Method for estimating viscosity

1. A method for estimating the viscosity of a liquid in an aspirating or dispensing probe, comprising:

measuring a reference pressure which is the pressure inside a metering tip when no dispense or aspirate is occurring (P ref );

dispensing or aspirating a liquid having a volume of air between the liquid and the pumping mechanism of the probe;

stopping the aspirate or dispense;

measuring the pressure (P stop ) inside the tip at a time (t) which is the time when the aspirate or dispense is stopped;

measuring the pressure (P stop′ ) inside the tip at a time (t′) after t;

calculating a relative pressure difference ( p ) which is the difference between P stop and P ref ;

calculating the rate of pressure change ({dot over ( p ) at P stop by {dot over ( p =(P stop′ −P stop )/(t′−t); and

estimating viscosity as a function of p and {dot over ( p .

2. The method according to claim 1 , wherein P ref is ambient pressure or P ref is the pressure when the tip is in the fluid and is effected by gravity and capillary forces.

3. The method according to claim 2 , wherein P ref is substantially the same when P ref is determined by ambient pressure or pressure when the fluid is inside the tip.

4. The method according to claim 1 , wherein the estimating further comprises using the equation:

μ

=

K

⁢

p

_

p

_

.

wherein μ is viscosity.

5. The method as claimed in claim 4 , wherein K is pre-determined by a calibration curve generated using different viscosities for the fluid being aspirated or dispensed.

6. The method as claim in claim 1 , further comprising:

calculating a rate of pressure rise

(

R

)

=

P

stop

+

3

-

P

stop

P

stop

-

P

ref

;

and

estimating viscosity as a function of R, wherein P stop+3 is the pressure measured 3 time indexes after the aspirate or dispense is stopped.

7. The method as claimed in claim 6 , wherein viscosity is estimated as a function of R using a non-linear calibration curve.

8. The method as claimed in 7 , wherein the viscosity is estimated by:

μ

=

ⅇ

{

-

1

β

3

⁡

[

β

2

+

ln

⁡

(

β

1

R

-

β

0

-

1

)

]

}

wherein β 0 , β 1 , β 2 , and β 3 are constants determined by a non-linear calibration curve.

9. The method as claimed in claim 6 , wherein viscosity is estimated as a function of R using a linear calibration curve.

10. The method as claimed in claim 6 , wherein the viscosity is estimated by

μ

=

B

⁢

⁢

0

+

B

⁢

⁢

1

R

+

B

⁢

⁢

2

,

wherein B0, B1 and B2 are determined by a linear calibration curve.

11. The method as claimed in claim 1 , wherein t′ is measured 15 milliseconds after t.

12. The method as claimed in claim 1 , wherein the amount of liquid being aspirated or dispensed is ≦10 μL.

13. The method as claimed in claim 1 , wherein the amount of liquid being aspirated or dispensed is ≦5 μL.

14. The method as claimed in claim 1 , wherein the amount of liquid being aspirated or dispensed is ≦2 μL.

15. The method as claimed in claim 1 , wherein the liquid is a sample of bodily fluid.

16. The method as claimed in claim 15 , wherein the bodily fluid is one or more of whole blood, plasma, serum, urine or saliva.

17. The method as claimed in claim 1 , wherein the liquid is one or more of a reagent, wash fluid, calibrator fluid or control fluid.

18. A method for aspirating a selected amount of liquid comprising:

providing an aspirating probe which comprises a probe tip and a pump, wherein the probe tip and pump are in fluid communication;

measuring a reference pressure which is the pressure inside a metering tip when no dispense or aspirate is occurring (P ref );

moving the tip into the liquid, whereby a volume of gas is located between the top of the liquid and the pump, wherein the reference pressure is measured before or after the tip enters the liquid;

actuating the pump for a duration or distance which corresponds to a selected volume of liquid to be aspirated;

stopping the aspirate or dispense;

measuring the pressure (P stop ) inside the tip at a time (t) which is the time when the aspirate or dispense is stopped;

measuring the pressure (P stop′ ) inside the tip at a time (t′) after t;

calculating a relative pressure difference ( p ) which is the difference between P stop and P ref ;

calculating the rate of pressure change ({dot over ( p ) at P stop by {dot over ( p =(P stop′ −P stop )/(t′−t);

estimating viscosity as a function of p and {dot over ( p ; and

compensating for viscosity by either notifying an operator if the estimated viscosity is outside a predetermined range, or adjusting the actuation of the pump based on the estimated viscosity.

19. The method for aspirating a selected amount of liquid as claimed in claim 18 , wherein the pump is a piston pump and wherein the piston of the piston pump is moved a predetermined distance which corresponds to a selected volume of liquid to be aspirated.

20. A method of analyzing a sample for an analyte, comprising:

providing a source of a sample;

providing an aspirating probe which comprises a probe tip and a pump, wherein the probe tip and pump are in fluid communication;

measuring a reference pressure which is the pressure inside a metering tip when no dispense or aspirate is occurring (P ref );

moving the tip into the liquid, whereby a volume of gas is located between the top of the liquid and the pump, wherein the reference pressure is measured before or after the tip enters the liquid;

actuating the pump for a duration or distance which corresponds to a selected volume of liquid to be aspirated;

stopping the aspirate or dispense;

measuring the pressure (P stop ) inside the tip at a time (t) which is the time when the aspirate or dispense is stopped;

measuring the pressure (P stop′ ) inside the tip at a time (t′) after t;

calculating a relative pressure difference ( p ) which is the difference between P stop and P ref ;

calculating the rate of pressure change ({dot over ( p ) at P stop by {dot over ( p =(P stop′ −P stop )/(t′−t); and

estimating viscosity as a function of p and {dot over ( p ; and

compensating for viscosity by either notifying an operator if the estimated viscosity is outside a pre-determined range, or adjusting the actuation of the pump based on the estimated viscosity.