|Publication number||US7994416 B2|
|Application number||US 11/037,719|
|Publication date||Aug 9, 2011|
|Priority date||Jun 17, 2003|
|Also published as||US7084342, US20040255998, US20050155640, WO2005001402A2, WO2005001402A3|
|Publication number||037719, 11037719, US 7994416 B2, US 7994416B2, US-B2-7994416, US7994416 B2, US7994416B2|
|Inventors||William C Schuh|
|Original Assignee||Watlow Electric Manufacturing Company|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (33), Non-Patent Citations (3), Referenced by (15), Classifications (27), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a divisional of U.S. patent application Ser. No. 10/463,135 filed on Jun. 17, 2003 now U.S. Pat. No. 7,084,342.
The present invention relates generally to thermocouples and more particularly to apparatus and systems for partially compensating the cold junction of a thermocouple system using low cost semi-compensated conductors.
Thermocouples are analog temperature sensors that utilize the thermoelectric properties of two dissimilar materials, typically metals, to generate an EMF in proportion to a temperature gradient across a material inhomogeneity. Common thermocouples used in temperature measurement comprise two metal wires of different thermoelectric properties called thermoelements connected at one end to form a “hot junction” also known as a “measuring junction”. The other ends of the wires are connected to instrumentation such as a voltmeter to measure the EMF produced by the thermocouple. The wires are connected to the instrumentation at a known reference temperature to form a “reference junction” or a “cold junction”. For the most precise measurements it is desirable that the only material inhomogeneity in the measurement circuit occurs at the measurement junction where the two dissimilar materials are joined.
Because it is undesirable to have any other EMF sources between the measurement junction and the reference junction, it is important that there is a minimal temperature gradient across the thermoelectric material and the electrical instrumentation leads. Thermocouple materials are specialized alloys while electrical instruments typically utilize common metals such as copper, nickel, gold, beryllium copper, aluminum, and a variety of plating materials. This material inhomogeneity at the reference termination can lead to significant errors unless care is taken to minimize temperature gradients in this region or to accurately characterize the gradients that exist.
In addition, thermocouple assemblies must use expensive hardware in the connection schemes at the termination end. Rather than using common contact materials such as copper, nickel, gold and others that are readily available, thermocouple connectors are made from more expensive thermocouple materials to minimize any inhomogeneity in the connector. A connector using thermocouple materials for the pins and sockets is referred to as a compensated connector. A compensated connector is designated to work only with a specific thermocouple type thus limiting the utility of the electronic instrumentation to only one type of sensor.
In thermocouple temperature measurement it is important to accurately establish the temperature of the reference junction in order to determine the temperature of the measured junction. In industrial temperature measurement, a temperature sensor such as an RTD, thermistor, diode, transistor, or an IC chip type sensor measures the cold junction. In almost all instances there will exist a temperature gradient between the cold junction sensor location and the location of the thermocouple leadwire termination. This temperature gradient is usually traversed by non-thermocouple wires or circuit board traces that generate little or no EMF. The result of this is an error in the temperature measurement that is approximately equal to the size of the temperature gradient.
A solution to the problem of a temperature gradient existing between the cold junction compensation (CJC) sensor and the thermocouple termination would be to use compensated materials for the lead connection and terminals. For example, a copper/constantan thermocouple (Type T) could use circuit board and terminals made from copper for the positive leg and constantan for the negative leg. This solution, however, has several negative aspects. The first and most obvious negative impact is the system is now only suitable for type T thermocouples because other types of thermocouples would experience an error from the different EMFs generated by the material differences. A second problem is a more practical problem of material availability. While copper is commonly available for terminals and printed circuit board traces, the other material, constantan, is not. Electrical hardware made from constantan or other common thermocouple alloys would be costly and would not be readily available.
Accordingly, there is a need for compensating the cold junction of a thermocouple system using low cost materials for system components accommodating various thermocouple types.
The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention is directed to a thermocouple system for partially compensating the cold junction of the system using low cost semi-compensated materially similar conductor pairs of system components, thereby enabling multiple thermocouple types to be universally applied to a single thermocouple monitoring system. The present invention further accommodates the application of a variety of conductor materials providing improved mechanical, electrical and thermal properties when used in connector terminals, pins, screws or other such system interconnection components while maintaining lower component costs and greater material availability.
Current trends in electronics packages for thermocouples are leading toward a next generation thermocouple monitoring system capable of universally accommodating various types of thermocouple materials. Beneficially, the present invention takes a big step toward meeting this need in the thermocouple industry by establishing a system comprising interconnection conductors providing cold junction compensation compatible with multiple thermocouple material types.
In conventional thermocouple systems, there are a number of interconnection junctions between the thermocouple and the monitoring instrumentation. For example, the thermocouple system may have junctions between the thermocouple leadwires and a pair of male connector pins, between the male connector pins and a pair of female connector pins, between the female connector pins and a printed circuit board, and potentially between the printed circuit board and the voltmeter.
Each of these junctions offers an opportunity for producing a material inhomogeneity and an EMF (e.g., a voltage) corresponding to the thermal differential across the materials. Ideally, the EMF produced corresponds to that of the particular thermocouple. This problem is commonly addressed by either providing materials that generally match those of the thermocouple, called “fully compensated” system, or simply use a common unmatched conductor material throughout all the junctions after the thermocouple, forming a substantially “uncompensated” system. Generally speaking, all of the aforementioned junctions except the measuring junction are in the relative vicinity of the cold junction. Therefore it is the cold junction which is typically either fully compensated by matched material use or substantially uncompensated. The present invention addresses the benefits of semi-compensated conductors for a partially compensated thermocouple system having multiple thermocouple types.
According to one aspect of the invention, a set of electrical connector pins for a thermocouple includes two materially similar conductor pairs, each conductor pair having conductors composed of a different material, and carried by an electrically insulating connector housing. The different materials of the conductor pairs provide a partial compensation to the thermocouple EMF developed between the hot junction and the cold junction when engaged thereto for the different type thermocouples. The conductors of each pair are operable to engage with two thermoelement conductors that form a thermocouple of differing types. The thermocouples provide a hot junction electrical interconnection therebetween at one end and are coupled to a cold junction at another end.
According to another aspect of the invention, an electrical connector is configured for mating to a thermocouple. The electrical connector includes two materially similar elongated metallic conductor pairs, each conductor pair composed of two different metals. The different metals of each of the elongated metallic conductor pairs provide a partial compensation to the thermocouple EMF developed at the cold junction of the respective thermocouple when engaged thereto. The conductor pairs are operable to engage with two other mating pairs of connector conductors electrically connected to two different thermocouple types to provide a hot junction electrical interconnection therebetween at one end and couple to a cold junction at another end. Also included is an electrically insulating connector housing through which each conductor is carried with each conductor positioned to project from the connector housing.
According to yet another aspect of the invention, an electrical connector is configured for mating to a thermocouple. The electrical connector includes two elongated metallic conductors. Each of the conductors is composed of a different material. The different metals of the conductors are adapted to provide a partial compensation to the thermocouple EMF developed at the cold junction of the thermocouple when engaged thereto. The conductors are operable to engage with two other mating connector conductors electrically connected to a thermocouple and provide a hot junction and electrical interconnection therebetween at one end and couple to a cold junction at another end. Also included is an electrically insulating connector housing. Each conductor is carried by the connector housing and is positioned to project from the connector housing.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and embodiments of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
The present invention will now be described with reference to the attached drawings, wherein like reference numerals are used to refer to like elements throughout. The invention relates to a thermocouple measurement system for partially compensating the cold junction of the system using materially similar pairs of system components comprising low cost conductors, thereby enabling multiple thermocouple types to be universally applied to a single thermocouple monitoring system.
Conventionally, expensive thermoelements may be used to fully compensate the junctions of a TC system. Alternately, conventional systems may not use any cold junction compensation rather than absorb the high cost of thermoelement based connection hardware. The thermocouple system of the present invention, rather, strikes a midway approach to either of these extremes, and in the process further presents a solution that provides a thermocouple system that is equally suited to a variety of TC material types. In order to better appreciate one or more features of the invention, several exemplary implementations of the temperature monitoring system, the thermoelements, some of the semi-compensation conductors and the benefits of each are hereinafter illustrated and described with respect to the following figures.
Referring back to
For example, the K type thermocouple is comprised of a nickel-chrome (+) thermoelement (typically in the form of a wire) joined to a nickel-aluminum-silicon (−) thermoelement. Nickel-chrome is the more positively polarized type K thermoelement (KP) comprising Nickel and Chromium, while the more negatively polarized Nickel-aluminum-silicon thermoelement (KN) comprises Nickel, Aluminum and Silicon. This TC combination provides a Seebeck Coefficient of approximately 0.041 mV/° C. over a 0-1260° C. temperature range. This may best be appreciated in the following figures.
The KP notation on both sides of junction 410 a, and the KN on both sides of junction 410 b indicate that no material inhomogeneity has taken place at these junctions between the TC leadwire and the male connector pins. The fully compensated TC system 400 has another junction 425 a, 425 b between the male connector pins KP 420 a and KN 420 b and a pair of female connector pins KP 430 a, KN 430 b, again with no material inhomogeneity. Finally, the fully compensated TC system 400 has a junction 435 a, 435 b between the female connector pins KP 430 a, KN 403 b and a printed circuit board. The printed circuit board PCB typically has copper printed circuit traces 440 a, 440 b that are eventually connected to a voltmeter 450. This final female connector to PCB junction 435 is the only junction in the system with a material inhomogeneity or a material transition 455.
Although there is a material transition 455 at this junction 435 a, 435 b, because a CJC sensor is utilized at this junction establishing a known reference temperature, the cold junction is said to be fully compensated to this point.
Given these temperature differentials and the TC materials used, the EMF produced in each section of the fully compensated system of
Area Material Temp differential EMF TC Portion K T1 = (100 − 30)° C. = 70° C. =2.893 mV TC Male Conn. K T2 = (30 − 25)° C. = 5° C. =0.203 mV PCB Female K T3 = (25 − 20)° C. = 5° C. =0.202 mV Conn. PCB CJC Portion CJC T4 = (20 − 0)° C. = 20° C. =0.798 mV TOTAL = =4.096 mV
Since a type K thermocouple produces 4.096 mV at 100° C., there is no error produced by the fully compensated TC system having a single material inhomogeneity 455 after that of the TC measurement junction 405, and then only at the cold junction.
Given the same temperature differentials, and the TC materials used in accordance with the uncompensated system of
Area Material Temp differential EMF TC Portion K T1 = (100 − 30)° C. = 70° C. =2.893 mV TC Male Conn. Cu T2 = (30 − 25)° C. = 5° C. =0.000 mV PCB Female Cu T3 = (25 − 20)° C. = 5° C. =0.000 mV Conn. PCB CJC Portion CJC T4 = (20 − 0)° C. = 20° C. =0.798 mV TOTAL = =3.691 mV
Since a type K thermocouple produced 3.691 mV due to the thermal EMFs representing 90° C. while the actual temperature is 100° C., there is an error of 10° C. produced by the uncompensated TC system of
A better solution to these extremes is the use of common materials to create semi-compensated terminations, for example, in the conductors or terminal pins of the connectors discussed. Semi-compensated electrical hardware of the present invention generate a portion of the EMF expected from common thermocouple types over any temperature gradient existing between the CJC sensor and the electrical termination. For instance, electrical hardware using copper for the positive leg connections and nickel for the negative leg connection yields a combination Cu/Ni Seebeck coefficient of:
This Cu—Ni combination produces about half the EMF output of a type K thermocouple that produces roughly 0.041 mV/° C. near room temperature. Thus the semi-compensated materials would correct about ½ of the EMF of a type K thermocouple. While this is not a perfect compensation it has the advantage of being made from common materials and being applicable to a variety of other thermocouple types. A type J thermocouple has around 0.06 mV/° C. and so the semi-compensated conductors (e.g., pins, terminals, etc.) would recover around a third of any error associated with a temperature gradient across a junction or connector inhomogeneity.
By contrast to either the fully compensated or the uncompensated TC system, semi-compensation is provided by the choice of the conductors used between the TC portion 460 and the PC board CJC portion 475, that is, in what is termed herein, the “semi-compensated portion”, or SC portion. The SC portion is further defined and bounded by at least two junctions having material inhomogeneities (e.g., 645 and 655 of
For example, in
Using the same temperature differentials as before, and the TC materials and conductor materials of the semi-compensated system of
Area Material Temp differential EMF TC Portion K T1 = (100 − 30)° C. = 70° C. =2.893 mV TC Male Conn. Cu/Ni T2 = (30 − 25)° C. = 5° C. =0.112 mV PCB Female Cu/Ni T3 = (25 − 20)° C. = 5° C. =0.112 mV Conn. PCB CJC Portion CJC T4 = (20 − 0)° C. = 20° C. =0.798 mV TOTAL = =3.915 mV
Since a type K thermocouple produces 3.915 mV due to thermal EMFs representing 95° C. while the actual temperature is 100° C., there is an error of 5° C. produced by the semi-compensated TC system of
In another semi-compensation example of the present invention illustrated in
Using the same temperature differentials as before, and the TC materials and conductor materials of the semi-compensated system of
Area Material Temp differential EMF TC Portion K T1 = (100 − 30)° C. = 70° C. =2.893 mV TC Male Conn. K T2 = (30 − 25)° C. = 5° C. =0.203 mV PCB Female Cu/Ni T3 = (25 − 20)° C. = 5° C. =0.112 mV Conn. PCB CJC Portion CJC T4 = (20 − 0)° C. = 20° C. =0.798 mV TOTAL = =4.006 mV
Since a type K thermocouple produces 4.006 mV due to thermal EMFs representing 97.5° C. while the actual temperature of 100° C., there is an error of only 2.5° C. produced by the semi-compensated TC system of
Thus, several electrical and economic benefits of using common semi-compensating conductor materials for the junctions of a single TC type measurement system have been shown. However, further economic and user advantages are available in providing a TC monitoring system, in accordance with the present invention, wherein the same pair combination of conductor materials is used for a TC system having multiple TC types. In particular, it is advantageous for the user of the system to be able to plug any number of a list of thermocouples into the same thermocouple receptacles, as well as less expensive for the manufacturer to supply a connector, screws or pins, for example, with common and more readily available hardware materials.
To avoid the electrical loss of uncompensated hardware, and the high cost and poor material availability of fully compensated hardware, the present invention attempts to provide a system using materially similar pairs of common conductor materials for receiving multiple TC types. For example, as will be further shown in association with the following figures, two various types (e.g., type J, T, K, N, S) of thermocouples may be both interconnected into a TC system using two pairs of Cu vs. Ni semi-compensating conductor hardware.
There are a variety of material combinations for semi-compensated conductors or pins including aluminum vs. nickel with a Seebeck coefficient of around 0.019 mV/° C., or palladium vs. platinum at 0.006 mV/° C. Thus, a variety of semi-compensation levels are available to accommodate various requirements of the system as shown in the following figure.
In addition, the specific combination of semi-compensation conductors may be selected in accordance with an aspect of the present invention to best coordinate with the range of Seebeck coefficients of the thermocouples used, to provide a particular level of compensation desirable for the TC system application. If, for example, a TC System uses a type J and a type K thermocouple, another set of semi-compensated conductors could be selected for the TC system, which provided a Seebeck coefficient midway between that of the type J and K thermocouples, yet had good cost, mechanical, electrical and thermal properties similar to those of the Cu/Ni combination, a higher level of semi-compensation may be attained for the TC system at a low cost.
For example, an Iron/Nickel combination would be the best choice from
Another junction terms a cold junction 950 a, 950 b having a material inhomogeneity is formed where the female pins 940 a, 940 b (e.g., Cu/Ni pins) attach to the traces 955 a, 955 b (e.g., typically copper traces), respectively, of the PCB 920. Near, and usually between the junctions 950 a,950 b, where the female pins 940 a, 940 b join the PCB 920, a cold junction compensation CJC sensor 960 (e.g., an RTD, thermistor, diode, transistor, or an IC chip type sensor) is mounted to detect the temperature of the cold junction 950 a, 950 b for CJC temperature correction of the TC system. PCB traces 965 attach to the CJC sensor 960 for external detection of the ambient temperature at the cold junction 950 a, 950 b. Knowing the CJC temperature is usually critical to the accurate ambient temperature compensation of the system, so CJC sensor 960 may also be potted or otherwise thermally bonded near the cold junctions 950 a, 950 b.
Thus, in the example of
Although a male plug/pins has been shown and described in the examples and figures of the invention in association with a thermocouple, and a female receptacle/pins has been used in association with a PCB, a female plug/pins used in association with the thermocouple, and a male receptacle/pins used in association with the PCB or wiring to another such circuit is also anticipated in the context of the present invention.
A cold junction 1050 a, 1050 b having the second material inhomogeneity is formed where the male pins 1040 a, 1040 b (e.g., Al/Ni pins) attached to the traces 1055 (e.g., typically copper traces) of the PCB 1020. A cold junction compensation CJC sensor 1060 (e.g., an RTD, thermistor, diode, transistor, or an IC chip type sensor) is mounted near the PCB junctions 1050 a, 1050 b of the male pins 1040 a, 1040 b to detect the temperature of the cold junction 1050 a, 1050 b for CJC temperature correction of the TC system 1000.
Thus, in the example of
The pin headers of
Each TC of the multiple TC type system 1100 is coupled to one of the materially similar conductor pairs 1121, wherein one of the conductors of each pair 1121 is composed of a material different than the thermoelectric materials of the TC portion 1110. System 1100 also comprises a cold junction compensation portion CJC 1130 using the same conductor material throughout all the conductor pairs, for example, conductor pairs 1131 all comprise one material type (e.g., Cu, Ag, Au). The CJC portion 1130 further comprises one or more thermal sensors (not shown) near the junction of the semi-compensation portion 1120 and the CJC portion conductor pairs 1131, wherein multiple semi-compensated temperature measurement outputs 1140 may be provided.
System 1200 further comprises a semi-compensation portion 1220 having materially similar conductor pairs, for example, a pair of Cu and Ni conductors 1221 comprise the semi-compensating conductor pairs (e.g., Au/Ni, Cu/NI, Pd/Pt) that are materially similar for al the TC types 1210 a used in the system 1200. Each TC of the multiple TC type system 1200 is coupled at junction 1222 to one of the materially similar conductor pairs 1221, wherein one of the conductors of each pair 1221 is composed of a material different than the thermoelectric materials of the TC portion 1210. In the illustration, the SC portion 1220 also exemplifies a TC male connector 1220 a and TC female connector 1220 b having metallic conductors, or other such conductive hardware for engaging forming a junction 1225 therebetween.
System 1200 also comprises a cold junction compensation portion CJC 1230 using the same conductor material throughout all the conductor pairs, for example, conductor pairs 1231 all comprise one material type e.g., Cu, Ag, Au). The CJC portion 1230 further comprises one or more thermal sensors (not shown) near the junction 1235 of the semi-compensation portion 1220 and the CJC portion conductor pairs 1231, wherein multiple semi-compensated temperature measurement outputs 1240 may be provided. For example, the multiple semi-compensated outputs 1240 may be coupled to a multiplexing analog to digital converter ADC 1250 for further measurement processing, output, or display. The connector configuration of
In one example, the semi-compensated TC system 1300 of
From this point, the terminal conductors 1325 a, 1325 b and 1330 a, 1330 b are joined to conductive traces 1350 a, 1350 b and 1355 a, 1355 b of the PCB 1345. Once the terminal conductor is joined to the PCB conductive traces, the remaining portion of the system may be described similar to that of
The TC system 1400 of
Using the temperature differentials indicated as before, and the TC materials and conductor materials of the semi-compensated system of
Area Material Temp differential EMF TC Portion K T1 + T2 = (100 − 25)° C. = 75° C. =3.096 mV Screw TS Cu/Ni T3 = (25 − 20)° C. = 5° C. =0.112 mV Conductor PCB CJC CJC T4 = (20 − 0)° C. = 20° C. =0.798 mV Portion TOTAL = =4.006 mV
Since a type K thermocouple produces 4.006 mV due to thermal EMFs representing 97.5° C. while the actual temperature is 100° C., there is an error of only 2.5° C. produced by the semi-compensated TC system of
Although the invention has been illustrated and described with respect to one or more embodiments, implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”
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|U.S. Classification||136/230, 439/259, 374/179, 439/638, 439/657, 136/236.1, 136/224, 374/181, 136/227, 374/208, 439/626, 439/727, 439/913, 136/235, 439/641, 136/232|
|International Classification||G01K, H01L35/12, H01L35/02, H01L35/06, H01L35/30, H01L35/04, G01K7/13, H01L35/28|
|Cooperative Classification||Y10S439/913, G01K7/13|
|Apr 29, 2005||AS||Assignment|
Owner name: WATLOW ELECTRIC MANUFACTURING COMPANY, MISSOURI
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:SCHUH, WILLIAM C.;REEL/FRAME:016184/0860
Effective date: 20050426
|Jan 21, 2015||FPAY||Fee payment|
Year of fee payment: 4