Four Problems to Be Noted in the Use of Flammable Gas Detection Alarms

In recent years, flammable gas detection alarms have been widely used in various industries. The large maintenance volume of flammable gas alarm systems and their common non-reported and misreported issues often plague the maintenance personnel of the meters. Therefore, it is important to regularly check the flammable gas alarms. As the first-line verification personnel, the following issues should be noted in the flammable gas alarm verification.

First, verify the environmental conditions. Because it is an electronic instrument, the errors in its verification mainly come from the verification environment:

1. Ambient temperature. During the verification process, changes in temperature before and after the test will significantly change the vitality of the molecule, and the increased temperature will increase the risk. The temperature has no direct influence on the flammable gas alarm itself, but when the temperature is below 0°C, the inertness of the standard gas is increased, and the standard gas molecular activity is not active enough. When the temperature is low, the instrument display value will have a large error. I think the temperature Below 0 °C is not suitable for the verification work.

2. It requires good ventilation. For the open diffusion of combustible gas, if there is a lack of good ventilation, it is easy to make the combustible gas content in the air close to or reach the lower explosion concentration, resulting in inaccurate test results.

3. No interference with the measured component. In the process of testing combustible gas alarms, attention should be paid to preventing electromagnetic interference. There are three main factors affecting the combustible gas alarm in the electromagnetic environment: human body static electricity, narrow pulse groups on the power supply and other input and output lines, and electromagnetic interference in the air.

Second, verification of flow control. In the verification process, since combustible gas alarms are mostly catalytic combustion sensors, the size of the flow rate directly affects the accuracy of the verification results. The alarm can maintain high sensitivity within a certain flow range, too high or too low will have a greater impact on the sensitivity. When the flow rate is less than the measurement requirements, the sensor's original surface can not reach the true concentration, and the combustible gas can not be replenished in time after the combustion of the combustible gas, and the combustion can not continue, so the sensitivity is very low. After the gas flow increases, the thermal effect increases accordingly, and the sensitivity gradually increases. When the gas flow continues to increase, excessive airflow will increase the heat dissipation of the components, which in turn reduces the sensitivity. When the flow exceeds the measurement requirements, the accuracy decreases. When the general instrument is designed, the manufacturer has selected the optimal gas flow through trial and error. JJG693-2011 "Combustion gas detection alarm verification procedures" also mentioned flow control can be in accordance with the technical specifications, but in the actual verification of a wide range of combustible gas alarms, we can not guarantee that can find reliable technical specifications, so in accordance with the years The verification experience, I think the flow control is a range, as long as the pressure is not the probe that is too much traffic on the line. Most of the normal checks are controlled at 500ml/min, and individual alarms are required to be around 350ml/min. Of course, at the same time, it is necessary to keep the flow rate stable and uniform, and it must not be ignored. Otherwise it may cause fluctuations in the test indication. Excessive flow can also damage the sensor. When the gas flow rate is small, the bypass flowmeter must be used for the inhaled alarms. Otherwise, the inaccuracy of the verification results will be easily caused.

Third, the main controller does not match the on-site detector. In general, combustible gas alarms are connected by the main controller and the detector in a sub-line system. The detectors are distributed in multiple outlets. The main controller centrally controls multiple monitoring outlets, and the gas concentration of each detector is independently displayed. When the concentration of the detector gas reaches or exceeds the preset alarm value, the main controller issues an alarm to remind the on-duty personnel to take safety measures so as to ensure the safe production of the enterprise. The author often finds that the main controller and the on-site detector show a far cry from the on-site inspection, which results in the tester not knowing how to judge whether the equipment is qualified or not. I think that this situation should be adjusted to adjust the zero and the range potentiometer of the main controller. The output should be related to the gas concentration and should be adjusted if it fails. The main controller can display the detectors on site separately, that is, they can be adjusted separately. If only the value on the detector is adjusted to ignore the value on the master, then the master controller is dummy and will not perform any monitoring.

Fourth, calibration calibration cover. The verification procedures stipulate that the diffusion instrument should have a special calibration mask, and each manufacturer has its own special calibration mask. However, in the actual verification, the verification personnel will replace the self-sealed bags and self-made calibration masks. I think the verification time is still to use the original factory as much as possible. Dedicated is better, because the calibration cover's inner space and air intake mode (bottom air intake and side air intake) have some influence on the two indicators of the instrument's response time and indication error. Of course, no matter what type of calibration hood is used when testing combustible gas alarms, one principle must be adhered to. That is, it is forbidden that the calibration hood is too tight and there is no overflow port to generate positive pressure, which will affect the verification results. It is also forbidden to be too loose, leading to the entry of outside gases, making the actual value of the standard gas smaller, affecting the verification results.