Abstract: Portable gas detection and/or monitoring apparatus including a case having a substantially cylindrical wall and two opposite end caps sealably connected to the cylindrical wall, at least one of the end caps being removable from the cylindrical wall, and at least a portion of the cylindrical wall being transparent. A gas detection and/or monitoring unit is mounted in the case and includes gas sensing means in sealed fluid communication with the ambient atmosphere outside the case. Data processing means is operatively connected to the gas sensing means, and data storage means and information display means is operatively connected to the data processing means.