Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
  • G01N13/02—Investigating surface tension of liquids
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
  • G01N13/02—Investigating surface tension of liquids
  • G01N2013/0241—Investigating surface tension of liquids bubble, pendant drop, sessile drop methods

Definitions

• the present invention relates to a method and an instrument for measuring surface tension by detecting liquid drops formed at the end of a capillary.
• the surface tension can indicate the substances existing in the liquid and the concentrations thereof.
• the analysis can then be used to for example control the concentrations of different substances in the liquid.
• a problem existing today is that it is not possible to continuously measure surface tension in a dynamical and exact way.
• the conventionally used methods for measuring surface tension are based on an external analysis of the surface tension, i.e. that a sample must be taken from the system in order for an analysis thereof will be performed. This method results in that there long time has to pass before an answer can be obtained.
• the disadvantages of such methods are naturally plural, and thereamong the following can be mentioned: - Time consumption. Because of the fact that it is not possible to perform the measurement inside the process it takes a longer time.
• U.S. patent 4,942,760 a device for measuring surface tension is disclosed, in particularly for measuring on liquids under relatively high pressures and temperatures. Liquid is injected with a high pressure from a cylinder pump into a closed space in which, by a camera, a picture is captured of a formed drop. Using this device the surface tension cannot be determined continuously or periodically at repeated small time intervals. Therefore it is not suited for for example process control.
• U.S. patent 4,646,562 a method is disclosed for continuously monitoring surface activity and surface tension of a liquid. The time intervals between the times when drops fall off from a nozzle are measured and evaluated.
• drops are formed at the end of a capillary in a closed space.
• the pressure in the closed space or the difference between this pressure and the pressure in the capillary is measured and evaluated to determine the surface tension.
• a capillary is used, in which a liquid is slowly flowing and at the end of which drops of the liquid are formed and fall off.
• the point of the capillary is located inside a closed space containing the fluid so that the drops fall off in this space.
• a suitable device for detecting growth and/or release of liquid drops a measured signal is generated.
• This detecting device can include an electrical sensor, an acoustical sensor or preferably an optical sensor.
• a sub-atmospheric pressure is maintained by pumping liquid away from the closed space.
• the difference between the sub-atmospheric pressure and the ambient pressure that can be relatively small causes new liquid to flow to the point of the capillary.
• a suction pump is then connected to the closed space, to a liquid outlet thereof, in order to transport the liquid away.
• the fluid or gas contained in the closed space thereby separates in nearly mechanical manner a liquid drop at the end of the capillary from the liquid sucked away by the pump.
• Vibrations, pulsations and other interference derived from the pump when it is operating will therefore not significantly influence the liquid drop or at least such vibrations, pulsations and interference during the time period when the liquid drop is being formed, is growing and finally falls off are strongly reduced. Also, the liquid flow in the capillary is not significantly affected.
• the fluid or gas in the closed space then works as a pulse attenuator. Told in another way, the flow velocity of the liquid in the supply conduit fluctuates less during each drop forming period compared to known methods. It implies that the simple pump having a pulsating flow behavior can be used, this allowing the use of low cost and/or industrially reliable and robust pumps.
• the velocity of the liquid flow though the capillary up to the point thereof at which the drops are formed can be controlled by varying the sub-atmo- spheric pressure generated by the pump.
• the sensor used in the detection of the growth and falling off of liquid drops is advantageously designed to detect two positions, a first position when a liquid drop during its growth at the end of the capillary reaches a given or predete ⁇ nined size, and a second position when the drops falls off from the end of the capillary.
• the time periods which are measured between the reaching of the second positions i.e. the times when a plurality of successive liquid drops fall off from the point or outlet of the capillary, are divided in two portions by also measuring the times when the first positions is reached.
• the relationship or quotient between the lengths of these two portions of the time periods can be used for calculating the surface tension of the liquid.
• the time periods for the growth of a plurality of successively drops are each measured and are, due to another measurement for each drop, divided in two sub-periods. It appears that the measured value of the surface tension of a liquid calculated from these two time lengths is not significantly negatively affected by unintended and unknown changes of the flow velocity of the liquid when it is being fed to the closed space, i.e. the flow velocity inside the capillary.
• An optical sensor for using the measurements of the liquid drops can for example include a photosensitive cell, a photosensitive switch, an optical distance meter, an photo-electric cell device consisting of a light source together with an associated sensor, a light absorbance meter consisting of a light source together with an associated sensor, a refraction meter consisting of a light source together with an associated sensor, a reflection meter consisting of a light source together with an associated sensor, an optical thermal sensor or a picture sensor provided to form an image or picture of the growth and falling off of drops.
• An electrical sensor can for example include an electrostatic contact sensor, a conductive sensor, an inductive sensor, a permeability sensor, a capacity sensor or a magnetic sensor.
• An acoustic sensor can for example include a sound distance meter.
• the signal is evaluated during a longer time period comprising one or more cycles which each one includes that a drop is being formed and that it falls off from the end of the capillary to give a value of the surface tension.
• the method and instrument are thus based on the detection of growth and/or falling off of liquid drops from an end of a capillary. Due to the fact that the drops for which a detection is made are formed in a fluid trapped in a closed space or volume, a high operational security is obtained.
• - Fig. 1 is a schematic view of the central parts of a measurement instrument, based on a photo- electric cell device, for measuring the surface tension of a liquid by detecting two positions in the growth and falling off of liquid drops at the end of a capillary inside a closed space connected to a suction pump,
• Fig. 2a is a diagram showing the measured signal as a function of time when a plurality of dr.ips are successively being formed at the end of a capillary and during the growth and falling off thereof pass a photo-electric cell device consisting of a light source together with an associated sensor,
• Figs. 2b - 2e are schematic pictures illustrating the growth and falling off of drops at the end c f a capillary in four different positions
• FIGs. 3a and 3b are diagrams showing the flow velocity as a function of time through a suction pump and through a capillary, respectively, liquid drops being formed and falling off at the end of the capillary,
• Fig. 4 is a block diagram of a measurement instrument for measuring surface tension
• FIG. 5 is a block diagram of an alternative embodiment of a measurement instrument for measur- ing surface tension.
• Fig. 1 are shown the central parts of an assembly or installation or measurement instrument for detecting or measuring surface tension, dynamically as well as statically between a liquid and a gas by detecting two positions in the growth and falling off of liquid drops at the end of a capillary, or generally between a first liquid having a higher, generally a considerably higher, density and a second fluid having a lower density that can be gas or liquid.
• the liquid, the surface tension of which in relation to the fluid is to be measured is supplied with a substantially constant flow velocity from a source, not shown in this figure, through a capillary 3 into a chamber 7.
• the capillary has at its free end portion an end surface at which drops of the liquid are being formed and from which they fall off.
• the end surface can for example advantageously be located horizontally whereas the end portion can extend in a vertical direction, as has been indicated above.
• a pump 31 sucks liquid away from the chamber 7 and creates in that way a sub-atmospheric pressure for the fluid contained in the chamber 7. It forces liquid to flow through the capillary 3 and to form, at the mouth of the capillary, drops 11 which one after another falls down towards the bottom of the chamber 7.
• the drop is not influenced by any mechanical interference of any kind.
• the suction pump 31 at the outlet of the chamber 7, the drop 11 at the end of the capillary 3 is, because of the fluid trapped in the chamber 7, completely mechanically separated from the pump 31 in the case where the fluid is a compressible gas.
• This mechanical separation implies that possible pulsations and fluctuations in the flow through the pump 31, are prevented from interfering with the drop 11.
• This attenuation of pulsations is among other things necessary to allow the use of industrially suited and chemically resistant pumps, for example hose pumps.
• the signal recorded by the photo-electric cell device 5, 5' will vary periodically, see the description hereinafter.
• the signal from the photo-electric cell device 5, 5' is recorded for two or advantageously several periods, i.e.
• the photo-electric cell device 5, 5' is placed so that it can detect the drop at the end of the capillary 3, when it has a size sufficient to break the light ray of the photo-electric cell device 5, 5'.
• the light ray of the photo-electric cell device 5, 5' will thus be broken from the time when the drop reaches some size until it falls off. Thereupon, the light ray of the photo-electric cell device 5, 5' will be unbroken until the next drop reaches the size when it breaks the light ray of the photo-electric cell device 5, 5'. From this signal the surface tension can be derived using calculations performed for example by the evaluation device 12.
• the unbroken drop time to which is defined as the time that it takes from the time that a drop 11 falls off from the end of the capillary 3 until the next drop 11 breaks the light ray of the photo-electric cell device 5, 5' is calculated.
• broken drop time tb which is defined as the time period that it takes from the time when a drop 11 breaks the light ray of the photo-electric cell device 5, 5' until the time when the drops falls off from the end of the capillary 3 is calculated.
• time periods are functions of the size of the capillary, the velocity with which liquid is flowing to the capillary opening, the viscosities and densities of the liquid and the fluid and the dynamical surface tension ⁇ d yn between the liquid forming drops and the fluid in which the drops are being formed.
• the influence of the viscosity on these times can be considered to be negligible. If the density of the liquid can be considered to be constant, the influence of the density thus does not have to be considered.
• the fluid in the system is assumed to be the same one over time and also the diameter of the capillary, at the end of which the drops are formed. Remaining factors influencing these measured times are the flow velocity and the dynamical surface tension between the liquid and the fluid.
• a measured signal is obtained similar to that illustrated in Fig. 2a. From the signal the surface tension can be determined in the same way as been described above by a calibrating method using liquids having known characteristics.
• Fig. 2b a drop is schematically shown that has not yet had time to break the ray path 0 between the parts of the photo-electric cell device 5, 5'.
• Fig. 2c a drop is illustrated that precisely breaks the ray path of the photo-electric cell device 5, 5'.
• Fig. 2d a drop is illustrated when it continues to grow while the ray path of the photo-electric cell device 5, 5' is broken.
• Fig. 1 a measured signal is obtained similar to that illustrated in Fig. 2a. From the signal the surface tension can be determined in the same way as been described above by a calibrating method using liquids having known characteristics.
• Fig. 2b a drop is schematically shown that has not yet had time to break the ray path 0 between the parts of the photo-electric cell device 5, 5'.
• FIG. 3 a an example of the flow velocity as a function of time for the flow through the pump 31 is shown.
• Fig. 3b the corresponding flow velocity as a function of time for the flow through the capillary is shown, in the case where the fluid 7 is a compressible gas, which mechanically separates or isolates the pump 31 from the capillary 3 and in that way acts as a pulse attenuator between them.
• a measurement instrument or measurement installation based on the method described above with reference to Fig. 1 can for example be designed as illustrated in the block diagram of Fig. 4.
• a pump 13 pumps liquid, for which a measurement is to be made, from a monitored bath 15 containing liquid to a container or a reservoir 17 placed on a horizontal level above that of the chamber 7.
• the upper surface of the liquid always has a constant height which 25 is achieved using some kind overflow device such as a spillway.
• the spillway overflow includes a partition wall 19 in the reservoir 17 which divides it in a first chamber 21 filled with liquid up to the upper edge of the partition wall and a second chamber 23.
• the second chamber has a bottom outlet, which by a conduit 25 returns overflowed liquid to the monitored bath 15.
• the first chamber 21 has also a bottom outlet, which through a conduit 21 including a first controlled valve 30 29 that is connected therein and is the on/off type, is connected to the inlet conduit for liquid to the capillary 3.
• the second pump 31 is connected to the outlet 9 of the chamber 7. Also, an aerating connection 4 is provided that is connected to a first end of a second valve 37 of on/off type, the other end of which is open to the ambient air.
• Flow meters 32 and 33 are arranged in the supply conduit to the capillary 3 and in the outlet conduit from the closed space 7, respectively.
• valves When using the installation according to Fig. 4 for measuring surface tension the valves have the positions illustrated. The first valve 29 is then in such a position that liquid is supplied to
• the second valve 37 is set so that the chamber 7 is completely separated from the surrounding air.
• the first valve 29 is set to stop the supply of liquid to the capillary 3 and the second valve 37 is opened to connect the chamber to the surroundings.
• the pump 31 empties the chamber 7 from possible liquid remaining from an earlier measurement.
• the valves 29, 37 are set to their opposite positions and o then the instrument is ready to provide measurement data.
• the photo-electric cell device 5 was of IR- type, the model 303 from the company Omron. It was supplied with a voltage of 24 V DC and provided a signal of 0 - 19 V.
• the chamber 7 had an inner volume of 16.5 ml.
• the signal from the sensor 5 was provided to electronic circuits, compare the evaluation unit 12 in Fig.
• hose pumps 47, 49 were included, each having two channels and manufactured by the company Watson Marlow Alitea and particularly designed to fulfill the requirements of the measurement installation.
• a first side of the first hose pump 47 was used to pump liquid from the monitored bath 15 to a cross flow filter 51, from which a larger amount of 5 liquid was returned to the bath by being conducted to the second chamber 23 in the reservoir 17, whereas a smaller amount, pumped by the first side of the second pump 49, was supplied to the first chamber 21.
• the second side of the first pump was used to pump liquid from the second chamber 21 in the reservoir to the monitored bath.
• the second side or channel of the second pump 49 replaces the suction pump 31 in the embodiment of Fig. 4 and pumps liquid from the 0 closed space 7 to the second chamber in the reservoir 17.
• This channel or side of the second hose pump 49 caused a flow of approximately 1 ml liquid per minute through the chamber.

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• Optical Measuring Cells (AREA)
• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
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• Measuring Fluid Pressure (AREA)

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• 2003
  • 2003-02-07 SE SE0300329A patent/SE0300329D0/xx unknown
• 2004
  • 2004-02-09 WO PCT/SE2004/000169 patent/WO2004070360A1/en active Application Filing
  • 2004-02-09 EP EP04709393A patent/EP1595134A1/de not_active Withdrawn

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