Compensating lead-wire

It is important that the thermistor or RTD used for measuring the cold junction be physically located at the cold junction, as the temperature of the cold junction is often different from that of the room, generally because of heat leakage from the furnace. Special wire, referred to as compensating lead-wire,... 

The high cost of platinum prevents the use of compensating leads of the same metal in the case of a rare-metal couple but inexpensive lead wires of copper and an alloy of nickel-copper are now available for use with the platinum-platinum 90, rhodium 10 couple. These lead wires do not compensate individually but taken together they compensate to within 5°C. for a variation of 200 C. in the couple-lead wire junctions. Since the compensating lead wires for the rare metal couple do not compensate individually both terminals on the head of the couple should be always as nearly as possible at the same temperature. The copper wire of the compensating leads is connected to the platinum-rhodium wire of the couple and the copper-nickel wire is connected to the platinum wire of the couple, i.e., alloy wire to pure metal in each case. The cold junction is then located at the indicator end of the compensating leads. The temperature of this junction may be controlled if necessary by one of the methods described above. Copper wires are carried from this point to the indicator. 

Wiring Diagrams of Thermocouple Installations.— Figure 11 illustrates a simple thermoelectric installation for a rare-metal couple. The couple is properly protected by a porcelain or quartz tube and if necessary by an outer tube of iron, chromel, fireclay, etc. From the head of the couple compensating lead wires are carried to the bottom of a pipe driven 10 ft. under ground. From the bottom of the pipe copper lead wires are carried to the indicator. 

Figure 13 illustrates a multiple thermocouple installation connected to a single indicator. Compensating lead wires are carried from the couples to a conveniently located cold-junction box. The temperature of this box is thermostatically controlled. From the cold-junction box copper wires are carried to the terminal block and selective switch illustrated. A common return has been employed between the cold-junction box and the switchboard. In general it is preferable to use individual return wires for each couple. The switchboard illustrated is designed for six couples. By pressing one of the buttons shown any desired couple is connected directly to the indicator. 

Use of Junction Box.—Figure 14-illustrates a wiring diagram for a multiple-couple installation which is very useful in saving compensating lead wire and in... 

The platinum or gold lead wires C d and T e in the thermometer are constructed of as nearly the same resistance as possible, and the copper lead wires CC and TT must also have an equivalent resistance. The battery B is connected between the ratio arms ri and of the bridge and to the compensating lead wire cd. A sensitive... 

For the connection of a thermocouple to the measurement instrument, the best way is to use leads made of the same materials (extension leads) these avoid lead junction errors. For economic reasons, however, cheaper alloys having similar e.m.f. output, at least over a limited temperature range close to room temperature, are often used. These compensating leads are often supplied by the same producers of the thermocouple wires. 

Fig. 3.27. The Pask-Plesch reaction calorimeter, approximately to scale. A phials of reagents, B phial magazine, C cold finger (not essential), D phial breaker, E vessel of calorimeter, F heater, Gj Pt wires of the conductivity probe, Gj terminals, H vacuum jacket, thermometer probe, terminals from thermometer probe and the compensating leads, K tap for evacuation of pseudo-Dewar space or admitting air, M magnetic pusher, T main tube,...

Thermistors have the desirable characteristics of small size, narrow spans, fast response (their time constant can be under 1 second), and a very high sensitivity. They do not need a cold-junction compensation, errors due to contact or lead-wire resistance are insignificant, and they are well suited for remote temperature sensing. They are inexpensive, their stability increases with age, and they are the most sensitive differential temperature detectors available. 

PRTs can have either two, three, or four lead wires. Besides the Mueller bridge setup, two-wire and three-wire bridges are available [24]. A two-wire (Wheatstone) bridge is the least accurate, because the lead-wire resistances are not accounted for. A three-wire bridge (shown in Fig. 16.14) allows for the compensation of lead-wire resistances by adding the resistance of the third lead wire to the leg of the bridge, where the known variable resistor resides. Hence,... 

The lead wires of these thermocouples are usually made of 0.35 to 0.5 mm. diameter wire. Sometimes it is undesirable to have thermocouple wires sufficiently long to bring the connections (which also form the cold junctions) directly to constant temperature. In this case the so-called compensators are used. These may be considered lead wire lengtheners. Such compensating wire may be ordered from the companies that supply thermocouples. 

Finally, another solenoid was wound with lead wire without NbZr end compensation. It was 8 in. long and consisted of two layers, each with 120 turns of 0.060-in.-diameter wire of 99.99% purity. The diameter of the inside layer was f in., that of the outside layer 1 in. Forty-two taps on each layer permitted voltage measurements across individual turns and groups of turns in the intermediate state. 

Many types of sensors and transducers have particular signal conditioning requirements. For example, thermocouples require cold-junction compensation for the thermoelectric voltages created where the thermocouple wires are connected to the data acquisition equipment. Resistive temperature devices (RTDs) require an accurate current excitation source to convert their small changes in electrical resistance into measurable changes in voltage. To avoid errors caused by the resistance in the lead wires, RTDs are often used in a 4-wire configuration. The 4-wire RTD measurement avoids lead resistance errors because two additional leads carry current to the RTD device, so that current does not flow in the sense, or... 

When compared with thermocouples, RTDs have higher accuracy, better linearity, long-term stability, do not require cold-junction compensation or extension lead wires and are less susceptible to noise. However, they have a lower maximum temperature limit and are slower in response time in applications without a thermal well (a protective well filled with conductive material in which the sensor is placed). 

The multiloop controller contains a variety of func tion blocks (for example, PID, totalizer, lead/lag compensator, ratio control, alarm, sequencer, and Boolean) that can be soft-wired together to form complex control strategies. The multiloop controller, as part of a DCS, communicates with other controllers and man/machine interface (MMI) devices also on the DCS network. 

In practice, the limit at high frequencies is controlled by the inductance in the circuit, (0L. The influence of this on the impedance (in contrast to that of the capacitance) increases with an increase in frequency. The difficulty is that the inductance that becomes significant when the frequency exceeds, say, 104 cps, is often more an irrelevant inductance, not one caused by the electrode process. Thus, it may arise because of some contribution from the wire connections to the cell and their interaction with the surroundings. Hence, very short leads to the cell should be used. It is possible to build circuits that compensate for the inductance effects, but usually the practice is to keep the frequency within the 10 kilocycle/s upper range, so as to make coL negligible. 

Three-lead Wheatstone Bridge Method.—Figure 27 illustrates the wiring diagram for a simple Wheatstone bridge and thermometer with the Siemens three-lead compensation. 

Before the robot starts to lay and contact the leads, a camera, integrated in the wiring tools, is positioned above the required position of the component designated for wiring. Each component is identified and its precise position is controlled by vision marks in the shape of three cylindrical cavities in the die casting of the IDC connector. Any deviation of the actual position from the target position is calculated and transmitted as compensation value to the robot s evaluation program. 

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