US3827305A

US3827305A - Adjustable pipette

Info

Publication number: US3827305A
Application number: US00300215A
Authority: US
Inventors: W Gilson; R Gilson
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-10-24
Filing date: 1972-10-24
Publication date: 1974-08-06
Anticipated expiration: 1991-08-06
Also published as: FR2203680B1; JPS4975170A; FR2203680A1; GB1439659A; JPS5713338B2

Abstract

An adjustable pipette includes an elongated body defining a barrel portion and a plunger cavity. A plunger unit is mounted for reciprocation in the body and includes a plunger portion extending from the barrel portion into the plunger cavity, and further includes an operating portion extending rearwardly out of the body. The plunger unit includes a stop disposed within the barrel portion, and an adjustable rear stop for varying the volume of a pipetted sample is carried on a threaded shaft received in a nut fixed in the body. An indicator including rotatable indicia-carrying rings is coupled to the shaft for providing an indication of the volume setting of the pipette. A forward stop for movement of the plunger unit is provided by an overtravel member engaged by the plunger stop member. Travel of the plunger unit past the forward position compresses an overtravel spring, and a seal assembly is compressed by the overtravel spring into sealing engagement with the plunger portion of the plunger unit. A sleeve carried within the threaded shaft around the operating portion of the plunger unit is adjustable relative to the shaft in order to align the indicator relative to the shaft. A lock arrangement is provided for locking the shaft in position to prevent inadvertent misadjustment of the volume.

Description

United States Patent 1191 1111 3,827,305 Gilson'et al. Aug. 6, 1974 ADJUSTABLE PIPETTE [57] ABSTRACT Inventorsi Warren 4801 sheboygan An adjustable pipette includes an elongated body de- R0136" Gllson, 4 Franklin fining a barrel portion and a plunger cavity. A plunger both of Madlson, 53705 unit is mounted for reciprocation in the body and in- [22] Filed: Oct 24 1972 cludes a plunger portion extending from the barrel portion into the plunger cavity, and further includes PP 300,215 an operating portion extending rearwardly out of the body. The plunger unit includes a stop disposed within 52 11.s.c1. 73/425.6, 222/43 the h P and adjustable Stop for [51 1m. 01. c0111 1/14 vary'hg the vhhhe a plpehedl Sample carried on [58] Field of Search 73/425.6, 425.4 P; a threaded shelft received in a hut fixed in the body- 128/234 218 F; 141/27; 222/340, 341 43; An indicator including rotatable indicia-carrying rings 277/102 187 is coupled to the shaft for providing an indication of the volume setting of the pipette. A forward stop for [56] References Cited movelment of the plungierbunirt is provided by an ogertrave mem er enga e yt e unger sto mem er. UNITED STATES PATENTS Travel of the plung r unit past the forwafd position 2,530,909 ll/l950 R1ggs 222/309 X compresses an overtrave] Spring, and a Sea] assembly a i 'figzi 43 2 is compressed by the overtravel spring into sealing en- 3'49s135 3 1970 seitz et al. If 31173/42515 gagemem the P i Portia" ofthe plunger 3I6I3I952 10/1971 Gilmont 73/425.6 x A Sleeve camed the threaded Shaft around the R27,637 5/1973 Roach 73/425.6 Operating Portion Of-Ihe Plunger unit is adjustable Primary ExaminerRichard C. Queisser Assistant Examiner-Daniel M. Yasich Attorney, Agent, or FirmMason, Kolehmainen, Rathburn & Wyss ative to the shaft in order to align the indicator relative to the shaft. A lock arrangement is provided for locking the shaft in position to prevent inadvertent misadjustment of the volume.

11 Claims, 4 Drawing Figures ADJUSTABLE PIPETTE The present invention relates to improvements in adjustable pipettes.

Traditional pipettes have not been able to meet modern requirements in that they are not designed rapidly to transfer liquid samples having precise volumes. As a result many attempts have been made to provide improved pipetting devices of various constructions which can conveniently be used to transfer samples rapidly and accurately. Examples of devices of this type can be found in U.S. Pat. No. 3,244,009 Tietje et al. and U.S. Pat. No. 3,506,164 Weichselbaum et al.

Although such devices provide samples of precise quantity, they suffer from several disadvantages. A primary disadvantage is that such devices are not capable of providing samples of continuously variable size. At the most, presently available devices are adjustable by the user between two or a few discrete volumes. In addition, no clear indication is provided of the volume setting, and this can result in errors or at least inconvenience in use.

Precise adjustment together with accurate volume indications have been available in the past with buretting devices. However, attempts made heretofore to adapt these features to pipettes have not been sucessful. For example, U.S. Pat. No. 3,013,435 Rodrigues, Jr. discloses a burette described as capable of being used as a pipette. Although this device can be continuously adjusted, the adjustment mechanism is extremely complex and awkward to use. Moreover, although this device provides an indication useful for buretting, the indication does not correspond consistently with the actual volume setting when the device is used for pipetting. Consequently, the Rodrigues, in device not only would be difficult or impossible to manufacture but also falls far short of solving the problems noted above with heretofore proposed pipetting devices.

Among the important objects of the present invention are to provide improvements in adjustable pipettes; to provide a pipette having a simplified and improved arrangement for continuous yet precise adjustments in volume; to provide a pipette having a reliable, consistent, and accurate volume indicator; to provide a pipette wherein the indicator can be aligned relative to the volume adjustment to assure accuracy; to provide an improved seal assembly for pipettes; to provide a pipette including structure for locking the volume adjustment in position; and to provide an improved adjustable pipette of sturdy and compact construction capable of economical manufacture.

In brief, an adjustable pipette constructed in accordance with the present invention includes an elongated body defining a barrel portion and defining a plunger cavity adjacent the forward end of the body. A plunger unit is mounted for reciprocation in the body and includes a plunger portion extending from the barrel portion of the body into the plunger cavity, together with an operating portion extending rearwardly out of the body. A seal assembly is carried by the body and seals against the plunger portion. The plunger unit includes a stop engaging member mounted on the plunger unit and disposed within the barrel portion of the body. A forward stop in the body is engaged by the stop engaging member upon forward movement of the plunger unit to a forward stop position.

In order to adjust the sample volume, a hollow threaded shaft engages a nut fixed in the body and includes a forward end disposed within the barrel portion of the body and a rear end extending rearwardly out of the body. A rear stop is associated with the forward end of the threaded shaft and is engaged by the plunger unit stop engaging means upon rearward movement of the plunger unit to a rear position. An indicator arrangement mounted with the body is keyed to the shaft for providing an indication in response to rotation of the shaft indicative of the position of the rear stop and thus of the sample size.

The present invention together with the above and other objects and advantages may be better understood with reference to the embodiment of the invention illustrated in the accompanying drawing, wherein:

FIG. 1 is a partly sectional side view of an adjustable pipette embodying the features of the present invention;

FIG. 2 is an enlarged, fragmentary sectional view of the adjustable pipette of FIG. 1 taken along the line 2-2 of FIG. 1;

FIG. 3 is an end view of the adjustable pipette partly taken in section along the line 3-3 of FIG. 2; and

FIG. 4 is an exploded, fragmentary perspective view of parts of the seal assembly of the adjustable pipette.

Having reference now to the drawing, there is illustrated an adjustable pipette generally designated by the reference numeral 10 and constructed in accordance with the principles of the present invention. The pipette 10 includes a body designated as a whole by the reference numeral 12 within which there is reciprocably supported a plunger unit 14. In accordance with one feature of the invention, there is provided a novel seal assembly generally designated as 16 providing an air tight seal between the body 12 and the plunger unit 14. A volume adjustment assembly generally designated as 18 functions in accordance with another feature of the invention to permit reliable and accurate smallincrement volume adjustments, and volume settings are clearly indicated in accurate fashion by a volume indicator assembly generally designated as 20.

Proceeding now to a more detailed description of the structure of the adjustable pipette 10, the body 12 includes a main housing 22 together with a nose member 24. The main housing 22 defines a generally cylindrical barrel portion 26 and an enlarged cavity 28 for enclosing the volume indicator assembly 20. The rear end of the main housing 22 is closed by a cap 30. The cap may be attached in any suitable manner, and preferably is snapped into place by a protuberance and notch arrangement 32 (FIG. 2).

Mounting of the nose member 24 to the main housing 22 is accomplished by means of a nut 34 threaded onto the forward end of the main housing 22 over the barrel portion 26. The nose member 24 includes an enlarged rearward end section 36 defining an overtravel chamber 38. A mounting rib 40 is held firmly against the forward end of the main housing 22 by a flange 42 of nut 34, and a spring retainer 44 is captured between the rearward end of the nose member 24 and a shoulder 46 defined by a counterbore in the forward end of the barrel portion 26 of the main housing 22.

Outwardly of the enlarged end 36, the nose member 24 includes an elongated section 48 defining a plunger cavity 50. In order to avoid contamination of samples from prior samples, preferably the forward end of the nose member 24 is provided with frictionally mounted, tapered tip members such as the member 52 illustrated in FIG. 1. An opening 54 in the nose member 24 provides communication between the interior of the tip 52 and the plunger cavity 50. The interior of the tip 52 is preferably large enough so that entire samples may be held therein without contact with the nose member 24, and preferably the tip member 52 is made of suitable non-wetting plastic material to facilitate complete discharge of samples.

Advantageously, the overall size and configuration of the adjustable pipette 10 are such as to facilitate its use. The slender configuration permits insertion of the nose member 24 into narrow receptacles. The adjustable pipette 10 can readily be held in the hand, and the main housing 22 is provided with a trigger guard-type finger handle 56 having a generally C-shape or D-shape and substantially completely encircling the forefinger of a hand grasping the main housing 22. The arrangement is such that in use the adjustable pipette can be held suspended upon a forefinger by the finger handle 56 in position naturally to be grasped by the hand with the thumb in position for use over the rearward end of the pipette 10.

Samples are drawn into and discharged from the tip member 52 by means of reciprocation of the plunger unit 14 within the body 12. The plunger unit 14 includes a circular cylindrical plunger portion 60 having a forward end extending into the plunger cavity 50 defined within the nose member 24. Extending rearwardly from the plunger portion 60, and preferably integral therewith, is an operating rod portion 62 terminating well outside of the rearward end of the body 12. A thumb button 64 is attached to the outer end of the operating rod portion 62 by a screw retainer 66.

In its rest position the plunger unit 14 is urged rearwardly by a spring 67 (FIG. 2) against an adjustable rear stop, described hereinafter and comprising part of the volume adjustment assembly 18. The spring 67 is held in compression within the barrel portion 26 of the housing 22 between the spring retainer 44 and a spring retainer and stop member 68. The member 68 is located over a shoulder 70 between the plunger portion 60 and operating rod portion 62 of the plunger unit '14, and is press fit to the plunger portion 60 or is otherwise permanently attached to form part of the plunger unit 14. Member 68 is provided with a rearwardly facing stop surface 72 held against the adjustable rear stop in the rest position of the plunger.

In order to draw up a sample into the tip member 52, the thumb button 64 is depressed to move the plunger unit 14 forward. This movement is accommodated by compression of spring 67. Member 68 includes an elongated small diameter forward end portion 74 comprising a forward stop surface. Forward movement of the plunger unit 14 results in movement of stop portion 74 through a central opening 76 in spring retainer 44 until it strikes a forward stop and overtravel member 78 mounted in the overtravel chamber 38 against the forward side of spring retainer 44. In this position, the plunger portion 60 of the plunger unit 14 is extended into the plunger cavity 50. Tip member 52 is immersed in a sample material, and the thumb button 64 is released. Plunger unit 14 is moved rearwardly by the force stored in spring 67 to retract the plunger portion 60 in the plunger cavity 50, and suction within the plunger cavity 50 communicated to the interior of the tip member 52 draws a sample of predetermined size into the tip member 52.

in order to discharge the sample from tip member 52, the thumb button 64 is once again depressed to move the plunger unit 14 in a forward direction. Upon the discharge stroke, the member 68 again moves forwardly until surface 74 contacts the forward stop and overtravel member 78. This member is held in the position illustrated in FIG. 2 by means of an overtravel spring 80 held in compression between the forward side of member 78 and the rearward side of a spring retainer and pressure member 82 disposed at the forward end of the overtravel chamber 38. During a discharge stroke, the plunger unit 14 is moved through an overtravel movement wherein the forward stop portion 74 of the member 68 drives the forward stop and overtravel member 78 against the force of the spring 80 into the overtravel chamber 38. As a result of this overtravel movement the plunger portion 14 moves further into the overtravel chamber 50 during the discharge stroke than during the intake stroke. Thus, it is assured that the entire sample is fully discharged from the tip 52. In order to provide a positive, easily detected stop at the end of the intake movement, the spring 80 is preferably substantially stiffer than the spring 67.

In accordance with a feature of the invention, the seal assembly 16 of novel construction provides a reliable, airtight, low friction seal between the body 12 and the plunger unit 14. Moreover, seal assembly 16 is spring loaded by over-travel spring 80 and thus spring 80 performs a dual function.

More specifically, as shown in FIGS. 2 and 4, the seal assembly 16 includes a toric seal member 86 disposed within a counterbore cavity 88 in the forward end of the spring retainer and pressure member 82. Seal member 86 includes a cylindrical collar portion 90 intimately encircling the plunger portion 60 and a flange section 92 engaged by the member 82. A resilient O- ring 94 is held around the collar 90 between the flange 92 of the seal member and the forward wall of the overtravel cavity 38. The size of the O-ring 94 relative to the size of the toric seal member 86 and cavity 88 is such that the O-ring 94 is somewhat compressed by the force of the spring 80. The toric seal member is of somewhat soft and flexible plastic material, and the cylindrical collar portion 90 of the seal member 88 is resiliently forced by the spring loaded O-ring 94 inwardly into sealing relation with the plunger portion 60. The use of a smooth metal plunger unit 14 and of a flexible plastic seal member 86 assures not only a good seal, but also very low sliding friction during reciprocation of the plunger unit 14.

An important advantage of the adjustable pipette 10 of the present invention is that it can be easily adjusted in a continuously variable manner and with great preci sion and accuracy. The volume adjustment assembly 18 provided in accordance with the present invention includes a hollow threaded shaft having an inner end disposed within the barrel portion 26 of the housing 22 and having an outer end spaced outwardly of the cap 30. The shaft 100 is received in an internally threaded nut member 102 fixed against rotational and longitudinal movement by means of a locking pin 104. Nut 102 is received in the rearward end of the barrel portion 26 and includes a flange 106 received in the forward end of the enlarged cavity 28. A retaining nut 108 is threaded onto the innermost end of the shaft 100 and is then looked in place, as by deformation. The purpose of the retaining nut 108 is to prevent the shaft 100 from being inadvertently withdrawn through the nut member 102.

Rotation of the shaft 100 is facilitated by the provision of a knurled head or cap 110 threaded over the outer end of the shaft 100 and locked into position by means of a pair of screw retainers 112. Slidably received over the operating rod portion 62 of the plunger unit 14 within the hollow threaded shaft 100 is an inner sleeve or shaft 114. The purpose of sleeve 114, described more fully hereinafter, is to permit adjustment of the volume indicator assembly 20 in relation to the volume adjustment assembly 18. The forward end of the sleeve 114, designated by the reference numeral 116, extends slightly beyond the forward end of the threaded shaft 100 and forms the forwardmost part of the volume adjustment assembly 18. Thus the forward end 116 of sleeve 114 is engaged by the stop surface 72 of the member 68 and provides the rearward stop for motion of the plunger unit 14.

Adjustment of the sample volume of pipette is accomplished by rotation of the head 110 resulting in reciprocation of the threaded shaft 100 and of the sleeve 114 due to threaded engagement with the nut member 102. This reciprocation moves the rearward stop 116 toward and away from the forward stop 78 thus adjusting the stroke of the plunger member 14 and the volume of a sample.

The volume adjustment assembly 18 also includes provision for locking the unit in position for any desired sample size. For this purpose there is provided a locking ring 118 threaded upon the shaft 100 to the rear of the cap 30 of the main housing 22. Once the shaft has been rotated to position it in the desired longitudinal position, the locking ring 118 is threaded inwardly firmly against the cap 30 thus locking the shaft 100 against further rotation.

In order to prevent movement of the locking ring 118 more than a slight distance away from the cap 30, there is provided a retaining pin 120 extending outwardly from cap 30 into a circular groove 122 in the underside of locking ring 118. If an attempt is made to unscrew the locking ring 118 away from the cap 30, the retaining pin 120 is engaged as illustrated in FIGS. 2 and 3 by a stop pin 124 extending across the groove 122 in a radial direction. The arrangement is such that rotation of the locking ring 118 in the opposite direction away from the position illustrated in FIG. 3 will result in tightening of the ring 118 against the cap 30 in less than a full revolution.

In accordance with another feature of the invention, the volume indicator assembly provides an exact and easily perceived readout of the volume setting of the volume adjustment assembly 18. Arranged in stacked relationship within the enlarged cavity 28 between cap and flange 106 are a series of indicator rings 130, 132 and 134, each encircling the adjustment shaft 100. In the illustrated embodiment of the invention the

rings

130, 132 and 134 express the volume setting in units, tens and hundreds, respectively, of microliters. It will be understood that fewer or more rings graduated in any desired units may be provided.

In order to couple the indicator assembly 20 with the volume adjustment assembly 18, the threaded shaft 100 includes a groove 136 extending throughout its length.

The units ring includes an inwardly extending projection 138 received in the groove 136 to the end that the ring 130 rotates with the shaft 100 but permits the shaft 100 to move longitudinally relative to the ring. The tens ring 132 and the hundreds ring 134 are free for relative rotation independent of the threaded shaft 100.

Movement of the

rings

130, 132 and 134 relative to one another is accomplished in a conventional manner. As a result of one full revolution of the units ring 130, a single pair of gear teeth 140 engage a spur gear 142 mounted on a shaft 144 extending axially through the length of the enlarged cavity 28 of the housing 22. The spur gear 142 engages continuous gears 146 on the tens ring 132 in order to move it a single increment of a tenth of a revolution. Similarly, each full rotation of the tens ring 132 results in incremental movement of the hundreds ring 134 by virtue of a single pair of gear teeth 148, an additional spur gear 150 and continuous gear teeth 152 on the hundreds ring 134.

As appears in FIG. 1, there is provided a transparent window 154 for viewing of the indicator rings 130, 132 and 134. Preferably the entire main housing 22 is formed of clear plastic with the window area 154 being polished for clear visibility. Any other window structure may be used if desired. Each of the

rings

130, 132 and 134 is provided with numerical indicia, and if desired the units ring 130 may be provided with fractional graduations registering with an index line 156 on the window 154.

In order to assure accuracy of the indication provided by the assembly 20, there is provided a zero adjustment feature. In the initial fabrication and assembly of the pipette 10, the

rings

130, 132 and 134 are mounted so that a zero indication appears when the shaft 100 is threaded inwardly approximately until the stop portion 74 is in contact with the forward stop 78 in its rearward position. In order precisely to adjust the zero position, the funtion of the slleeve 114 comes into play. The head 110 is rotated until the indicator assembly 20 provides precisely at zero indication. At this point the screw retainers 112 are loosened and the head 110 can be rotated while the shaft 100 remains stationary. During such rotation the reading of the indicator 20 is not changed. However, rotation of the head 110 advances or retracts the sleeve 114 relative to shaft 100 to precisely locate the plunger assembly 14 at the zero volume position. When this position has been reached, the screw retainers 112 are tightened once agin and the zero adjustment is completed.

The operation of the adjustable pipette 10 will be apparent to those skilled in the art in light of the above detailed description. In use, the pipette is first adjusted through manipulation of the volume adjustment assembly 18 to the desired volume. This is accomplished by rotation of the head 110 to cause reciprocation of the shaft 100 thus setting the position of the rear stop 116. When the desired volume as appearing at the window 154 has been attained, the locking ring 118 is rotated firmly against the cap 30 in order to prevent further inadvertent rotation of the shaft 100.

To draw up a sample, the thumb button 64 is depressed to drive the plunger portion 60 of the plunger unit 14 forwardly into the plunger cavity 50 through a precisely predetermined distance until the stop portion 74 strikes the forward stop 78. Inadvertent overtravel is avoided due to the stiffness of the overtravel spring 80. At this point the tip member 52 is immersed in a sample material and the thumb button 64 is retracted to draw up the sample.

The sample is discharged by once again depressing the thumb button 64 to move the plunger portion 60 into the plunger cavity 50. During the discharge stroke the plunger unit 14 is moved into the overtravel range by forcing the forward stop and overtravel member 78 forwardly into the overtravel chamber 38 against the force of the stiff spring 80. This overtravel movement assures complete discharge of the sample. When the thumb button 64 is released, the plunger unit 14 re turns to its rest position with the surface 72 against the rear stop 116.

Although the invention has been described with reference to details of the illustrated embodiment, it should be understood that such details do not limit the invention which is defined in the appended claims.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. An adjustable pipette comprising in combination;

an elongated body defining a barrel portion and defining a plunger cavity adjacent the forward end of said body;

a plunger unit mounted for reciprocation in said body and including a plunger portion extending from said barre-l portion into said plunger cavity and an operating portion extending rearwardly out of said body;

first biasing means in said barrel portion urging said plunger unit in a rearward direction;

said body defining an overtravel chamber disposed between said barrel portion and said plunger cavy;

a seal assembly resiliently carried in the overtravel chamber in said body in sealing relation with said plunger portion;

said plunger unit including a stop engaging member permanently mounted thereon and disposed within said barrel portion;

an overtravel member mounted for movement in said overtravel chamber and having a forward stop surface engageable by said stop engaging member upon forward reciprocation of said plunger unit to a forward stop position;

second biasing means in said overtravel chamber for urging said overtravel member against the rearward end of said overtravel chamber;

an internally threaded nut fixed in said body against rotational and axial movement;

a hollow shaft threaded through said nut for rotational and reciprocal movement and including a forward end disposed within said barrel portion and a rear end extending rearwardly out of said body;

said operating portion of said plunger unit extending through said shaft;

rear stop means reciprocal with the forward end of said shaft upon rotation of said shaft in said nut and engageable by said stop engaging member upon rearward reciprocation of said plunger unit to a rear stop position; and

indicator means in said body coupled to said shaft for providing an indication in response to rotation of said shaft indicative of the position of said rear stop means.

2. The pipette of claim 1, said second biasing means being substantially stiffer than said first biasing means to provide a positive feel of the forward stop position.

3. The pipette of claim 1, said overtravel chamber having a forward wall, said seal assembly being compressed between said second biasing means and said forward wall around said plunger portion.

4. The pipette of claim 3, said seal assembly including a retainer member pressed by said second biasing means toward said forward wall and having an axial central opening receiving said plunger portion, a counterbore cavity in the forward side of said retainer member cooperating with said forward wall to define an annular chamber surrounding said plunger portion, and compressible sea] means in said annular chamber.

5. The pipette of claim 4, said compressible sealing means including a flexible cylindrical collar surrounding said plunger portion, and an O-ring surrounding said collar, said O-ring being sized to deform when compressed by said second biasing means to force said collar inwardly against said plunger portion.

6. The pipette of claim 1, a locking member threaded upon said shaft to the rear of said body, said locking member being threadable to a locking position against the rear of said body selectively to lock said shaft against rotation.

7. The pipette of claim 6 including retaining means on said body and on said locking member for preventing rotation of said locking member through a complete revolution from the locking position.

8. The pipette of claim 1, said indicator means including a plurality of indicator rings encircling said threaded shaft within said body, indicia on said indicator rings, a window means in said body for viewing of said indicia, one said ring being coupled to said threaded shaft for rotation therewith while permitting reciprocal movement of said shaft through said rings, and drive means coupled between said rings for controlling the relative movement thereof in response to rotation of said shaft.

9. The pipette of claim 8, said body being formed at least in part of clear plastic, said window means comprising a region of said plastic having a polished surface.

10. The pipette of claim 8, the position of said rear stop means being adjustable relative to said shaft for adjustment of said rings relative to the rear stop position of said plunger unit.

11. The pipette of claim 10, a cylindrical sleeve slightly longer than said shaft slidably received within said shaft around said operating portion of the plunger unit, a head member threaded on the rearward end of said shaft and engaging the rearward end of said sleeve, the forward end of said sleeve forming said rear stop means, and said head being rotatable on said shaft to reciprocate said sleeve relative to said shaft.

Claims

1. An adjustable pipette comprising in combination; an elongated body defining a barrel portion and defining a plunger cavity adjacent the forward end of said body; a plunger unit mounted for reciprocation in said body and including a plunger portion extending from said barrel portion into said plunger cavity and an operating portion extending rearwardly out of said body; first biasing means in said barrel portion urging said plunger unit in a rearward direction; said body defining an overtravel chamber disposed between said barrel portion and said plunger cavity; a seal assembly resiliently carried in the overtravel chamber in said body in sealing relation with said plunger portion; said plunger unit including a stop engaging member permanently mounted thereon and disposed within said barrel portion; an overtravel member mounted for movement in said overtravel chamber and having a forward stop surface engageable by said stop engaging member upon forward reciprocation of said plunger unit to a forward stop position; second biasing means in said overtravel chamber for urging said overtravel member against the rearward end of said overtravel chamber; an internally threaded nut fixed in said body against rotational and axial movement; a hollow shaft threaded through said nut for rotational and reciprocal movement and including a forward end disposed within said barrel portion and a rear end extending rearwardly out of said body; said operating portion of said plunger unit extending through said shaft; rear stop means reciprocal with the forward end of said shaft upon rotation of said shaft in said nut and engageable by said stop engaging member upon rearward reciprocation of said plunger unit to a rear stop position; and indicator means in said body coupled to said shaft for providing an indication in response to rotation of said shaft indicative of the position of said rear stop means.

4. The pipette of claim 3, said seal assembly including a retainer member pressed by said second biasing means toward said forward wall and having an axial central opening receiving said plunger portion, a counterbore cavity in the forward side of said retainer member cooperating with said forward wall to define an annular chamber surrounding said plunger portion, and compressible seal means in said annular chamber.