Oxygen sensors are often placed in lift shaft wells to measure the atmosphere prior to entry. Problems arise due to the pressure changes in the left shaft well due to the air movements associated with the lift.resulting in false alarms . There is also an issue associated with safe work permits for access into the lift shaft area when the oxygen sensor requires calibration or replacement. The GDA 4500 system uses a vacuum pump ( dc brushless) , a flow fail mechanism and a sensor to extract the gas via a 6mm ID tubing . The Nylon tube is fitted with a filter element to provide dust filtering and prevent mycological build up in the sampling line.