Thread Content
Common Units %LEL: Refers to the percentage of the lower explosive limit of a flammable gas, expressed as a percentage. It is the lowest concentration at which a combustible gas, when mixed with air, explodes upon encountering a minimum ignition energy. This unit is generally used for combustible gases; examples include hydrogen (H2), carbon monoxide (CO), methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), ethylene (C2H4), propylene (C3H6), butylene (C4H8), acetylene (C2H2), propyne (C3H4), butyne (C4H6), and so on. %VOL: Refers to the volume ratio concentration of a gas mixture, also expressed as a percentage; it is a general term used to describe the proportion of gases in a mixture. It is commonly used to detect gases such as carbon dioxide (CO2), oxygen (O2), nitrogen (N2), and argon (Ar). PPM: It is an abbreviation for part per million, meaning one part in a million; it is a dimensionless value that indicates the amount of a certain substance in one million parts of a mixture. It is commonly used in units dealing with toxic and harmful gases, such as hydrogen sulfide (H2S), ammonia (NH3), hydrogen fluoride (HF), hydrogen chloride (HCl), chlorine (CL2), and so on. Most gas detectors measure gas concentrations in terms of volume concentration, while some environmental protection agencies require that air pollutant concentrations be expressed in units such as mg/m3. Therefore, our **standard specifications also adopt mass concentration units (such as mg/m3, μg/m3). Unit Conversion: To convert between %LEL and %VOL, it is necessary to know in advance the lower explosive limit of the gas in question. For example, the lower explosion limit of methane is 5.0% VOL; that is, 100% LEL = 5% VOL. From this, it can be deduced that 1% LEL = 5% VOL × 1% = 0.05% VOL. Note: When the concentration of flammable gases in the air reaches its lower explosive limit, we consider the explosion hazard associated with flammable gases in that area to be 100 percent. Conversion between %VOL and PPM: Since %VOL is expressed as a percentage, while PPM is expressed as a part per million, the conversion between them is very simple. 1%VOL = 10,000 PPM. Conversion between PPM and %LEL: A direct conversion between PPM and %LEL is not possible; it is necessary to first convert %LEL to %VOL, and then use %VOL to convert to PPM. For example, the lower explosion limit of methane is 5.0% VOL; using the conversion method between %LEL and %VOL, 20% LEL = 1% VOL = 10,000 PPM. Conversion between mg/m3 and PPM: The value of mass concentration in mg/m3 changes depending on environmental conditions such as temperature and pressure; therefore, it is necessary to measure both temperature and pressure during actual use. ● mg/m3 = M/22.4 × PPM × (273.15/(273.15+T)) × P/101325, where M is the molecular weight of the gas (g/mol), T is the temperature of the gas (°C), and P is the pressure (Pa). Note: Under standard conditions, the molar volume of any gas is approximately 22.4 L/mol. Common Terms Terminology Sensitivity: Refers to the degree to which a gas detector can detect changes in gas concentration. Detection accuracy: refers to the overall performance, including the correctness and precision of the values displayed by the gas detector (or the values indicated by the measurement results). Alarm accuracy: refers to the difference between the alarm set value and the gas concentration at the time the alarm is triggered, or a value expressed as a percentage of this difference relative to the alarm set value. Repeatability (precision): The consistency of the various measurement values obtained when repeatedly testing the same gas to be detected using the same method under the same conditions and within a relatively short period of time. Response time: The time required to keep the indicated value within a specific range of the final value when there is an instantaneous change in the concentration of the test gas.