Expansion of common fumigant gas concentration detection technology
This site2017-04-06 18:17:22
Commonly used fumigant gas concentration detection technology; space concentration detection is to ensure the success of fumigation treatment and prevent accidental poisoning; (a) thermal conductivity detector; thermal conductivity detector has been widely used in plant quarantine fumigation treatment; l The principle of the thermal conductivity detector; in the thermal conductivity detector, the detection sensor is a thermal conductivity cell; 2 the detection of the fumigant gas concentration; the detection process is divided into the following steps: preheating (before testing, open the instrument; (l) Instrument calibration; all instruments have been calibrated before leaving the factory; (2) Maintenance and maintenance; the instrument is in operation. Detection of spatial concentration is an important means to ensure the success of fumigation treatment and prevent accidental poisoning. It mainly includes two aspects: the detection of the space fumigant gas concentration during fumigation and the fumigation environment concentration after fumigation of scattered gas. The former is to detect the fumigant gas leakage and ensure the success of fumigant gas concentration detection, which belongs to high concentration detection. There are many types of instruments that can perform this test, such as thermal conductivity meters, gas detectors, infrared analyzers, test tubes, gas chromatographs, electrochemical toxic gas detectors, halogen lamps or halogen leak detectors. The environmental concentration test is a low concentration test, the purpose of which is to determine whether the fumigant gas concentration in the working environment has fallen below the safe threshold concentration. The instruments that can be used for this test are poison tubes, electrochemical toxic gas detectors, infrared analyzers, gas chromatographs, halogen lamps or halogen leak detectors. (1) Thermal conductivity detector Thermal conductivity detectors have been widely used in the detection of fumigant gas concentrations in phytosanitary fumigation treatments. This type of instrument is mainly used to determine the gas concentration of fumigants such as methane, sulfuryl fluoride, etc. However, it cannot detect the concentration of phosphine gas, and it cannot detect the gas concentration of each single component of mixed fumigants, unless the cargo is The adsorption rates of the components of the mixed fumigant are the same. l Principle of thermal conductivity detector In the thermal conductivity detector, the detection sensor is a thermal conductivity cell. A general thermal conductivity cell has four cells, and a thermistor wire is installed in each cell. A Wheatstone bridge is formed by these four thermistor wires, and each of the two resistor wires forms an arm of the bridge. One arm of the Wheatstone bridge is used as a reference arm, and the cell where the thermistor wire is located is filled with dry air: the other arm of the bridge is used as a measurement arm, and the gas sample to be tested flows through the two parts that make up the arm during detection. Small room. If a mixed gas of fumigant gas and air is introduced into the detection chamber, the equilibrium temperature of the arm will inevitably be changed, which will further cause the arm resistance and current to change. At this point the Wheatstone bridge is out of balance and has a voltage output. The output voltage of the Wheatstone bridge is directly proportional to the level of the fumigant gas concentration. Amplifying this output voltage and displaying it on the display, we can directly read the fumigant gas concentration. 2 Detection of fumigant gas concentration The detection process is divided into the following steps: preheating (before testing, turn on the air pump of the instrument and display the power switch to allow the instrument to run and warm up); adjust the gas flow rate (adjust the gas flow rate through the flow meter to the specified flow rate of the instrument) ); Zero adjustment (if the displayed value is not zero after the instrument is warmed up, you can use the zero adjustment button to perform zero adjustment); Detection (After the instrument is adjusted, connect one end of the detection tube to the instrument's air inlet to detect The other end of the tube is inserted into the fumigation space. When the display value of the instrument is stable during the test, the value can be recorded. After all the test work is completed, the instrument should be run for a period of time to return the instrument to zero, and then shut down. (L) Calibration of the instrument. All instruments are calibrated before leaving the factory. However, as the number of uses increases and the time increases, the detection accuracy of the instrument will change. In order to ensure the accuracy of the detection value of the instrument, the thermal conductivity fumigant concentration detector should be calibrated at least once a year. (2) Care and maintenance. The instrument should be handled lightly during operation, because the hot wire element in the thermal conductivity cell will be easily damaged if it encounters severe vibration after heating. Even during normal transportation, it should be handled gently. Charge each time after each use. The charging time is 12-16 hours. During each use, electricity should be saved. Try to avoid deep discharge to extend battery life. Wherever possible, use AC power. During use, it is strictly forbidden to pump water into the instrument to avoid damage to the thermal conductivity cell and the sampling pump. If a similar situation occurs, immediately run the instrument for several hours until it can be adjusted to zero. After the sampling pump is turned on, it is strictly forbidden to block the suction and exhaust ports of the instrument with hands and other objects to avoid damaging the sampling pump. (II) Halogen leak detection lamp and halogen leak detector Both halogen leak detection lamps (Australian lamps) and halogen leak detectors are available on the market. The detection principle of the halogen leak detection lamp is: the halogen compound decomposes in the flame, and reacts with the oxidized copper to form a copper halide, which makes the flame a unique color. From the difference in flame color, the concentration of a certain halide fumigant gas in the air can be roughly judged. The halogen leak detector uses semiconductor sensors to detect halogen compounds such as methane. The sensitivity of this instrument is particularly high, and its minimum detectable concentration is below 10mg / kg. However, as the number of uses increases, the sensor is susceptible to poisoning, so the sensor must be replaced at any time. The instrument has no specific concentration display and cannot be used to directly measure the concentration of a gas. However, during the detection process, as the concentration increases, the alarm sound from the leak detector becomes louder. The halogen leak detector is small in size, easy to carry, and has no open flame, so it is safe to use, easy to operate, and requires no special maintenance. Due to the high sensitivity of halogen leak detectors and halogen leak detectors, they can be used not only to detect leaks, but also to determine whether trace amounts of fumigants harmful to human health are still toxic in the fumigation place after ventilation gas. (Three) detection tube Detecting tubes are those glass tubes with graduations that contain indicator compounds that can react with specific fumigant gases and produce color changes. At present, there are test tubes on the market that detect low concentrations of methane, phosphine and ethylene oxide in various concentration ranges. The detection tube is very convenient to use. When using it on the spot, first break the sealing heads at both ends of the detection tube, then insert one end into the air inlet of the hand pump or electric sampling pump, and connect the other end to the poison sampling tube, and Start the electric sampling pump or hold the hand pump evenly several times according to the requirements of the instructions. Remove the test tube from the sampling pump. After the indication of the chemical agent in the test tube stops changing color, you can read the test result. The detection accuracy of the detection tube is directly related to the use of the manual air pump technology, that is, directly related to the accuracy of the gas sample collection. If you strictly follow the requirements of the instruction manual, you can make the detection accuracy reach 70-90% of the average value of multiple test results. It is worth noting that the storage life of the test tube at room temperature is about two years; if the temperature is higher than 30 ℃, the failure rate is more critical. Direct sunlight can also accelerate its failure. Under low temperature conditions, especially when the temperature is below freezing, the display reading is unreliable, and it should be warmed to room temperature before use. The detection tube may also be sensitive to multiple fumigant gases or other organic gases, so attention should be paid to the actual detection. (Four) gas chromatograph The gas chromatograph is the instrument with the highest detection sensitivity, the most accurate detection result, and the widest application range among the fumigant gas concentration detection instruments. It can be used not only for the detection of high and low concentrations, but also for the detection of the gas concentration of each component of the fumigant. Gas chromatographs are available either as large gas chromatographs for laboratory use or as portable gas chromatographs for indoor and outdoor use. The Canadian-made PHOTO VAC 10S series portable gas chromatographs are small in size. The entire instrument is housed in an aluminum anodized box and weighs approximately 12 kg. It mainly includes photoelectric ion detector, chromatographic column (packed column or capillary column, can be used in the field for more than 8 hours), lead-acid sealed battery (can be used for instruments for more than 8 hours), an electromagnetic valve system that controls automatic sampling, Microcomputer system for automatic control and chromatographic data processing. The instrument has a high degree of automation and can perform automatic sampling, automatic sampling, etc. After the program is set, it can also automatically sample and analyze automatically at regular intervals, and it can also automatically calibrate the instrument. (5) Colorimetric method or chemical titration method l The principle of australian-potassium colorimetry is that methane can be hydrolyzed by potassium hydroxide-ethanol solution at room temperature to produce potassium aluminate. The effect of potassium aluminate and silver nitrate on the formation of silver australian turbidity can be determined by colorimetry. The sensitivity is 10 ug / ml. 2 The principle of phosphine-aluminate hinge colorimetry is that phosphine is oxidized by potassium permanganate in sulfuric acid solution of potassium permanganate to generate phosphoric acid. Phosphoric acid and aluminate hinge form aluminophosphonate, which is then reduced to aluminum blue by a reducing agent, which can be colorimetrically determined. The sensitivity of this method is 21 ug / ml.