Gas detection operate in the following manner: An air sample is drawn through a tube containing a reagent, causing a colour change. The concentration is then read from the length of the colour stain in the reagent. The advantages of detection tubes over other analytical methods are simplicity of use, rapid response, low cost, and very low maintenance. Because each batch of Honeywell tubes is pre-calibrated, no calibration equipment is necessary. Errors are prevented by directly marking the calibration information on each tube, and accuracy is further ensured by controlling the volume of air sampled.