DE1926509C

DE1926509C - Device to compensate for changes in pressure when measuring low flow rates of liquid

Info

Publication number: DE1926509C
Authority: DE
Inventors: Frank Dipl.-Phys. 7500 Karlsruhe Wichmann
Original assignee: Siemens AG
Current assignee: Siemens AG
Publication date: 1972-10-26

Description

schaltung ist es vorteilhaft, eine möglichst dünnwandige Meßkapillare zu verwenden. In diesem FaI! kann die Meßkapillare jedoch zusätzlich zu einer elektrischen Widerstandsänderung, die durch eine Änderung des Durchflusses der Flüssigkeit zustande kommt, eine elektrische Widerstandsänderung erfahren, die aus einer Deformation der Meßkapillare infoige einer Druckänderung in der Flüssigkeit resultiert. Dieser Effekt macht sich besonders bei kleinen Durchflußmengen und hohem Druck als Verfälschung des Meßergebnisses bemerkbar.circuit, it is advantageous to use a measuring capillary that is as thin as possible. In this case! However, the measuring capillary can in addition to a change in electrical resistance caused by a Change in the flow of the liquid occurs, experience a change in electrical resistance, which results from a deformation of the measuring capillary infoige a pressure change in the liquid. This effect turns out to be a falsification, especially with small flow rates and high pressure of the measurement result noticeable.

Der Erfindung liegt die Aufgabe zugrunde, eine Einrichtung der eingangs erwähnten Art derart auszugestalten, daß in der Flüssigkeit auftretende Druckänderungen einen möglichst geringen Einfluß auf das Meßergebnis ausüben.The invention is based on the object of designing a device of the type mentioned at the beginning in such a way that that pressure changes occurring in the liquid have the least possible effect on the Exercise measurement result.

Diese Aufgabe wird erfindungsgemäß dadurch gelöst, daß die Vergleichskapillare mit der Meßkapillart flüssigkeitsleitend verbunden, sonst aber allseitig verschlossen ist. "° This object is achieved according to the invention in that the comparison capillary is connected to the measuring capillary in a liquid-conducting manner, but is otherwise closed on all sides. "°

Weitere Merkmale der Erfindung ergeben sich aus den Unteransprüchen.Further features of the invention emerge from the subclaims.

Die Erfindung wird durch die in der Zeichnung dargestellten Ausführungsbeispiele näher erläutert.The invention is explained in more detail by the exemplary embodiments shown in the drawing.

In den Fig. 1 bis 3 sind jeweils eine Meßkapillare R1 und eine Vergleichskapillare R 2 als Bestandteil einer Brückenschaltung BS dargestellt. Eine Änderung der Flüssigkeitsströmung in der von einer Flüssigkeit FL durchströmten und von konstantem Strom beheizten Meßkapillare R1 macht sich zunächst als Temperaturänderung ihrer Wandung bemerkbar. Die Temperaturänderung führt ihrerseits zu einer elektrischen Widerstandsänderung der Meßkapillarenwandung, da der elektrische Widerstand ER 1 der Meßkapillare R1 einen möglichst hohen Temperaturkoeffizienten besitzt. Der Meßkapillarenwandung wird je.nach Strömungsgeschwindigkeit der Flüssigkeit FL mehr oder weniger Wärme entzogen. Ist die Differenz zwischen der Temperatur der einfließenden Flüssigkeit FL und der Temperatur der ♦<> Meßkapillare Λ 1 in undurchströmtem Zustand konstant, so kann in der Brückenschaltung BS, in die als elektrischer Kompensationswiderstand ER 2 die Wandung der Vergleichskapiilare R 2 eingeschaltet ist, sowohl der elektrische Widerstand ER 1 der Meßkapillarenwandung als auch die Widerstandsänderung erfaßt werden, welche bei einer Strömungsschwankung der Flüssigkeit FL hervorgerufen wird. In die beiden übrigen Zweige der Brückenschaltung BS sind ohmsche Widerstände R 3 und K 4 geschaltet. Über die eine Diagonale a, b wird der Brückenschaltung BS von einer Konstantstromquelle Q über einen regelbaren Vorwiderstand KV ein Brückenspeisestrom zugeführt, während in der zweiten Diagonale c\ d ein Ausschlaginstrument A liegt. Dieses Ausschlaginstrumcnt A zeigt bei Brückenverstimmung, d. h. bei einer Änderung des Widerstands ER 1, eine dem Durchfluß der Flüssigkeit FL durch die Mcßkapillare R 1 proportionale Meßgröße an.1 to 3 each show a measuring capillary R 1 and a comparison capillary R 2 as part of a bridge circuit BS . A change in the flow of liquid in the measuring capillary R 1 through which a liquid FL flows and is heated by a constant current is initially noticeable as a change in the temperature of its wall. The temperature change in turn leads to a change in the electrical resistance of the measuring capillary wall, since the electrical resistance ER 1 of the measuring capillary R 1 has as high a temperature coefficient as possible. Depending on the flow rate of the liquid FL, more or less heat is withdrawn from the measuring capillary wall. If the difference between the temperature of the inflowing liquid FL and the temperature of ♦ <> measuring capillary Λ 1 in undurchströmtem state constant, so can be used in the bridge circuit BS is switched in the as an electrical compensation resistor ER 2 the wall of the Vergleichskapiilare R 2, both of the electrical resistance ER 1 of the measuring capillary wall as well as the change in resistance can be detected, which is caused by a flow fluctuation of the liquid FL. Ohmic resistors R 3 and K 4 are connected in the two remaining branches of the bridge circuit BS. A bridge feed current is fed to the bridge circuit BS from a constant current source Q via a controllable series resistor KV via one diagonal a, b , while a removal instrument A is located in the second diagonal c \ d. In the event of a bridge misalignment, that is to say if the resistance ER 1 changes, this deflection instrument A displays a measured variable proportional to the flow of the liquid FL through the measuring capillary R 1.

Nach Fig. 1 sind die Meß- und Verglcichskapillaren R 1 und Λ 2 in der Brückenschaltung BS aus einem einzigen wärme- und elektrischen Strom leitenden Rohr R hergestellt. In der geometrischen Mitte des Rohres R befinde.t sich ein Abflußrohrs. Die Flüssigkeit FL strömt in Pfeilrichtung in den Eingang der Meßkapillare R1 ein und durch das Abflußrohr S wieder heraus. Innerhalb der Vergleichskapillare R 2 befindet sich dieselbe Flüssigkeit FL wie in der Meßkapillare R 1. Druckänderungen, die in der Flüssigkeit FL innerhalb der Meßkapillare R1 auftreten, werden auch auf die Flüssigkeit FL innerhalb -der Vergleichskapillare R 2 übertragen. (Das gleiche gilt für Meß- und Vergleichskapillare R1 bzw. R 2 gemäß Fig.2 und 3.) Das blinde Ende der Vergleichskapillare RI ist mit einem Hahn Wl abgeschlossen. Dieser Hahn H\ wird bei der Reinigung des Rohres R geölT-net. (Das gleiche gilt für den Hahn der Vergleichskapillare R 2 gemäß F i g. 2 und für die Hähne H 3 und HA der VergleichskapilJare R2 gemäß Fig. 3.)According to FIG. 1, the measuring and comparison capillaries R 1 and Λ 2 in the bridge circuit BS are made from a single tube R which conducts heat and electrical current. In the geometric center of the pipe R there is a drain pipe. The liquid FL flows in the direction of the arrow into the inlet of the measuring capillary R 1 and out again through the drain pipe S. Inside the comparison capillary R 2 is the same liquid FL as in the measuring capillary R 1. Changes in pressure that occur in the liquid FL within the measuring capillary R 1 are also transferred to the liquid FL within the comparison capillary R 2 . (The same applies to measuring and comparison capillaries R 1 and R 2 according to FIGS. 2 and 3.) The blind end of the comparison capillary RI is closed with a stopcock Wl. This tap H \ is oiled when the pipe R is cleaned. (The same applies to the cock of the comparison capillary R 2 according to FIG. 2 and for the cocks H 3 and HA of the comparison capillary R2 according to FIG. 3.)

Der Eingang der Meßkapillare Ri in Fig. 1 bildet den Diagonalpunkt α, das blinde Ende der Vergleichskapiilare/? 2 beim Hahn Hl bildet den Di₁,-gonalpunkt b und die Anschlußstelle des Abflußrohres 5 bildet den Diagonaipunkt c der Brückenschaltung BS. The entrance of the measuring capillary Ri in Fig. 1 forms the diagonal point α, the blind end of the comparison capillary /? 2 at the cock Hl forms the Di ₁ , -gonal point b and the connection point of the drain pipe 5 forms the diagonal point c of the bridge circuit BS.

In F i g. 2 ist die Meßkapillare R 1 bifilar ausgeführt. Damit entfallt ein Abflußrohr5 nach Fig. 1. Durch die bifilare Ausführung der Meßkapillare R 1 können der Eingang und der Ausgang der Meßkapillare R1 auf gleiches Potential gelegt werden (Brükkendiagonalpunkt a), und der elektrische Widerstand ER 1 der Meßkapillare R 1 ergibt sich aus der Parallelschaltung der beiden Teilrohre der Meßkapillare R 1. Am Umkehrpunkt der beiden bifilaren Teilstücke der Meßkapillare R 1 ist die Vergleichskapillare R 2 elektrisch- und flüssigkeitsleitend angeschlossen. Sie ist an ihrem Ende mit dem Hahn H 2 abgeschlossen. Der Diagonalpunkt c der Brückenschaltung BS ist an der mechanischen Verknüpfungsstelle der Vergleichskapillare R 2 mit der Meßkapillare R 1 angeschlossen.In Fig. 2, the measuring capillary R 1 is designed bifilar. This eliminates a drain pipe 5 according to FIG. 1. Due to the bifilar design of the measuring capillary R 1, the input and the output of the measuring capillary R 1 can be placed at the same potential (bridge diagonal point a), and the electrical resistance ER 1 of the measuring capillary R 1 results from the parallel connection of the two tubes of the measuring tube 1. On reversal point R of the two bifilar portions of the measuring tube 1, the R Vergleichskapillare R 2 is connected electrically and fluidly. It is closed at its end with the valve H 2 . The diagonal point c of the bridge circuit BS is connected to the mechanical junction of the Vergleichskapillare R 2 with the measuring tube 1 R connected.

F i g. 3 zeigt eine weitere Ausführung, bei der die Meßkapillare R1 und die Vergleichskapillare R 2 bifilar ausgebildet sind. Die beiden Kapillaren R1 und R2 sind miteinander über eine Verbindungsleitung V an ihren Umkehrpunkten elektrisch- und flüssigkeitsleitend verbunden. An dieser Verknüpfungsstelle ist der Diagonalpunkt c der Brückenschaltung BS angeschlossen. Der Ausgang und der Eingang der Meßkapillare R 1 sowie der Ausgang und der Eingang der Vergleichskapillare R2 sind jeweils miteinander elektrisch verbunden. Die beiden elektrischen Verknüpfungspunkte bilden die Diagonalpunkte α und b der Brückenschaltung BS. Die Vergleichskapillare R 2 ist dabei mit den Hähnen//3 und H4 abgeschlossen.F i g. 3 shows a further embodiment in which the measuring capillary R 1 and the comparison capillary R 2 are bifilar. The two capillaries R 1 and R2 are connected to one another in an electrically and fluid-conductive manner at their reversal points via a connecting line V. The diagonal point c of the bridge circuit BS is connected to this junction point. The output and the input of the measuring capillary R 1 and the output and the input of the comparison capillary R2 are each electrically connected to one another. The two electrical connection points form the diagonal points α and b of the bridge circuit BS. The comparison capillary R 2 is closed with the taps // 3 and H 4.

Mit den in den F i g. 1 bis 3 dargestellten Einrichtungen zur Durchflußmessung läßt sich also eine Kompensation des Oruckeinfhisscs erreichen. Sämtliche mit der Flüssigkeit FL in Berührung stehende Teile sind aus demselben Material herstellbar. Hin besonderer Vorteil der Einrichtung liegt in der Vermeidung von Korrosionen innerhalb der Kapillaren «1 und R 2. With the in the F i g. 1 to 3 shown devices for flow measurement, a compensation of the Oruckeinfhisscs can be achieved. All parts in contact with the liquid FL can be produced from the same material. A particular advantage of the device lies in the avoidance of corrosion within the capillaries «1 and R 2.

Hierzu 1 Blatt Zeichnungen1 sheet of drawings

Claims

Patent claims:

1. Device for measuring a low throughput of a liquid under high pressure flowing through a measuring capillary, preferably for liquid chromatography, using a bridge circuit fed by a current source with two ohmic resistors and with two temperature-sensitive resistance elements, one of which with the flowing liquid and the other is in thermal contact with a stationary liquid, with a deflection instrument lying in the diagonal of the bridge circuit, the deflection of which is a measured variable derived from the throughput of the liquid, and the two temperature-sensitive resistance elements as liquid-filled measuring and comparison capillaries which consist of a material with a high temperature coefficient with respect to the electrical resistance, according to patent 1803 661, characterized in that the comparison capillary (R 2) with the measuring capillary (R 1) is liquid gkcitsleitcnd connected, but otherwise closed on all sides.

2. Device according to claim 1, characterized in that the removal instrument (A) between the mechanical junction (c) of the measuring and comparison capillary (R 1 and Rl) and the electrical junction (d) of the two ohmic resistors (R 3 and R 4) is switched.

3. Device according to claim 1 or 2, characterized in that the measuring capillary (R 1) and the comparison capillary (R 2) are the two halves of a tube (R) which is closed at the end and halfway with a drain pipe (.V) is given away.

4. Device according to claim 1 or 2, characterized in that the Meßkapillarc (R I) formed bifilar and at the reversal point (<) with the comparison capillary (R 2) is connected.

5. Device according to claim 2 and 4, characterized in that the input of the Meßkapillarc (/? 1) is electrically connected to its output, so that the electrical resistance (ERl) of the measuring capillary (RI) from the parallel connection of the resistances of the two measuring capillary halves is formed.

6. Device according to claim 4 or 5, characterized in that the comparison capillary (R2) is bifilar and is connected to the measuring capillary (R I) at the reversal point (c).

7. Device according to claim 6, characterized in that the input of the comparison capillary (R 2) is electrically connected to its output, so that the electrical resistance {ER2) of the comparison capillary (R2) is formed from the parallel connection of the resistances of the two comparison capillary halves.

H. Device according to one of the preceding claims, characterized in that the comparison capillary (R 2) is closed with a measure (Z /! Iv.w. 112) or with several taps (// 3 and 114) .

The subject of the main patent 1803 661 is a Device for the measurement of a low salinity of a liquid under high pressure, which flows through a measuring capillary, preferably for liquid chromatography, using a bridge circuit fed by a current source with two ohmic resistors and with two temperature-sensitive resistance elements, one of which with the flowing liquid and

ίο the other with a dormant liquid in thermal Contact is made, with a removal instrument in the diagonal of the bridge circuit, the magnitude of which is a measured variable derived from the throughput of the liquid, and where the two temperature-sensitive resistance elements as measuring and comparison capillaries filled with liquid are formed, which are made of a material with a high temperature coefficient with respect to the electrical Resistance.

Such a device is used to measure the passage of a liquid under a very high pressure (100 to 300 atmospheres) and or which flows at a very low flow rate (about 1 ml / min). The measuring device lit-gt z. B. on Separating column inlet of a liquid chromatographic analysis device. The liquid to be measured is guided in capillary-like lines that can withstand high internal pressure.

The way the device works is as follows:

Becomes a capillary, which contains a standing liquid, between its two ends of a Electric current flows through the wall of the capillary and the liquid contained therein heated because the capillary wall represents an electrical resistance. On the other hand, flows the If there is liquid in the capillary, the capillary wall changes depending on the specific heat and the temperature Throughput, heat is continuously withdrawn from the liquid. The cooling of the wall leads to a change their electrical resistance. With appropriate dimensioning (inner diameter and Wall thickness) of the capillary there is a wide range in which there is proportionality between the Amount of heat that is continuously given off to the liquid and the temperature difference that the Having capillary wall when moving compared to when the liquid is stationary. As in most cases the change in electrical resistance of the wall material caused by heat dissipation when used Capillary is also proportional to this temperature difference, the throughput is that of the The liquid flowing through the capillary is directly proportional to the change in resistance of the capillary wall material. This change in resistance is measured in a bridge circuit.

As further stated in the main patent, the device can be designed such that before Meßkapillarc a low resistance to their resistance Coiled pipe is connected, the so around a measuring and comparison capillary containing Piece of material is wound so that the liquid flowing into the measuring capillary is always the same Temperature like the piece of material and its surroundings. The measuring and the comparison capillary are arranged separately from one another in recesses of the piece of material.

In the interests of high sensitivity and a small setting time distance of the bridge