The sampling tube is identified by a series of air inlet holes on the tube. The sampling tube is always installed with the air inlet hole facing into the air flow. For duct widths greater than 18 inch, sampling tubes must be purchased separately. The recommended length is specified below. It is also recommended that the sampling tube length extend at least 2/3 across the duct width for optimal performance. Sampling tubes over 3ft long must be supported at the end opposite the duct smoke detector.