Technical background Piston-driven manually operated pipettes usually involve a so called secondary movement function of the piston. Thus, for instance, Fl patent specification 64752 (corresponding e. g. to EP specification 112887) discloses such a pipette. This pipette comprises a stop sleeve adjustable in the longitudinal direction of the piston rod and a stationary stop in the body, against which stop a primary spring presses the stop sleeve, thus determining the upper limit of the piston movement.

The upper part of the rod comprises a calibration nut with a flange, against which a secondary spring stronger than the primary spring presses a lower stop flange.

When pressed downwardly, the piston initially moves against the force of the primary spring until the lower stop flange hits the body stop. When the pipette tip is now immersed in a liquid and the piston is allowed to resume its upper position, a desired liquid volume is drawn into the pipette. When the liquid is to be removed, the piston is again first pressed against the force of the primary spring, however, with continued pressing against the force of the secondary spring as well, so that the liquid will be removed from the pipette as completely as possible.

Summary of the invention A pipette according to claim 1 has now been invented. The other claims define some embodiments of the invention.

In accordance with the invention, the pipette comprises an intermediate phase between the primary phase and the secondary phase, with the push button moving but the piston device being immobile during this intermediate phase. In this

manner, the piston, when pressed downwardly, will always stop exactly at the lower limit of the primary movement.

The invention provides improvements both in dosage precision and repeatability.

This also minimises the impact of individual pipetting manners ("hand") of pipette users.

Drawings The accompanying drawings pertain to the written description of the invention and relate to the following detailed disclosure of the invention. In the drawings - figure 1 is a lateral cross-sectional perspective view of the pipette - figure 2 is an enlarged view of the top of the pipette and - figure 3 illustrates the use of the pipette.

Detailed disclosure of the invention The pipette of the invention comprises a piston device and a push button accompanying the piston movement at a first phase. The first phase is followed by an intermediate phase, at which the push button moves while the piston remains immobile. The intermediate phase is followed by a second phase, at which both the piston device and the push button move.

The pipette comprises preferably force elements, which resist the movement of the piston device or the push button towards the lower position. The force elements preferably return the piston device or the push button, respectively, into the upper position. The forces generated by the force elements vary preferably at different phases, allowing the pipette user to perceive the differences between various phases. The force of the first phase is most preferably smaller than the force of the intermediate phase or the second phase, and the force of the intermediate phase is smaller than the force of the second phase.

The force element may consist e. g. of a spring, especially a coil spring, or e. g. a magnetic assembly. The spring of the first phase may be located between a support in the body and a stop in the piston device, pressing the piston device against the stop in the body. The spring of the second phase may also be disposed between the support in the body and the stop in the piston device,

pressing the piston device against the stop in the body. The stop and the support of the first phase determine the move distance of the piston during the aspiration movement. The location of the stop or the support may be variable, allowing adjustment of the volume. In that case, the pipette preferably comprises also a suitable display, e. g. a digital display, for indicating the volume. The stop or the support may be adjustable so as to allow calibration of the volume.

The push button and the piston device may be coaxially disposed, in particular with the push button having an axial hole, into which the upper end of the piston device is fitted.

The spring of the intermediate phase can be disposed between a stop and a push button in the piston device, especially about a rod in the piston device. In that case, the spring is preferably surrounded by a sleeve in the piston device, the push button comprising preferably an additional part disposed between the rod and the sleeve. The lower end of this part preferably comprises a protruding portion and the sleeve comprises accordingly an annular space, in which the protruding portion is adapted to move axially.

The upper end of the piston device may be such that rises at the intermediate phase above the upper surface of the push button, preferably through a hole provided in its supper surface, so that it can be perceived by the pipette user.

While a liquid is being drawn into the pipette, the push button can be pressed beyond the lower limit of the first phase, with the piston still remaining exactly at the lower limit. When using a conventional pipette, which consequently does not have any intermediate phase, one has to take special care not to press the piston into the secondary phase in order to avoid aspiration of too large a volume. By contrast, users that are aware of this risk and take care to avoid it tend to cause an inverse error. The invention allows the same user to attain substantially enhanced pipetting precision and repeat precision. In addition to this, each user has his/her individual pipetting manner ("hand"), which influences precision and repeat precision. The invention also reduces substantially the impact of such different pipetting manners.

A further example of the pipette of the invention is given below.

The pipette has a body 1 and a tip part 2 at the lower end of the body, and a tip container or tip for liquid is fixed to the lower end of the tip part. A sliding tip removal sleeve 3 is provided on top of the tip part. The removal sleeve is extended by a removal sleeve force element rod 4, which, with a view to reducing the necessary driving force, is forced into movement relative to a handle by means of a wheel 5 (cf. FI 92374 corresponding to e. g. EP specification 566939). A spring 6 for returning the tip removal sleeve is disposed between the force element rod and the body. A pusher 7 is provided at the upper end of the force element rod. A finger rest 8 is disposed in the upper part of the body.

A circular hole goes through the tip part 2, which hole is extended also by a hole through the body 1. The tip part has a cylinder, comprising a piston 9, which is extended by a rod 10. The lower portion of the rod is shaped as a sleeve and disposed on top of the upper part of the piston. The upper part of the rod penetrates through the body. A push button 11 is provided at the upper end of the rod. In the upper portion of the tip part, the hole has an enlargement, at the bottom of which an O-ring 12 is fitted for sealing the piston against the tip part. The O-ring is pressed from above by a sealing spring 13 disposed between the body and the tip part.

The piston rod 10 has a thread, on which a nut 14 is screwed, the rotation of the nut relative to the body being prevented by longitudinal guide bars and grooves.

Consequently, as the rod is rotated, the nut slides in the longitudinal direction of the rod. The body comprises an upper position support 15, against which the piston is pressed from below by a primary spring 16. In other words, the nut and the upper position support determine the upper limit of the piston movement. A secondary support sleeve 17 is disposed slidably in the upper part of the rod, the secondary support sleeve having an outer flange 18 at its upper end and an inner flange 19 at its lower end. Above the secondary support sleeve, a calibration nut 20 is threaded into a bore in the body. The secondary support sleeve is surrounded by a secondary spring 21, whose upper end presses the secondary support sleeve at its upper flange against the calibration nut.

At its upper end, the piston rod 10 comprises a necked knob rod 22, whose upper end 23 extends throughout the push button 11. A sleeve-like knob 24 is attached to the piston rod, with the knob rod extending through the knob. The bore has an enlarged space containing a fastener 25. The lower part of the knob is fitted

around the upper end of the large portion of the piston rod 10 and it comprises a necking 26 fitted against the upper end of this large portion. A supplementary knob 27 is disposed about the knob rod movably relative to this. The supplementary knob has an outer sleeve 28, which consists of longitudinal strips having at their lower ends projections 29 engaging the enlarged groove-like inner space in the knob. This allows the supplementary knob to be fitted into its position by pushing as the strips yield. Inside the outer sleeve, an intermediate spring 30 is disposed, which is stiffer than the primary spring 16 but weaker than the secondary spring 21. The lower end of the intermediate spring bears against the fastener and the necking of the knob and its upper end presses the supplementary knob into upper position, where the protrusions of the lower end of the outer sleeve of the supplementary knob bear against the upper end of the enlarged inner space of the bore in the knob. Inside the intermediate spring, an inner sleeve 31 shorter than the outer sleeve is disposed around the knob rod. The upper end of the supplementary knob has a larger knob flange 32, on top of which a shaped cap 33 is fixed, which has a bore for the knob rod.

When the piston rod 10 is pressed down with the push button 11, the weakest spring, i. e. the primary spring 16, is the first to yield. The secondary support sleeve 17 and the supplementary knob 27 with its intermediate spring 30 move along with the piston rod until the lower end of the knob 24 hits the inner flange 19 of the secondary support sleeve, and then the stronger intermediate spring starts resisting the movement. The push button can be pressed by a distance equalling the height of the enlarged inner space of the knob even also against the force that of the intermediate spring, without the piston moving. Besides from the increased resistive force, the lower limit of the primary movement is known from the upper end of the knob rod 22 rising from the upper surface of the cap 33, whereby the pipette user can feel it with his/her finger. With the pistons at the lower limit of the primary movement, the pipette tip is immersed in the liquid and the piston is allowed to resume its upper position, so that the liquid is drawn into the pipette.

When the liquid is removed from the pipette, the push button11 is first pressed against the force of the primary spring 16 and then against the force of the intermediate spring 30, until the lower end of the upper flange 32 of the supplementary knob 27 hits the upper end of the knob 24. At this phase, the force of the secondary spring 21 starts resisting the movement. When the push button is still pressed against this force, the piston, accompanied by the secondary support

sleeve 17, starts moving downwardly and the liquid will be removed with maximum precision.

Figure 3 further illustrates the reliable dosage assured by the invention. An intermediate region A is located between the lower limit of the primary movement and the upper limit of the secondary movement, and within this region the piston does not move, but is immobilised exactly at the lower limit of the primary movement. Consequently, movement of the push button 11 into this intermediate region does not influence the liquid volume to be aspirated. This allows not only for enhanced precision but also for enhanced repeat precision in particular.

The dosage volume is adjusted by rotating the push button 11, resulting in a shift of the position of the nut 14 on the piston rod 10. The dosage volume is calibrated by adjusting the lower limit of the aspiration movement with rotation of the calibration nut 18. (Similar volume control and calibration arrangements have been described e. g. in FI 64752 cited as a reference above and EP 73515).

The pipette also comprises a volume display. A first digital ring 34 is disposed about the piston rod 10 in co-rotation with this. After each full rotation, the first digital ring rotates a second digital ring 36 via a first transfer wheel 35, the second digital ring rotating accordingly a third digital ring 37 via a second transfer wheel 37. At the upper surface end of the first digital ring, a stop recess is provided for each ring reading, corresponding to a bossage in a non-rotatable ring spring 39.

The body 1 comprises a window showing the volume indicated by the digital rings.

(Similar volume indicating arrangements have been explained e. g. in FI specification 64752 mentioned above).