What standard gas is used for combustible gas detector verification?
Structure and principle of combustible gas detection alarm
The combustible gas detection alarm can measure the concentration of combustible gas within the lower explosion limit (LEL) in the air of working environment and give an alarm. According to the mode of use, it can be divided into three types: portable, mobile and fixed. According to the working principle, the instrument can be divided into catalytic combustion type, electrochemical type and semiconductor type. The combustible gas detection alarm consists of a detection part and an alarm indicator part.
Taking the most commonly used catalytic combustion type as an example, the measuring bridge consists of two adjacent branches consisting of a reaction element (or working element) RM 1 and a compensation element RM2, and the other two branches consist of two resistors with the same resistance, which are respectively connected to the corresponding power supply, concentration indicator and alarm device.
A certain amount of combustible gas will produce flameless combustion on its surface after passing through the reaction element in the electrified circuit, generating a certain amount of heat, changing the resistance value to make the bridge out of balance, generating an electrical signal, converting the generated analog signal into a digital signal through digital-analog and displaying it on the concentration indicator, and when it reaches a certain concentration, the connected alarm device will give an alarm sound.
Verification of combustible gas detection alarm
According to the requirements of verification regulations, the verification environment of combustible gas alarm is strict, because it is an electronic instrument, and its verification error mainly comes from the verification environment, which requires not only a certain ambient temperature and humidity, but also good ventilation and does not interfere with the tested components. This is because: first, due to the closed environment and lack of oxygen, the gas can not be completely burned on the surface of the combustion element, generating heat and changing the resistance of the combustion element; Secondly, due to the high concentration of combustible toxic and harmful gases, lack of oxygen and incomplete combustion, the gas concentration cannot be completely detected; Thirdly, although the concentration of harmful gas in the environment can be completely burned, because its concentration is higher than the measuring range of the instrument, the phenomenon that the sensitivity of the instrument increases after the reaction element is impacted by high-concentration gas for a period of time will greatly affect the correctness of the verification.
The specific identification steps are as follows:
(1) Appearance and power-on inspection: The combustible gas detection alarm shall be verified according to relevant requirements in case of different situations in use.
(2) Verification of indication error: it can only be verified after the instrument is powered on and preheated stably, because some sensors will be stabilized after a certain period of power on and preheating, during which the display pointer may swing, which is only a transitional state, not a detector failure, so ensuring sufficient preheating stability is also an important measure to ensure that the basic error is true and reliable; At the same time, when verifying the indication error, the full range of the instrument should be calibrated first, which can also achieve the purpose of saving standard gas. The verification steps of indication error are as follows:
First of all, first of all, adjust the zero point by introducing the zero point gas according to the flow rate required in the manual. When adjusting the zero point, the power supply voltage should meet the power supply requirements specified by the instrument, so that the adjusted electric zero point is also stable.
Second, adjust the calibration value. For different instruments, it is necessary to adjust different potentiometers on the instrument to achieve accurate scale values, and some indicating scales may need to be adjusted twice or three times.
Thirdly, according to the requirements of the verification regulations, the standard gases with concentrations of about 65438±00% LEL, 40%LEL and 60%LEL are respectively introduced to record the stable indication of the instrument, and the maximum absolute value is taken as the indication error according to the calculation results.
Fourthly, stable verification data should be obtained in the verification process. During instrument verification, low-concentration gas should be introduced first, and then high-concentration gas should be introduced gradually.
Fifthly, for standard gases with different concentrations, when verifying ventilation, the flow rate should be stable and uniform to achieve accurate and reliable verification values.
(3) Verification of repeatability: Because repeatability is done after the verification of indication error, we can't simply use the data of indication error, but do it again to get the numerical value. Under normal working conditions, zero gas is introduced to calibrate the zero point, and then the stability indication of the standard gas recording instrument with a concentration of about 40%LEL is introduced, which requires 6 times.
The result is less than 1/3 of the basic error of 40%LEL point, and can't be compared with 10%LEL point and 1/3 of the basic error of 60%LEL point.
(4) Verification of alarm error: each instrument has its corresponding alarm point, which has been set before leaving the factory, so it should be calibrated according to the alarm point set in the instrument manual. The standard gas concentration should be greater than the alarm set point concentration 1. 1 times, and generally 1.5 times. The concentration of this standard gas should not be too small or too large, otherwise it is not conducive to reading or directly affects reading.
(5) Verification of response time: generally, the purpose can be achieved according to the requirements of verification regulations, and at the same time, the stability and uniformity of each gas flow should be emphasized.
Gases that have a direct impact on human body, such as carbon monoxide and hydrogen sulfide, are often used in the verification of toxic and combustible gas detection and alarm devices. If carbon monoxide reaches 1.6× 10-9, it can cause headache and vomiting in 20 minutes and death in 2 hours. Therefore, in the verification room, we should not only pay attention to the influence of residual gas on verification, but also protect the influence of verification waste gas on human body, and increase necessary ventilation and exhaust equipment to ensure indoor air cleanliness.
Correct use of combustible gas detection alarm
When the concentration of combustible gas in the ambient air reaches or exceeds the alarm set value, the combustible gas detection alarm immediately sends out an alarm signal to remind people to take safety measures as soon as possible to prevent fire and explosion accidents. Now talk about the correct use of the commonly used fixed catalytic combustion combustible gas detection alarm.
(1) When using the detector, it should be noted that the grade of the dangerous place should be adapted to the explosion-proof mark of the instrument. The explosion-proof category, grade and group of detectors must meet the requirements of the category, grade and group of explosive gas mixture on site. It shall not be used in an environment that exceeds the permission of the explosion-proof mark, otherwise the explosion-proof effect cannot be achieved on site.
(2) The detector cannot be used in places containing sulfur, arsenic, phosphorus and halogen compounds. Because they will poison the detection elements in the detector, reduce the sensitivity of the detector, shorten the service life, and even make the detector fail. To detect combustible gases containing the above-mentioned elemental compounds, an anti-gas catalytic combustion detector or a semiconductor detector should be selected. Because catalytic combustion detector is explosive to hydrogen, electrochemical detector or semiconductor detector should be selected for hydrogen detection.
(3) The detector cannot be used under the environmental conditions that the concentration of combustible gas is higher than the lower explosion limit. Because the detection element used in the catalytic combustion detector is a carrier catalytic active element, when the concentration of combustible gas is higher than the lower explosion limit, the detection element in the detector will be burned out.
(4) Be careful not to make the detector accidentally enter water or be sprayed by steam. Because the detection element in the detector will affect its performance after water enters, if water enters unexpectedly, it is necessary to replace the detection element in the detector. Detectors installed outdoors should be equipped with rain cover.
(5) The installation position of the detector should be suitable for the density of the gas to be measured. Gases lighter than air always diffuse upwards, and the detector should be installed above the leakage source. The installation height should be 0.5 ~ 2m higher than that of the release source, and the horizontal distance from the release source should be appropriately reduced to less than 5m, so as to detect combustible gas as soon as possible. Gases heavier than air should be installed below the leakage source, and the installation height should be 0.3~0.6m higher than the ground. Too low is likely to cause damage to the detector due to rain splashing, while too high is beyond the height where gases heavier than air can easily accumulate.
(6) Before the instrument is put into operation, the working current (voltage) of the detector should be adjusted. The adjusted current (voltage) value should be within the range specified in the instrument manual to ensure the normal operation of the detector. After this current (voltage) is adjusted, it is generally unnecessary to move except for normal maintenance.
(7) When maintaining the instrument, it is not allowed to disassemble the detector on the spot when the instrument is powered on. Be careful when disassembling explosion-proof parts, and be careful not to damage the explosion-proof surface and contain dirt.
(8) Choose the installation position of the alarm correctly. The alarm is a non-explosion-proof part and is fixedly installed in a safe place. Its installation position should be chosen in a place convenient for observation and maintenance, and there should be no strong electromagnetic field around it that affects the normal work of the instrument.
(9) Set the alarm value of the alarm correctly. In general, the concentration range of combustible gas displayed by the alarm is 0 ~ 100% LEL. The alarm setting value is generally 20 ~ 30% LEL. For instruments with secondary alarm, the set value of primary alarm (upper limit) should be less than or equal to 20% LEL；; ; The setting value of secondary alarm (upper and lower limits) should be less than or equal to 50%LEL.
(10) The instrument shall be calibrated according to the calibration cycle, and the alarm function of the instrument shall be checked regularly at ordinary times. For the instrument with test button, start the test button of the alarm to check whether the alarm function of the alarm is normal.