Personal air sampling system and pump for use therein

Abstract

In a personal air sampling system, flow control is achieved by monitoring the back pressure created by a local flow restriction (eg. an adjustable valve) located at the sampler's outlet. The pressure drop across the restriction is monitored by a pressure sensor and is directly related to inlet flow rate. The use of an adjustable restriction on the outlet provides increased dynamic range of the flow measurement system without excessive loading to the pump. A signal conditioning circuit takes the signal from the pressure sensor and feeds it to an analogue to digital converter which in turn feeds it to a microprocessor. The microprocessor controls the pump drive circuitry such that a constant inlet flow rate is maintained. The user can select the desired operating flow rate to an accuracy such that an additional external flow measuring device is not required. A pump of the system makes use of non-circular diaphragms within the pump assembly to provide a compact, space saving design. Both upstream and downstream dampers are employed to reduce pulsation in the air flow.

1 . An air sampling system comprising a pump for drawing air through a sampling device, a local flow restriction downstream of the pump, a transducer for measuring a pressure drop across the restriction and for producing a signal indicative thereof, an input device for enabling a user to set a desired flow rate, and a controller containing a correlation between the pressure drop and the volumetric flow rate of air through the restriction, the controller being configured to control the pump to produce a said pressure drop corresponding to the desired flow rate set by the user.  
     
     
         2 . A system as claimed in  claim 1 , further comprising a display.  
     
     
         3 . A system as claimed in  claim 2 , wherein the display is integral with the pump.  
     
     
         4 . A system as claimed in  claim 1 , further wherein the controller is configured to calculating the actual flow rate.  
     
     
         5 . A system as claimed in  claim 4 , wherein the display is configured to display the actual flow rate.  
     
     
         6 . A system as claimed in  claim 4 , wherein the controller is configured to alert a user if the calculated actual flow rate differs from the flow rate set by the user by more than a predetermined amount.  
     
     
         7 . A system as claimed in  claim 1 , wherein the input device is integral with the pump.  
     
     
         8 . A system as claimed in  claim 1 , wherein the local flow restriction is adjustable.  
     
     
         9 . A system as claimed in  claim 8 , wherein the local flow restriction is a needle valve.  
     
     
         10 . A system as claimed in  claim 1 , comprising a temperature sensor for sensing the temperature of the air passing through the restriction, the controller adjusting the said correlation according to the sensed temperature.  
     
     
         11 . A system as claimed in  claim 1 , comprising a pulsation damper downstream of the pump.  
     
     
         12 . A system as claimed in  claim 11 , wherein the temperature sensor is located in the downstream pulsation damper.  
     
     
         13 . A system as claimed in  claim 1 , comprising a pulsation damper upstream of the pump.  
     
     
         14 . A system as claimed in  claim 1 , wherein the pump comprises a pumping chamber having a displaceable wall, a motor and a drive driven by the motor for reciprocating the wall, the wall having a major dimension and a minor dimension, and being disposed with its major dimension substantially parallel to a major dimension of the motor.  
     
     
         15 . A system as claimed in  claim 14 , wherein the major dimension of the motor is parallel to an axis of rotation thereof.  
     
     
         16 . A system as claimed in  claim 14 , wherein there are two pumping chambers, each with a displaceable wall disposed on opposite sides of the motor.  
     
     
         17 . A system as claimed in  claim 14 , wherein the drive comprises an eccentric mounted on a shaft of the motor.  
     
     
         18 . A system as claimed in  claim 14 , where the or each wall has substantially semicircular ends joined by straight sides.  
     
     
         19 . A system as claimed in  claim 14 , wherein the or each wall is a diaphragm.