It is known to use in-line measuring devices containing a magneto-inductive, measurement pickup for measuring the flow velocity and/or volume flow, e.g. volume flow rate, of an electrically conductive fluid flowing in a stream direction through a measuring tube of the measurement pickup. For this, the magnetically inductive sensor uses, mostly, diametrically facing, field coils of a magnetic circuit arrangement electrically connected to an exciter electronics of the in-line measuring device, to produce a magnetic field, which passes through the fluid within a given measuring volume at least sectionally perpendicularly to the direction of flow and which closes on itself essentially externally of the fluid. The measuring tube is composed, for such purpose, usually of non-ferromagnetic material, so that the magnetic field is not unfavorably affected. Due to the movement of the free charge carriers of the fluid in the magnetic field, an electrical field is produced in the measuring volume that runs perpendicularly to the magnetic field and perpendicularly to the direction of flow of the liquid, according to the magneto-hydrodynamic principle. An electrical voltage induced in the fluid is therefore measurable by means of at least two measurement electrodes spaced from each other in the direction of the electrical field, and by means of an evaluation electronics of the in-line measuring device connected to these electrodes. The induced voltage is, in turn, a measure for the volume flow rate. Serving for sensing the induced voltage can be, for instance, fluid-contacting, galvanic, or fluid-non-contacting, capacitive measurement electrodes. For conveying and coupling the magnetic field into the measurement volume, the magnetic circuit arrangement generally includes coil cores surrounded by the field coils. The coil cores are separated from each other, especially diametrically, along a periphery of the measuring tube, and are arranged with, in each case, a free end face located especially at positions where each is, in effect, a mirror image of the another. In operation, the magnetic field created by the field coils connected to the exciter-electronics is so coupled via the coil cores into the measurement tube, that it passes through the fluid flowing between the two end faces at least sectionally perpendicularly to the stream direction. Because of their high measuring accuracy, on the one hand, and the versatile applicability on the other hand, especially also in almost all usual nominal diameters, in-line measuring devices with such measurement pickups have become established over decades in almost all domains of industrial measurement technology. In-line measuring devices that measure flow velocities, and/or volume flow rates, of flowing fluids acoustically by means of ultrasonics, are often used as an alternative to such in-line measuring devices with magneto-inductive measurement pickups, at least in the case of non-conductive media.
Due to the required high mechanical stability demanded for measuring tubes used in such measurement pickups, the former—both for magneto-inductively, as well as for acoustically, measuring, measurement pickups—comprise most often an outer, especially metal, support tube of predetermined strength and diameter, coated internally with an electrically non-conductive, insulating material of predetermined thickness, the so-called liner. For example, the magneto-inductive measurement pickups described in U.S. Pat. No. 6,595,069, U.S. Pat. No. 5,664,315, U.S. Pat. No. 5,280,727, U.S. Pat. No. 4,679,442, U.S. Pat. No. 4,253,340, U.S. Pat. No. 3,213,685 or JP-Y 53-51 181 comprise, in each case, a measuring tube insertable fluid-tightly into a pipeline, and having a first, inlet end and a second, outlet end. The measuring tube, in each case, is comprised of a non-ferromagnetic support tube, as an outer casing of the measuring tube, and a tubular liner, accommodated in a lumen of the support tube and made of an insulating material, for conveying a flowing liquid isolated from the support tube
The liner, which is usually made of a thermoplastic, thermosetting or elastomeric plastic, or synthetic material, serves to chemically isolate the support tube from the fluid. In the case of magneto-inductive measurement pickups, wherein the support tube has a high electrical conductivity, for example through the use of metal support tubes, the liner serves also as electrical isolation, or insulation, between the support tube and the fluid, in order to prevent a short circuiting of the electrical field through the support tube. By a suitable design of the support tube, it is thus possible to adapt the strength of the measuring tube to the mechanical loads in particular application cases, while an adaptation of the measuring tube to the chemical and/or biological requirements of particular applications can be realized by means of the liner.
Because of its good workability, on the one hand, and its good chemical and mechanical properties, on the other hand, polyurethane in particular has become established as a material for liners of in-line measuring devices, in particular those with magneto-inductive measurement pickups. This is in addition to hard rubber or fluorine-containing plastics such as PTFE or PFA. Furthermore, liners of polyurethane have mostly good biological properties, in particular also in bacteriological regard, and are, as a result, also suitable for application in the case of aqueous fluids.
The polyurethanes used for the production of liners of the described kind are mostly elastomeric plastics, that are made on the basis of a flowable, especially liquid, multi-component system formed, directly before the processing, of reactive starting components. After the mixing, the obtained, multi-component system is applied onto the inner wall of the support tube pretreated with adhesive agent and left there to harden, or cure, to the liner within a predetermined reaction time. It is well known that polyurethanes are made by the polyaddition method from di- and poly-isocyanates and di- or poly-valent alcohols, for example butanediol. In such case, prepolymers, developed from aliphatic and/or aromatic ether- and/or ester-groups, as well as glycol- and isocyanate groups, can, for example, serve as a primary components that can react with the di- or poly-valent alcohol, supplied as a further primary component. As required, color-giving fillers, especially powdery or pasty ones, for example soot, pigments or reactive dyes can be added.
Used for the manufacture of liners of polyurethane is a so-called ribbon flow method, in which the previously prepared, flowable, multi-component system is evenly distributed on the appropriately moved, inner wall of the support tube by a corresponding pour, or spray, head of an application device, for example a low pressure or high pressure metering/mixing/pouring device. The necessary reaction time for the subsequent solidifying and hardening of the multi-component system can be set by the dosage of the starting components, and also, to a large extent, by a suitable controlling of the processing temperature. However, short reaction times of less than a minute, which are necessary for cost-effective production of the liner with a processing temperature of about room temperature, are obtained usually only through addition of a suitable catalyst, usually one containing heavy metal and/or amine, to the multi-component system. Here, in particular, tertiary amines and/or mercury are used as catalysts.
Considering that the catalyst itself remains essentially unchanged in the finished polyurethane, the latter has, as a result, inevitably also toxic, or at least physiologically not completely harmless, characteristics. Numerous investigations have also shown that especially such catalysts can, to a significant degree, be dissolved out of the liner, at least in the presence of water. As a result, the polyurethanes prepared with such catalysts used at present in in-line measuring devices are only suitable conditionally for applications with high hygienic requirements, e.g. for measurements in the field of drinking water, since high requirements for the chemical resistance of the fluid-touching components in the drinking water field and the physiological compatibility can, without more, no longer be fulfilled. Special attention in the drinking water field is placed on, among other things, meeting the maximal tolerable rate of migration (Mmax, TOC) regarding a total organic carbon content (TOC) and/or the specific migration limit values (SML) defined for toxicologically critical substances. Equally strict are the requirements regarding the effect of the liner on the external properties of drinking water, especially regarding the taste-, color-, turbidity- and/or smell-neutrality of the liner in the presence of water, as well as regarding the maximally tolerable chlorine demand rates (Mmax,Cl). Fortunately, beyond that, the possibility exists, as for example suggested in the not before-published German patent application DE 102005005195.2, to use as catalysts metal-organic compounds such as e.g. di(n-octyl)tin dilaurate. This has, among other things, the advantage that the polyurethane manufactured therewith has good physiological, organo-leptic and bacteriological characteristics and can thus also be quite suitable for applications in the drinking water field.
Because of the short reaction time set by means of the catalyst, the final blending of all the components used for the production of the multi-component system can thus however inevitably take place only immediately before the application of the multi-component system onto the support tube, for example through the use of in-line mixers. Considering, however that the catalyst, based on the entire multi-component system, generally constitutes only a very small volume, or mass, fraction of less than one percent, in the fabrication of such polyurethane liners, in an intermediate step of the manufacturing process, generally the catalyst, which, at least as regards quantity, serves as a secondary component, is mixed into the alcohol in concentration figured on the basis of the entire multi-component system, whereby a catalyst-alcohol mixture serving practically as an intermediate component of the multi-component system is formed from the two starting components alcohol and catalyst. The mixture, composed, at least, of the alcohol and the catalyst, formed in this way, is placed for subsequent use, in an appropriate storage container of the application apparatus specified above and held there in an amount, for example 20-50 liters, sufficient for the actual manufacturing of a series of liners.
In the production of the described measurement pickups in small and medium numbers, the output rate can lie for example in an order of magnitude of approximately 50-100 pieces per day, from which would result, depending on nominal width of the measuring tube, approximately 0.5-3 kg of the multi-component system per measurement pickup, a daily requirement for such catalyst-alcohol-mixtures between approximately 0.5 kg and about 2 kg. Due to the stored mixture of alcohol and catalyst, it is thus achieved that also the catalyst, especially also in the case of application of only very small amounts of the multi-component system, can be precisely measured in the final blending of all components with defendable technical complexity. Further, the multi-component system can in this way be mixed using a reduced number of storage containers compared to the total number of nominally used starting and/or intermediate components and by means of less branched, and, as a result, more simply built in-line mixers.
It has, however, been found that, in the use of such catalyst-alcohol-mixtures manufactured in advance, depending on composition, the case can arise that these can be quite highly reactive, and, as a result, chemically unstable, especially when using organo-metallic catalysts. This can especially be attributed to alcoholysis and/or solvolysis reactions occurring in such mixtures, as elaborated, for instance, in the article “Solvolytic Degeneration of Aliphatic Polyesteroligomers: Poly(Tetramethylene Adipate) Diol”, Mormann W., Wagner J., Laboratorium für Makromolekulare Chemie der Universität-GH Siegen (Laboratory for Macromolecular Chemistry, Siegen University-institute), FB (Faculty Branch) Aug. 8, 1987. Thus, for example, corresponding decay times of less than two days were experimentally determined for catalyst-alcohol-mixtures based on butanediol and di(n-octyl)tin dilaurate. Therefore, charges of such catalyst-alcohol-mixtures correspondingly mixed in advance for the production of polyurethane cannot always be kept over longer timeframes of preferably more than two weeks, technologically meaningful for the manufacturing process of liners. Inversely, the recurring daily requirement of such catalyst-alcohol-mixtures, for example in the production of the initially mentioned magneto-inductive measurement pickup, can be estimated only very inaccurately. This in particular also, because, for example, measurement pickups of the described kind are essentially manufactured “just-in-time” and thus the respective production process, especially also the manufacturing of the liner, is often done in the realm of short deadlines. Consequently, the production rate of measurement pickups per day and thus also the quantity of multicomponent system to be processed each day can vary to a significant extent, wherein the spread can, by all means, lie in the range of 100% or more. Thus, it can be a problem with the production and processing of multicomponent systems of the described kind using catalyst-alcohol-mixtures to have, on the one hand, always a charge of a sufficient quantity of the utilized catalyst-alcohol-mixture for the started production is always available, and, on the other hand, however, to consume this charge within a few days, in order, surely, to be able to avoid a decomposition while in the application apparatus and to prevent a rejected production otherwise caused thereby.
In view of the fact that the production process for such measurement pickups must be flexible to a high degree and consequently the daily requirement for the multicomponent system can be exactly assessed in advance only on a short timeframe, a disadvantage of catalyst-alcohol-mixtures of the described kind is to be seen in the fact that the available charge is measured almost inevitably either too large, wherein the surplus resulting from it is very complex and accordingly expensive to ultimately dispose of as hazardous waste, or as a result of insufficiently measured charge size an increasing need of measurement pickups in a short timeframe cannot be covered without special effort. Furthermore, a further disadvantage of such catalyst-alcohol-mixtures consists also of the fact that the application apparatus is to be cleaned unavoidably before each filling with a new charge, and thus is to be cleaned with much effort practically every day, in order to avoid an influencing of its chemical characteristics by possible residue of the expired catalyst-alcohol-mixture.