The sample being measured is pulled under an adjustable vacuum created by an air driven eductor. Flow rates are controlled by an integral needle value with the velocity being indicated on a flow meter. The sample is passed across the sensor and exhausted, along with instrument air, via the exhaust port. If the aspirator is positioned in a “safe” area, it is advisable to pipe the exhaust back to the area it has come from.Sample line dross can be cleared by pressing the line purge button, diverting full pressure instrument air down the sample line.