDIR):
NDIR sensors have a wide operating temperature range, from military-grade -55℃ to over 100℃, and are used to detect gases in outer space.
Catalytic combustion sensors (LEL):
LEL sensors have a wide operating temperature range, from -40℃ to 70℃, which is also the range required for China's fire certification.
Electrochemical sensors (EC):
Electrochemical sensors have a slightly narrower operating temperature range, generally from -20℃ to 55℃. Newer EC sensors can operate at -40℃. However, EC sensors cannot function in high-temperature environments for long periods due to the acidic or alkaline liquid inside them. In high-temperature environments, water will evaporate or increase rapidly, causing electrolyte loss or leakage. This ultimately causes the response time T90 to lengthen, the return-to-zero time to increase, sensitivity to decrease, or even a lack of response. For EC sensors, the most challenging conditions are high temperature and low humidity (HTLH) and high temperature and high humidity (HTHH).
Photoionization sensors (PID):
PID sensors have a narrow temperature range and do not perform as well as NDIR sensors. Generally, they are functional between -25℃ and 55℃.
Metal oxide semiconductor sensors (MOS):
MOS sensors have a wide temperature range, but temperature drift is quite significant. The magnitude of their sensitivity and temperature coefficient are comparable, making MOS sensors unsuitable for extreme temperature environments and limiting their use to standard room temperature conditions.
NDIR sensors have a wide operating temperature range, from military-grade -55℃ to over 100℃, and are used to detect gases in outer space.
Catalytic combustion sensors (LEL):
LEL sensors have a wide operating temperature range, from -40℃ to 70℃, which is also the range required for China's fire certification.
Electrochemical sensors (EC):
Electrochemical sensors have a slightly narrower operating temperature range, generally from -20℃ to 55℃. Newer EC sensors can operate at -40℃. However, EC sensors cannot function in high-temperature environments for long periods due to the acidic or alkaline liquid inside them. In high-temperature environments, water will evaporate or increase rapidly, causing electrolyte loss or leakage. This ultimately causes the response time T90 to lengthen, the return-to-zero time to increase, sensitivity to decrease, or even a lack of response. For EC sensors, the most challenging conditions are high temperature and low humidity (HTLH) and high temperature and high humidity (HTHH).
Photoionization sensors (PID):
PID sensors have a narrow temperature range and do not perform as well as NDIR sensors. Generally, they are functional between -25℃ and 55℃.
Metal oxide semiconductor sensors (MOS):
MOS sensors have a wide temperature range, but temperature drift is quite significant. The magnitude of their sensitivity and temperature coefficient are comparable, making MOS sensors unsuitable for extreme temperature environments and limiting their use to standard room temperature conditions.
How Does Ambient Temperature Affect Gas Concentration Measurement Results?
Non-dispersive infrared sensors (NDIR): Many factors affect the NDIR sensor signal's response to ambient temperature, including the light source spectrum, detector, filter temperature coefficient, op-amp temperature coefficient, resistor and capacitor temperature coefficients, and more. When these factors interact, NDIR temperature compensation becomes highly complex. This complexity is one reason NDIR sensors are expensive.
Catalytic combustion sensors (LEL): Let’s review the principle of LEL sensors: The catalytic bead is wound with platinum wire, and the resistance of the platinum wire changes with temperature. The higher the temperature, the greater the resistance. When the temperature of the platinum wire exceeds 400 degrees, the catalytic activity on the surface of the catalytic bead becomes significant. When the catalyst contacts methane, the methane begins to burn, transferring heat to the platinum wire, causing its resistance to increase.
With this principle in mind, the question becomes easier to answer: When the ambient temperature rises or falls, the temperature of the platinum wire in the catalytic bead also changes. Although catalytic beads are used in pairs to eliminate some temperature change effects, this is only effective at the sensor's zero point, and sensitivity will still vary with temperature.
Electrochemical sensors (EC): An oxidation-reduction reaction occurs inside EC sensors. For any chemical reaction, the reaction rate changes with temperature. The general rule is that lower temperatures reduce sensor sensitivity, while higher temperatures increase it. Among all electrochemical sensors, the galvanic oxygen sensor (oxygen cell) has the smallest temperature drift. From +20℃ to -20℃, its sensitivity decreases by only about 10%. The hydrogen sensor exhibits the largest temperature drift, with sensitivity decreasing by about 80% from +20℃ to -20℃.
Photoionization sensors (PID): PID temperature drift is primarily caused by the temperature drift of electronic components. The ionization energy of gases is nearly unaffected by temperature changes. These electronic components include op-amps, resistors, capacitors, and transformer coils. It should be noted that when the temperature drops below a certain threshold, the ultraviolet lamp (UV lamp) will not light up. More critically, the two components of the UV lamp—the lamp tube and the light-transmitting sheet—will crack due to differing thermal expansion coefficients. As a result, the UV lamp becomes unusable due to leakage.
Metal Oxide Semiconductor Sensors (MOS): MOS sensors exhibit significant temperature drift, which is one of the main reasons they are unsuitable for industrial safety and are only used in civilian environments. One might ask if temperature compensation can address the large temperature drift. Engineers know it is nearly impossible to make a sensor with a temperature drift of more than ±30% achieve an accuracy of ±3% at extreme temperatures.
In civilian environments, temperature fluctuations are not significant, typically ranging from 10℃ to 40℃, and high accuracy is not required. When only an alarm is required, MOS sensors are indeed a good, low-cost choice.
Catalytic combustion sensors (LEL): Let’s review the principle of LEL sensors: The catalytic bead is wound with platinum wire, and the resistance of the platinum wire changes with temperature. The higher the temperature, the greater the resistance. When the temperature of the platinum wire exceeds 400 degrees, the catalytic activity on the surface of the catalytic bead becomes significant. When the catalyst contacts methane, the methane begins to burn, transferring heat to the platinum wire, causing its resistance to increase.
With this principle in mind, the question becomes easier to answer: When the ambient temperature rises or falls, the temperature of the platinum wire in the catalytic bead also changes. Although catalytic beads are used in pairs to eliminate some temperature change effects, this is only effective at the sensor's zero point, and sensitivity will still vary with temperature.
Electrochemical sensors (EC): An oxidation-reduction reaction occurs inside EC sensors. For any chemical reaction, the reaction rate changes with temperature. The general rule is that lower temperatures reduce sensor sensitivity, while higher temperatures increase it. Among all electrochemical sensors, the galvanic oxygen sensor (oxygen cell) has the smallest temperature drift. From +20℃ to -20℃, its sensitivity decreases by only about 10%. The hydrogen sensor exhibits the largest temperature drift, with sensitivity decreasing by about 80% from +20℃ to -20℃.
Photoionization sensors (PID): PID temperature drift is primarily caused by the temperature drift of electronic components. The ionization energy of gases is nearly unaffected by temperature changes. These electronic components include op-amps, resistors, capacitors, and transformer coils. It should be noted that when the temperature drops below a certain threshold, the ultraviolet lamp (UV lamp) will not light up. More critically, the two components of the UV lamp—the lamp tube and the light-transmitting sheet—will crack due to differing thermal expansion coefficients. As a result, the UV lamp becomes unusable due to leakage.
Metal Oxide Semiconductor Sensors (MOS): MOS sensors exhibit significant temperature drift, which is one of the main reasons they are unsuitable for industrial safety and are only used in civilian environments. One might ask if temperature compensation can address the large temperature drift. Engineers know it is nearly impossible to make a sensor with a temperature drift of more than ±30% achieve an accuracy of ±3% at extreme temperatures.
In civilian environments, temperature fluctuations are not significant, typically ranging from 10℃ to 40℃, and high accuracy is not required. When only an alarm is required, MOS sensors are indeed a good, low-cost choice.
How can the influence of ambient temperature be eliminated?
Non-dispersive infrared sensors (NDIR): There are several ways to eliminate the influence of temperature on NDIR sensors, but the cost is relatively high. The first method is formula-based, the second involves table lookup, and the third uses constant temperature.
Catalytic combustion sensors (LEL): Temperature compensation is generally not required. LEL sensors account for temperature drift during the development process. From -20℃ to 50℃, the zero point and range point exhibit a temperature drift of 1%-3% LEL, which is acceptable to users.
Electrochemical sensors (EC): Temperature compensation is required and can generally be achieved using a quadratic or cubic equation. The independent variable is temperature, measured in ℃, and the dependent variable is the normalized temperature coefficient, measured in %.
Photoionization sensors (PID): Temperature compensation is required, similar to EC sensors, and can generally be achieved using a quadratic or cubic equation.
Metal oxide semiconductor sensors (MOS): Temperature compensation is necessary, but it should be noted that even with compensation, accuracy remains limited. Due to the inconsistency of MOS sensors, achieving accuracy requirements with a unified temperature compensation formula may be difficult. The best approach is to compensate MOS sensors individually, using a quadratic equation or a lookup table. However, both methods are relatively expensive.
Catalytic combustion sensors (LEL): Temperature compensation is generally not required. LEL sensors account for temperature drift during the development process. From -20℃ to 50℃, the zero point and range point exhibit a temperature drift of 1%-3% LEL, which is acceptable to users.
Electrochemical sensors (EC): Temperature compensation is required and can generally be achieved using a quadratic or cubic equation. The independent variable is temperature, measured in ℃, and the dependent variable is the normalized temperature coefficient, measured in %.
Photoionization sensors (PID): Temperature compensation is required, similar to EC sensors, and can generally be achieved using a quadratic or cubic equation.
Metal oxide semiconductor sensors (MOS): Temperature compensation is necessary, but it should be noted that even with compensation, accuracy remains limited. Due to the inconsistency of MOS sensors, achieving accuracy requirements with a unified temperature compensation formula may be difficult. The best approach is to compensate MOS sensors individually, using a quadratic equation or a lookup table. However, both methods are relatively expensive.
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