A smoke detector is a converter that converts the volume fraction of a gas into a corresponding electrical signal. The probe conditioners the gas sample according to the smoke detector, generally including the display part of the instrument for filtering out impurities and interfering gases, drying or refrigeration processing.
Types of smoke detectors
Semiconducting smoke detectors; This type of detector accounts for 60% of all the smoke detectors, which is divided into conductivity type and non-conductivity type according to its mechanism, and can be divided into surface type and volume control type in conductivity type.
(1) SnO2 semiconductor belongs to a typical surface-type gas-sensitive element, and its sensing working principle is that SnO2 is an n-type semiconductor material. When the voltage is increased, the temperature of the semiconductor material rises, and the adsorbed oxygen receives electrons in the semiconductor to produce O2 or O2 primary gases H2, CO, CH4, so that the surface resistance of the semiconductor decreases, the conductance rises, and the conductance change is proportional to the gas concentration. NiO is a p-type semiconductor with oxidizing gas that reduces conductance and is sensitive to O2. ZnO semiconductor sensors also belong to this type.
a. The conductivity detector element is divided into surface sensitive type and volume control type, and the surface sensitive sensing material is SnO2+Pd, ZnO ten Pt, AgO, V205, metal phthalein, Pt-SnO2. The surface-sensitive smoke detector can detect various combustible gases C0, NO2, and Freon. The smoke detector of the sensing material Pt-SnO2 can detect the gas as combustible gas CO, H2, CH4.
b. The volumetric control sensing material is Fe2O8, la1-SSrxCOO8, TiO2, CoO-MgO-SnO2 body sensor, which can detect gases of various combustible gases, CO, NO2 Freon.
Volumetric control semiconductor smoke sensors detect liquefied petroleum gas, alcohol, air-fuel ratio control, and combustion furnace exhaust gas.
(2) The volumetric control type is a change in conductivity caused by lattice defect changes, and the conductance change is proportional to the gas concentration.
Fe2O8 and TiO2 belong to this type and are sensitive to combustible gases.
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(3) Hot wire detector, which is based on the use of thermal conductivity change semiconductor detector, also known as hot wire semiconductor detector. It is coated with SnO2 layer on the Pt wire coil, Pt wire in addition to the role of heating, which has the role of detecting temperature changes. Increase the voltage semiconductor becomes hot, the surface layer absorbs oxygen, so that the concentration of free electrons decreases. When the combustible gas exists, due to the increase of the concentration of free electrons consumed by combustion, the thermal conductivity increases with the increase of the concentration of free electrons, and the heat dissipation rate increases accordingly, so that the temperature of the Pt wire and the resistance value decreases, and the change of the resistance of the Pt filament has a linear relationship with the gas concentration.
This type of detector is small, stable, anti-toxic, can detect low concentration gases, and plays an important role in the detection of combustible gases.
(4) Non-conductivity FET smoke detector, Pd-FET. It has field effect transistor sensor, using Pd to absorb Hz and diffuse to reach the interface of semiconductor Si and Pd, reduce the work function of Pd, which is sensitive to H2 and CO.
Non-conductivity FET smoke sensor is small, easy to integrate, multi-functional, and is a promising smoke detector.
Solid electrolyte smoke sensor
This sensor element uses ion to make solid electrolyte separator conduction and is called electrochemical cell. It is a selective sensor that can be divided into cation conduction and anion conduction. As the most explored type of electrolyte smoke detector, zirconia solid electrolyte sensor uses the theory that the potential difference between the two batteries on both sides of the separator is equal to the potential of the concentration battery. Stable chromium oxide solid electrolyte detector has been successfully used in the determination of oxygen in molten steel and the measurement of engine air-fuel ratio composition.
In order to make up for the lack of conductivity of solid electrolytes, a gas-sensitive film has been coated on solid electrolytes in recent years to link the number of gas molecules present in the surrounding environment with the number of movable particles in the medium.
Contact combustion smoke detector
Contact combustion detectors are suitable for the detection of flammable gases H2, CO and CH4. When combustible gas contact surface catalyst Pt/Pd, combustion will occur with heat breaking, and combustion heat is related to gas concentration. This type of detectors can be widely applied and is praised for its small size, simple structure and good stability, but the poor selectivity will be its main disadvantage.
Electrochemical Smoke detector:
There are two commonly used types of electrochemical smoke detectors
(1) Potentiostatic electrolytic detector
It is to ionize the measured gas under the specified electric field, and measure the gas concentration by the electrolytic current flowing through, this detector has high sensitivity, and the detection gas can be selected by changing the potential, which is of great significance for the detection of toxic gases.
(2) Galvanic battery smoke detector
In KOH electrolyte solution, Pt-Pb or Ag-Pb electrode constitutes a battery, which has been successfully used to detect O2, but the main disadvantage is the water permeability and moisture absorption, also the electrode can be poisonous.
(3) New sensors with gas optical properties
The optical fiber temperature detector is one of this type, and the catalyst is coated on the top of the optical fiber to react with the gas and become hot. The ambient temperature changes, causing the ambient temperature of the fiber to change. The use of optical fiber temperature measurement has reached the degree of productization, and the detection of gas is also successful.
In addition, detectors that use other parametric changes to measure gas composition are constantly being developed, such as surface acoustic wave sensors that detect SO2, NO2, H2S, NH3, H2 and other gases with high sensitivity.
Smoke detector in toxic and harmful, flammable, explosive, carbon dioxide gas and other gas detection industry has a wide range of applications. As environmental pollution has always been one of the most concerned topics in the country and even the world, people live in the environment has always suffered very serious damage, we need to establish an environmental supervision mechanism to protect the environment, thus the construction of the IoT has become necessary. As a necessary sensor for environmental monitoring, smoke detector also help to build the environmental IoTs.
