Patent Description:
Thermometers with resistance elements often include so-called thin-film sensor elements, Resistance Temperature Detectors (RTD). Typically, such a sensor element has a carrier substrate that is provided with lead wires and is metallically coated on a front surface. A metallic coating may also be available on the rear surface. Platinum elements are often used as sensor elements, which are also commercially available under the designations PT10, PT100, and PT1000, among others. In many cases, the sensor elements are encapsulated or embedded, preferably in pure ceramic powders, and the connecting leads are guided in a guide tube to electronics, for example a temperature transmitter. Details of industrial platinum resistance thermometers can be found, for example, in the European standard EN60751.

Various measuring methods for determining the temperature by means of a resistance element have become known from the prior art. A transmit signal, usually in the form of an electric current, is impressed into the sensor element and a receive signal, usually in the form of a voltage dropped across the sensor element, is detected, and evaluated with respect to temperature.

In the simplest case, the temperature is determined by means of a temperature-dependent resistance, in which the sensor element is simply contacted via two connecting wires. The disadvantage of this solution is that the resistance of the connecting wires is included in the temperature determination as an error. According to another method, the sensor element is contacted via three connecting wires. By tapping the falling voltage in pairs between two of the three connecting wires in each case, a resistance of the connecting wires or the connecting lines can be largely compensated, if it can be assumed that the three connecting wires have the same resistance. It is also known to measure the temperature with four connecting wires.

RTD Pt100 temperature sensors are widely used as sensor elements in process monitoring. They are the most common used standard temperature sensors in the market. In certain application of process automation very long sensors are required to reach the zone where the temperature should be detected and/or monitored. Such temperature sensors can have a total length up to a hundred of meters.

In such an application, the sensor cable must ensure the necessary robustness. Usually, it is made by an MgO cable with an external metal protection sheath. The protection sheath is usually made of stainless steel or nickel alloy. A certain number of conductive wires, in most applications copper wires, form the connection wires for connecting the temperature sensor to electronics.

The best-known solution of measuring the temperature in the case of very long temperature probes consists in measuring the electric resistance of the Pt100 probe by a <NUM>-wire terminal sensing. This so-called <NUM>-point probe method consists in injecting a current by using two wires and measuring the voltage by using the remaining two wires as it is schematically described in <FIG>. The advantage of this measuring method is that the measurement is not affected by the resistance of the connection wires between the probe and electronics of the temperature probe and therefore the measurement is independent of the length of the connection cable.

Another very common measuring method requested by the market is the <NUM>-wire terminal sensing, <NUM>-wire connection, or <NUM>-point probe method.

Generally, this method is requested to reduce the costs of the temperature measuring device, or for design reasons: indeed, in probes with multiple sensing elements (two or more), the reduction of the number of necessary wires in the cable has the benefit to make the cable more compact (smaller diameter and so less invasive) or allows to increase the number of possible measuring points in the same cable. As a matter of example, <CIT> discloses a sheathed RTD, wherein temperature measurement is performed using the above-mentioned <NUM>-point probe method.

Compared to the <NUM> wires, this method has an essential limit: the measurement can compensate the resistance of the connection wires without any additional error only if the resistance of the cable is the same in all three cables used in the measuring device.

This limit can easily be demonstrated by analyzing how the measurement is performed - see <FIG>:
In this case the resistance of the Pt100 probe is the result of the resistances of two loops:
In one circuit the resistance Rc1 is measured between the common connection point C and the point <NUM>:<MAT>.

Then the measurement is made between points <NUM> and <NUM>:<MAT>.

Under the assumption that the three connection resistances are the same the result is calculated as:<MAT>.

Other calculations methods are possible and applied by different measuring devices, but the result is always the same: the measuring device can measure the Pt100 without any error only if the three cables have the same resistance. This becomes clear when considering the circuit shown in <FIG>. <MAT> (in R2 the current is zero)<MAT><MAT>.

In general, this method is a good compromise, but it assumes that the resistances of the connection cables are the same. This is usually not true. Unfortunately, the wires of an MgO cable do not have the same resistance. This is caused by the manufacturing drawing process of the wires. In these components it is common to have a resistance difference of about <NUM>-<NUM> % of the total value that is depending on the length of the cable. For very long sensors, the total resistance could reach <NUM> Ohms or more depending also on the diameter of the cable, i.e., of the diameter of the internal wires.

The difference of resistance among the wires of the cable can negatively affect the measurement accuracy that can be over the requested limits.

The measurement error of a Pt100 probe can be as follows if a calibration check in ice + water reference bath at <NUM> is made:<MAT>.

These measuring errors of the different classes are generally not achievable with very long MgO cable constructions if using the known <NUM>-wire methods.

It is an object of the invention to provide a temperature probe operating according to a three-wire method, which enables highly accurate temperature measurement. Additionally, it is an object of the invention to provide a method of manufacturing such a temperature probe.

To achieve this object, the invention comprises a temperature probe for determining the temperature according to the three-point probe method with a sensor element providing temperature values, wherein a three-wire line several meters long, consisting of a first connecting line, a second connecting line and a third connecting line, is connected to the sensor element, wherein the connecting lines are made of a first material and serve to transmit energy and the measured temperature values, wherein a conductive element having a certain length and diameter and made of a second material is inserted in each of the second connecting line and the third connecting line, the resistivity of said second material is at least five times higher than the resistivity of the first material, and wherein the length and/or diameter of the two inserted conductive elements is/are dimensioned in such a way that the second connecting line and the third connecting line have substantially the same resistance as the first connecting line.

The solution according to the invention is particularly applicable in connection with <NUM>-wire cables of quite long temperature probes to ensure a preferably high accuracy class: The resistance compensation of the connecting lines is preferably reached by inserting quite short pieces of a conductive material having a higher resistivity than the material of the connecting lines in usually two of the three wires of a <NUM>-wire cable. By inserting conductive elements of a certain length and/or diameter into two of the three wires, it is achieved that the resistance of each of the three connecting wires is equal.

According to the claimed invention it is proposed that the resistivity of the second material is at least five times higher than the resistivity of the first material. Preferably, the connecting lines are made of copper, and the inserted conductive elements are made of constantan. The resistance compensation is achieved by selecting the right material for the conductive element. To achieve an effective construction, a good compensation and a short length, the material must have an electrical resistivity much higher than the original wires. Most of the wires of the MgO cables are made of copper. An analysis done by comparing different materials and the resistance values that must be compensated, leads to consider Constantan as the preferred material of the conductive elements. Constantan has a high resistivity compared to e.g. copper and good and robust mechanical properties.

According to an embodiment of the temperature probe the inserted conductive elements made of the at least one second material are arranged within a transition bushing of the probe. In the transition bushing, two sections of the three-wire cable are connected together. To provide extended temperature probes, a transition bushing is generally used to connect the MgO cable to a flexible extension cable. The resistance compensating conductive elements are inserted between the end sections of the corresponding wires of the MgO cable and the flexibele extension cable. They can be connected by any of the known methods, for example: welding, brazing, soldering, or crimping. Every connection can be protected by an additional Kapton or thermo-shrinking insulation cable to isolate it from the other connections. Finally, the complete bushing may be sealed by a resin potting.

According to an alternative design of the temperature sensor, the conductive elements made of at least a second material are inserted in a connection area through which the three-wire line can be connected to external electronics: the conductive elements that compensate for the differences in resistance of the connection lines are attached to the terminals to which the connection lines of the main cable are connected. Depending on the length of the main cable, this can be the MgO cable or the flexible extension cable. The two wires into which the conductive elements are inserted may be stripped and interrupted. The conductive elements are inserted between the connecting wires and the terminals. Again, the connections can be welded, brazed, soldered, or crimped. The wires are insulated from each other, e.g., with heat shrink tubing. Additional shrink tubing insulation can be applied to protect the connection.

According to another alternative design of the temperature sensor, the conductive elements made of at least a second material are inserted in a connection area. Preferably this connection area is arranged within the flexible extension cable. The two wires into which the conductive elements are inserted may be stripped and interrupted. The conductive elements are inserted between the connecting wires and the terminals. Again, the connections can be welded, brazed, soldered, or crimped. The conductive elements and the wire connections are attached to the terminals directly like in the previous embodiment or by using a rigid support as a reinforcement.

According to an embodiment of the temperature probe, the resistance of each of the two conductive elements inserted in the second connecting line and in the third connecting line is designed in such a way that the temperature probe provides measured values with a predetermined measurement accuracy. For example, the accuracy class may be A or B. It is further provided that the sensing element is a Resistance Temperature Detector-RTD - element, preferably a platinum measuring resistor PT100. Any other appropriate sensor element may be used in connection with the inventive solution.

With regard to the method of manufacturing a temperature probe for determining the temperature according to the three-point probe method with a sensor element, preferably designed as a platinum measuring resistor, which provides temperature measured values, wherein a three-wire line several meters long, consisting of a first connecting line, a second connecting line and a third connecting line, is associated with the sensor element, wherein the connecting lines are made of a first material with a predetermined specific resistance and serve for transmitting energy and for transmitting the measured temperature values, the method comprises:.

In a development of the method the conductive elements are welded, brazed, soldered, or crimped for insertion into the corresponding connecting lines.

The invention is explained in more detail with reference to the following figures.

The different prior art solutions of temperature probes <NUM> and the corresponding methods for measuring the temperature are already described in <FIG>.

For temperature sensors <NUM> with resistance thermometer elements <NUM>, for example a Pt100, MgO cables <NUM> are usually used. A cable length of more than <NUM> is often required to measure the temperature in a remote location. Further requirements are a predetermined high measuring accuracy (e.g., class A) and the use of a <NUM>-wire line. Due to the technical properties of the MgO cable, it is difficult, or in some cases impossible, to reach the requested accuracy class. The problem is that the inner connecting wires <NUM>, <NUM>, <NUM> of an MgO cable <NUM> usually do not have the same resistance. The manufacturers generally declare an accuracy among the wires <NUM>, <NUM>, <NUM> of a three-wire cable <NUM> of about <NUM>,<NUM> Ohm/m on a typical <NUM> MgO cable <NUM> with a wire resistance of about <NUM>,<NUM> - <NUM>,<NUM> Ohm/m.

Corresponding experimental investigations have confirmed that statistically a difference in resistance of wires <NUM>, <NUM>, <NUM> with a standard deviation of about <NUM>% of the total measured value can be expected.

<FIG> shows, as an example, a table visualizing the measurement error as a function of the length of a three-wire cable <NUM> of a temperature probe <NUM> with three connecting wires <NUM>, <NUM>, <NUM>. In particular, the table shows the maximum cable length above which a required accuracy class A or B for the temperature measurement can no longer be maintained. So, by considering very long temperature probes <NUM> we have a situation like it is shown in <FIG>: The measurement values leave a given accuracy class as soon as the connecting lines <NUM>, <NUM>, <NUM> exceed a certain length.

According to the inventive temperature probe <NUM> the differences of the resistances of the three connecting lines <NUM>, <NUM>, <NUM> is compensated by adding an additional resistance. Preferably, the resistances of two of the three wires <NUM>, <NUM>, <NUM> are equalized to the resistance of the connecting line (for example <NUM>) with the highest resistance. The inventive temperature probe <NUM> is simple and inexpensive to manufacture, as the compensation method is less invasive, but provides a high accuracy of the temperature measurement. A piece of a conductive element <NUM>, <NUM> with a higher resistivity and the determined dimensions is needed to modify the resistance of the remaining two connecting lines <NUM>, <NUM> in such a way that each of the connecting lines <NUM>, <NUM>, <NUM> has the same resistance.

<FIG> shows a schematic view of the inventive temperature probe <NUM> determining the temperature according to the three-point probe method. <FIG> shows a table of the resistivity of different conductive materials.

The calculation of the linear resistance of a connecting line <NUM>, <NUM>, <NUM> is quite simple: Linear wire resistance = material resistivity / wire section.

By doing the calculation using a standard Constantan wire with a diameter between <NUM>,<NUM> and <NUM>,<NUM> we can compensate the resistance differences between the three connecting lines <NUM>, <NUM>, <NUM> of a MgO cable <NUM> by adding a conductive element <NUM>, <NUM> of <NUM> to <NUM> of a Constantan wire.

Whereby the length of the conductive element <NUM>, <NUM> is calculated by:<MAT>.

In the following the steps for compensating resistance differences on the three wires is described:.

<FIG> shows a first embodiment of the temperature probe <NUM> according to the invention. The focus is on the attachment of the conductive elements <NUM>, <NUM> to two or at least one of the connecting wires <NUM>, <NUM>, <NUM>. The conductive elements <NUM>, <NUM> made of the at least one second material, for example constantan, are arranged in a transition bushing <NUM> of the temperature probe <NUM>. Such a transition bushing <NUM> serves to connect two different sections <NUM>, <NUM> of the three-wire cable <NUM>.

For extended temperature probes <NUM> such a transition bushing <NUM> is generally used to connect the MgO <NUM> cable to a flexible extension cable <NUM>. The compensating conductive elements <NUM>, <NUM> are inserted between the end sections of the corresponding wires of the MgO cable <NUM> and the flexible extension cable <NUM>. They can be connected by any of the known methods, for example: welding, brazing, soldering, or crimping. For electrical insulation, each joint may be protected by an additional Kapton or heat shrink insulating sleeve or cover <NUM>. Finally, the complete bushing <NUM> may be sealed by a resin potting <NUM>.

<FIG> shows a view on a second embodiment of the inventive temperature probe. According to this alternative design of the temperature probe, the conductive elements <NUM>, <NUM> made of at least a second material are inserted in a connection area <NUM> through which the three-wire cable <NUM> can be connected to external electronics <NUM>: the conductive elements <NUM>, <NUM> that compensate for the differences in resistance of the connection lines <NUM>, <NUM>, <NUM> are attached to the terminals <NUM> to which the connection lines <NUM>, <NUM>, <NUM> of the main cable <NUM> are connected. Depending on the length of the main three-wire cable <NUM>, this may be the MgO cable <NUM> or the flexible extension cable <NUM>. The two wires <NUM>, <NUM>, <NUM> into which the conductive elements <NUM>, <NUM> of a determined design are inserted may be stripped and interrupted. The conductive elements <NUM>, <NUM> are inserted between the connecting wires <NUM>, <NUM>, <NUM> and the terminals <NUM>. Again, the connections can be welded, brazed, soldered, or crimped. For electrical insulation, each joint may be protected by an additional Kapton or heat shrink insulating sleeve or cover <NUM>. Additionally, heat shrink tubing insulation <NUM> can be applied to protect the connections.

Claim 1:
Temperature probe (<NUM>) for determining the temperature according to the three-point probe method with a sensor element (<NUM>) providing temperature values, wherein a three-wire line (<NUM>) several meters long, consisting of a first connecting line (<NUM>), a second connecting line (<NUM>) and a third connecting line (<NUM>), is connected to the sensor element (<NUM>), wherein the connecting lines (<NUM>, <NUM>, <NUM>) are made of a first material and serve to transmit energy and the measured temperature values, wherein a conductive element (<NUM>, <NUM>) having a certain length and diameter and made of a second material is inserted in each of the second connecting line (<NUM>) and the third connecting line (<NUM>), the resistivity of said second material is at least five times higher than the resistivity of the first material, and wherein the length and/or diameter of the two inserted conductive (<NUM>, <NUM>) elements is/are dimensioned in such a way that the second connecting line (<NUM>) and the third connecting line (<NUM>) have substantially the same resistance as the first connecting line (<NUM>).