Chemical dosimeter having a constant flow air sampling pump

An improved dosimeter for individual use that has an electric motor, a power source, an exhaust port and a filter means in which particles or vapors present in an air stream being pumped through the dosimeter are collected on the filter means;

• the improvement in use therewith comprises
• a variable drive pump tubularly connected to the filter means and coupled to the electric motor draws the air stream through the filter means;
• an air reservoir connected to the pump into which the air stream is being pumped retains excess air supplied by the pump and maintains a constant flow rate of the air stream;
• an orifice comprising an adjustable needle valve being positioned in a tube attached to the air reservoir and to the exhaust port, wherein the air stream is pumped through the orifice and thereby creates an air pressure drop;
• a differential pressure switch positioned in a tube connected to the exhaust port and in parallel to the orifice is activated by a change in the air pressure drop of the air stream and creates a low voltage electrical input signal;
• an integrator circuit electrically connected to a power source and to the pressure switch uses the low voltage input signal generated by the pressure switch and integrates this signal;
• an amplifier circuit electrically connected to the power source and connected in series to the integrator circuit and to the electric motor which amplifies the signal generated by the integrator circuit and feeds the amplified signal to the electric motor, thereby controlling the speed of the motor driving the pump in relationship to the signal generated by the pressure switch to maintain the air stream at a constant flow rate.