Abstract:
A liquid dosing arrangement has a liquid supply system with a first pressure sensor for measuring a first pressure p 1 ; a liquid delivery system, with a second pressure sensor for measuring a second pressure p 2 ; and an injection valve, which is arranged so that the liquid is supplied through a restricting member in the injection valve and let through the restricting member by a pulsed opening and closing valve mechanism. The pressure drop, and the dimensions of the restricting member in the injection valve, are selected to make the velocity ν of the liquid flowing through the restricting member is high enough to make the flow of liquid through the liquid delivery system independent of the viscosity of the liquid.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a liquid dosing arrangement, and more specifically to an arrangement for maintaining flow accuracy without the use of a flow sensor when an injection valve is used for dosing liquids.  
         [0003]     2. Description of the Prior Art  
         [0004]     There is often a need for dosing liquids with good accuracy, especially in the field of medicine, e.g. during intravenous infusion or epidural anesthesia. A number of different systems exist on the market, e.g. syringe infusion pumps, peristaltic pumps and hydrostatic infusion devices. All these methods have both advantages and drawbacks with respect to for example performance, security, complexity and pricing. Syringe infusion pumps for example give good accuracy, but are quite expensive. It is therefore advantageous to instead use a simple and cheap injection valve for dosing liquid.  
         [0005]     An injection valve releases liquid in short pulses from a pressurized liquid source. The flow of liquid can be controlled by varying the pulse frequency and/or the pulse width of the liquid pulses. The flow however varies significantly, depending on parameters such as the ambient temperature, the inner diameter of the tubing, the radius of curvature of the tubing, counter pressure from the patient, or occlusions in the catheters. The ambient temperature causes the viscosity of the liquid to change—the viscosity of water for example decreases 50% when the temperature changes from 10° C. to 40° C. The counter pressure from the patient depends on parameters such as blood pressure, and also varies due to changes in hydrostatic pressure as the patient moves. Therefore an accurate flow sensor is required in order for the flow to be controlled. Accurate flow sensors are however quite expensive and have a limited flow range, and since they are also difficult to sterilize, they are hardly suitable for the intended applications.  
         [0006]     There is therefore a need for a method and an apparatus for using an injection valve for accurately dosing liquids without the use of an accurate flow sensor.  
       SUMMARY OF THE INVENTION  
       [0007]     To achieve this objective the present invention provides a liquid dosing arrangement having a liquid supply system in which a first pressure sensor is arranged for measuring a first pressure p 1 ; a liquid delivery system in which a second pressure sensor is arranged for measuring a second pressure p 2 ; and an injection valve, located between the liquid supply system and the liquid delivery system, the liquid being supplied through a restricting member of the injection valve, and being let through the restricting member by a pulsed opening and closing valve mechanism with opening pulses having frequency f o  and width t o , and the pressure drop over the injection valve being Δp=p 1 −p 2 ; characterized in that the pressure drop Δp, and the dimensions of the restricting member in the injection valve, are selected to make the velocity ν of the liquid flowing through the restricting member high enough to make the flow Φ of liquid through the liquid delivery system essentially independent of the viscosity of the liquid.  
         [0008]     The invention thereby makes it possible, with a comparatively simple and inexpensive device, to achieve an accurate flow control irrespective of downstream counter pressure as well as variations in temperature and geometric variations of the tubing, and without the use of accurate flow sensors.  
         [0009]     In a preferred embodiment, the restricting member in the injection valve is an orifice in an orifice plate. The diameter of this orifice is preferably less than 300 μm. If a higher flow rate is desirable several orifices may be used, each with a diameter of typically less than 300 μm.  
         [0010]     The liquid dosing arrangement preferably further comprises an expansion chamber downstream of the injection valve, preferably located immediately downstream of the injection valve.  
         [0011]     A low pass filter can be arranged to filter the signal from the second pressure sensor. In a preferred embodiment this signal is then fed to a pressure regulating system which is arranged to feed a regulating signal to a pressure regulator in order to regulate the first pressure p 1  in the liquid supply system.  
         [0012]     The liquid dosing arrangement preferably further has an alarm which is arranged to be activated when the second pressure p 2  measured by the second pressure sensor exceeds a certain preset value p max , or when the flow measured by a simple monitoring flow sensor in the liquid supply system deviates beyond certain preset values.  
         [0013]     The present invention further provides an apparatus for intravenous infusion comprising a liquid dosing arrangement according to the invention.  
         [0014]     The present invention also provides a method of dosing liquid including the steps of providing a liquid dosing arrangement comprising a liquid supply system, a liquid delivery system and an injection valve; measuring a first pressure p 1  in the liquid supply system by the use of a first pressure sensor; and measuring a second pressure p 2  in the liquid delivery system by the use of a second pressure sensor; characterized in selecting the pressure drop Δp=p 1 −p 2  over the injection valve, and the dimensions of the restricting member in the injection valve, so that the velocity ν of the liquid flowing through the restricting member becomes high enough to make the flow Φ of liquid through the liquid delivery system essentially independent of the viscosity of the liquid.  
         [0015]     The method of dosing liquid according to the invention preferably further includes the step of providing an expansion chamber downstream of the injection valve, preferably immediately downstream of the injection valve.  
         [0016]     In a preferred embodiment, the method further includes the steps of filtering the signal from the second pressure sensor in a low pass filter; feeding the signals from the first and second pressure sensors to a pressure regulating system; and feeding the output of the regulating system as a regulating signal to a pressure regulator in order to regulate the first pressure p 1  in the liquid supply system.  
         [0017]     The method preferably further includes the step of activating an alarm when the second pressure p 2  measured by the second pressure sensor exceeds a certain preset value p max , or when the flow measured by a simple monitoring flow sensor in the liquid supply system deviates beyond certain preset values. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  schematically illustrates a liquid dosing system comprising an injection valve according to one embodiment of the present invention.  
         [0019]      FIG. 2   a  schematically illustrates an orifice plate which is a part of the injection valve in  FIG. 1 .  
         [0020]      FIG. 2   b  is a side view of the orifice plate in  FIG. 2   a.   
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     The pressure Δp over an injection valve depends on the density and the viscosity of the liquid flowing through the valve as follows:  
         Δ   ⁢           ⁢   p     =         k   1     *       ρ   *     v   2       2       +       k   2     *     η   ⁡     (   T   )       *   v           
    ν=the velocity of the liquid flowing through the restricting member     ρ=the density of the liquid (which depends very little on the temperature)     η(T)=the dynamic viscosity of the liquid (which depends very strongly on the temperature)     k 1 , k 2 =constants (k 1  is normally about 1-1.5, assuming that the kinetic energy is lost in eddies downstream the abrupt restriction)    
 
         [0026]     For values of ν which are high enough, the viscosity part becomes negligible, since it only depends on ν, while the kinetic pressure drop depends on ν 2 . For values of ν which are high enough, the pressure drop Δp thus becomes virtually independent of the viscosity η(T), and accordingly also of the temperature T.  
         [0027]     For high enough values of ν we therefore have:  
               Δ   ⁢           ⁢   p     =         k   1     *       ρ   *     v   2       2       ⇒           
     ⁢           v   =           2   *   Δ   ⁢           ⁢   p         k   1     *   ρ                             
 
         [0028]     The flow Φ through an injection valve depends on the velocity ν of the liquid and the diameter d 1  of the restricting member in the following way:  
             Φ   =         π   *     d   1   2       4     *                 v   =         π   *     d   1   2     *     2         4   *       k   1           *         Δ   ⁢           ⁢   p     ρ                   
 
         [0029]     Since the density ρ of the liquid is almost constant regardless of the temperature, Φ thus becomes proportional to √{square root over (Δp)}, which can be kept constant by a very simple form of pressure regulation. For high enough values of ν, Φ is therefore independent of all temperature and time dependent pressure drops downstream of the injection valve. Also, the fact that Φ is proportional to √{square root over (Δp)} results in that a regulating error in Δp of x % only leads to an error in Φ of x/2%.  
         [0030]     A high velocity ν of the liquid through the restricting member in the injection valve is accomplished by letting the restricting member have a small diameter, and letting Δp be high. Δp should therefore be regulated to a value high enough, and the diameter of the restricting member selected to be small, with the additional condition that a desired maximum flow Φ max  can be accomplished and that certain practical additional conditions are met, such as the limitation that Δp can never be higher than the available pressure p in  of the employed pressure source.  
         [0031]     The practical application of this will now be described with reference to  FIG. 1 .  FIG. 1  shows a liquid dosing system having a pressurized liquid source  1 , an injection valve  2  and a liquid delivery system  3 . The liquid delivery system  3  communicates, via a suitable liquid delivery interface with a schematically indicated patient to deliver liquid to the patient which, in the patient exhibits pressure Pp(t). The pressurized liquid source  1  can for example be an elastic container  12  under pressure in the form of a so called bag-in-bottle. The source of the driving pressure p in  can for example be the compressed air that is usually available from tapping sources in the walls in hospitals, but of course any suitable pressure source can be used. The driving pressure p in  is in hospital environments usually about 2-8 Bar. It is possible to use other methods of liquid feeding, such as mechanical pumps or other types of mechanical pressurizing means, instead of a bag-in-bottle. However, the described bag-in-bottle type liquid source is preferred due to its simplicity and to the ease of maintaining the sterility of the liquid in critical applications.  
         [0032]     The injection valve  2  releases liquid in pulses having frequency f o  and width t o  from the pressurized liquid source  1 . The pressure drop over the injection valve  2  is Δp=p 1 −p 2 . If the driving pressure p in  is constant and no regulation is effected, the pressure p 2  varies significantly, depending on parameters such as the ambient temperature, the radius of curvature of the tubing, counter pressure from the patient, or occlusions in the catheters. This results in an undesirable strong variation of the flow Φ, since this as explained above depends on the pressure drop Δp.  
         [0033]     According to the invention the liquid dosing system therefore further has pressure sensors  4  and  5 . Pressure sensor  4  measures the pressure p 1  in the liquid dosing system  1 , and pressure sensor  5  measures the pressure p 2  immediately after the injection valve  2 . In the configuration shown in  FIG. 1  the bag  12  must be very pliable and its wall material not stretched in order for the pressure sensor  4  to measure the correct liquid pressure inside the liquid container. The signal from pressure sensor  5  is filtered through a low pass filter  7 , and then fed to a pressure regulating system  15  together with the signal from pressure sensor  4 . The pressure regulating system  15  outputs a regulation signal to a pressure regulator  6  which regulates the pressure p 1  in order to keep Δp=p 1 −p 2  constant and equal to a certain preset value Δp ref . Δp ref  is preferably selected to the highest possible value which is practical in the chosen application. In a hospital environment a suitable Δp ref  can for example be 3 Bar.  
         [0034]     The injection valve  2  preferably has a restricting member in the form of at least one circular hole  22  drilled in an orifice plate  21 , as shown in  FIG. 2   a , but other restricting members can also be used. The restricting member should be “short”, which is accomplished by the orifice plate  21  being thin, i.e. the thickness b in  FIG. 2   b  being small. A thin restricting orifice plate  21  also reduces any contribution to the pressure Δp from the viscosity part. The diameter d 1  of the hole(s)  22  in the orifice plate  21  should be, as explained above, as small as possible, with respect to parameters such as Δp, the desired maximum flow Φ max , the characteristics of the liquid, and other practical considerations. The hole(s)  22  can of course have any suitable shape, not necessarily circular. The flow Φ is accomplished by opening the injection valve  2  fully during short pulses, having frequency f o  and width t o . Variation of the flow is accomplished by varying either t o  or f o , or alternatively a combination of both.  
         [0035]     The liquids used in hospitals are often water-based diluted solutions. Within the limitations explained above the velocity ν of the liquid will be the same for the same pressure drop Δp ref , frequency f o  and width t o . If for a specific type of injection valve the thickness b of the orifice plate is about 0.2 mm, Δp ref  is set to 3 Bar and the valve is opened for 2 ms every 10 seconds, the velocity ν will be 20 m/s. If the injection valve has one hole having a diameter d 1  of 100 μm, Φ max  will then be about 160 μl/s and Φ min  will be about 0.03 μl/s.  
         [0036]     If the diameter d 1  of the hole is only 50 μm, Φ max  will only be about 40 μl/s and Φ min  will be about 0.01 μl/s.  
         [0037]     If the diameter d 1  of the hole is 200 μm, Φ max  will be about 630 μl/s and Φ min  will be about 0.13 μl/s.  
         [0038]     If the diameter d 1  of the hole is 300 μm, Φ max  will be about 1400 μl/s and Φ min  will be about 0.28 μl/s.  
         [0039]     If the injection valve instead has four holes, each having a diameter d 1  of 100 μm, Φ max  will be about 630 μl/s and Φ min  will be about 0.13 μl/s.  
         [0040]     These values are of course just examples of working embodiments, and they are in no way limiting to the scope of the invention. There are many different types of injection valves and they all have specific characteristics, for example concerning the amount of viscous pressure drop in the valve mechanism, and the type of liquid used also affects the flow, so in practice an empiric optimization of the relevant parameters must be done, based on the above explained principles.  
         [0041]     It is also possible to use an injection valve having an orifice plate with a hole having a diameter d 1  which is larger than 300 μm, but with a larger diameter the advantages of the claimed solution become less apparent.  
         [0042]     A problem with liquid delivery systems such as shown in  FIG. 1  is that they normally employ long plastic tubing with a small inner diameter and rigid walls. This causes them to have a very high analog “inductance”, i.e. “resistance” to quick flow changes.  
         [0043]     The pressure drop p 2  over the tubing is a function of the flow change according to:  
               p   2     =       Φ   .     *   L   ⁢           ⁢   where                 L   =       4   *   ρ   *   l       π   *     d   2   2                   
 
 d 2 =the inner diameter of the tubing 
 
 l=the length of the tubing 
 
 ρ=the density of the liquid 
 
         [0044]     If the pulse time to is short, this results in that virtually no liquid flow at all has time to occur, and Φ thus does not become proportional to t o  when the pulse time is increased.  
         [0045]     One way of counteracting this effect is to place an expansion chamber  14  immediately downstream of the valve  2 , with a compliance of C=ΔV/Δp, adapted so that the pressure increase Δp becomes sufficiently small for the volume ΔV that is obtained at the longest time t o  that will be used in the pulse train. An expansion chamber  14  with a suitable shape and adequate compliance (caused by e.g. a spring load or the elasticity of the chamber itself) is therefore shown in  FIG. 1  positioned immediately downstream of the injection valve  2 , in order to absorb the pressure transients from the pulsed operation. Now, the effect of the inertia of the liquid in the tubing is eliminated and the mean flow through the tubing will be essentially proportional to the pulse width. Any remaining ripple in the pressure signal p 2  is filtered away in the low pass filter  7 , which should have a long time constant (typically 1-10 s).  
         [0046]     Viscous pressure drop or kinetic pressure drop between the container  12  containing the liquid and the injection valve  2  is also undesirable. Therefore the tubing  13  from the container  12  to the injection valve  2  should be as short as possible and have a sufficiently large inner diameter. Otherwise the pressure regulator  6  cannot manage to keep Δp at a constant value.  
         [0047]     Pressure sensors  4  and  5  are preferably of a type that is simple, inexpensive and easy to clean. They may also be of a disposable type.  
         [0048]     The pressure regulating system  15  can be designed in any suitable way, but preferably has a comparator  8  which calculates the pressure drop Δp, and another comparator  9  which compares the pressure drop Δp with the preset value Δp ref . It is advantageous to also include an amplifying device  10  for amplifying the error signal before feeding it to the pressure regulator  6 .  
         [0049]     In a preferred embodiment, the liquid dosing arrangement further has an occlusion alarm  11 , which is activated when p 2  exceeds a certain preset value p max . The occlusion alarm  11  can be used to interrupt the flow by lowering p 1  to zero, discontinue the pulsing of the valves, etc.  
         [0050]     When a liquid dosing arrangement according to the invention is used for infusion, the pressure sensor  5  and the occlusion alarm  11  are already present in the system, since this is a requirement for infusion. The pressure sensor  5  is normally placed immediately after the valve.  
         [0051]     A suitable flow alarm  11  at instances such as major valve leakage or errors in the regulation, which causes the flow measured by a flow sensor  16  in the liquid supply system  3  to deviate beyond certain preset values, can also be required for certain applications. In this case, a very simple type of flow sensor  16  of pressure drop type (with lesser accuracy) can be used for this function. The alarm  11  can be the same alarm as used for the occlusion alarm, or be a separate device.  
         [0052]     Although modifications and changes may be suggested by those skilled in the art, it is the invention of the inventor to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of his contribution to the art.