At some point, most atmospheric monitors will display a “negative” or minus reading for a flammable gas or toxic contaminant. First of all, it is not actually possible for an atmosphere to contain a “negative amount” of a substance. These negative readings usually result from improper use of the monitor.
Most monitors will “Field Zero” or “Fresh Air Calibrate” its sensors when powered on. Because of this, it is very important to power on the unit in a clean, fresh air environment away from confined spaces, running equipment or other possible contaminants. Otherwise, the monitor may falsely calibrate based on the contaminant that is present.For example, a monitor that is powered on in an atmosphere that contains 10 ppm of a contaminant and then moved to fresh air may display a reading of minus 10 ppm. Even more troublesome, if that same monitor is then brought to a confined space that actually contains 25 ppm of the contaminant, it may display a reading of only 15 ppm. As you can see, this could easily lead to the improper selection of PPE for the entrant and result in a confined space emergency.
As always, it is very important to consult with the manufacturer of your particular atmospheric monitor in order to determine how to use it properly. Don’t take any chances with this critical part of preparing for confined space entry or rescue operations.