Toxic and flammable gas leaks pose a constant threat to industrial plants, their employees, and those who live nearby. Continuous, reliable operation of equipment and your familiarity with gas detection systems are also critical.
There are some common units and common terms involved when using gas detectors. Do you know what they are?
Common Units
%LEL: refers to the percentage of the lower explosion limit of flammable gas, expressed as a percentage. It is the lowest concentration at which flammable gas explodes when it encounters minimum ignition energy when mixed with air. This unit is generally used for combustible gases, such as: hydrogen (H2), carbon monoxide (CO), methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), ethylene (C2H4), propylene (C3H6) , butene (C4H8), acetylene (C2H2), propyne (C3H4), butyne (C4H6), etc.
%VOL: refers to the volume specific concentration of the mixed gas, also expressed as a percentage. This is a general term used to describe the gas mixing ratio. Commonly used to detect carbon dioxide (CO2), oxygen (O2), nitrogen (N2), argon (Ar) and other gases.
PPM: is the abbreviation of part per million in English, which means one millionth, dimensionless, the number of parts of a certain substance in a mixture of one million parts. Commonly used in units with toxic and harmful gases, such as hydrogen sulfide (H2S), ammonia (NH3), hydrogen fluoride (HF), hydrogen chloride (HCL), chlorine (CL2), etc.
The gas concentration measured by most gas detectors is volume concentration, and some environmental protection departments require the concentration of air pollutants to be measured in units such as mg/m3, so our national standards have also adopted mass concentration units (such as mg/m3 , μg/m3).
Unit Conversion
Conversion of %LEL and %VOL:
To convert between %VOL and %LEL of a certain flammable gas, you need to know the lower explosion limit of the gas in advance.
For example: the lower explosion limit of methane is 5.0%VOL, that is, 100%LEL=5%VOL, then it is further deduced that 1%LEL=5%VOL×1%=0.05%VOL.
Note: When the concentration of combustible gases in the air reaches its lower explosion limit, we call the explosion risk of the combustible gas environment in this place 100%.
Conversion of %VOL and PPM
Since %VOL means percentage and PPM means parts per million, the conversion relationship between them is very simple.
1%VOL=10000PPM
Conversion of PPM and %LEL
There is no direct conversion between PPM and %LEL. You need to convert %LEL into %VOL first, and then convert %VOL and PPM.
For example: the lower explosion limit of methane is 5.0%VOL. Refer to the conversion method of %LEL and %VOL, 20%LEL=1%VOL=10000PPM.
Conversion of mg/m3 and PPM
The value of mass concentration mg/m3 will change with changes in environmental conditions such as temperature and air pressure (pressure). During actual use, temperature and pressure need to be measured simultaneously.
● mg/m3=M/22.4×PPM×(273.15/(273.15+T))×P/101325
Among them, M is the gas molecular weight (g/mol), T is the gas temperature (℃), and P is the pressure (Pa).
Note: Under standard conditions, the molar volume of any gas is approximately 22.4L/mol.
Terminology
Sensitivity: refers to the degree to which a gas detector senses changes in gas concentration.
Detection accuracy: refers to the comprehensive performance including the correctness and accuracy of the value displayed by the gas detector (or the value indicated by the measurement result).
Alarm accuracy: refers to the difference between the alarm set value and the gas concentration when the alarm starts, or a value expressed as a percentage of the difference and the alarm set value.
Repeatability (accuracy): When the same detection target gas is repeatedly measured using the same method under the same conditions and within a relatively short period of time, the measured values obtained are consistent.
Response time: When the concentration of the test gas changes instantaneously, the time required for its indicated value to be controlled within a specific range of the final value.