Gas detection equipment

First, the term "Parts Per Million" (PPM) concentration measurement unit is generally used in the field of gas detection. For example, hydrogen sulfide containing 1 PPM in mixed air means that one unit volume of hydrogen sulfide is contained in one million unit volumes of gas.

Flammable limits are divided into Lower Explosive Level and Upper Explosive Level. The unit of LEL and UEL is usually a percentage and refers to the percentage of a gas contained in air (or oxidant). In an environment lower than the LEL, combustible gas is too small to burn. When the concentration of combustible gas in the environment is higher than that of UEL, too much gas cannot be burned. The LEL value and UEL value of various combustible gases can be obtained in related data.

Threshold Limit Values ​​(TLV) The TLV indicates that when the content of a certain gas in the air is less than this threshold, the health of workers who are fully and continuously exposed to the environment will not be damaged. When referring to this value, it must be based on the national standard and the latest correction value should be used. TVL includes the following two departments:

Uniform Threshold (TLV-TWA) This value represents the time-weighted uniform concentration value in the environment. When most workers work for 8 hours a day and 40 hours a week, they will not have health problems.

The TLV-STEL parameter is defined as a 15-minute weighted average value. The concentration of a harmful gas in the workplace must not exceed its specified threshold at any time during a working day, even during this day. The weighted uniform value reaches a uniform threshold. The number of times during the day that exceeds the uniform threshold and falls below the instantaneous threshold must not exceed 4 times, and the duration of each must be less than 15 minutes.

Dangerous Concentration (IDLH) If the worker does not wear a gas mask or lacks escape experience and the concentration of gas in the work environment reaches a dangerous concentration, 30 minutes of detention will cause permanent damage to the human body or weaken the body's health (for example, decreased visual acuity). ).

The RS485 serial bus demarcates the form of a double-ended electrical interface. Its standard is a double-ended transmission signal. If one of the lines is a logic 1 state, the other is a logic 0. Because the voltage loop is bidirectional differential, common mode interference in the transmission loop can be suppressed, which greatly improves the communication performance.

Explosion range In a mixture of combustible gas and air, the concentration range between the lower explosive limit and the upper explosive limit of a combustible gas is called the explosion range.

Town gas generally includes natural gas, liquefied petroleum gas, and artificial gas.

Response time Under the premise of the test, the time from when the detector is exposed to the measured gas to when it does not indicate the value is indicated. Normally, the time to reach 90% of the unspoken value is read as the response time.

Recovery time Under the premise of the test, the time from the detector's disengagement of the measured gas to the restoration of the monitoring state. Normally, the time to restore to 10% of the indication value is read as the recovery time.

Zero gas Clean air or nitrogen without measured gases or other interfering gases.

Scale gas composition, concentration, and accuracy are all known gases.

Explosive atmospheres and explosion-proof electrical equipment contain explosive mixtures and are called explosive atmospheres. The electrical equipment that is designed and manufactured according to the prescribed conditions without causing explosion of the surrounding explosive mixture becomes an explosion-proof electrical equipment for explosive atmospheres.

Explosion-proof signs The state has definitive delineated various explosion-proof types of explosion-proof electrical equipment used in explosive atmospheres. d IICT6 d denotes explosion-proof type for flameproof type, II denotes factory electrical equipment, and C denotes maximum experimental safety gap for explosive gas mixtures or The tightest level of the minimum ignition current ratio (A, B, and C levels) and T6 indicates the maximum level of the maximum surface temperature allowed (85°C)

Bus and splitter buses and splitters are concerned with the connection of controllers and detectors. If each detector requires a wire to complete the communication with the controller, this connection is called a split connection. If several detectors can share a wire to complete the communication with the controller, this connection is called a bus connection.

Binary In the bus system, the encoding of the device on the bus is binary, 8 is high, 1 is low, dial to the ON side is 0, OFF side is 1, the encoding formula is as follows: code number = 1 × N1 2 × N2 4 × N3 8×N4 16×N5

After the sensor warm-up sensor is powered on, the output value is not disordered, and the sensor waits for the warm-up period during the period when the output value is not disturbed.

Sensor poisoning When the sensor is in the power-on state, if the sensor is exposed to gas whose concentration is far beyond its range, the sensor output value may remain high. Some poisoned sensors recover after a period of time, and some are not recoverable.

Second, the common combustible gas-related properties Gas name Molecular formula weight (air = 1) TLV-TWA (PPM) TLV-STEL (PPM) TLV-IDLH (PPM) LEL (V%) HEL (V%) Hydrogen H2 0.0695 4 75 Ammonia NH3 0.58 25 35 500 15 28 Carbon monoxide C0 0.976 25 1500 12.5 74 Hydrogen sulfide H2S 1.115 4.3 45 Chlorine CL2 0.5 1 30 Methane CH4 0.554 5 15 Ethane C2H6 1.035 3 12.5 Ethylene C2H5 0.975 2.7 36 Propane C3H8 1.56 2 9.5 Propylene C3H6 1.49 2.4 10.3 Butane C3H6 2.01 800 1.9 8.5 Butene-1 C4H8 1.937 1.6 10 Butene-2 ​​C4H8 1.94 1.8 9.7 Butadiene C4H6 1.87 2 20000 2 12 Isobutane (CH3) 3CH 2.068 1.8 8.4

Third, the explosion limit of combustible gas and air mixture

The explosion limits of combustible gases and air mixtures are related to the following factors: the type and chemical nature of combustible gases; the purity of combustible gases; the average of combustible gases and air mixtures; the form, energy and ignition position of the ignition source; Geometry and dimensions; temperature, pressure, and humidity of combustible gases and air mixtures.

Fourth, gas detector classification According to the test object classification, there are flammable gas (including methane) detection alarm, toxic gas detection alarm, oxygen detection alarm. Classification according to the detection principle, combustible gas detection catalytic combustion type, semiconductor type, thermal conductivity type and infrared absorption type; toxic gas detection electrochemical type, semiconductor type; oxygen detection electrochemical type. Classified by type of use, portable and stationary. According to the location of use, there are conventional type and explosion-proof type. Classified by function, there are gas detectors, gas alarms, and gas detection alarms. Classified by sampling method, with diffusion and pumping