EP0078109A1

EP0078109A1 - Liquid dispenser

Info

Publication number: EP0078109A1
Application number: EP82305200A
Authority: EP
Inventors: Roger Abraham Bunce
Original assignee: University of Birmingham; National Research Development Corp UK
Current assignee: University of Birmingham
Priority date: 1981-10-23
Filing date: 1982-09-30
Publication date: 1983-05-04
Also published as: EP0078109B1; JPS5880541A; DE3278622D1

Abstract

In medical tests on biological fluids it is frequently necessary to dispense a quantity of fluid from a first vessel (21) into a second vessel (30) in order to investigate the fluid. As well as accurate dispensing, there is.often the requirement that none of the fluid should escape into the atmosphere; as in the instance when the fluid is blood serum infected with dangerous micro-organisms.

The invention provides a first vessel (21) having a cap (6) with two nozzles (1) and (2) extending from it, the nozzles being typically of equal length and internal diameter. For dispensing, the first vessel (21) when inverted rests on the second vessel (30). The fluid flows down one nozzle (1) and air up the other (2) until the fluid level in the second vessel (30) reaches the nozzles, when flow stops. On removal of the first vessel, no further liquid escapes from the nozzles, provided they are proportioned, according to the invention, in correct relation to the fluid to be dispensed, and to the ambient temperature.

Description

This invention relates to liquid dispensers, particularly to liquid dispensers integral with caps and most particularly to those for dispensing blood serum or plasma.
Generally before analysis can be performed on a blood specimen contained in a capped blood tube the serum or plasma must be separated from the red cells. This is usually achieved by centrifugation followed by removal of the blood tube cap and pipetting or pouring of the serum. This may produce dangerous aerosols and pouring from conventional vessels has been declared unsafe in the 'code of practice of the Prevention of infection in clinical Laboratories and Postmortem Rooms' (The Howie report).
Recently improvements have been made by using a chemically inert gel separating medium of density intermediate between the red cells and serum; using a powdered or granular clotting agent which may be powdered glass, kaolin or plastic beads; and coating the insides of tube and cap with a silicon ester to prevent red blood cells remaining inside the tube and cap.
It is an object of the present invention to provide a liquid dispenser which will dispense a measured volume of blood serum safely.
According to the invention, a liquid dispenser comprises a first vessel to contain liquid to be dispensed, and a dispenser cap adapted to sealably fit over an open end of said first vessel, said cap having at least two nozzles of predetermined geometry, adapted to dispense, in use, a volume of liquid of predetermined properties from the first to a second vessel, such that dispensing of the liquid is terminated predeterminably.
Preferably, in use, the open terminations of the nozzles are spaced away from the base of the second vessel; and in one embodiment the spacing is achieved by a combination of nozzles protruding into the second vessel when the cap is arranged to rest on the lip of the second vessel.
The nczzles are preferably cylindrical tubes protruding from the cup, all to the same distance; arranged to extend substantially longitudinally of a common axis of the cap and the first vessel to which it is sealed.
In another embodiment the nozzles may extend both outward from and inward into the first vessel from the base of the cap transverse of the open end of the first vessel; and each nozzle may extend to equal distances inward and outward. The nozzles may be made frusto-conical with the larger diameter inward of the said base. The nozzles may be arranged to be concentric with one another.
In another optional embodiment the nozzles are three in number with axes which do not all lie in a single plane.
In further optional embodiments the nozzles may extend to different distances from the cap base; and may have internal diameters different one from another.
The cap may comprise two separate concentric portions, the nozzles being part of the inner portion, and the outer portion being arranged to secure the cap sealably to the first vessel. The outer portions may be provided with a transverse frangible membrane isolating the nozzles from the interior of the first vessel, and which can be broken by relative displacement of the inner and outer portions to place the nozzles in communication with the interior of the first vessel.
The liquid dispenser may be arranged to be initially partially evacuated to enable liquid to be drawn into the first vessel for subsequent dispensing.
The first vessel and its cap may each be provided with a projecting member, the members co-acting as a safety catch device such that the cap can be readily fitted to the first vessel but cannot be removed therefrom accidentally.
The nozzles of the liquid dispenser may be arranged to be of such dimension that dispensing is terminated predeterminably whcn the liquid is human blood serum at usual ambient temperatures. In which case the nozzles are preferably circular cylindrical, of equal length outward from the cap base, and have an internal diameter in the range 2.5to 3.5 millimetre; while the internal length of the nozzles is not less than 8.8 millimetre.
The invention extends to a method of constructing a liquid dispenser which includes the step of arranging for a given liquid in given ambient temperature conditions and for given dimensions of the liquid dispenser the length of nozzles through which the liquid may be dispensed, such that when a desired quantity has been dispensed from the inverted liquid dispenser under gravity, and the liquid dispenser is removed away from the dispensed quantity of liquid, no further liquid will emerge from said nozzles until the liquid dispenser has been righted and reinverted. The method is particularly applicable when the nozzles are cylindrical and circular in bore and the liquid is human blood serum.
The invention will now be described, by way of example only, and with reference to the accompanying drawings of which:-

figure 1 is a cross-sectional side elevation of a liquid dispenser;
figure 2 is a cross-sectional side elevation of a centrifuged blood tube fitted with a liquid dispenser
figure 3 is a cross-sectional side elevation of a liquid dispenser having wholly protruding nozzles with a sample container in position;
figure 4 is a cross-sectional side elevation of a blood tube and liquid dispenser having partially protruding nozzles;
figure 5 is a cross-sectional side elevation of a blood tube and liquid dispenser having concentric nozzles;
figure 6 is a cross-sectional side elevation of a blood tube and liquid dispenser having partially protruding tapered nozzles;
figure 7 is a cross-sectional side elevation of a blood tube and liquid dispenser having three partially protruding nozzles;
figure 8is a cross-sectional side elevation of a blood tube and liquid dispenser having unequal length protruding nozzles;
figure 9 is a cross-sectional side elevation of a blood tube and liquid dispenser having unequal diameter partially protruding nozzles;
figure 10 is a cross-sectional side elevation of a wadded two piece liquid dispenser;
figure 11 is a cross-sectional side elevation of a wadless two piece liquid dispenser;
figure 12 is a diagrammatic cross-section of a blood tube and liquid dispenser assembly before nozzle severance;
figure 13 is a diagrammatic cross-section of an assembly after nozzle tip severance;
figure 14 is a diagrammatic cross-section of an assembly with a sample container in position;
figure 15 is a diagrammatic cross-section of an inverted assembly and sample container before flow commencement;
figure 16 is a diagrammatic cross-section of an inverted assembly and sample container immediately before release of the first air bubble;
figure 17 is a diagrammatic cross-section of an inverted assembly and sample container during flow;
figure ₁8 is a diagrammatic cross-section of an inverted assembly and sample container after flow cessation;
figure 19 is a diagrammatic cross-section of an inverted assembly removed from a sample container after flow cessation;
figure 20 is a diagrammatic cross-section of an upright assembly after flow cessation;
figure 21 is a diagrammatic cross-section of an assembly after nozzle clearance; (one tube might still be full)
figure 22 is an isometric view of an assembly showing a wedge system;
figure 23 is a diagrammatic cross-section of an assembly and sample container before flow commencement in wetted nozzle condition;
figure 24 is a diagrammatic cross-section of an assembly and sample container after serum has run into nozzles in wetted nozzle condition;
figure 25 is a diagrammatic cross-section of an assembly and sample container after serum in nozzles has unbalanced and flowed to end of one nozzle; and
figure 26 illustrates an optional means for supporting a liquid dispenser while dispensing.

Figure 1 shows a liquid dispenser in the form of a cap of resilient plastics material provided with a first nozzle 1 and a second nozzle 2 the distal ends 3 of the nozzles 1 and 2 are closed and the basal ends 4 are open. An annular lip 5 projects upward from the base 6 and a second annular lip of smaller radius 7 projects co-axially in a like manner.
A third annular lip 8 projects downward from the base 6 and a fourth annular lip of larger radius 9 projects in a like manner. The fourth annular lip 9 has a fast multistart femal thread moulded in its inner surface. Before the first dispensing the cap is cut with scissors, or a cutter like a cigar cutter, along a line indicated by 10 and the severed tip portion 11 discarded.
Figure 2 shows a cap 20 such as that described above with reference to figure 1 fitted to a blood tube 21, ie the first vessel, containing serum 22, a separating gel 23, and coagulated red cells 24 the remaining volume in the blood tube 21 being occupied by a volume of air 25.
Figure 3 shows in more detail how the third annular lip 8 forms a seal with the blood tube 21, the lengths of lips 8 and 9 are such that the cap 20 may be partially unscrewed without breaking the seal. A sample container 30 is located with its rim between annular lips 5 and 7. In this embodiment the nozzles 1 and 2 are flush with the base 6 and are identically dimensioned.
Figure 4 shows a second embodiment having two nozzles 40 and 41 of identical dimensions which protrude both above and below a base 42.
Figure 5 shows a third embodiment having two concentric nozzles 50 and 5₁ protruding from a base 52.
Figure 6 shows a fourth embodiment having two nozzles 60 and 61 of idential lengths protruding both above and below a base 62..The nozzles are frusto-conical having their larger diameter ends below the base 62.
Figure 7 shows a fifth embodiment having three nozzles 70, 71 and 72 of identical lengths and diameters protruding both above and below a base 73 the nozzles are arranged so that their axes do not fall in a single plane.
Figure 8 shows a sixth embodiment having two equidiameter nozzles 80 and 81 flush with a base 82. Nozzle 80 is longer than nozzle 81 and accordingly protrudes above the base 82 to a greater distance.
Figure 9 shows a seventh embodiment having two nozzles 90 and 91 protruding both above and below a base 92. The nozzles are equilength and nozzle 90 is of smaller diameter than nozzle 91...
It will be appreciated that these seven embodiments are only examples of the many different combinations of nozzle geometries which fall within the scope of this invention and that features of different embodiments may readily be combined to produce further variations.
Figure 10 shows a modification to the invention in which the dispensing cap is constructed from two parts. A screw cap 100 having an axial orifice through which a resilient member 101 protrudes, being an interference fit with cap 100, and having two nozzles formed within itself as two cylindrical ducts 102. The resilient member broadens out below the cap 100 to form a wad 103 which is seated on the neck 104 of the.blood tube 21 upon tightening of the screw cap 100.
Figure 11 shows another modification of the invention in which a screw cap 110 screws directly onto the blood tube 21; formed in the centre of the screw cap 110 is a deep recess 111 which may have weaknesses b_lilt into it at 112, for example a frangible membrane. A cylindrical member 113 having two nozzles 114 and 115 is a close fit in the recess 111. Downward pressure on the cylindrical member 113 causes the recess 111 to rupture at weakness 112 and, the cylinder 113 to move from the broken line to the continuous line position and the nozzles to connect the inside of the blood tube 21 and the environment.
It will be obvious to one skilled in the art that although the invention has been described with reference to a screw cap, it is equally possible to use a push-fit cap or a snap-fit cap.
The invention is utlized in a sequence of operations hereinafter described: a blood sample is taken from a patient in a conventional manner and subsequently transferred to a blood tube containing a silica gel. The blood tube is then sealed with a screw cap containing two nozzles and spun in a centrifuge to separate-the red cells from the serum.
Figure 12 is a diagrammatic representation of such a spun blood tube 120, with a cap 121, and containing red cells 122 which have migrated to the bottom of the blood tube, separating the red cells from the less dense serum 123 is a separating gel 124 which forms a barrier between the red cells 122 and the serum 123. The cap 121 has two nozzles 125 and 126 protruding therefrom, terminally sealed by tips 127 and 128.
Typically the blood tube 120 is 14.5mm diameter and 94mm long. The depth of the red cells 122 is 32mm, the separating gel 124 is 14mm and the serum is 32mm. Thus the air space above the serum has an initial depth of 16mm.
After centifugation the tips 127 and 128 of the nozzles 125 and 126 are cut off with scissors, or cigar type of cutter. The scissors may be of the type which retain the severed portion. This type of scissor minimises the risk of contamination of the human operator. Either cutting device could have a stop to gauge the length of nozzles.
An alternative, and at present less conventional, system is able to sample the patient's blood directly. The blood tube containing gel 124 and having a resilient cap 121 affixed, is supplied with its contents at apartial vacuum. A double ended needle is inserted into the patient and then through the resilient cap 121 whereupon the partial vacuum causes a sample of blood to be drawn into the blood tube 120. Thereafter the needle is removed from the patient and cap and the cap and blood tube is centrifuged in the same manner as has been described above.
Figure 13 shows the blood tube 120 and cap 121 with unused, dry, open nozzles 125 and 126 after the nozzle tips have been removed.
Figure 14 shows a sample container 140 emplaced over the nozzles 125 and 126. The blood tube and sample container assembly is subsequently manually inverted to the position shown in figure 15.
The serum 123 flows to the capped end of the blood tube ₁20 under the action of gravity. The gel 124 is sufficiently viscous to stay in place and thereby retain the red blood cells 122 at the end of the blood tube 120 remote from the cap 121.
The probable mechanism of flow will now be described using the nomenclature tabulated below:
The mechanism of flow of serum 123 into the nozzles 125 and 126 depends upon whether the nozzles have previously been used, hereinafter referred to as the wet condition, or are unused, hereinafter referred to as the dry condition.
The mechanism for flow in the dry condition may be explained with reference to figure 16. Initially the serum 123 flows down only one nozzle 125 and an air bubble 160 is formed at the blood tube end of the other nozzle 126.
Consider the equilibrium at the bubble surface 161, here surface tension forces are resisting the differential pressure across the surface 161. In the extreme case.the differential pressure is equal to the surface tension forces, eg when the bubble is about to break free from the nozzle. So, as shown in 'Intermediate Physics" by C J Smith.
P _s is assumed to be the pressure acting at the base of the air bubble 160. Additionally, as the movement of serum 123 down the nozzle 125 takes place quickly, heat transfer is negligible so the expansion of gas in volume 162 may be assumed to be adiabatic.
therefore:
Also from static pressure head considerations ignoring surface tension effects in the nozzle containing serum, as for an unwetted nozzle the meniscus is flat, ie infinite radius,
As the serum 123 is substantially incompressible then from volume considerations:
also
and V₁ ⁼ a_t(h_air + Δh_d) ...................... equation 6 Now substituting equation 3 into equation 1
and substituting equation 2 into equation 7
substituting equations 5 and 6 into equation 8
simplifying
For a graphical solution,
Now considering the equilibrium of the air bubble 160. It is assumed that the bubble is spherical. This is a valid assumption if the nozzle radius is small and the angle of contact between the serum 123 and the nozzle 126 is zero.
For equilibrium the upward force = weight of serum displaced
For the bubble 160 to detach from the end of the nozzle 126 the upward force must overcome the surface tension force. The bubble may be experimentally determined to be attached to the nozzle at its internal diameter, so the surface tension force is given by: 2 r . So for detachment to occur
therefore
After the first air bubble 160 has detached, serum 123 flows down the nozzle 125, thereby filling it, and continues to flow out of the nozzle and into the sample container 140 as shown in figure 17.
Serum continues to flow into the sample container 140 and air bubbles 170 continue to be formed and released from the nozzle 12o until the level of serum 123 in the sample container 140 reaches the lower end of the nozzle 126,whereafter serum is drawn into the nozzle 126 as shown in figure 18. This action prevents further bubbling and hence stops further flow of serum 123.
The blood tube and cap assembly are gently removed as shown in figure 19. Surface tension between the serum 123 and the ends of the nozzles 125 and 126 is sufficient to prevent the serum flow out of the nozzles.
It is found experimentally that as the bore of the two nozzles increases the liquid equilibrium becomes more susceptible to mechanical shock. The variation in pressure P _S which causes the effect of surface tension to be overcome and the liquid to run out is due to two main effects First, the differential pressure head due to a sideways acceleration or retardation temporarily imposing a greater head of liquid above one nozzle than the other, and secondly, a pressure change due to the inertia of the column of liquid in the blood tube acted upon by a vertical acceleration or retardation; that is the effect of a temporarily increased or decreased head of liquid above both nozzles. Also the change in volume of air above the serum in the inverted blood tube, and hence the distortion of the free liquid surfaces at the end of the nozzles depends upon the initial volume of said air. Thus prediction of the stability of the said free liquid surfaces under mechanical shock is complex; especially when the chock is produced by human handling, in which the accelerations applied to a blood tube cannot be standardised. However, with a nozzle diameter of 2 millimetre spillage is avoided readily. The blood tube and cap assembly when righted is shown in figure 20. The serum 123 falls to be immediately above the gel 124, with the exception of that serum which remains in the nozzles 125 and 126. This serum may be removed, as shown in figure 21, by unscrewing the cap 121 slightly in order to reduce the air pressure 210 and therefore cause the serum in at least one nozzle to be drawn into the blood tube. The assembly is now ready to dispense a further volume of serum if required. Even if only one nozzle is cleared this nevertheless prevents serum from being undesirably expelled due to a change ambient temperature or pressure. Change in temperature of any gas contained in the blood tube should be avoided as far as possible by eliminating unnecessary handling of the blood tube in use, so conducting heat to the contents and expelling serum to the environment.
In order to prevent the cap 121 being inadvertantly removed during the nozzle clearing operation a wedge form 220, as shown in figure 22, is moulded or glued to the outside of the blood tube 120 and a tab 221 extends from the cap 121. The orientation of the wedge 220 allows the tab 221 to ride up its inclined surface during screwing on, but to be prevented from being completely removed accidentally. The tab and wedge may be regarded as specific examples of coacting projecting members of a safety catch device.
Further serum samples may be dispensed by repeating the operational sequence Figure 23 shows a blood tube and sample container assembly which has been inverted. The following is an example of a practical design using blood serum and the following data.

L_d is to be calculated

First calculating the nozzle length for dry dispensing using equation 13: the radius of the air bubble R = 2·368
Now if Δh_d ranges from 0.038 to 0.039mm by increments of 0.0001 mm and y₂ may be calculated from each value of Δh_d using equations 10 and 11. From a graph
Δh_d = 0.03867mm is where y_l= y₂ so substituting in equation 4 and dividing by a
So then the length of the nozzle for the dry condition must be greater than this ie L_d > 8.7702mm.
For the example given, the calculations have been based on a liquid dispenser with dry nozzles. However, a second dispensing is equally satisfactory, ie with wet nozzles, with the same prederminable termination. If the nozzles have previously been treated internally with a wetting agent a different calculation would have to be made, because the equilibrium position of the liquid may be different; that is, the liquid would enter the nozzles by a small distance.
In practice the nozzles have to be longer than the theoretical predictions to overcome the wetability forces between the liquid to b e dispensed and the material of the dispenser. These are very difficult to predict and are dependent upon molecular cleanliness. This is well illustrated by the way rain drops follow irregular patterns down a window pane and is probably the main cause of any inconsistancy in dispensing especially with small diameter nozzles.
For use in dispensing serum the optimum internal diameter of the nozzles should be between 3.0 and 3.4 mm. This is based on experimental evidence using standard nylon tubing. If 2.0 mm diameter was used dispensing was inconsistant and sometimes the dispenser had to be tapped on the bench to initiate pouring.
The theory predicts the minimum overall length of the nozzles from hydraulic considerations. The distance the nozzle projects from the outer end face of the cap in relation to the receiving vessel determines the volume dispensed. To adjust the sample volume indep- ently of the overall length of the nozzle the nozzle may be sunk into the cap (Fig 4); however, for a given size of the dispensing vessel, this reduces the volume available for dispensing. In this example the delivery volume was 2 ml and the nozzles were made 20 mm long overall.
When designing a serum dispenser it should be made compatable with centrifuge technology. Some types of centrifuge do not allow their buckets to swing freely, eg, the tubes are retained at 45⁰. In this case, long nozzles can undergo a permanent bending during centrifugation and need to be more solidly constructed than short nozzles. It is therefore desirable to minimise the length of the nozzles by using design techniques described.
The minimum spacing between the nozzles was found to be 7 mm for 4.76 mm 0/D 3.00 mm I/D nozzles. When dispensing, at a shallow angle to the horizontal, liquid moves down one tube and up the other. If the nozzles are too close together liquid is sucked up with the air and pouring stops. Also, if the spacing is made smaller a web of serum forms between the tubes and when the dispenser is removed from the dispensing vessel this can be a source of contamination.
The material used in the construction of the Liquid Dispenser is not critical. Nylon and PTFE have been used to make the nozzles, polythene for the cap and glass or plastic for the vessel. Five liquids have been dispensed: human blood serum, reconstituted equine serum, water, methanol and oil. All worked well, but the oil was obviously dispensed much slower because of the greater viscosity. Varying the amount of liquid in the dispenser did not appear to affect dispensing.
The invention has been described so far in relation to an arrangement in which when liquid is dispensed from a blood tube 21 into a sample container 30 the blood tube is located on the lip of the container to determine the depth of sample dispensed. For use with any other size of sample container 30A the blood tube may be provided with a depth gauge in the form of rods 260 arranged in the same direction as the nozzles, as shown in Figure 26 to locate the nozzles a required distance from the bottom of container 30A. The blood tube may be supported in relation to the rods 260 by a clamp 262.
If it is important to minimise the size of the nozzles pouring can be made more consistant by coating the inside of the nozzles with a dryed film of wetting agent such as BRIJ 35 (R.T.M.) (30% solution) having a dilution of 1000:1. If the diameter is increased above 3.4 mm there is a danger that the dispenser may continue to pour after it is lifted clear of the receiving vessel.
The foregoing assumes that liquid dispensing will, in the main, be conducted at a substantially constant temperature of 22^oC. However, it is usual to store blood specimens, before test, at a temperature of 4 C, and it should be stated that a liquid dispenser according to the invention also operates satisfactorily at the latter temperature in terminating dispensing predeterminably.

Claims

1. A liquid dispenser characterised by comprising a first vessel (21) to contain liquid to be dispensed, and a dispenser (20) adapted to sealably fit over an open end of said first vessel, said cap having at least two nozzles (1) and (2) of predetermined geometry, adapted to dispense in use a volume of a liquid of predetermined properties from the first to a second vessel, such that dispensing of the liquid is terminated predeterminably.

2. A liquid dispenser according to Claim 1 characterised in that a required volume of liquid is dispensed having the open terminations of the nozzles (1) and (2), in use spaced away from the base of the second vessel.

3. A liquid dispenser according to Claim 2 characterised in that spacing is achieved by a combination of nozzles (1) and (2) protruding into the second vessel when the cap (20) is arranged to rest on the lip of the second vessel.

4. A liquid dispenser according to any one of the preceding claims characterised in that the nozzles (1) and (2) are cylindrical tubes protruding from the cap (20), all to the same distance.

5. A liquid dispenser according to any one of Claims 2 to 4 characterised in that the nozzles (1) and (2) are arranged to extend substantially longitudinal of a common axis of the cap (2) and the first vessel (21) to which it is sealed.

6. A liquid dispenser according to any one of the preceding claims characterised in that the nozzles (40) (41) extend both outward from and inward into the first vessel (21) from the base (42) of the cap transverse of the open end of the first vessel.

7. A liquid dispenser according to Claim 6 characterised in that each nozzle (40) (41) extends to equal distances inward and outward.

8. A liquid dispenser according to Claim 7 characterised in that the nozzles (60) (61) are frusto-conical with the larger diameter inward of the said base.

9. A liquid dispenser according to any one of the preceding claims characterised in that the nozzles (50) (51) are concentric.

10. A liquid dispenser according to any one of Claims 1 to 8 characterised by having three nozzles (70) (71) (72) the axes of which do not lie in a single plane.

11. A liquid dispenser according to any one of the preceding claims characterised in that the nozzles (80) (81) extend to different distances from the cap base.

1₂. A liquid dispenser according to any one of the preceding claims characterised in that the nozzles (90) (91) have internal diameters different one from another.

13. A liquid dispenser according to any one of the preceding claims characterised in that the cap comprises two separate concentric portions, the nozzles being part of the inner portion (101), and the outer portion (100) being arranged to secure the cap sealably to the first vessel (21).

14. A liquid dispenser according to Claims 13 characterised in that the outer portion (110) has a transverse frangible membrane (112) isolating the nozzles from the interior of the first vessel (21), and which can be broken by relative displacement of the inner (113) and outer portions to place the nozzles in communication with the interior of the first vessel.

15. A liquid dispenser according to any one of the preceding claims characterised in that it is initially partially evacuated to enable liquid to be drawn into the first vessel for subsequent dispensing.

16. A liquid dispenser according to any one of the preceding claims characterised in that the first vessel and the cap are provided each with a projecting member (220) (221), the members coacting as a safety catch device such that the cap can be readily fitted to the first vessel but cannot be removed therefrom accidentally.

17. A liquid dispenser according to any one of the preceding claims characterised in that the nozzles (1) (2) are of such dimensions that dispensing is terminated predeterminably when the liquid is human blood serum at usual ambient temperatures.

18. A liquid dispenser according to Claim 17 characterised in that the nozzles (1) (2) are circular cylindrical, of equal length outward from the cap base and have an internal diameter in the range 2.5 to 3.5 millimetre.

19. A liquid dispenser according to claim 18 characterised in that the internal length of the nozzles (1) (2) is not less than 8.8 millimetre.

20. A method of constructing a liquid dispenser according to any one of the preceding claims characterised in that it includes the step of arranging for a given liquid in given ambient temperature conditions and for given dimensions of the liquid dispenser the length of nozzles (1) (2) through which the liquid may be dispensed, such that when a desired quantity has been dispensed from the inverted liquid dispenser under gravity, and the liquid dispenser is removed away from the dispensed quantity of liquid, no further liquid will emerge from said nozzles until the liquid dispenser has been righted and reinverted.

21. A method according to Claim 20 characterised in that the nozzles are cylindrical and circular in bore and the liquid is human blood serum.

22. A method of dispensing liquid characterised in that it includes dispensing a quantity of liquid from a liquid dispenser according to any one of Claims 1 to 19.