The Uniphos brand of detector tubes are used with the precision ASP-40 piston hand pump. The air sample is drawn through the narrow tube containing a specific gas sensing chemical causing a color change.  The length of color stain gives the concentration which is read directly from the measurement scale printed on the tube. The piston style pump and tube configuration often have the measurement based on a single pump stroke.   Uniphos detector tubes provide a simple measurement of chemicals in gas samples. Since the tubes are pre-calibrated, no calibration is necessary and hence are absolutely free of maintenance.   More than 250 different Uniphos Gas Detection Tubes are available for over 150 different chemicals and concentration ranges.