Issues with the setting of alarm values for toxic gases
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It started because I saw a post (Figure 1) discussing the alarm values that should be set for benzene, and since there were some controversies in that post, I went to check GB-50493 (Figure 2). According to Figure 3, the PC-TWA (time-weighted average permissible concentration) for benzene is 6 mg/m3, which is approximately 1.85 ppm, while the PC-STEL (short-term exposure permissible concentration) is 10 mg/m3, or about 3 ppm. The specifications state that the first-level alarm setting for toxic gases should be less than or equal to 100% OEL (Occupational Exposure Limit), while the second-level alarm setting should be less than or equal to 200% OEL. This led me to wonder whether OEL refers to MAC (Maximum Allowable Concentration), PC-TWA, or PC-STEL. I searched for information on OEL (Figure 4), and according to that figure, OEL seems to refer to PC-TWA. However, I saw in various posts that many companies set the first-level alarm value for benzene at 3 ppm and the second-level alarm value at 6 ppm, which implies that OEL in their case refers to PC-STEL. Therefore, I’m confused – could someone tell me which concentration OEL actually refers to in these specifications?
The specification also has a second part: if the alarm value is set at 1.85, the measuring range of the instrument is too large, resulting in insufficient accuracy
I looked at 50493 again, and there was a passage in it that said
The poster’s screenshot makes it clear; just follow the regulations strictly