Existing Problems in Selecting Toxic and Hazardous Gas Detectors
There are numerous problems in the selection of various gas detectors, which are reflected in the following points:
① More emphasis is laid on combustible‑gas detection than toxic‑gas detection.
② Greater importance is attached to gases that may cause acute poisoning than those that may lead to chronic poisoning.
Bitter lessons from explosion accidents caused by numerous combustible‑gas leaks have drawn great attention to combustible‑gas detection. In petrochemical and chemical plants, the vast majority of hazardous‑gas detectors are LEL detectors. However, merely equipping LEL detectors is far from sufficient to truly protect workers' safety and health.
Admittedly, most volatile hazardous gases are combustible. Nevertheless, catalytic‑combustion LEL detectors are not the optimal choice for all combustible gases. They are specially designed for methane detection and show poor performance for other substances. Therefore, the detectable lower‑limit concentration of combustible gases other than methane is far higher than their permissible concentration. For instance, it is extremely dangerous to rely solely on combustible‑gas detectors for hazardous toxic gases such as benzene and ammonia. The lower explosive limit of benzene is 1.2 %, and its correction factor on an LEL detector is 2.51. That means the reading of a methane‑calibrated LEL detector for benzene is only 40 % of its actual concentration. The minimum alarm concentration of benzene detectable by LEL is 10 %LEL = 10 % × 1.2 % × 2.51 = 3.0 × 10⁻³, which is nearly 600 times higher than the permissible concentration of benzene at 5 × 10⁻⁶. Similarly, the alarm concentration of ammonia measured by an LEL detector is 1.5 × 10⁻², approximately 600 times higher than its permissible concentration of 2.5 × 10⁻⁵. Accordingly, selecting dedicated toxic‑gas detectors according to target gases is much safer and more reliable than simply adopting LEL detectors.
In addition, sufficient attention is paid to gases causing acute poisoning such as hydrogen sulfide and hydrocyanic acid, while insufficient attention is given to gases triggering chronic poisoning including aromatic hydrocarbons and alcohols. In fact, hazards posed by the latter to workers' health and safety are no less serious than those of acutely toxic gases. They may induce cancers and other latent diseases and impair workers' life expectancy and physical condition. Apart from cognitive factors, the previous lack of qualified detectors for low‑concentration organic gas measurement on the market is another major contributing factor.
