Calibration coefficient for fixed combustible gas detectors

Refer to JJG 693-2011 \"Test Regulations for Flammable Gas Detection and Alarm Devices\"

There is a formula that needs to be converted, and it’s quite complicated. With hundreds of different flammable gases available on the market, it’s not feasible to calibrate each one using standard gases; this would be impractical for manufacturers. Usually, methane is used as a standard gas to calibrate other flammable and explosive gases, but conversion factors are needed to determine the alarm thresholds. There are two ways to express the concentration of pollutants in the environmental atmosphere (air): 1. Mass concentration: the mass of pollutants per cubic meter of air, expressed in mg/m3. 2. Volume concentration: the volume of pollutants in one million cubic meters of air, expressed in ppm. The gas concentrations measured by most gas detection instruments are given in terms of volume concentration (ppm). According to China’s regulations, particularly those set by the environmental protection authorities, gas concentrations must be expressed in terms of mass concentration units (such as mg/m3). Our **standard specifications also use mass concentration units (such as mg/m3) for this purpose. What is the relationship between these two units of gas concentration, mg/m3 and ppm? How is the conversion done during that time? Using the mass concentration unit (mg/m3) as a way to express air pollutant concentrations enables the easy calculation of the actual amount of pollutants. However, the mass concentration is related to the temperature and pressure of the gas being measured, and its value changes as these environmental conditions change ； During actual measurement, it is necessary to determine both the temperature of the gas and the atmospheric pressure. When using ppm to describe pollutant concentration, this problem does not arise because a volume ratio is employed. Conversion between concentration units ppm and mg/m3: Calculate using the following formula: Mass concentration in mg/m3 = Molecular weight of gas / 22.4 * ppm value ** (Pressure/Ba / 101325). Here, M represents the molecular weight of the gas, ppm represents the measured volume concentration, T is the temperature, and Ba is the pressure. When humidity is high, such as at 100% relative humidity, another factor—the molecular weight of the gas—is required. Combustible gas detectors – Concentrations related to ppm and unit conversions for concentrations (I). Solution concentration: Solution concentration can be divided into three categories: mass concentration (such as mass percentage concentration), volume concentration (such as molar concentration, equivalent concentration), and other forms of volume concentration. 1. Mass percent concentration: The concentration of a solution is expressed as the percentage of the mass of the solute relative to the total mass of the solution, and it is denoted by the symbol %. For example, 25% glucose injection means that 100 milliliters of the injection contains 25 grams of glucose. Mass percent concentration (%) = Mass of solute / Mass of solution × 100%. 2. Volume concentration: (1) Molar concentration – The concentration of a solution is expressed as the number of moles of solute per liter of solution; this is denoted by the symbol mol. For example, if 1 liter of concentrated sulfuric acid contains 18.4 moles of sulfuric acid, then its concentration is 18.4 mol. Molar concentration (mol) = number of moles of solute / volume of solution (liters). (2) Equivalent concentration (N) – this unit is hardly used these days and has been phased out, but it was still frequently mentioned in books from the 1950s. The concentration of a solution, expressed as the number of equivalent grams of solute per liter of solution, is called equivalent concentration and is denoted by the symbol N. For example, if 1 liter of concentrated hydrochloric acid contains 12.0 equivalents of HCl, then its concentration is 12.0 N. Equivalent concentration = number of gram-equivalents of solute / volume of solution (in liters). 3. Mass-volume concentration: The concentration expressed as the mass of solute per unit volume (1 cubic meter or 1 liter) of solution is called mass-volume concentration, and it is denoted by the symbols g/m3 or mg/L. For example, if 1 liter of chromium-containing wastewater contains 2 milligrams of hexavalent chromium, then the concentration of hexavalent chromium is 2 milligrams per liter (mg/L). Mass-volume concentration = mass of solute (in grams or milligrams) / volume of solution (in cubic meters or liters). 4. Formulas for converting concentration units: 1) Equivalency concentration = 1000 × mass percent concentration / E; 2) Mass percent concentration = equivalency concentration / 1000 × E; 3) Molar concentration = 1000 × mass percent concentration / M; 4) Mass percent concentration = mass-volume concentration (mg/L) / 1000; 5) Mass-volume concentration (mg/L) = 1000 × mass percent concentration. 5. ppm represents a percentage by weight; ppm = mg/kg = mg/L. That is, 1 ppm = 1000 ug/L, and 1 ppb = 1 ug/L = 0.001 mg. Here, E represents the gram-equivalent of the solute ； d—Specific gravity of the solution ； M—molar mass of the solute ； (II) Gas concentration: For pollutants in the atmosphere, their concentration there is commonly expressed in terms of volume concentration and mass-volume concentration. 1. Volume concentration: Volume concentration is expressed as the volume of pollutants per cubic meter of air, in cubic centimeters or ml/m3. The commonly used unit for expressing this is ppm, where 1 ppm = 1 cubic centimeter per cubic meter = 10-6. In addition to ppm, there are also ppb and ppt. The relationships between them are as follows: 1 ppm = 10-6, which means one millionth; 1 ppb = 10-9, which means one billionth; 1 ppt = 10-12, which means one trillionth. Additionally, 1 ppm = 10^3 ppb = 10^6 ppt. 2. Mass-volume concentration: The concentration expressed as the mass of pollutants per cubic meter of air is called mass-volume concentration, with units of milligrams per cubic meter or grams per cubic meter. Its conversion relationship to ppm is as follows: X = M.C/22.4; C = 22.4X/M. Here, X represents the concentration of the pollutant in milligrams per cubic meter ； C—Concentration of pollutants expressed in ppm ； M—the molecular weight of the pollutant. From the above formula, the following relationship can be obtained: 1 ppm = M/22.4 (mg/m3) = 1000·M/22.4 ug/m3. Example 1: Determine the ppm concentration of hydrogen fluoride at 30 mg per standard cubic meter under standard conditions. Solution: The molecular weight of hydrogen fluoride is 20; therefore, C = 30.22 × 4/20 = 33.6 ppm. Example 2: Given that the concentration of sulfur dioxide in the atmosphere is 5 ppm, determine its concentration value in mg/Nm3. Solution: The molecular weight of sulfur dioxide is 64. X = 5.64/22.4 mg/m3 = 14.3 mg/m3. 3. Conversions between ppm, ppb, and mass concentration in soil, animals, plants, and solid waste: 1 ppm = 1 mg/kg = 1000 ug/kg; 1 ppb = 1 ug/kg = 10-3 mg/kg; 1 mg/kg = 1 ppm = 1000 ug/kg; 1 ug/kg = 1 ppb = 10-3 ppm

It’s a small effort; we’re all in the same field, so it’s only right to help

Generally, methane and propane are used; in some cases, propane is used for calibration. When the metrology institute comes to conduct inspections, methane is used, so you must calibrate again using methane as well, haha.

The lower explosion limit for propane is 2.2% v/v. As long as the lower explosion limit of the gas you are measuring is higher than 2.2% v/v, it can be calibrated directly without any adjustments; it’s simply a matter of dealing with a gas that has a higher lower explosion limit, meaning the alarm threshold will be lower. After all, it’s not a gas analyzer, so it’s impossible to measure the gas concentration with precision. If the lower explosion limit is below 2.2% v/v for propane, adjustments are necessary. For example, in the case of benzene-based gases, the coefficient can be adjusted according to the ratio of their lower explosion limits. In general, the first-level alarm threshold for most flammable gases is set at 25% LEL, and all devices are calibrated using propane. Even if the lower explosion limit of a certain gas is half that of propane, the actual alarm threshold will still be 50% LEL, which is above the lower explosion limit and sufficient to serve as a warning. Therefore, many quality inspection laboratories choose to use propane for calibration purposes to simplify the process

This post was last edited by zxb1990 on 2016-7-2 at 22:57. I can tell you for sure that no manufacturer uses any formula for calculations; all the data is obtained through experiments. Let me give you an example: if 0.6% (29% LEL) propane is used as the calibration gas, then the gauge calibrated in this way will be most accurate for measuring propane. If 2.5% (50% LEL) methane is passed through this gauge, a certain value will be obtained (let’s say 90). We can see that this value has increased by a factor of 1.8, and this is the conversion factor between methane and propane. Of course, this value isn’t determined by calibrating just one gauge; it requires multiple tests before an average value can be obtained. From the example above, it can be seen that for a gauge calibrated with propane, if the gas being measured on-site is methane, the reading will be amplified by a factor of 1.8. Therefore, if the gauge shows 18, the actual methane concentration should be 10 (of course, both values are in LEL%). If you have any questions, feel free to contact me via the in-app messaging system

May I ask where the HCl alarm in the hydrochloric acid storage tank is installed – on the ground or at a high position on the top of the tank? ? ?

This post was last edited by Azure123 on 2016-12-7 at 16:16. If there are multiple gases to be detected in the testing environment, meaning that the possible explosive gas is a mixture (for example, 30% hydrogen and 40% methane), then should hydrogen or methane be chosen as the gas to be monitored?