【If it’s to be solved, it must be solved thoroughly】Testing of portable combustible and toxic gas detectors【LEL】
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First, let me explain the basic features of my portable detector: it is a three-in-one gas tester that can measure the ppm levels of Ⅰ) CO and H2S； Ⅱ) Volume percentage of oxygen content (20.9% as measured in an environment with proper ventilation) ； Ⅲ) Testing of the LEL of flammable and explosive gases (calibration using methane or pentane gases at 25% LEL or 50% LEL). Regarding item I above, there are low-concentration alarms, high-concentration alarms, TWA readings, and STEL readings. I have the following questions: 1. For item I, regarding the TWA readings and STEL readings, TWA represents the 8-hour average exposure limit, while the STEL reading represents the exposure limit for a short period of 15 minutes. In a toxic gas environment, as long as the concentration exceeds the lower limit, it will issue an alarm ； If it exceeds the limit, it will trigger an alarm and vibration. However, the TWA and STEL values remain essentially at zero. Why is that? How to understand it? 2. Item III is the percentage of the explosive gas relative to its lower limit. It is calibrated using methane. The instructions state that LEL is used to test flammable and explosive gases, but can it measure all types of flammable gases? Can ammonia be detected? If other explosive gases can be measured, and they are calibrated using methane, then based on the readings, it’s not possible to determine the exact concentration, right? Can only determine that there are flammable and explosive gases in the space? I’m quite confused about this; my language skills aren’t good enough, so please be understanding!
For the first item, to determine whether it is accurate or not, you need to use a standard gas for testing; if the reading is zero under normal conditions, it indicates no leakage, which is normal. Thirdly, combustible gas detection: in theory, it can detect almost all organic VOC vapors, including H2 and NH3, etc. However, the values displayed range from 0 to 100% LEL, where LEL refers to the lower explosive limit of methane, which is 5%. That is, the actual corresponding volume concentration range is 0-5% LEL. Different gases have different RAN heating values, and accordingly, the measured values also vary. That is, the detection values for 3% methane and 3% H2 are different. However, by using methane as the national standard and setting the alarm levels at 25% and 50% LEL, the risk of explosion can be basically avoided. I’m not sure if the original poster will understand it this way
I can understand part of it; I’ll think about it more and explore it further! :handshake{:1_90:}
I stayed there for a moment in a high-alarm environment; the high alarm was both glowing and vibrating. Then the TWA and STEL readings were almost zero, remaining practically unchanged; it was indeed a measurement taken in an environment with concentrations above the limits. Do I really have to wait 8 hours just to get the TWA values? ? Seeking advice from experts! !
It is normal for this TWA to have no response. If you set the alarm for a short period of time and average it over 8 hours, indeed nothing can be detected. If you are unsure whether the instrument can still be used effectively, you can try passing 3% methane through it for a period of time (at least 3 minutes), and the STEL value should fluctuate.
Uh, got it! O(∩_∩)O