Saturday, January 22, 2011
Weak current College】 【gas sensor principle】.
Gas detector for key part is a gas sensor. Gas sensor from principle can be divided into three main categories:
A, use the physical and chemical properties: such as semiconductor gas sensor type (surface control, volume control, surface potential), catalyzed flaming type, solid thermal conductivity type and so on.
B, using the physical nature of the gas sensor: heat-, light interference, infrared absorption, etc.
C, using electrochemical gas sensors: Fixed potential electrolysis, Caleb cutting, battery type, diaphragm ion electrode, fixed electrolyte type and so on.
According to harm, we will toxic gas into flammable and toxic gas. Because of their nature and harm, its detection means.
1. flammable gases combustible gas is a petrochemical and other industrial applications encounter most dangerous gases, it is mainly a paraffin and other organic gases and some inorganic gases:
Such as carbon monoxide, and so on. Combustible gas explosion must have certain conditions: some concentration of combustible gas, a certain amount of oxygen and enough heat to ignite their fire, this is the explosion of the three elements (as shown by the explosion triangle) are indispensable, that is, the lack of any one of these conditions do not cause fires and explosions. When the combustible gas (steam, dust) and oxygen mix and reaches a certain concentration, it meets a certain temperature of ignition source will explode.
We put the combustible gas explosions yuhuoyuan concentration of the concentration limit of explosions, hereinafter referred to as explosion limits, general use% said. In fact, the mixture is not in any proportion will explode and you want to have a concentration range. As shown in the figure on the right part of the shadow. When the combustible gas concentrations below the LEL (minimum explosive limit) (combustible gas density less than) and its concentration above the UEL (maximum explosion limit) (oxygen deficiency) will not explode.
Variety of combustible gas in the LEL and UEL is different, this is great when the calibration equipment. For security purposes, generally we should in combustible gas concentration for 25% of the LEL or below and alert you when 50%, 25% LEL here, is called the low limit alarm, and 50% of the LEL called high limit alarm. This is what we will be combustible gas detector also known as the cause of the LEL detector.
It is important to note that LEL detector on 100% is not a combustible gas concentrations reach gas volume of 100% is reached 100% of the LEL, that is equivalent to the minimum of combustible gases, if the lower explosion limit is methane, 100% LEL = 5% of the volume level (VOL). Detection of combustible gases can use semiconductor, catalyzed flaming type (anti-poisoning), heat-and infrared sensors.
2. toxic gas detection at present, for specific toxic gas detection, we use the most dedicated gas sensors. Detecting gas sensor main semiconductor, electrochemical and electrolytic cell type.
-Fixed potential electrolysis, Caleb cutting, battery type, diaphragm ion electrode, fixed electrolyte type and so on semiconductor sensor for high sensitivity and low resolution. This principle of sensor has almost been eliminated, and the user selects this kind of sensor to extremely seriously. Electrolytic cell sensor solid electrolyte gas sensor using solid electrolyte gas sensing materials do gas sensors. The principle is gas sensing materials produced by gas ions to form EMF EMF to measurement, the measuring gas concentrations.
Because of this high conductivity sensor, sensitivity and selectivity, and has been widely used, almost into an petrochemistry, environmental protection, various industries, second only to the metal oxide semiconductor gas sensor. Such as measuring, measuring of H2S YST-Au-WO3 NH + 4CaCO3 of NH3. Electrochemical sensor is currently widely used in the detection of gas sensors. It leverages oxidation-reduction reaction, through different electrolytes can detect dozens of toxic gases.
According to the quality of the electrolyte life generally 2 to 4 years. Electrochemical sensors consisting of: two reaction electrode-working electrode and the electrode and a reference electrode placement in particular electrolytes (as shown), and then in the reaction between the electrode sufficient voltage, so that through heavy metal catalyst coated films to be logging gas for oxidation-reduction reaction, then through the instrument of circuit system measuring gas electrolysis when the current, then including microprocessor calculates the concentration of gas.
Currently, electrochemical sensors are widely used in the detection of inorganic toxic gas sensor, can detect the specific gas sensors include: carbon monoxide, hydrogen, sulphur dioxide, nitric oxide and nitrogen dioxide, ammonia, chlorine, hydrogen cyanide, ethylene oxide, hydrogen chloride and so on.
3. volatile organic compound detection on volatile organic compound detection there is also a Photoionization detection (PID photo-ionizationdetector short), it can measure low-ppm (1) organic toxic gas and vapor concentration.
PID can detect most of volatile organic compounds (VOC, hereinafter referred to as volatileorganiccompound) simple, PID can measure carbon number from 1 (for example, CH2Cl2) to 10 (naphthyl) of volatile organic compounds. PID can be used for all types of use, production, storage and transport of various organic compounds enterprise safety and health. At the same time, it can also be used for environmental protection industry emergency accidents, industrial hygiene consulting, security checks, inspections and so on fields.
4. oxygen detector oxygen also in industrial environments, especially in closed environments need to pay great attention to factors. Generally we will more than 23.5% oxygen content is referred to as excess oxygen (the oxygen-rich), when it is easy to a risk of explosion; and oxygen content below 19.5% oxygenLess than (anoxic), this can easily happen, coma and even workers suffocation risk of death. Normal oxygen content should 20.9%. Oxygen detector is an electrochemical sensor.