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The test readings of electrochemical flue gas analyzers are “positively correlated” with the flow rate of the gas being sampled. Any analyzer that cannot ensure that the on-site gas flow rate equals the calibration flow rate in the laboratory cannot guarantee the accuracy of on-site tests! Unfortunately, most such instruments, both domestically and internationally, are unable to do this. Flue gas analyzers designed using the \"potentiostatic electrolysis method\" (electrochemical sensors), whether they are domestic or imported instruments, often encounter the problem of inaccurate measurements during use; that is, tests with standard gases in the laboratory yield good results, but accurate readings cannot be obtained on-site. And this issue is difficult to detect in actual use. Because the concentration of the reference gas in the laboratory is known, while the concentration of a certain gas to be measured in the flue gas at the site is unknown. Without other methods for comparison, it is impossible to determine whether the measured value is accurate, too high, or too low. Based on comparisons from on-site tracking tests and theoretical analysis, there are two main factors that affect the accuracy of on-site testing: 1) Positive pressure in the flue increases the airflow rate, while negative pressure in the flue reduces it; 2) Dust accumulation reduces the airflow rate. Both of these factors are directly related to the airflow rate measured by the instruments at the site. These two factors that affect testing are difficult to simulate in the laboratory: the laboratory provides gas at positive pressure (without a gas bag transition) or at zero pressure (with a gas bag transition), whereas the smoke in the flue is either at positive pressure or at negative pressure ； And it often fluctuates in size. In extreme cases, some flues also have a high negative pressure (for example, the negative pressure at the sintering machine head of Baosteel = 20 kPa). The standard gases in the laboratory are pure and free of dust, whereas the flue gas at the site often contains dust at varying concentrations, which to some extent clogs the dust-filtering materials and increases the resistance in the gas collection pipelines. **Standard HJ/T 57-2000 issued by the Environmental Protection Administration emphasizes that: \"Changes in the gas flow rate have a direct impact on the test readings of the instruments.\" **The \"Technical Specifications for Environmental Protection Acceptance Monitoring of Completed Construction Projects in the Thermal Power Generation Industry\" issued by the National Environmental Monitoring Center also state that: \"Monitoring instruments using the potentiostatic electrolysis method have very strict requirements regarding the sampling flow rate; the displayed monitoring data is proportional to changes in the sampling flow rate. When the sampling flow rate of the instrument decreases (for example, when the negative pressure in the flue is greater than the instrument’s ability to withstand such pressure), the monitoring data decreases significantly.\" To reduce measurement errors during use, the operating flow rate of the instrument should be equal to the flow rate used during calibration.” Why does a change in the gas flow rate directly affect the instrument’s test readings? Since electrochemical sensors are extremely sensitive to changes in flow rate, electrochemical flue gas analyzers draw flue gas from the flue through a sampling gun; after condensation and dehydration, the gas flows at a constant speed through each electrochemical sensor. Sensors convert the concentrations of the various gas components detected into electrical signals of corresponding amplitudes. Taking sulfur dioxide as an example: sulfur dioxide in the flue gas diffuses through the sensor’s membrane and enters the electrolytic cell, where it undergoes an oxidation reaction at the potentiostatic working electrode. This results in the generation of a limiting diffusion current i. Within a certain range, the magnitude of this current is proportional to the concentration of sulfur dioxide. That is, under specified operating conditions, the number of electron transfers Z, Faraday’s constant F, the diffusion area S, the diffusion coefficient D, and the thickness of the diffusion layer δ are all constants; therefore, the concentration of sulfur dioxide C can be determined from the limiting current i. The limit current i is amplified and converted from analog to digital before being sent to a microcomputer for processing. It should be noted that the electron transfer number Z is in a \"positive correlation\" with the flow rate V passing through the sensor; therefore, the magnitude of the limiting current is also in a \"positive correlation\" with the flow rate V. **Standard HJ/T57-2000 issued by the Environmental Protection Administration emphasizes that: \"Changes in the gas extraction flow rate directly affect the test readings of the instrument.\" In the laboratory, a standard gas with a known concentration C is pumped into the sensor at a constant flow rate V; the output current I1 is measured, and the proportionality constant k is calculated using the following formula and stored within the instrument. At the site, flue gas of unknown concentration is pumped to the sensor at an equal flow rate V in the laboratory, and the output current I2 is measured. Then the flue gas concentration is given by: x = k*I2. The single-board computer collects, calculates, and displays the concentration values at predetermined time intervals (usually 1 second), and then calculates and stores the average value as well as related data based on the ‘sampling period’ set by the user. For this reason, after the instrument is calibrated in the laboratory, the following three situations may occur during field testing: positive pressure in the flue, which increases the flow rate of the gas being sampled and results in higher readings; negative pressure in the flue, which reduces the flow rate of the gas being sampled and leads to lower readings; or dust clogging the filtering material, thereby increasing the resistance in the sampling pipeline and also resulting in lower readings. How can these practical conditions, which are difficult to simulate in the laboratory, be addressed? In response to the common issue of negative pressure in domestic testing environments, many foreign instruments have increased the power of their vacuum pumps, and their promotional materials highlight features such as \"high-power pumps\" and \"resistance to negative pressure of xx kPa\". For example, a German company emphasizes this “feature” repeatedly in its newly launched series of flue gas analyzers: the product description states that it features “a high-power sampling gas pump that can operate normally in high-negative-pressure environments”. The product 2 description emphasizes: “Equipped with a high-power sampling gas pump, it is capable of sampling flue gas under high negative pressure.” The product 3 description states: \"It features strong suction power, with two high-powered air pumps built in; it can achieve an extreme vacuum level of -60 kPa and can still operate normally when the negative pressure in the flue is at -15 kPa.\" It also has a rapid response time, as the high-power exhaust pumps ensure that the sensors are in full contact with the smoke, thereby increasing the reaction speed. Domestic similar instruments have also followed suit. In some tender notices for flue gas analyzers, some also provide special descriptions. For example, one bidder wrote: “...Main specifications for the smoke analyzer: It must be equipped with a high-performance DC vacuum pump (capable of achieving 0.7 liters per minute even when the resistance is 20 kPa)...” It is clear that the purchaser is already aware of the issue related to resistance to negative pressure. What is not known is that in the field of smoke detection, “negative pressure resistance of xx kPa” is a misleading parameter! Increasing the pump’s suction power ensures that smoke can be drawn out even at a \"xx kPa negative pressure,\" but the flow rate of the extracted air is **lower than the flow rate measured in the laboratory under identical conditions!\" This measure can only prevent the inability to exhale; it still does not resolve the issue of \"lower negative pressure reducing the airflow rate.\" Therefore, whether it is a high-power pump or a low-power pump, as long as there is negative pressure in the flue, the measurement value will definitely be low. In fact, the flow rate of the gas being sampled doesn’t need to be high or low; what’s important is that the flow rate on site is equal to the flow rate calibrated in the laboratory. In other words, as long as the flow rate of the gas sampled on site is not equal to the value calibrated in the laboratory, the test results will definitely be inaccurate.