Introduction to Gas Detection Instruments

one. 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 LEL, combustible gas is too small to burn. When the concentration of combustible gas in the environment is higher than UEL, then 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. Reference to this value must be based on the national standard, and the latest correction value should be used. TVL includes the following two parts:

TLV-TWA This value represents the time-weighted average concentration in the environment. The vast majority of workers do not have health problems when they work in this environment for 8 hours a day, 40 hours a week.

The TLV-STEL parameter is defined as a weighted average of 15 minutes. 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 average reaches the average threshold. The number of times in a day that exceeds the average 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 specifies the form of a double-ended electrical interface. The 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, greatly improving 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 test conditions, the time from when the detector contacts the gas being measured to when it reaches a stable indication. Normally, the time to reach a stable value of 90% is read as the response time.

Recovery Time The time during which the gas is removed from the detector to recover the monitored state under test conditions. Normally, the time to recover 10% of the stable indication value is read as the recovery time.

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

The standard gas composition, concentration, and accuracy are all known gases.

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

Explosion-protection signs There are definite regulations for various explosion-proof types of explosion-proof electrical equipment for explosive atmospheres. In d IICT6, d denotes the explosion-proof type as explosion-proof type, II denotes the electrical equipment for the factory, and C denotes the maximum experimental safety gap or minimum for explosive gas mixtures. Ignition current ratio (A, B, C three) the most severe level, T6 represents the maximum level of maximum surface temperature allowed (85 °C)

Bus and splitter buses and splitters are concerned with the connection of controllers and detectors. If each detector needs 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 the high bit, 1 is the low bit, the dial to the ON side is 0, the OFF side is 1 and the encoding formula is as follows: code number = 1 x N1 2 x N2 4 x N3 8×N4 16×N5

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

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.

two. Common Combustible Gas Related Properties Gas Name 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

three. Explosion limit of combustible gas and air mixture

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

four. The classification of gas detectors is classified according to the object of detection. There are flammable gas (including methane) detection alarms, toxic gas detection alarms, and oxygen detection alarms. 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, there are diffusion type and pump suction type.