Method and apparatus for dispensing precise aliquots of liquid

1. A pipette controller comprising:

a pipette holder adapted to operatively connect a pipette to the pipette controller;

a pump having a vacuum port and a pressure port, the pump pneumatically connected to the pipette holder;

an aspirate valve that controls airflow between the vacuum port and the pipette holder;

a dispense valve that controls airflow between the pressure port and the pipette holder;

a piston chamber;

an aliquot dispense pump including a piston having a shaft that extends into the piston chamber, the shaft defining a stroke length; and

an aliquot check valve that connects the pipette holder and the aliquot dispense pump; wherein

the aliquot check valve opens to allow airflow into the pipette holder upon engagement of the aliquot dispense valve;

a piston chamber pressure sensor that determines air pressure inside the piston chamber;

an atmospheric pressure sensor that determines atmospheric air pressure; and

a pipette pressure sensor that determines pipette air pressure.

2. The pipette controller of claim 1 , wherein the stroke length is defined by a movable threaded stop located on the shaft.

3. The pipette controller of claim 2 , further comprising:

a threaded stop control, wherein the threaded stop control is rotatable to move the threaded stop.

4. The pipette controller of claim 3 , further comprising:

an aspirate check valve that connects the piston chamber to an atmosphere; wherein

the aspirate check valve opens to allow airflow from the atmosphere into the piston chamber.

5. The pipette controller of claim 1 , further comprising:

a stepper motor that drives the aliquot dispense pump.

6. The pipette controller of claim 5 , further comprising:

an aliquot volume control operable to select an aliquot volume; and

a processor; wherein

the processor controls the stepper motor to deliver a number of steps required for the selected aliquot volume.

7. The pipette controller of claim 6 , wherein:

the processor controls the stepper motor to deliver successive aliquots.

8. The pipette controller of claim 7 , wherein:

the successive aliquots are of different aliquot volumes.

9. The pipette controller of claim 1 , wherein:

the pipette controller corrects the number of steps required for a selected aliquot volume based on the air pressure of at least one of the piston chamber pressure sensor, the atmospheric pressure sensor, or the pipette pressure sensor.

10. The pipette controller of claim 9 , wherein:

the number of steps is determined by a value in a lookup table.

11. The pipette controller of claim 9 , wherein:

the number of steps is calculated by formula.

12. The pipette controller of claim 6 , further comprising:

an orientation sensor that measures an angle of the pipette connected to the pipette holder relative to vertical;

wherein the pipette controller corrects the number of steps required for a selected aliquot volume based on the angle of the pipette.

13. The pipette controller of claim 1 , wherein:

the pipette controller is a handheld device.