Pipette

Abstract

An outer tube has a hollow elongated member, attached to one end of the tube in a coaxial relationship. A rod slidable in the tube causes a replaceable tip, mounted on the hollow elongated member, to fill with fluid when the rod is moved in a first direction and discharges the fluid when the rod is moved in the opposite direction. A first spring surrounds the rod and exerts a force between a stop, mounted at an intermediate position within the tube, and the rod to bias the rod in the first direction. A second spring surrounds the rod and is interposed between the stop and the inner end of the hollow elongated member. A first limiting member is mounted on the rod to compress the second spring when the rod is moved in the opposite direction beyond a predetermined limit.

What is claimed is: 
     
       1. A syringe type of measuring instrument including: an outer tube; a rod slidable in said tube; a hollow elongated member attached to one end of said tube in coaxial relationship therewith for receiving said rod and defining a chamber which, when its volume changes, causes a replaceable tip to fill with fluid when said rod is moved in a first direction with respect to said tube and causes discharge of fluid from the replaceable tip when said rod is moved in the opposite direction; a stop member mounted in said tube at an intermediate position therein; a first spring member surrounding said rod and exerting a force between said stop member and said rod to bias said rod along the longitudinal axis of said tube and in said first direction; a second spring member surrounding said rod and interposed between said stop member and the inner end of said hollow end member; and a first limit member mounted on said rod in position .Iadd.to engage said stop member when the rod is moved in the first direction and .Iaddend.to compress said second spring member when said rod is moved in said opposite direction beyond a predetermined limit. 
     
     
       2. The syringe type of measuring instrument defined in claim 1 in which said elongated hollow member is threadably attached to said one end of said outer tube. 
     
     
       3. The measuring instrument defined in claim 1 and which includes a push button attached to the end of said rod remote from said elongated hollow member, said push button protruding beyond the corresponding end of said outer tube. 
     
     
       4. The measuring instrument defined in claim 1 and which includes a stop washer positioned in said tube against said stop member and interposed between said stop member and one end of said second spring, and a further limit member mounted on said rod on the opposite side of said washer from said first limit member in position to engage said stop washer and limit the movement of said rod in said first direction. 
     
     
       5. The measuring instrument defined in claim 1 and which includes an insert member interposed in said outer tube between the inner end of said elongated hollow member and said second spring member. 
     
     
       6. The measuring instrument defined in claim 1 and which includes a support bracket interposed between said stop member and the end of said first spring member, a pair of switch contacts mounted on said support bracket, and a bridging electrically conductive member movable with said rod to establish electrical contact between said switch contacts when said rod is moved to a limit position in said opposite direction. 
     
     
       7. The measuring instrument defined in claim 1 and which includes electrical switching means mechanically coupled to said rod and actuable when said rod is moved to a predetermined axial position with respect to said outer tube. 
     
     
       8. A syringe type of measuring instrument including: an outer tube; a rod slidable in said tube; a knob attached to one end of said rod and protruding beyond the corresponding end of said outer tube; an elongated hollow member threadably attached to the other end of said tube in coaxial relationship therewith for receiving said rod, said elongated hollow member defining a chamber which, when its volume changes, causes a replaceable tip to fill with fluid when said rod is moved in a first direction with respect to said tube and for discharging fluids from the disposable tip when said rod is moved in the opposite direction; an annular stop member mounted in the bore of said tube at an intermediate position therein; a first spring member surrounding said rod and exerting a force between said annular stop member and the inner end of said knob to bias said rod in said first direction; a second spring member surrounding said rod and interposed between said annular stop member and said elongated hollow member; a stop washer positioned in said tube against said annular stop member and interposed between said annular stop member and the inner end of said second spring; a first retainer ring mounted on said rod between said washer and said knob and in position to engage said washer and compress said second spring member when said rod is moved in said opposite direction beyond a predetermined limit; and a second retainer ring mounted on said rod on the opposite side of said washer from said first retainer ring and in position to engage said washer and bias said washer against said annular stop member to limit the movement of said rod in said first direction. 
     
     
       9. The measuring instrument defined in claim 8 and which includes an insert member interposed in said outer tube between the inner end of said elongated hollow member and the end of said second spring member. 
     
     
       10. The measuring instrument defined in claim 8 and which includes a support bracket interposed between said annular stop member and the end of said first spring, a pair of switch contacts mounted on said support bracket and insulated from one another, and a bridging electrically conductive member integral with said knob and movable with said rod to establish electrical contact between said switch contacts when said rod is moved to a limiting position in said opposite direction.