Liquid delivery system

Abstract

A liquid delivery system comprises a first section 202 connected to a second section 204 by a first coupling 270 and a third section 206 connected to the second section by a second coupling 230. The f

Description

The present invention relates to a liquid / ink delivery system having particular application to research and development work relating to pumping of low volumes of corrosive and / or delicate liquids and facilitating the fast assembly and disassembly for enabling fast access to constituent parts for cleaning and changing to a different liquid. A liquid delivery system comprises wetted path parts and non-wetted parts and the former are not typically easily accessible for thorough cleaning. For sensitive liquids, to avoid contamination when changing to a new liquid a simple flush procedure may not be sufficient, it is often necessary to fully soak and clean individual wetted parts. Furthermore, it is desirable to minimise use of disposable parts which need replacing each time a new ink is used. Additionally, the space available to test inks in a laboratory may not be large and it is often desirable to integrate a test liquid delivery system into an existing equipment or test arrangement, with very little space available to do so. A system meeting these needs and able to deliver a smooth flow without large pressure excursions is desirable. The system design should scale to production levels so that re-optimisation of liquids does not need to be done after changing from a research liquid delivery system to a production liquid delivery system. For many liquids this means it should be possible to recirculate the liquid in the same manner, with the same pump design in both development and production equipment. Existing low volume ink delivery systems may provide one or more of the requirements above but fail to provide others. For example, a low volume may be achievable, but the recirculation flow rate is limited to low flow rates. A fast changeable reservoir may be provided but is disposable requiring an additional consumable part. A low volume may be delivered but with relatively poor pressure stability if a diaphragm pump is used; if a gearpump off-the shelf is used, it is likely to be prone to overheating the ink due to bearing friction. According to a first aspect of the present invention there is provided a liquid delivery system according to accompanying claim 1. Preferred features are set out in the dependent claims. In particular, the fast locking coupling may comprise one of the following: a bayonet coupling a screw fitting, the screw having a diameter of substantially the same dimension of the second section, a latching sleeve, a force coupling, a magnetic coupling, and a plurality of clips. Preferably both the fast locking couplings are selected from this list and further preferably they are both of the same type. The fast locking coupling is preferably a bayonet coupling and more preferably a bayonet coupling with four spigots. The liquid delivery system preferably comprises a heater, which allows it to pump liquids having a wider range of viscosities and meet requirements of printheads. The liquid delivery system preferably further comprises a second reservoir in the second section which is in fluid communication with the first reservoir. The second reservoir is preferably arranged concentrically with the first reservoir to provide a compact construction. The liquid delivery system may further comprise a gas connection to the reservoir and, in that case, the reservoir may be a gas-tight reservoir. The gear pump is preferably arranged such that a proportion of the liquid pumped through the inlet is arranged to flow around the gears and leak from the gear pump via at least an aperture to be returned to the reservoir. The proportion is preferably at least 0.5% and more preferably at least 5%. The proportion is also preferably less than 25%, more preferably less than 15% and still more preferably less than 10%. This aspect of the present invention addresses at least some of the above needs by permitting ready dismantling for cleaning. It also optionally provides for a gearpump without spigots which is selflubricating. The pump may be reversible which is an advantage over diaphragm pumps. The liquid delivery system proposed enables liquid recirculation where the minimum amount of liquid needed for the full function of the liquid delivery system is less than 20ml and preferably around 10ml. Inkjet printing is increasingly enabling a wider range of inks (or other materials) to be jetted. Some of these inks have very high value per kilo, for example, gold, platinum and silver inks and there is a need to minimize ink wasteage and this includes using as little ink for development tests as possible. Inkjet printing is expanding as an industry at a fast rate and many new inks of every kind are developed than before. This is because there are many new printheads and applications that all require subtle tweaks to the inks to optimize print quality. Typically, inks are initially made at laboratory scale and are valuable at least in the sense that it takes a significant amount of chemistry expertise and effort to produce them. In this scenario, i.e., from an ink development point of view, there is also a need to use less ink. Additionally, an existing inkjet integrator or print application developer may wish to assess a number of existing inks quickly to see which performs best in jetting trials, etc. Existing ink delivery systems either require too much ink for testing, and / or are too slow to change from one ink to another, and / or do not provide a good correspondence of results achieved at low volume ink scale with a low volume ink system to results achieved with a higher volume productionready ink system, and are too hard to clean thoroughly. A low volume ink / fluid delivery system which allows minimal quantities of ink to be used for testing, which allows fast and easy access for cleaning, allows fast filling and changing of one ink to another, and is scalable, low pulsation, is required. Existing devices on the market, (e.g. megnajet labjet,) cater for recirculation for low flow printheads but not high flow printheads. The design does not use low pulsation pumps nor do they include an integrated easy-to-clean heater which would allow for the most accurate temperature control. Currently, there are no low pulsation low volume ink delivery systems on the market. By facilitating the disassembly of the pump, a gear pump according to the first aspect of the present invention allows for rapid dismantling, without tools, permitting access to clean the pump. Preferred embodiments of the pump according to the first aspect of the invention are of limited size and pumping capacity, having a reservoir capable of holding no more than 20ml of liquid to be pumped. Such pumps not only provide ease of cleaning, but minimize waste of the liquid being printed. The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a block diagram of a liquid delivery system according to embodiments of the present invention, Figure 2(a) shows a front view of a pump according to a first embodiment of the present invention Figure 2(b) shows a side view of a pump according to the first embodiment of the present invention, Figure 2(c) shows a first section through a pump according to a first embodiment of the present invention, Figure 2(d) shows a second section, orthogonal to the first, through the pump of Figure 2(a), and Figure 3 shows a side view of the pump of Figure 2, separated into discrete sections. The terms used to describe the orientation of the pump, such as above, below etc. refer to the orientation shown in the figures and are not intended to place any limit on the relative location of the constituent parts of the pump. Figure 1 shows a block diagram 100 of a pump according to embodiments of the present invention. A motor 102 is arranged to rotate in either direction, although rotation in just one direction is possible. The motor 102 is mounted above a reservoir 104 through which passes a driveshaft 106 coupled to the motor. The reservoir is mounted above a gear and sensor block 108 and the pump arrangement is coupled via send and return lines to a printhead 110. The printhead may be a piezo or thermal printhead or other types of printhead, particularly those variants requiring ink recirculation such as valve jets or electrohydrodynamic printheads, for example. The gear and sensor block 108 includes a gear pump 112 and temperature and pressure sensors 114 on both the send and return lines. An air pump 116 is provided to control the air pressure in the reservoir (i.e. the pressure of the gas, typically air, above the ink in the reservoir). The air pump is preferably a peristaltic pump. Control electronics 118 receives the outputs of the temperature and pressure sensors 114 and controls the speed and direction of the motor as well as the air pump 116. It also provides a safety cut-off, should it be detected that the pump is running dry or DPmax or MPmax are exceeded, for example. The motor 102 is separable, by way of a quick release connection, from the reservoir 104. The reservoir 104 is also separable, by way of a quick release connection, from the gear and sensor block 108. The pump will be described in greater detail below. By providing a return line from the printhead back to the reservoir, recirculation is possible. However, this may be used only periodically (or even not at all) while a suitable meniscus pressure is maintained at the printhead. In the following description, the terms ink and liquid-to-be-pumped are used interchangeably. Inkjet printing is now used for the deposition many materials which would not conventionally be regarded as inks but are suitable for inkjet printing, such as: 1) biological liquids, drugs etc. jetted onto patches, into DNA assays / wells for analysis 2) 3D printing materials either for the support or build material - some are clear, some are coloured 3) clear liquids used as pre or post treatment agents for printing or other applications 4) clear varnishes / overcoats / sealing layers 4) inkjet printed adhesives (thermoset or UV-curable, etc.) 5) photoresists 6) some paints 7) functional fIuids / liquids, often called inks generically but are used for PCB manufacture / other electronic component manufacture e.g. displays, solar cells, capacitors, transistors, antennae, etc. 8) oils for food application development 9) other microfluidics applications Some of these liquids may require heating to be printed effectively. Embodiments of the present invention include a heater to provide this functionality. Figures 2(a) to 2(d) show a front view, a side view, a front sectional view and a side sectional view respectively of a pump according to the first embodiment of the present invention. The same reference numerals are used for the same parts but some reference numerals are omitted from some figures (even if the relevant integer is visible) for clarity. Figure 2(a) shows a front view of a pump according to a first embodiment of the present invention. The pump comprises a lower first section 202 coupled to a middle second section 204 which in turn is coupled to an upper third section 206. The three sections may be separated from each other very quickly by means of a first bayonet coupling 260 between the first and second sections and a second bayonet coupling 230 between the second and third sections. The lower section comprises a pumping and sensor block, the mid section comprises a reservoir and the upper section comprises a motor having an indirect coupling for a driveshaft which is linked to a gear pump in the lower section. These are explained in more detail with reference to Figures 2(b) to 2(d) below. Also visible in the Figure are an outlet line 296, a return line 298 and two thumbwheels 290 which will be described further with reference to Figures 2(b) and 2(d). Figure 2(b) shows a side view of a pump according to the first embodiment of the present invention. The pump comprises three sections 202, 204, 206, the first section including a gear and sensor block 294, the second section including the reservoir 248 and the third section including the motor 210. The Figure further shows a vacuum port 239 for connection to an air pump (116, Figure 1). The vacuum port is connected to the reservoir 248 at a point above the level of the liquid to be pumped. The mid section 204 includes a cage 250 to protect the reservoir 248. Two thumbwheels 290 are also visible. These are provided to permit rapid separation of a heater 292 from a gear and sensor block 294. This is more clearly apparent in the sectional view of Figure 2(d). Figure 2(c) shows a front section 200a through a pump according to the first embodiment of the present invention and Figure 2(d) shows a second section 200b, orthogonal to the first, through the pump. The pump is divided into a lower, or first section 202, a mid, or second section 204 and an upper or third section 206. The upper section comprises a motor 210 having an output shaft 212 which is coupled to the first side of a magnetic coupling 214. A second side of the magnetic coupling 214 is connected to a drive shaft 216. The magnetic coupling may comprise a first section in the shape of an inverted cup which is connected to the motor and a second cylindrical section which fits within the cup. The first and second sections either comprise magnets and magnetic materials or carry magnets and magnetic materials such that the sections will rotate together. The purpose of the magnetic coupling is to isolate the motor and output shaft 214 from the reservoir to be discussed below. An air tight seal 237 approximately in the shape of a top hat is provided between the first and second sections of the magnetic coupling and is made of non-magnetic material. This permits the reservoir to be pressurized (with positive or negative pressure) as is required in certain inkjet printing applications. Further details of a suitable magnetic coupling are available in the present applicant's earlier-filed application no. GB2302024.1. The upper section of the pump is coupled to the mid section of the pump by a bayonet coupling 230 having four spigots. Two of the spigots 232, 236 are shown in cross section in Figure 2(c) while the other two spigots 234, 238 are shown in cross section in Figure 2(d). The second section 204 of the pump comprises a first reservoir 242 which is cylindrical in shape and surrounds the drive shaft 216, and a second reservoir 244 which is also cylindrical in shape and surrounds the first reservoir. The first reservoir is formed by a glass cylinder 246 and the second reservoir is formed between the glass cylinder 246 and a glass cylinder 248. A reservoir top 240, made from acetal plastic, includes two concentric circular grooves arranged to accept the upper edges of the cylinders. The two reservoirs are sealed at the top by O-ring seals 241 and are protected by a structural cage 250. The reservoir top 240 also includes a hole through which the drive shaft 216 passes. The reservoirs are maintained gas-tight by seals arranged around and between the two sections of the magnetic coupling 214. The cage 250 is of any suitable design to protect the reservoirs and which allows a user to see the reservoirs in use. The cage 250 in this embodiment is a 3-D printed component and may advantageously be embossed with a manufacturer's logo or instructions to operators such as a type of ink to be used. In one embodiment the inner reservoir 242 is 48mm high and has a diameter of 15mm. The outer reservoir has an inner diameter of 30mm. The purpose of the double reservoir arrangement is to increase the volume of gas (typically air, but not necessarily) above the liquid in the reservoir. A larger volume of gas provides an improved damping effect that reduces the degree to which pressure pulses generated by the pumping gears are apparent at the output of the pump. The two reservoirs are linked by a channel in the first section described below. In another embodiment only a single reservoir is provided, in other words the inner cylinder 246 is omitted. Alternatively, or in addition, another damper may be provided such as a volume capable of holding gas coupled to the output line of the pump. Such a volume may comprise a membrane damper. Damping is particularly important for a low volume system that does not have long pipe runs to reduce pressure fluctuations. The concentric reservoir arrangement is a compact and effective solution but other damping elements may be added. The mid section 204 and the lower section 202 are also coupled by a bayonet fitting 260 having four spigots of which two spigots 262, 266 are shown in Figure 2(c). The lower section comprises a pump top 270 having concentric grooves to receive the lower ends of the cylinders 246, 248 and also comprising a hole through which the drive shaft 216 passes. The concentric grooves in the pump top also contain O-rings to provide an air-tight seal when the pump is assembled. Beneath the pump top 270 is a top plate 272 of the gear pump body which also comprises a gear block 274. The block includes a figure-of eight portion which contains a drive gear 276 and a driven gear 278. The gear pump is shown end-on in Figure 2(c) so that the drive gear 276 is visible. As an alternative, the pump top 270 could be omitted if the reservoirs comprise cylinders which are open at the top but closed at the bottom to resemble a beaker. A hole for the driveshaft would be required in the base of each reservoir. Another alternative would be to replace the pump top with a flat gasket seal. A flat gasket seal may also be used at the top of the reservoir(s) as an alternative to reservoir top 240 and O-rings 241. The two cylinders are preferably marked with a permanent graticule or ruling or other code such as a QR code for information or identity purposes. For example, an ink code could be included to reduce the possibility that a particular reservoir is ever filled with the wrong ink. A user could be provided with a small scanner to identify the correct contents of the reservoir which is particularly useful where several pumps and different inks are in use. While glass reservoirs have been described, the skilled reader will appreciate that other materials may be substituted according to particular requirements, for example if a corrosive liquid that reacts with glass is to be pumped. The lower section 202 further comprises a heater 292, shown in Figure 2(d) to maintain the liquid at the desired temperature. The heater comprises a Kapton heating pad 291 and a serpentine heating block 293 through which the liquid to be heated passes. The heating pad and the serpentine block are attached to the gear and sensor block 294 by three thumb-wheels 290 for ease of removal. The heater can therefore be removed for ease of cleaning of the pump and the heating pad 291 never makes direct contact with the pumped liquid. The heater is behind the pump in the view shown in Figures 2(a) and 2(c). In this embodiment the liquid is heated in both the outward and return pumping directions, each of which is provided with a temperature sensor. In alternative embodiments the liquid may only be heated in the outward direction. The lower section further comprises pressure and temperature sensors. A pressure sensor is provided in each of a supply pressure sensor well 282 and a return pressure sensor well 280. A supply temperature sensor 284 is shown in Figure 2(d). A return temperature sensor is also provided but not visible in the figure. The sensors are coupled to a sensor connector 288 for connection to the control circuitry (118, Figure 1). In use, liquid in the inner reservoir 242 is pumped through the gear pump into the outer reservoir from where is passes to the output 296 of the pump. The roles of the two reservoirs may be reversed if desired. The gear pump is preferably designed and dimensioned to permit some leakage of liquid from the pump back into the reservoir. This provides both lubrication and reduces the likelihood that heating of the liquid by the gear pump will cause premature curing of ink being pumped. A leakage of less than 25% and greater than 0.5% of the pump input is preferred. A leakage of less than 15% or even less than 10% is further preferred. A minimum leakage of greater than 5% is still further preferred. More details can be found in the present applicants' earlier patent application referenced above. The gears are preferably spigot-free to minimize heating of the pumped liquid through friction. While the figures show straight-cut gears, the use of helical gears is preferred to reduce pressure fluctuations in the output of the pump. Herringbone gears may equally be employed. The gear pump is shown in the Figures at the lowest point in the reservoir but this need not be the case in practice. However, if the gear pump is arranged at a higher level, then there will be a level of liquid in the reservoir that cannot be pumped which is particularly problematic for a low-volume delivery system. While an external gear pump is shown, the present invention is equally applicable to an internal gear gump, i.e. one in which one of the gears rotates inside the other. While a bayonet fitting with four spigots is shown, other fitting techniques may be applied. A screw thread could replace each bayonet with approximately three or four turns required to join the sections of the pump. The screw thread has a diameter of the same order as the width of the pump (to distinguish it from a fitting having multiple screws around the periphery). One drawback to the screw fitting is that it may place additional stress on electrical connectors such as the cable for driving the motor 210 and the pipe (not shown) connecting the vacuum port 239 to the air pump (116, Figure 1). Other fitting techniques include a push-fit such as one typically used to join pressure hoses, having an outer ring that can be moved axially along the pump to release the connection. A magnetic coupling may alternatively be used. The two couplings 230, 270 may be of different types, although it is preferred that they are the same for ease of manufacture and dismantling. A force-fitting having one or more sprung nodules may also be used. It is important, however, to provide sufficient force to adequately compress the seal(s) but not subject the pump structure to excessive amounts of force, which may cause damage. For this reason, the bayonet coupling is preferred. While the described embodiment shows a pair of reservoirs, in alternative embodiments only one reservoir is required. While the embodiment shows cylindrical reservoirs, alternative shapes are possible such as square or hexagonal. However, a cylindrical shape is preferred for reasons of strength and availability. Since the pump may not be provided with a level sensor, it is preferred that the reservoir is transparent, or at least sufficiently transparent for a user to determine the level of liquid in the reservoir. Where the liquid is light-sensitive, such as UV-curable inks, a UV-blocking layer may be placed around the reservoir. The UV blocking layer preferably still permits visible light to pass, thus allowing a user to check the level of ink in the reservoir. In some embodiments a level sensor may be provided although this is not usually necessary for low volume designs used in experimental arrangements as an operator will typically be paying close attention. While the pump has been shown in an upright orientation, the skilled reader will appreciate that it will still function even if not strictly vertical. Alternatively, the motor drive may be arranged to the side of the reservoir, although this is less preferred as the liquid will wet the driveshaft side of magnetic coupling 214 and this may have a deleterious effect on the coupling over time. Inkjet printing arrangements are often provided with a bypass valve. While not shown in the described embodiments, a bypass valve could be provided for the inventive pump. This would have two key benefits: A higher flow through the heater which helps to ensure that the ink remains hot even if long pipe runs are used (slow flow allows the ink to cool) It permits removal of air from the system. Firstly the pump is run forwards, then stopped and run backwards. This sequence can be repeated until all the air is eliminated. Before starting fluid re-circulation operation, liquid to be pumped must be introduced into the reservoir 242 via return line 298. This enables the removal of the liquid from a low point in the circuit. This may be a manual or automatic fill procedure During the fill operation the control system must operate the vacuum pump to maintain the correct pressure inside the system by removing air volume as fluid volume in introduced. A manual fill procedure may be performed using a syringe containing the liquid to be pumped which syringe is removed or may remain in place. The syringe may conveniently be attached to T piece in return line. An automated fill could be performed by a motorized syringe pump that may also remain or be removed. This permits a switch from manual to automatic filling as desired. At start up, the control electronics (118, Figure 1) is arranged to operate the drive motor 210 so that the pump will suck ink from the reservoir 242 and pump it via the heater block 293 and outlet pressure sensor 282 to the output port 296. The drive motor will typically be driven with a current that ramps gently upwards from zero. The controller will activate the air pump (116, Figure 1) until an appropriate level of vacuum is provided within the reservoir 242. The appropriate level of vacuum is inferred from the sensors 280 &282 in the manifold. The heater 291 is activated (possibly in response to an input from an operator) and ink will flow via the heater block 293 and manifold back to the reservoir. The temperature of the ink will thus increase and the control electronics (118, Figure 1) will monitor the output of the temperature sensors in the manifold to determine whether it is sufficiently hot to commence printing operations. Once the correct operating temperature has been attained (typically user-settable via a user interface), the control electronics will inform the operator via the user interface and de-activate the heater 291 to prevent the ink from becoming too hot. Printing may then commence. During operation the control electronics will then: ® activate and de-activate the heater to ensure that the ink is provided within the correct temperature range to the print head (110, Figure 1). Alternatively the heater may be arranged to maintain a base heating level to which additional heating is added as required. ® monitor the pressure of the ink at the manifold 280, 282 and alter the operation speed of the motor 210 as necessary, as well as controlling the air pump (116, Figure 1) to ensure that the correct level of vacuum is maintained in the reservoir (inferred from the meniscus pressure at the printhead manifold) The gear pump may run at speeds of up to 5000 r.p.m. To clean the pump, it is preferably run briefly with cleaner instead of ink or other liquid to be pumped, then dismantled and the wetted parts placed in a suitable cleaning agent. The O-rings located in the reservoir top 240 or pump top 270 may be cleaned or replaced as desired. Since O-rings are available at very low cost they may consequently be discarded and replaced on cleaning of the pump. The majority of the pump components may be made from acetal with the exception of the heating block 293, screws etc. Alternatively, PEEK may be employed, particularly when exotic inks or liquids are being pumped because PEEK is more chemically resistant than acetal. The skilled reader will appreciate the range of suitable materials that may be employed for a given choice of materials constituting the liquid delivery system. Figure 3 shows a front view of a pump according to embodiments of the present invention, separated into the first section 302, the second section 304 and the third section 306. One key thing to note is that the driveshaft 316 is predominantly located within the second section 304. The inner portion of the magnetic coupling 214 protrudes slightly from the top of the second section. This is the isolator that sits over the ring magnet and ferrule on the drive shaft. The magnet cup with arc magnets inside is the magnetic coupling part which is coupled to motor and is part of the third section is not visible in this figure. A lower portion of the driveshaft 316 protrudes from the bottom of the second section and has a coupling or coupling portion to engage with the drive gear (276, Figure 2). In this case it is a portion of the stainless steel driveshaft having a pentagonal cross section. Since the three sections are so readily separated, the procedure may be automated, i.e. a robot is provided that can grip one section and turn an adjacent section to separate the bayonet or screw fittings. A robot may equally be arranged to separate other types of fittings. The robot may also be arranged to perform cleaning steps and re-assemble the pump.

Claims

1. A liquid delivery system comprising a first section connected to a second section by a first coupling and a third section connected to the second section by a second coupling, the first, second and third sections being mutually separable to effect a thorough cleaning of the pump,the third section comprising a rotary drive means and an indirect coupling for a drive shaft,the second section comprising a first reservoir for liquid to be pumped, and a driveshaft passing between the third section and the first section, andthe first section comprising a gear block, a drive gear and a driven gear located within the gear block, the drive gear coupled to the driveshaft, the first section further comprising, a pump inlet and pump outlet,at least one of the first and second coupling being a fast locking coupling.

2. A liquid delivery system as claimed in claim 1, wherein the fast locking coupling is selected from:a bayonet couplinga screw fitting, the screw having a diameter of substantially the same dimension of the second section,a latching sleeve,a force coupling,a magnetic coupling, anda plurality of clips.

3. A liquid delivery system as claimed in claim 2, wherein the plurality of clips comprise over-centre latches.

4. A liquid delivery system as claimed in claim 2 or claim 3, wherein both first and second couplings are selected from:a bayonet couplinga screw fitting, the screw having a diameter of substantially the same dimension of the second section,a latching sleeve,a force coupling,a magnetic coupling, anda plurality of clips.

5. A liquid delivery system as claimed in any one of the claims 1 to 4, wherein the first and second coupling are of the same type.

6. A liquid delivery system as claimed in claim 1 wherein at least one of the first and second couplings comprise bayonet fittings.

7. A liquid delivery system as claimed in claim 6, wherein the at least one bayonet fitting comprises four spigots.

8. A liquid delivery system as claimed in any preceding claim, wherein the second section further comprises a second reservoir, in fluid communication with the first reservoir.

9. A liquid delivery system as claimed in claim 8, wherein the second reservoir is arranged concentrically with the first reservoir.

10. A liquid delivery system as claimed in any one of the preceding claims, further comprising a gas connection to the reservoir.

11. A liquid delivery system as claimed in claim 10, wherein the reservoir is a gas-tight reservoir.

12. A liquid delivery system as claimed in any preceding claim, further comprising a heater.

13. A liquid delivery system as claimed in any preceding claim whereby, in use, a proportion of the liquid pumped through the inlet is arranged to flow around the gears and leak from the gear pump via at least an aperture to be returned to the reservoir.

14. A liquid delivery system as claimed in claim 13, wherein the proportion of the liquid that leaks is greater than 0.5% of pump flow.

15. A liquid delivery system as claimed in claim 13 or claim 14, wherein the proportion of the liquid that leaks is less than 25% of pump flow.

16. A liquid delivery system as claimed in claim 13, claim 14 or claim 15, wherein the proportion of the liquid that leaks is less than 15% of pump flow.

17. A liquid delivery system as claimed in any one of the claims 13 to 16, wherein the proportion of the liquid that leaks is less than 10% of pump flow.

18. A liquid delivery system as claimed in any one of the claims 13 to 17, wherein the proportion of the liquid that leaks is greater than 5% of pump flow.

19. A liquid delivery system as claimed in any preceding claim, wherein the indirect coupling comprises a magnetic coupling.

20. A liquid delivery system as claimed in any preceding claim, wherein the pump may be driven in either direction.

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