Patent Application
20230266219
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This invention relates generally to the field of capillary viscometers, and in particular to a capillary viscometer capable of direct, in-situ flow measurement by thermal flow sensors.
The present invention relates to the field of viscometers, and more specifically to a capillary viscometer. A capillary viscometer is a measuring device for recording the viscosity of a diluted sample. In its simplest form, it consists of a glass capillary with two markings. The sample to be examined is made to flow through the capillary by means of gravity. The viscosity is determined by measuring the transit time of the meniscus from one mark to the next. The measurement is carried out once with pure solvent, then with the sample. The ratio of the transit times then corresponds to the relative viscosity. The time measurement is carried out manually with the aid of a stopwatch or electronically with suitable oil-sensitive sensors. Optical measurement of the flow velocity is difficult, if not impossible, with media that are not translucent. The accuracy of the measurement also depends on surface effects on the capillary wall. The measurement method also requires the use of glass capillaries, which are subject to little mechanical stress.
A differential capillary viscometer is a special form of capillary viscometer and is suitable for continuous measurement of viscosity. As a first example, U.S. Pat. No. 4,463,598 issued to Haney in 1984 entitled “Capillary Bridge Viscometer” discloses a viscometer that includes a bridge with two capillaries in series to measure the relative viscosity of a solute in a solvent. The Haney device operates on the principle of creating a differential pressure across the capillary bridge that is a function of the viscosity of the second liquid relative to a known first liquid viscosity reference. The Haney device has been sold extensively thru a product line known as Viscotek and is now part of Malvern Panalytical. However, the product line has not become a market leader due to other products having similar accuracy but at a lower cost.
The use of differential pressure measurement, however, has significant drawbacks. First, the pressure transducers used must be extremely sensitive, thus resulting in some fragility. Second, the system must always be completely free of air bubbles to provide reliable results. Also, the overall performance of such a device is highly dependent on the performance of the attached pump that delivers the solvent and sample segment through the system. Yet another disadvantage of prior art based on a Wheatstone bridge is that the bridge itself, when flowed through by pure solvent alone, must be in as perfect equilibrium as possible. This requirement raises the cost to manufacture and has been shown to limit the success of viscometers sold using a Wheatstone bridge and pressure transducers. Due to the high sensitivity of the pressure transducers used and the associated extremely limited dynamic range, the range of application of such a device is relatively limited in terms of the possible flow rates.
Another prior art example is U.S. Pat. No. issued to Huebner et al in 1987 entitled “Capillary Viscometer” and discloses a viscometer that measures viscosity by sensing electrical resistance changes in hermetically sealed electrical resistors spaced apart at two separate levels in the tube in which the liquid meniscus is to be detected. Huebner's device has been sold worldwide by Schott Instruments GmbH. However, such prior art devices are not considered ideal since the sample itself is disturbed because of electrical current passing through during measurement of resistance.
Yet another prior art example is U.S. Pat. No. 7,213,439 issued to Trainoff in 2007 entitled “Automatic Bridge Balancing Means and Method for a Capillary Bridge Viscometer”. Trainoff s device used a thermally controlled stage connected within one arm of a bridge of a capillary bridge viscometer so that the bridge can be balanced in situ to provide highly accurate measurement signals. This device included a thermal control technique in order to provide more accurate viscosity measurements. Trainoff s device was assigned to Wyatt Technology Corporation and became one of Wyatt's main instrument product lines. However, these Wyatt devices are typically expensive compared to other viscometers.
As a final prior art example, U.S. Pat. No. 10,551,291 issued to Murphy et al. and assigned to Malvern Panalytical entitled, “Balanced Capillary Bridge Viscometry” discloses a viscometer that uses four hydraulic paths with adjustable flow restrictors and a proprietary balancing device to measure viscosity. This prior art example also has high-cost drawbacks as well due to the complexity of the design and components used.
Clearly, there is a need to provide a viscometer that employs more advanced technology to maintain high accuracy yet be designed such that they are affordable to the entire scientific community and also are designed for ease of maintenance such that most repairs can be done onsite without sending back to the factory. These desired market specifications shall be met by the present invention.
It is a primary object of the present invention to provide a capillary viscometer that replaces volume or pressure measurement by a direct, in-situ flow measurement by thermal flow sensors.
It is yet another object of the present invention to provide a differential capillary viscometer that uses a direct, in-situ flow measurement by thermal flow sensors using two separate capillaries connected to a component providing flow separation to the two capillaries.
It is a final object of the present invention to provide a differential capillary viscometer that uses a direct, in-situ flow measurement by thermal flow sensors using four separate capillaries connected in a Wheatstone bridge configuration.
Referring now to the drawings and in particular
Referring next to
The present invention aims to replace the volume or pressure measurement of prior art viscometers by a direct, in-situ flow measurement using a thermal flow sensor. This solution offers a significant improvement by allowing the use of capillaries of any type and material and provides significant accuracy improvements through a direct flow velocity measurement technique.
The second embodiment of a differential viscometer is shown in
an inlet capillary (50);
a flow splitter (commonly referred to as a T fitting) (70);
a pressure stable vessel (90);
a first measuring capillary (10a);
a first thermal flow sensor (20a);
a first outlet capillary (52a);
a second measuring capillary (10b);
a second thermal flow sensor (20b); and
a second outlet capillary (52b)
The operation of the differential viscometer according to
In-situ measurement using thermal flow sensors also offers significant advantages over previous prior art solutions, The change in flow within the system is the primary information that results when a sample segment is introduced into the system. Thus, the immediate measurement of the change in flow is directly related to the change in viscosity in the system and is therefore an unbiased method. Furthermore, the measurement of the flow is much simpler than the indirect determination by the system pressure and does not require a balanced system, nor is it sensitive to disturbances such as air bubbles.
One skilled in the art of making viscometers could create other embodiments of the present invention. Some additional examples are described here without the use of additional figure drawings. In yet another embodiment of a differential viscometer, the two separate outlet capillaries 52a and 52b can be brought together by means of another flow splitter 70 to create a common outlet capillary 52c. Also, the position of the thermal flow sensors 20a and 20b can be modified, taking into account that thermal flow sensor 20a is always positioned downstream of the vessel 90. Also possible is to measure the total flow F at the inlet or at the common outlet and to realize only one of the two partial flows F1 and F2. All these variants lead to the same result.
Yet another significant advantage is that a differential viscometer of this design can easily be combined with other measuring devices such as devices for determining the sample concentration or the light scattering of the sample. The aim can be the determination of the intrinsic viscosity or the absolute molar mass. The combination can be in the form of a separate module or as an integral part of the viscometer. Indeed, the ability to integrate the present invention into other measuring instruments gives the present invention a significant advantage over prior art stand-alone viscometers.
A final embodiment of a viscometer is shown in
The function of the viscometer 300 according to
In the other branch (flow F2), however, the sample segment flows undisturbed through the measuring capillary 10b. Thus, the apparent resistance of the two branches (flows F1 and F2) is changed, because in the first branch (flow F1), the measuring capillaries 10a and 10c still have pure solvent flowing through them, while the measuring capillary 10b has sample flowing through it. According to Hagen-Poiseuille's law, the partial flows F1 and F2 will therefore behave inversely to the viscosity change in measuring capillary 10b and change accordingly. As a result, the pressure at the flow splitters 70b and 70c will no longer be identical and thus a transverse flow F3 will occur, which will be detected by the thermal flow sensor 20b. When the sample then reaches the vessel 90b, it will also be strongly diluted here, the measuring capillary 10d will then flow through as almost pure solvent and thus equilibrium will be restored in the system and the flow F3 will become zero again. Thus, the specific viscosity of the sample is determined from the two flow signals of thermal flow sensor 20a and thermal flow sensor 20b.
One skilled in the art of making viscometers could create yet other embodiments of the present invention shown in
