Abstract
A suction unit has been developed and tested which will automatically dvaw air into filter samplers at a preset constant rate in spite of wide variutions in the resistance of the collecting media. Two models are described, one having a fixed flow rate of 10 cfm and the other being adjustable between 1 and 15 cfm. Components were selected to make them portable and to enable them to operate under adverse field conditions. Flow control is achieved b y using a vacuum operated valve ahead of the pump to compensate f o r upstream pressure variations and maintain constant pumping conditions. In performance tests the units kept the floui comtant within 5% while the pressure drop at the intake was increased from 0 to nearly 8 in. of Hg.