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The invention pertains to a method for measuring a liquid flow and to equipment to apply this method.

Known methods for liquid flow yield measuring, thus the measuring of the volume or mass per second, are based upon the measuring of a reservoir to be filled 5 with liquid flowed out during a given period or the weighing of the mass flowed out in a given period. In particular for small flow rates to be measured in real time known methods are too unaccurate, not real time but integrating in time, too time consuming and too sensitive for noise.This holds for e.g. medical infusion systems where often very small quantities are involved. Deviations from the stipulated flow can have rather 10 desastreous medical consequences and should be corrected or avoided.

It is an object of the invention to disclose a method which works accurate for relatively small flow rates. It can be realised in a simple way, using known elements mainly and hence reliable to produce.

A method as introduced in the preamble is according to the invention 15 characterized in that the use is made of optical imaging means for volume measuring of a liquid flowed through an outlet. The use of optical imaging and image processing means enables the measuring of a volume flow rate in real time with a high accuracy and results in a flexible measuring set-up to take into account other relevant parameters like environmental back-pressure. For the volume determination with optical means use 20 is made of, preferably digital, image acquisition and image processing tools. Then image registration, processing and presentation can be executed on-line, fast, accurate and clear. Any transformation from volume into mass, using the density can be performed either.

In a preferred embodiment the growth of drops growing at a nozzle due to 25 the flow is part of the measuring method. This growth is measured by processing of the drop image and calculating continuously the real time, instantaneous, drop volume. By

measuring as well the volume of the drops as the number of drops falling off from the outlet in a unit of time an apparatus can be used for a wide range of flow rates. Also the volume of the falling drops can be measured as well after leaving the outlet once they have a stable spherical shape. In a preferred embodiment for the optical registration use is made of a detector matrix like a CCD chip, a classical video camera, an open RAM memory or other light sensitive devices. Such a detector is arranged along the path of the growing and or falling drops.

Although it may be supposed that as well a building up drop as a falling drop do have cylindrical symmetry perpendicular to the direction of falling it can be preferred in a further embodiment to make use of image acquisition in more directions. In this situation more acquisition devices can be applied as well the image from different directions is projected on the detector using optical tools, like mirrors. In such an arrangement corrections can be made for asymmetry and/or vibrations in the drops. Hence the accuracy of the measuring can be improved.

Determination of the volume growth of the drops in combination with counting the number of fallen drops in a unit of time can be used for flow rate measuring and for controlling and adjusting any flow or flow rate as long as drop formation exists. The total delivered volume is calculated from the sum of volumes of all the fallen drops. To minimize the absolute error in the yield of the flow the drops are collected in a container where the position of the meniscus will be measured optically. Knowing the diameter of that container the increase of level in a unit of time is a measure for the flow rate as well. Potentially substantial evaporation during the growth of drops or level increase at very small flow rates can be compensated by adapting the partial vapour pressure in the measuring room by adjusting the temperature just above the condensation point. Attention should be paid to avoid condensation on the optical parts of the instrument. i a further preferred embodiment the container for the collected drops has the shape of a transparent cylinder. Once filled this cylinder acts as a cylinder lens and hence the level of the meniscus can be determined easily due to the difference in imaging properties between the filled and empty part of the container. An illumination

slit is preferably positioned parallel to the axis of the cylinder opposite to the image acquisition device in the back focal plane of the cylinder lens formed by the filled container. In case of an empty container the container acts as a window so the slit will not be magnified. This changes in the case of a full container where the slit will be magnified onto the pick-up device.

In a further embodiment the container is at the bottom closed by a valve. Down stream of the valve there is a border to define the zero reference level in the container. To ensure a reproduceable meniscus and to reduce the possibility of droplets at the inner wall of the container it can be desireable to roughen the innerside of the container. The surface is wettened better by this treatment. Such a roughening does not hamper the meniscus determination due to the fact that the liquid in the container will smooth the surface.

Determination of the increase of level of the content in the container in a unit of time can be used for flow rate measuring and for controlling and adjusting any flow or flow rate.

The change in size of a volume caused by a flow is thus optically measured. Based on the geometrical shape the content of the volume is calculated continuously. The change of the content in a unit of time is a measure for the flow. Good shapes from which the content can be measured are free hanging drops or measu- ring beakers. The increase of the drop size or meniscus level in the beaker are simple parameters due to their symmetry. The measurement of the size of falling drops can be used for a cross-check on the integrated yield over a longer time.

With the above described automated measuring methods a feed-back controlled and adjusted infusion pump can be realised. Also existing pumps can be measured and checked for all the relevant parameters.

This measuring method can be applied in a wide range of instruments meant for dosing, measuring or controlling and regulating. Two applications are described for measuring and dosing purposes. The measuring of the flow rates of pumps to be checked can be executed by measurements on drops as well measuring beakers. A pump with a continuous flow can be designed using as well the drop measurement as the meniscus level detection for feed-back of the real time yield of the flow to adjust the speed of the pump and hence the flow.

A method as disclosed is particularly applicable in an apparaus for an accurate determination of liquid flows or liquid flow rates such as for example in medical infusion apparatus.

In a preferred embodiment an appartus according to the invention is carried out to check the yield for dosing medical treatment or diagnosis.

In a further embodiment an apparatus according to the invention is carried out to control and/or adjust a flow rate of a dosing system of e.g. a medical infusion set.

The invention will be described in more detail with reference to the drawings in which:

Figure 1 shows schematically a measuring set-up for drop measurement according to the invention;

Figure 2 shows a measuring set-up for optical meniscus level detection, Figure 3 shows an example of a flow-feed-back regulated pump, Figure 4 shows a further measuring set-up according to the invention, and

Figure 5 a plural container holder for such a set-up. A measuring set-up as represented in Figure 1 shows schematically a support 1 with, close to an outlet 2 of a tubing 3 of a pump 4 to be checked an image pick-up device 5 on which an image 6 of a drop 7 at the outlet is projected by a lens 28. A prism 8 projects an image 9 of a falling drop 10 also on the device 5. Similar set¬ ups 11 can be positioned under an angle with the described set-up for minimizing the influence of asymmetries. The images are processed by an image processor 12 which calculates the content and represents the value or reprocessed results on a presentation peripheral 13. A scetch of an equipment for meniscus level detection is given found in

Figure 2. In a container 14 falling drops or outstreaming liquid can be collected with a meniscus 15. Opposite to an image pick-up device 5 at the rear site of a preferrably round container 14 a light bar 16 is positioned in the back-focal plane 17 of the filled container. A filled part 18 of the container projects a magnified image 19 on the pick-up device or bend an image 19a of the bar totally besides the pick-up device if the optical axes 20 of bar, container and pick-up device do not coincide. In both cases the boundary between the image of the bar, not influenced by the unfilled part 21 of the °

container, and the image, influenced by the filled part, can easily be detected by an image processor 12 which calculates the volume increase and represents the value or reprocessed results on a presentation peripheral 13.

In Figure 3 the former 2 set-ups, a set A for the drop measurement and a set B for the meniscus detection, are combined for regulation of a flow itself. Set-up A regulates the driving force needed for meeting the required flow. A processor unit 22 steers a valve 23 or the speed of a pump 24. Set-up B detects the meniscus level to let compensate the back pressure at the outlet 25 of the complete device. Pressure sensors 26 in the outer world and a drop chamber 27 can also support the control strategy due to their even faster sampling speed.

A measuring set-up as represented in Figure 4 shows schematically a support 32 with, an arm 33 mounted close to a outlet 34 of for example a medical infusion system 36. The set-up is provided with an infusion pump 37, a first image pick-up device 38 being double shaped here and a second image pick-up device 40 positioned along side a path 39 for falling drops 43, also being douϊbe shaped here. With the help of a holder 42 the outlet 34 can easily be positioned with regard to the pick-up device 40. A container 44 for falling drops is provided at a bottom side of the support 32. The container 44 is preferably formed to enable measuring of the volume o the liquid stored therein for example by meniscus determination. The container 44 can also be provided with weight measuring means and with means for counting the number of falling drops 43. The pick-up devices 38, 40 and 46 are connected to a registration and processing unit 48. In this unit drop growth can be measured on line for signals derived from the pick-up device 38, thus a time dependent flow-rate can be determined on-line.

It is also possible to count the number of falling drops. In the arrangemen measuring signals of the pick-up device 40 can be used to determine the volue of the falling drops. Counting the number of falling drops and measuring the volume thereof can be combined to realize a flow-rate determination. From the container 44 subsequently the integrated volume growth of a row of stored drops can be determined by measuring the total weight thereof. To the registration and processing unit 48 a display 50 is added to show the meausring results. The display 50 can be a monitor, a priter or a writer.

The arrangement as shown indicates two pick-up devices at each measuring location. These devices are positioned mutually under a given angle, preferably under 90° in order to avoid deviations in the measuring due to non-spherical cylindrical geometryu of the drops. Such a non-symmetry can easily occur especially . during growth of the drops. The number of pick-up devices and the angle enclosed can be chosen in dependency to the amount of non-symmetry of the drops.

Figure 5 shows an embodiment of a support 60 for a number of containers 44. The support 60 comprises a rotatable disc 64 mounted about an axis 62 being directed parallel to the path of falling drops in an apparatus in function. A number of holes 66 are provided in the disc 64 to enclose containers 44 which can subseqeuntiy be positioned in the path of falling drops. Container 44 can be of different size adapted to the magnitude of the flow-rate to be determined.

For a continuous measuring the containers are provided with a valve 70 on a rear side 68 in order to enable emptying of the containers. Opening and closing of the valves 70 can be correlated with the position of the container with respect to the path of felling drops 39.