Factors Affecting the Readings of Gas Sensors: Gas Flow

Gas is a fluid, and its flow is caused by a pressure difference. The larger the pressure difference is, the higher the flow rate becomes. For gas sensors, what matters is not the total pipeline flow rate but the flow rate near the sensor’s air inlet. The cavity volume near the sensor’s air inlet is generally less than 10 ml. Therefore, the recommended flow rate is usually measured in ml/min.

When discussing flow, it's important to explain two ventilation modes: diffusion mode and pumping mode. Diffusion mode, as the name suggests, relies on the natural diffusion of air into the sensor. This static diffusion mode does not depend on flow rate but solely on natural diffusion. Pumping mode uses a gas pump to increase gas flow across the sensor surface, thereby improving response speed.
 

Non-Dispersive Infrared Sensor (NDIR): 200 ml/min to 2 L/min. The larger the flow is, the faster the response time becomes. A large flow rate is beneficial only for NDIR sensors; however, dust and water removal are necessary.
Catalytic Combustion Sensor (LEL): 200 ml/min to 1 L/min. The catalytic beads are sensitive to flow rate; however, a wind shield covers the beads, and powder metallurgy sintered sheets or multi-layer steel mesh sheets are placed outside the wind shield to block airflow.
Electrochemical Sensor (EC): 200 ml/min to 1 L/min. The air inlet of the electrochemical sensor regulates the flow rate, ensuring that the internal catalytic capacity far exceeds the amount entering the sensor. When the external airflow changes, the sensor reading remains relatively stable. However, it should be noted that a large flow rate can remove moisture from the EC sensor, potentially causing it to fail within 30 days.
Photoionization Sensor (PID): 200 ml/min to 1 L/min. PID sensors operate using pump suction mode. The advantage of pump suction mode is that it can remove volatile organic compounds (VOCs) from the measured gas. If VOCs accumulate on the UV lamp and electrode for an extended period, a layer of sludge can form, necessitating cleaning of the UV lamp and electrode.
Metal Oxide Semiconductor Sensor (MOS): 200 ml/min to 1 L/min. MOS sensors are typically heated, and a large flow rate can dissipate heat, reducing the temperature.
 

Generally speaking, the flow rate has little effect on the measurement results over a short period. If the flow rate is within the range of 200 ml/min to 1 L/min, the change in reading should be less than 3%. Of course, this change is not an outcome that can be achieved effortlessly. The structural design of the gas cap and air plate is quite specific.
 

Non-dispersive infrared sensor (NDIR): Generally, there is no need for any restrictions. NDIR sensors need to respond quickly, and the diffusion speed of the airflow entering the sensor should be maximized.
Catalytic combustion sensor (LEL): LEL sensors typically feature explosion-proof designs, and sintered sheets inherently block airflow. Because the sensitivity of the LEL sensor is relatively high, the effect of airflow on the catalytic combustion sensor can be nearly ignored.
Electrochemical sensor (EC): It is essential to control the impact of airflow on the EC sensor and ensure that the airflow is perpendicular to its normal direction. For non-adsorbent gases, the flow rate should be controlled within the range of 200 ml/min to 600 ml/min. For adsorbent gases, the flow rate must be controlled within the range of 500 ml/min to 1,000 ml/min. Never allow airflow to flow parallel to the sensor's normal direction; otherwise, the airflow will directly enter the sensor's diffusion hole, altering its sensitivity, reproducibility, and response time.
Photoionization sensor (PID): Similar to NDIR sensors, the diffusion speed of airflow entering the sensor should be maximized.
Metal oxide semiconductor sensor (MOS): General MOS sensors have a heating function, and airflow can dissipate heat, causing the operating temperature of the MOS to change and, consequently, altering the sensor's sensitivity. Therefore, it is essential to prevent large airflow from directly contacting the surface of the MOS sensor. Similar to the LEL sensor, using a sintered sheet is a good choice.
 

The gas sensor measures the concentration of gases. When gas is compressed, the relative concentration of the gas does not increase, but the absolute concentration does. This means that the number of gas molecules in a unit volume increases. Therefore, when the relative concentration of the gas remains constant, an increase in gas pressure will lead to a corresponding increase in the gas sensor reading. Before introducing the pressure range of the sensor, it is important to briefly explain the unit of gas pressure: the most common unit is 'atmospheric pressure,' which is equivalent to 0.1MPa, 100kPa, or 1,000hPa. This is the pressure exerted by a 10-meter high water column at the bottom of a container.
 

Non-dispersive infrared sensor (NDIR): The pressure range of NDIR sensors can be quite broad, ranging from 0 to 2 atmospheres. This range depends on the pressure resistance of both the light source and the detector. If the light source is a glass bulb, it is pressure-resistant. The infrared detector is sealed with a metal shell and an infrared filter, making it subject to pressure resistance concerns as well.
Catalytic combustion sensor (LEL): The LEL sensor is a physical sensor with a broad pressure range and can operate effectively between 0 and 2 atmospheres. However, note that the higher the gas pressure, the lower the concentration of combustible gas the LEL sensor can measure. If the sensor's output voltage exceeds the specified range, it may be damaged.
Electrochemical sensor (EC): Since the EC sensor contains liquid, its pressure range is relatively narrow, typically around 1 ± 0.2 atmospheres. If the pressure is too high, the sensor may experience gas or liquid leakage. If the pressure is too low, the electrolyte may "spill" out from the top of the sensor, leading to sensor failure.
Photoionization sensor (PID): The pressure range of PID sensors is typically narrow, around 1 ± 0.1 atmospheres. The narrow range is mainly due to the UV lamp being encapsulated in a glass tube and crystal, which are not pressure-resistant. Particularly at extreme temperatures, PID sensors are not pressure-resistant.
Metal Oxide Semiconductor Sensor (MOS): MOS sensors are solid-state devices with no internal liquids, making them more pressure-resistant. They can operate effectively between 0 and 2 atmospheres.
 

Regardless of the type of gas sensor, it measures the absolute concentration of the gas being measured. When pressure increases, physical and chemical changes occur, resulting in a higher reading.