Patent Number: 062193988
Section: description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT As illustrated in FIG. 1, a heated junction thermocouple level measurement system 10 comprises a heated junction thermocouple cable arrangement 12 and a plurality of sensors in the probe 42. As best shown in prior art FIGS. 2 and 3, five separate wires A, B, C, D and E must be connected to each sensor assembly 14. Two of the wires, A and C, comprise thermocouple conductors of a first polarity, usually positive. A positive thermocouple conductor is typically formed from a Ni.sub.90 /Cr.sub.10 thermocouple alloy sold under the tradename Chromel.RTM.. A negative thermocouple conductor is typically formed from a Ni.sub.95 Al+Mn+Si.sub.5 thermocouple alloy sold under the tradename Alumel.RTM.. The first thermocouple conductor A of the first polarity forms a component of heated thermocouple 16 while the second conductor C of the same polarity forms a component of the unheated thermocouple 18. A third wire B comprises a thermocouple conductor of the opposite polarity. The conductor B is typically shared between both the heated and unheated thermocouples 16, 18. The remaining two wires D and E are typically nickel clad copper and act as positive and negative power conductors providing electricity to heater coil 20. To prevent inaccurate readings, the heated and unheated thermocouples 16, 18 of each sensor 14 are physically displaced from one another so that heat from the heater coil 20 positioned next to the heated thermocouple 16 does not affect the voltage generated by the unheated thermocouple 18. The heated and unheated thermocouples 16, 18 are monitored for both the absolute temperatures of the thermocouples as well as the differential temperature between the two thermocouples making up a sensor 14. The net voltage generated by each of the thermocouples 16, 18 are a function of the temperature difference between them. The heated thermocouple 16 will generate a voltage representative of its temperature. The unheated thermocouple 18 will also generate a voltage representative of its temperature. When liquid coolant surrounds both the thermocouples 16, 18 the heat generated by the heater coil 20 will be transferred to the surrounding coolant. Therefore, the temperature of both thermocouples will remain essentially identical. Since the voltage produced by the heated thermocouple opposes the voltage produced by the unheated thermocouple, the net voltage should be small. When there is an absence of liquid coolant surrounding both the thermocouples 16, 18 the heat produced by the heater coil 20 does not transfer as well to the surrounding gaseous coolant. As a result, the heated thermocouple temperature will rise above the unheated thermocouple temperature and a much larger net voltage results between the two thermocouples 16, 18. The thermocouple wires A,B and C have an outer diameter in the range of approximately 0.01 to 0.02 inches and more specifically an outer diameter of 0.015 inches. The prior art heater coil wires D and E have an approximate diameter of 0.040 inches. The five wires A through E associated with each of the eight sensors 20 are typically combined into a single cable C.sub.1 through C.sub.8 as best shown in prior art FIG. 2. The outer diameter of cables C.sub.1 through C.sub.8 is approximately 0.25 inches. In turn one or more cables C.sub.1 through C.sub.8 are secured within a conduit 30. Typically, each conduit 30 is formed from a flexible metal hose or sheath sold under the tradename Penflex.RTM.. If all eight cables are secured within a single conduit 30 the outer diameter of the resulting conduit is more than 0.75 inches. In direct contrast to the teachings of the prior art, the present invention significantly reduces the number of wires from forty to twenty-eight, as shown in FIG. 3. The sixteen separate heater coil conductors D and E traditionally associated with cables C.sub.1 through C.sub.8 of the prior art are reduced to a total of four wires D' and E'. The wires representing power conductors D' and E' have a diameter in the range of approximately 0.06 to 0.09 inches and more specifically approximately 0.08 inches. The thickness of wires D' and E' permit greater power to be transmitted through the wires. One set of wires D' and E' are isolated within a separate cable C.sub.9 while the other set of wires D' and E' are isolated within a separate cable C.sub.10. An advantage of having the wires D' and E' separated from the sensor wires A, B, and C is that the wires D' and E' have a tendency to interfere with the sensor signals if the five wires are shared within a single cable. Cables C.sub.9 and C.sub.10 have an approximate diameter of approximately 0.31 inches. The remaining eight cables C.sub.1 ' through C.sub.8 ' only have three wires, namely the thermocouple conductors represented by wires A, B and C. As a result, the corresponding cables C.sub.1 ' through C.sub.8 ' have an outer diameter of approximately 0.11 inches. It is advantageous to have cables C.sub.1 ' through C.sub.8 ' with a smaller diameter, namely 0.11 inches as compared to approximately 0.25 inches. Cables C.sub.1 ' through C.sub.8 ' are more easily bent and manipulated than the prior art five wire cables C.sub.1 through C.sub.8. If all ten cables are secured within a single conduit 30' the outer diameter of the resulting conduit is approximately 0.55 inches, significantly less than the greater than 0.75 inches required in the prior art. The mass is reduced by approximately 30%. Having a smaller and lighter conduit promotes installation and handling particularly in the "hot" area of a nuclear reactor. The decrease in conduit size and mass is achieved even though the outer diameter of wires D' and E' is at least three and preferably four times greater than that of wires A, B, and C. Further, unlike the prior art, the eight heater coils 20 are broken into two sets of heater coils, one set provided electricity by wires D' and E' of cable C.sub.9 and the other set provided electricity by wires D' and E' of cable C.sub.10. As best shown in FIG. 5, each set of the heater coils 20 is ganged in parallel. The wires D' and E' are very strong and able to resist potential damage while also providing adequate power to the heater coils 20. Even if one of the heater coils fails, the use of wires D' and E' still permits the remaining heaters to function. The total resistance in the circuit R(T) is typically equal to the resistance of each heater divided by the number of heaters or R(H)/4. Thus, if one of the heaters coils 20 fail and becomes an open circuit, the total resistance increases to R(H)/3. The power to the heaters is only reduced by 25% with current decrease according the formula I=V/R and can be compensated for if necessary by increasing voltage. As best shown in FIG. 1, having fewer wires also promotes the ability to connect various cables together between containment penetration connector 40 and heated junction thermocouple probe 42. A containment conduit 44 includes a 28 pin or socket connector 46 and 48 at opposite ends, connector 46 mating with penetration connector 40. In turn, containment conduit 44 mates with a bridge conduit 50, the bridge conduit having a 28 pin or socket connector 52 and 54 at opposite ends, connector 52 mating with connector 48. Bridge conduit 50 also mates with a head lift rig conduit 56, the lift rig conduit having a 28 pin or socket 58 and a 40 pin or socket 60. In a preferred embodiment, the lift rig conduit 56 acts as a transition conduit, allowing a prior art probe 42 with a 40 pin or socket 62 to mate with a corresponding 40 pin or socket 60. Socket 60 has jumpers 64 between select pins 66 in the backshell of the connector to provide sixteen pins or sockets for the power conductors which are reduced to four in the cable conduit 56. Alternatively, the mating socket may have the jumpers. The transition between a 40 pin or socket connector to a 28 pin or socket connector preferably takes place as close to the "hot" zone of the reactor vessel penetration by probe 42 as possible. By having fewer conductors in the cable near the reactor vessel, the time necessary and problems associated with completing the connection or disconnection operation are significantly decreased. There is less likelihood of bending or breaking pins when handling bridge conduit 50 and head lift rig conduit 56. The disclosed embodiments and examples are given to illustrate the present invention. However, they are not meant to limit the scope and spirit of the present invention. Therefore, the present invention should be limited only by the appended claims.