Poisonous and harmful gas detection principle and classification

For various types of production sites and testing requirements, selecting the right gas detector is a must for everyone involved in safety and hygiene. Here we will introduce some specific situations for your reference.

Confirm the gas type and concentration range to be detected:

The gas types encountered by each production department are different. When selecting a gas detector, all possible conditions must be taken into consideration. If methane and other less toxic alkanes are present, selecting the LEL detector is undoubtedly the most appropriate. This is not only because of the simple principle and wide application of the LEL detector, but also its convenient maintenance and calibration features. If there are toxic gases such as carbon monoxide and hydrogen sulfide, a specific gas detector must be selected to ensure the safety of workers. If there are more organic toxic and harmful gases, considering the low concentrations that may cause human poisoning, such as aromatic hydrocarbons, halogenated hydrocarbons, ammonia (amines), ethers, alcohols, fats, etc., you should choose the light that was introduced in the previous chapter. Do not use an LEL detector to ionize the detector, as this may result in personal injury or death.

Determine the use of the occasion:

The type of gas detector is different depending on the industrial environment.

A) Fixed Gas Detection Protocol:

This is a detector that is used more often in industrial installations and production processes. It can be installed at specific detection points to detect specific gas leaks. The fixed detector is generally a two-body type. A sensor and a detection head composed of a transmission are integrally installed at the inspection site. Secondary instruments including a circuit, a power supply, and an alarm display device are integrally installed in a safe place to facilitate monitoring. Its detection principle is the same as described in the previous section, but it is more suitable for the continuous and long-time stability required by the fixed test in terms of technology and technology. They also need to be selected according to the type and concentration of gas in the site, and they should also be careful to install them on the site where the specific gas is most likely to leak, for example, to select the most effective height of the sensor installation according to the specific gravity of the gas.

B) Portable Gas Detector:

Because the portable instrument is easy to operate and small in size, it can be carried to different production sites. The electrochemical detector is powered by alkaline batteries and can be used continuously for 1,000 hours. The new LEL detector, PID and compound instruments use rechargeable batteries (some The use of non-memory helium-hydrogen or helium-ion batteries makes them generally capable of continuous operation for nearly 12 hours. Therefore, the use of such instruments in various types of factories and health departments is becoming more and more widespread.

If it is in an open place, such as an open work shop using such an instrument as a safety alarm, a diffuse gas detector can be worn on the fly, because it can continuously, real-timely and accurately display the concentration of toxic and harmful gases on site. Some of these new instruments are also equipped with vibration alarm attachments to avoid audible alarms in noisy environments, and computer chips are installed to record peaks, STELs (short-term exposure levels for 15 minutes) and TWA (average of 8-hour statistical weights). ) Provide specific guidance for worker health and safety.

If you enter a confined space, such as a reaction tank, storage tank or container, sewer or other underground pipelines, underground facilities, closed agricultural granaries, railway tankers, shipping cargo tanks, tunnels, etc., the personnel must be tested before entering. , but also to test outside the confined space. In this case, a multigas detector with a built-in sampling pump must be selected. Because the gas distribution and gas types in different parts of the confined space (upper, middle, and lower) are quite different. For example, the proportion of combustible gases in the general sense is light. Most of them are distributed in confined spaces. The proportion of carbon monoxide and air in the confined space is almost the same. Generally, they are distributed in the confined space, while heavier gases such as hydrogen sulfide exist in the confined space. The lower part of the space (as shown). At the same time, oxygen concentration is also one of the types that must be detected. In addition, a detector that can detect organic gases is also needed if the possible volatilization and leakage of organic substances in the tank are taken into consideration. Therefore, a complete closed-space gas detector should have a built-in pumping function so that it can detect non-contact, sub-sites with multiple gas detection functions to detect hazardous gases in different spatial distributions, including inorganic gases and organic gases. Prevents oxygen-deficient or helium-rich instruments that do not affect worker's work. Only in this way can the absolute safety of workers entering the confined space be guaranteed.

In addition, after entering the confined space, it is necessary to continuously test the gas components in order to avoid changes in concentration of volatile organic compounds or other toxic and harmful gases caused by personnel entry, sudden leaks, and temperature changes.

If it is used for emergency accidents, leak detection and patrols, instruments with pumping, short response time, high sensitivity, and high resolution should be used. This can easily determine the location of the leak. In the case of industrial hygiene tests and health surveys, instruments with data recording and statistical calculations and functions that can be connected to computers are very convenient to use.

At present, with the development of manufacturing technology, portable multi-gas (compound) detectors are also a new choice for us. Since this detector can be equipped with multiple gas (inorganic/organic) detection sensors on a host computer, it has the characteristics of small size, light weight, corresponding rapidity, and multiple gas concentration display at the same time. More importantly, the price of pumped composite gas detectors is lower than that of multiple single diffusion gas detectors and is more convenient to use. It should be noted that when selecting such detectors, it is best to select an instrument that has the ability to switch each sensor individually to prevent the use of one sensor from affecting other sensors. At the same time, in order to avoid clogging of the suction pump due to water ingress, it is also safer to select a smart pump design with a pump-off alarm.

I. Introduction of toxic and harmful gases commonly used in the production process The factors that can cause harm to the health and life of property and people in the production process can be broadly divided into three aspects: physics, chemistry and biology. Among them, chemical factors have the greatest harm. The toxic and harmful gases are the most common and most common part of chemical factors. Therefore, this section focuses on the knowledge of toxic and harmful gases.

According to hazards, we classify poisonous and harmful gases into combustible gases and toxic gases. Toxic gases are divided into three categories, irritation gas, suffocating gas and acute poisoning organic gas, according to their different mechanisms of action on the human body.

Among them, irritant gases include chlorine, phosgene, diphosgene, sulfur dioxide, nitrogen oxides, formaldehyde, ammonia, and ozone. The action of irritating gases on the body is characterized by a strong irritant effect on the skin and mucous membranes, some of which also have a strong corrosive effect. The degree of damage to the body by irritating gases is related to its solubility in the water and the site of action. In general, water-soluble chemicals, such as chlorine, ammonia, and sulfur dioxide, rapidly stimulate the eyes and upper respiratory tract, causing eye and upper respiratory tract irritation; and water-soluble chemicals such as light. Gas, nitrogen dioxide, and other effects on the lower respiratory tract and alveoli are more obvious. The severity of lesions caused by irritant gases, in addition to the nature of the chemical itself, is most closely related to the concentration and timing of exposure to chemicals. Short-term exposure to high concentrations of irritating gases can cause severe acute poisoning, while chronic exposure to low concentrations can cause chronic injury. Acute irritant gas poisoning usually occurs first in the eye and upper respiratory tract irritation symptoms such as conjunctival hyperemia, tearing, runny nose, throat dryness, cough, chest tightness and other symptoms, and then these symptoms can be reduced or disappeared, after a few hours to 3 days After the incubation period, the symptoms reappear suddenly and aggravate rapidly. In severe cases, chemical bronchial pneumonia and pulmonary edema may occur. The symptoms include severe coughing, slightly white or pink foam, dyspnea, and cyanosis. The symptoms may be due to pulmonary edema or acute Respiratory distress, etc. lead to disability.

Asphyxiating gases include gases such as carbon monoxide, hydrogen sulfide, hydrogen cyanide, and carbon dioxide. The hypoxia of the tissue cells resulting from the entry of these compounds into the body varies. After carbon monoxide enters the body, it mainly binds to the hemoglobin of the red blood cells to form carboxyhemoglobin so that the red blood cells lose their ability to carry oxygen, and thus the tissue cells do not get enough oxygen. After the hydrogen cyanide enters the body, the cyanide ion directly acts on the cytochrome oxidase, which causes it to lose its ability to transfer electrons. As a result, the cells cannot take up and utilize oxygen and cause intracellular suffocation. Methane itself has no obvious poisoning to the organism, and the hypoxia caused by the histiocytes is actually an anoxic asphyxia due to the decrease of oxygen concentration in the inhalation gas. The role of hydrogen sulfide in the body is numerous. Hydrogen sulfide binds to ferric iron in oxidized cytochrome oxidase and inhibits the activity of cellular respiratory enzymes, leading to the hypoxia-induced hydrogen sulfide in tissue cells that binds to the thiol group of glutathione, inactivates glutathione and aggravates it. Hypoxia of tissue cells In addition, high concentrations of hydrogen sulfide through a strong stimulation of olfactory nerves, respiratory mucosal nerves, and carotid sinus and aortic body chemokines, leading to respiratory paralysis, and even sudden death.

The organic solvents for acute poisoning include n-hexane and methylene chloride. The above organic volatile compounds, like the above inorganic toxic gases, may also cause harm to the human respiratory system and nervous system, and some may cause cancer, such as benzene. Since organic compounds are mostly flammable substances, most of them have been tested for explosiveness before detection of organic compounds, but the lowest explosion limit of organic compounds is far greater than its MAC (maximum allowable concentration in space) value. In other words, the detection of the toxicity of organic compounds is necessary and necessary.

The hazards of flammable gases are mainly the explosion caused by gas combustion, thus causing damage to property and human life. However, the explosion of flammable gas must meet certain conditions. A certain amount of combustible gas, enough oxygen, and an ignition source. The above three conditions are indispensable. The concentration of a gas that causes a combustible gas to explode is usually referred to as the lowest explosion limit, which is generally expressed as LEL. Different combustible gases have different LELs. Therefore, the detection of combustible gas generally detects its LEL.

Second, the detection principle and classification of toxic and harmful gases The key component of the gas detector is the sensor. Gas sensors can be divided into three categories from the principle:

A) Gas sensors using physicochemical properties such as semiconductors, catalytic combustion, solid heat conduction, photoionization, etc.

B) Gas sensors using physical properties: such as thermal conductivity, light interference, infrared absorption, etc.

C) Gas sensors using electrochemical properties: current type, potential type, etc.

The following will introduce several commonly used detectors for toxic and harmful gas detection to introduce their principles.

For the detection of common combustible LELs, catalytic combustion detectors are now generally used. Its principle is as follows: The core of the sensor is a Wheatstone bridge. One of the bridges has a catalyst. When it comes into contact with combustible gas, the combustible gas burns on the bridge with the catalyst. The resistance of the bridge arm changes. The resistance of the remaining bridge arms does not change, causing the output of the entire circuit to change. This change is proportional to the concentration of the flammable gas, thus enabling the detection of flammable gases. It can be seen from the above principle that the combustible gas is detected by this method, which is based on catalytic combustion, so its resolution is low. The resolution of this method is generally 1% LEL, about 100PPm. Therefore, this method cannot be used for the detection of organic gas toxicity.

For the detection of common toxic gases, especially inorganic toxic gases, special sensors are generally used for detection. Determined and quantitatively tested. Most of these sensors are electrochemical sensors. Electrochemical sensors are generally in the form of three electrodes. The target gas reacts on the working electrode, the generated current forms a loop through the counter electrode, and the reference electrode provides a suitable bias value for the working electrode. The sensor reacts selectively with the catalyst of the reference electrode and the working electrode, that is, a qualitative reaction. The current produced by the circuit is proportional to the concentration of the gas, enabling a quantitative reaction. The general oxygen sensor is a two-electrode sensor. His detection principle is similar to that of the three electrodes, except that the output of a three-electrode sensor is more stable and has a longer life.

For the detection of the toxicity of organic volatile gases, the method of detecting tubes was generally used in the past. However, since the types of detection tubes are limited, the accuracy is not high, and the operation is troublesome, the actual application is affected. At present, the most advanced detection method in the world is the photoionization detection method. Its principle is that a target gas is ionized by an ultraviolet lamp, and ions collect and form a current through a sensor, and the current is proportional to the concentration of the target gas, thereby realizing the Since quantitative detection of organic volatile gases is based on ion-level detection, this method has high resolution and fast response time. The resolution of this method reaches 0.1PPm, up to 1PPb. In principle, it can be known that any organic matter that can be ionized can be detected by the instrument, and substances that cannot be ionized cannot be detected. Since the IE of common inorganic gases in most books is high, no interference is detected. Most organic gases can be ionized, so the detector is a broadband detector for organic volatile gases. The high accuracy, wide detection range, short response time, and easy operation make this instrument particularly suitable for applications in the fields of safety and industrial hygiene.

Principles and Applications of Poisonous and Harmful Gas Detection Electrochemical sensors Electrochemical sensors can detect all types of toxic contaminants that come into confined spaces and work during them. At present, the number of specific substance sensors having a small response to disturbance components is still not many, about 20 or so. Others are broadband pollutant sensors. Electrochemical sensors are characterized by their small size, low power consumption, good linearity and repeatability, and long life.

On the market, not only can a single gas detector be installed with a specific electrochemical sensor, but also a compound enclosed space detector containing oxygen, flammable and explosive gases, and one to three electrochemical sensors.

Specific gas electrochemical sensors include the following: Diffusion-type diaphragms that can penetrate gas but cannot penetrate liquids; Acidic electrolyte tanks (usually sulfuric acid or phosphoric acid); Sensing electrodes; Measuring electrodes; Reference electrodes (triode design Some sensors also include a filter that filters out interference components.

Sensor electrodes can catalyze some special reactions. With the different sensors, the substance to be tested will undergo an oxidation or reduction reaction on the electrode and generate a positive or negative potential difference with respect to the measurement electrode. The two-electrode system means that the potential of the measuring electrode must be kept constant, and in fact, because the reactions occurring at both electrodes will polarize the electrode, it also limits the concentration range that they can detect.

The three-electrode design is different. The instrument measures the potential change between the reference electrode and the sensor electrode. Since the reference electrode does not participate in the reaction, it maintains a constant potential. At this time, the change in potential is directly related to the change in concentration. . The current produced by the sensor is directly proportional to the gas concentration and has a wide linear measurement range.

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