Abstract:
A reusable cable apparatus for interfacing disposable temperature probes with a patient monitor. One reusable cable connects a thermocouple probe to the patient monitor and has a phone plug connection wherein a cold junction and cold junction compensation is located. A second reusable cable connects a thermistor probe to the patient monitor through a similar phone plug. Circuit means in the patient monitor receives either phone plug and produces an output signal which is substantially the same regardless of whether the thermocouple and first reusable cable or thermistor and second reusable cable are connected. The effect of secondary thermocouple junctions in interconnecting connector shells is substantially eliminated by connecting lead wires to contact pins such that the secondary junctions are physically close to one another.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates to temperature sensing instrumentation and, more particularly, to improved apparatus for connecting temperature sensor probes of the thermistor and thermocouple type to a patient monitor apparatus. 
     2. Description of Related Art 
     Thermocouple and thermistor probes for use with temperature sensing apparatus are known, in the prior art. In U.S. patent application No. 107,989, cold junction compensation circuitry for a thermocouple is disclosed. While the subject matter of that patent application has provided advantageous improvements, it has appeared desirable to the inventors to further improve the ability of instruments to accept either thermistor or thermocouple type probes. 
     SUMMARY OF THE INVENTION 
     According to the invention an interconnecting cable apparatus is disclosed which permits connection of either a disposable thermocouple or a disposable thermistor probe to an instrument simply by changing the interconnecting cable. According to a feature of the invention, the apparatus may include first and second reusable cables, each employing a connector at either end, one of the connectors adapted to plug into a temperature monitoring apparatus and the other adapted to connect to a respective temperature sensing element, i.e., a thermocouple probe or a thermistor probe. 
     In a preferred embodiment, the cable for connecting a thermocouple probe mounts the cold junction and cold junction compensation means in a phone plug connector. Means are supplied for isolating the cold junction and cold junction compensation means. As another feature, means are provided for minimizing the effects of secondary thermocouple junctions formed in the connectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The just-summarized invention will now be described in detail in conjunction with the drawings, of which: 
     FIG. 1 is a side sectional schematic view of a portion of an interconnecting cable according to the preferred embodiment; 
     FIG. 2 is a side sectional view illustrating the remainder of the cable of FIG. 1 and a cooperating disposable connector; 
     FIG. 3 is an electrical schematic diagram illustrating interconnection of the reusable cable of FIGS. 1 and 2; 
     FIG. 4 is an electrical circuit schematic equivalent of the circuit of FIG. 3; 
     FIG. 5 is a schematic diagram illustrating interconnection of a second reusable cable for use with a thermistor; and 
     FIG. 6 is a circuit schematic equivalent to the circuit of FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventors of carrying out their invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the generic principles of the present invention have been defined herein specifically to provide a readily manufacturable, reusable cable apparatus. 
     In U.S. patent application Ser. No. 107,989, a means for compensating the potential of a cold junction is described using a thermistor as the cold junction temperature sensor. The location of the cold junction and the cold junction temperature sensor is in the connector terminating the reusable extension cable mated to the disposable temperature probe connector, in turn connected to the hot junction as in FIG. 3 of said Application. 
     The connector topology of the preferred embodiment described hereafter achieves the following goals: 
     1. Cold junction compensation external to the measuring instrument and located in a reusable thermocouple interconnection cable. Compensation is achieved by use of a cold junction temperature sensor (thermistor in this case) in a circuit that produces a voltage that changes with equivalent slope (change in voltage versus temperature) opposite to that of the thermocouple cold junction (by default equivalent to that of the thermocouple hot junction). 
     2. Alternate connection to the instrument such that a disposable temperature probe of a different type may be connected to the same instrument through the same instrument interconnection interface. The different type of sensor may comprise a thermistor located at the temperature monitoring location. The resulting thermistor probe circuit eliminates all thermocouple junctions and exhibits a voltage change equivalent to that of the thermocouple sensor circuit comprised of the series connected potentials of the cold junction compensator, cold junction, and hot junction. 
     3. The connectors used on both ends of the cable are of conventional design and do not require thermocouple (e.g., copper, and constantan) materials in their construction. 
     4. Connection to the disposable connector is with conventional pin and socket connectors using nonthermocouple materials. 
     5. The construction allows cost effective manufacture of both the reusable extension cable and the disposable probe connectors. 
     FIG. 1 illustrates a connector or connector means according to the preferred embodiment including a housing 24. The connector of FIG. 1 employs a conventional 1/4-inch phone plug 11 in which a copper wire 28, a cold junction 12, a cold junction lead 27, and a cold junction temperature sensor 14 are housed. The phone plug 11 connects to a temperature monitoring instrument 23, for example, a patient monitor apparatus in a medical environment. 
     Connection of the copper wire 28 and lead 27 to a disposable probe sensor, e.g., containing hot junction 32 of FIG. 3, is accomplished with commercially available pin and socket contacts housed in respective conventional connector shells 13, 19 shown in FIG. 2. The commercially available hard connector shell 13 is preferably used to house the male contact pins 15, 16. The disposable connector 19 is insert molded to match the design of the shell 13. The interface between a reusable cable 17 and a disposable connector 19, therefore, is conventional in external mechanical (dimensional) design. 
     The phone plug 11 allows the reusable cable 17 to be disconnected from the instrument 23. Conventional thermocouple interconnection techniques employ a special connector in which the contact materials used to make electrical interconnection are of the same materials as the thermocouple. For example, in a typical conventional apparatus, one connector contact would have to be copper and the other constantan. The cold junction and cold junction compensator temperature sensors are then located in the interior of the instrument. These special connectors are usually quite costly and are specific in use with thermocouple-type probes. 
     As indicated above, the connector of the preferred embodiment uses a conventional 1/4-inch stereo phone plug 11, which is available from many sources (e.g., from Mouser Electronics, Mansfield, Tex.). Because the phone plug 11 does not employ thermocouple metal materials such as constantan, the cold junction 12 and cold junction temperature sensor 14 must be located outside the instrument, in this case within the housing 24 provided by the 1/4-inch phone plug 11. 
     The cold junction 12 is formed by connecting the constantan wire 27 of the extension cable 17 to a copper lead 29. The copper lead 29 is connected to one thermistor lead 22 at terminal &#34;C&#34; of the phone plug 11. The copper cable lead or wire 28 is connected to terminal &#34;B&#34; of the phone plug 11, while a second thermistor lead 30 is connected to terminal &#34;A&#34; of the phone plug 11. 
     The cold junction 12 is insulated with a plastic sleeve 33 and potted together with the cold junction temperature sensor (thermistor) 14 in a high heat conductivity epoxy 37. A metal sheath 35 surrounds the epoxy 37. The assembly within the metal sheath 35 assures that the temperature of the cold junction 12 matches the temperature of the cold junction temperature sensor 14 and that the entire assembly temperature is not appreciably affected by fast in ambient temperature changes by virtue of its combined thermal mass. The assembly is positioned in the 1/4-inch phone plug 11 within an air space 39 which further insulates the assembly from fast changes in ambient temperature. 
     The reusable cable/disposable probe connections are performed by a stock pin and socket connector design (Molex Inc., Lisle, Ill.). Such stock connectors typically employ brass pins and sockets. A thermocouple junction exists between the cable wires and the pin material and between the disposable probe wires and the pin material. However, as long as the temperature difference along the axis of the cable wire and probe wire is small, no net effect occurs by the inclusion of these secondary thermocouple junctions. The connector of the preferred embodiment reduces the axial temperature difference by soldering the lead wire, e.g. 27, inside the pin, e.g. pin 15, and welding the probe wire, e.g. 101, close to the open end of the contact, e.g. 103, such that the cable and probe wires and their associated secondary junctions to the pin/contact materials are physically close together when the two connectors 13, 19 are mated. The insert molded plastic body of the disposable connector 19 provides additional insulation and thermal mass around the secondary junctions, further insulating them from fast changes in ambient temperature. 
     The function of the instrument 23 with either a thermocouple or a thermistor probe is shown in FIGS. 3-6. 
     The thermocouple circuit (FIGS. 3, 4) comprises the series strung voltages of the thermocouple hot junction (Vh), thermocouple cold junction (Vc), and the voltage developed across the thermocouple cold junction compensator (Vcjc). As mentioned previously and described in the prior application, the elements of the cold junction compensator, i.e., circuit means 54, 56, 48, and thermistor 50 are chosen so as to produce a voltage drop across resistor 48 (Vcjc) that changes in opposition to the voltage of the cold junction 12. Since the cold junction 12 is physically comprised of the same materials as the hot junction 32, the cold junction compensator slope (voltage change versus temperature change) matches that of the input thermocouple type. 
     The much simpler thermistor circuit (FIGS. 5, 6) eliminates all junctions and simply measures the voltage drop across the resistor 48. The voltage across the resistor 48 (Vth) changes as a function of the temperature of the hot site thermistor temperature sensor 50&#39;. No changes in the internal circuits of the instrument are necessary because the change in voltage of the thermistor circuit matches the change of the compensated thermocouple hot junction. A connection 107 (FIG. 5) is made between parts B and C of the 1/4-inch phone plug 11, either where the disposable probe 50&#39; plugs into the end of the reusable extension cable, or at the end of the reusable cable where the phone plug 11 is attached (this can be achieved by hardwiring the B and C terminals together or by using a monoaural plug). Other manufacturers of reusable thermistor sensor use a monaural 1/4-inch phone plug, allowing compatability of the present invention with those sensors. 
     The ability of the present invention to accept either thermocouple or thermistor-type probes with only a change in the interconnecting cable, which is a significant advantage. 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. In particular, while constantan and copper have been disclosed as particular thermocouple materials in the foregoing discussion, those skilled in the art will appreciate that various other thermocouple materials may be used in various embodiments according to the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.