Although it is physically impossbile for the atmosphere to contain a negative quantity of a substance, a gas detector can display a negative reading on either the Lower Explosive Limit (LEL) or toxic channel. 

Negative readings are a result of the detector software interpreting the sensor output. LEL Sensors detect gas in a different way to toxic sensors but they both create a signal (electical output) that is directly porportional to the amount of gas detected. 

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