Discussion on the specifications for combustible gas detectors
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Question 1: In the list of measuring instruments subject to mandatory inspection, those used for detecting toxic, harmful, flammable, or explosive gases (and equipped with alarms) require formal approval. Assuming that the approval in question refers to that for methane detectors, other instruments that also need such approval include those for hydrogen sulfide, sulfur dioxide, carbon monoxide, carbon dioxide, smoke, and nitrogen oxides detected by chemiluminescence. According to the General Technical Requirements for Environmental Gas Detection and Alarm Devices in Workplaces GB12358-2006, the product label must bear a mark indicating compliance with metrological standards. Can it be assumed that the form certification obtained for the methane detector can also be applied to detectors for other gases? Such as hydrogen, propylene. Question 2: In Section 4.4 of Part 1 of GB15322-2019, regarding the response performance for detecting gases other than methane, propane, and carbon monoxide, for detectors of other flammable gases and vapors, one of methane, propane, or carbon monoxide must first be used as the reference gas for testing, and the required performance standards must be met. Then, in accordance with the target gases specified by the manufacturer or using an equivalent method, perform the range indication deviation test and response time test; the test results shall comply with the manufacturer’s specifications. According to GB15322 5.1.7, the test gas shall be prepared in accordance with the types of detection gases and alarm setpoints specified by the manufacturer. Unless otherwise specified for relevant tests, the test gas shall be composed of a mixture of flammable gas and clean air; its humidity shall be at normal levels, and the mixing error shall not exceed ±2% of the alarm setpoint. When using one of methane, propane, or carbon monoxide as the combustible gas for the test, the purity of the combustible gas should be no less than 99.5% ； For other types of detection gases claimed by the manufacturer, test gases using standard gas mixtures that meet the manufacturer’s requirements can be employed. Can it be assumed that using standard gases such as methane with a concentration greater than 99.5% during annual inspections allows for the measurement of other types of gases? Question 3: In JJG693-2011 gas reference materials, gas reference materials of the same type as the gas being measured by the instrument should be used. If the instrument does not specify the type of gas to be measured, isobutane or propane gas reference materials can be used. Can it be assumed that the same gas must be used for testing during annual inspections? For example, an alarm designed to detect propylene must be tested using propylene; if the gas to be detected is not specified, isobutane or another gas can be used for testing.
Question 1: The type approval for methane detectors does not apply to detectors for other gases, such as hydrogen and propylene. Different gases have distinct physical and chemical properties, therefore separate approval procedures are required for each type of gas. Question 2: According to the GB15322-2019 standard, for detectors designed to detect other flammable gases and vapors besides methane, propane, and carbon monoxide, it is necessary to use one of methane, propane, or carbon monoxide as the reference gas for testing purposes in terms of response time and other performance criteria, and such detectors must meet the specified performance requirements. Then further testing is carried out based on the target gas claimed by the manufacturer or using equivalent methods. Therefore, using standard gases such as methane with a concentration greater than 99.5% during annual inspections cannot measure other types of gases. Question 3: According to the provisions of JJG693-2011 for gas reference materials, it is permissible to use gas reference materials of the same type as the gas being measured by the instrument for calibration. If the instrument does not specify the type of gas to be measured, isobutane or propane gas standards can be used for calibration. Therefore, during the annual inspection, testing must be carried out using a gas of the same type as that detected by the alarm; if the testing medium is not specified, a gas standard of the same type as other media can be used for calibration. .
Thank you for the answer. However, the mandatory list of measuring instruments does not include any mandatory type approval requirements for other media; it’s not practical to obtain approval for each one
Expert explanations for questions 2 and 3: Since combustible gas detection alarms based on the principle of catalytic combustion respond to almost all hydrocarbons, their sensitivity varies greatly. For example, benzene and alcohol gases result in either a higher or a lower sensitivity of catalytic combustion sensors. To ensure accurate measurements by combustible gas detection alarms during installation and operation, as well as effective environmental monitoring, the types of gas reference materials used should match the components that the user or the environment to be monitored requires to be measured. Corresponding standard gases should also be utilized for the verification and calibration of the instruments. JJG693-2011 stipulates that if the instrument does not specify the type of gas to be measured, isobutane or propane gas reference materials can be used. It should be understood that neither the instrument nor the site being monitored specifies the type of gas being measured; therefore, isobutane or propane reference gases can be used. (It is recommended to include a statement in the certificate/report indicating that the verification/calibration results are valid only for the specific test object.) The instrument does not specify the type of gas being measured, but it is known that for the gases present in the area being measured, calibration and verification must be carried out using gas reference materials of the same type, as required by the regulations.
Regarding question two, combustible gas detectors based on the principle of catalytic combustion respond to most hydrocarbons; however, the sensitivity to different gases may vary. To ensure that combustible gas detection and alarm devices function accurately and effectively during installation and use, it is necessary to use gas reference materials identical to the components that need to be measured in the user’s requirements or in the environment being monitored, for the calibration of these devices. Regarding Question 3, JJG693-2011 provides guidance on the selection of gas reference materials. If the instrument does not specify the type of gas to be measured, isobutane or propane gas standards can be used for calibration and verification. It should be noted, however, that in such cases it is necessary to clearly state in the certificate report that the test and calibration results are valid only for the tested object. In summary, when neither the instrument nor the site being monitored specifies the type of gas to be measured, isobutane or propane gas reference materials can be used for calibration and verification. And if the instrument knows the type of gas present in the area being measured, it should use gas reference materials of the same type for calibration and verification in accordance with the relevant procedures. .
Selection of calibration standard gases: Combustible gas detection and alarm devices are used in various environments, and the types of combustible gases detected also vary. When calibrating an alarm, a suitable standard gas should be selected to ensure accurate value transfer. Selection is generally based on the following principles: (1) Choose a standard gas of the same type as the gas detected by the alarm, such as methane, hydrogen, propane, etc. (2) Select the most prominent gas component among those detected by the alarm as the standard gas. In some work environments, there are various flammable gas detectors; these detect mixtures of flammable gases. Catalytic combustion-type detectors do not measure the concentration of a specific gas, but rather the oxidation reaction that occurs when all flammable gases come into contact with oxygen in the air. For calibration purposes, it is not necessary to use all the standard gases present in the gas mixture being tested – it is sufficient to choose the most prevalent gas component in that environment as the standard gas. (3) Alternative gases may be used for calibration. Catalytic combustion alarms determine gas concentration by measuring the heat of combustion of combustible gases. Although the lower explosion limits of various gases differ, there is a corresponding relationship between the concentration of a gas and its heat of combustion. For most flammable gases, the product of their lower explosion limit concentration and their heat of combustion is approximately equal to a constant, generally ranging between 1050 and 1100. Therefore, using the extreme heat of combustion as a standard, the lower explosion limit concentration of most flammable gases measured by catalytic combustion alarms is approximately equal to that of isobutane. As a result, isobutane can be used as a reference gas to calibrate most catalytic combustion flammable gas alarms. For example, gasoline is a mixture of hydrocarbon compounds; isobutane is used as a standard gas for its calibration. However, for a few flammable gases such as hydrogen, carbon monoxide, and hydrogen sulfide, the product of their heat of combustion and lower explosive limit concentration differs significantly from that of isobutane. Therefore, when calibrating these gases, the same standard gases should be used instead of isobutane.