Extension wires

Temperature Measurement shift. Measurement not representative of process. Indicator reading varies second to second. Ambient temperature change. Fast changing process temperature. Electrical power wires near thermocouple extension wires. Increase immersion length. Insulate surface. Use quick response or low thermal time constant device. Use shielded, twisted pair thermocouple extension wire, and/or install in conduit. 

The second major area for the use of plastics in electrical applications is at the terminations of the conductors. The connectors that are used to tie the wires into the equipment using the power, or used to connect the wires to the power source, are rigid members with spaced contacts. These are designed to connect with a mating unit and to the extension wires. The other type of wire termination is terminal boards where there are means to secure the ends of the wire leading to the equipment and the internal wiring in the equipment. These termination units require ... 

In industrial applications it is not uncommon that the thermocouple must be coupled to the readout instrument or controller by a long length of wire, perhaps hundreds of feet. It is obvious from the differential nature of the thermocouple that, to avoid unwanted junctions, extension wire be of the same type, eg, for a J thermocouple the extension must be type J. Where the thermocouple is of a noble or exotic material, the cost of identical lead wire may be prohibitive manufacturers of extension wire may suggest compromises which are less cosdy. Junctions between the thermocouple leads and the extension wire should be made in an isothermal environment. The wire and junctions must have the same electrical integrity as the thermocouple junction. Because the emf is low, enclosure in a shield or grounded conduit should be considered. 

In order to protect the thermocouple against chemical or mechanical damage, it is normally enclosed in a sheath of mineral packing or within a thermowell (Fig. 6.24). Any material which contains the junction should be a good conductor of heat on the one hand, but an electrical insulator on the other. A potentiometric converter is frequently employed to convert the thermocouple signal to the standard 4-20 mA current range prior to further processing and control room presentation. The extension wires which connect the thermocouple element to the control room should have similar thermoelectric properties to those of the thermocouple junction wires. 

Extension wires having same thermoelectric properties as thermocouple elements... 

Low-temperature thermistors are usually made from nonstoichiometric iron oxides and have a resistance sensitivity of around 15% per Kelvin at 20 K.17 Thermistors act as ohmic conductors at any fixed temperature. Therefore, one advantage of using a thermistor is that ordinary copper wiring may be used to build the circuit reference junctions and special extension wires are not needed.14 Thermistors are generally quite stable to long-term fluctuations after an initial aging period. 

When using a measuring junction at a remote site relative to the reference junction, it is important to use the proper extension wire between the junctions. Usually, a wire with the same composition as the thermocouple wire itself, but not made to such a high specification, is used. 

A 1500-W tubular heater is wrapped around the reactor. The autoclave is placed inside a copper pipe, with Fiberfrax bulk fiber insulation packed between the copper pipe and the reactor. Four 1000-W quarter-circle ceramic heaters are placed around the copper pipe. The space between the copper pipe and the ceramic heaters is filled with aluminum balls for efficient and uniform heat transfer from the heaters to the copper pipe. The bottom of the copper vessel is closed with a copper plate. Beneath the copper plate, there are two heaters (a small one of 500 W and a larger one of 1000 W), which are tightly clamped to the bottom copper plate. The top of the copper pipe is similarly covered with another copper plate, and two slots are provided for the feed and product gas lines, coolant inlet and outlet lines, thermocouple extension wires, and the lead wires of the tubular heater. Two ring heaters (660 and 1000 W) are placed on the top copper plate. One thermocouple and a tape heater are placed on the shaft extension to minimize the heat loss through the shaft. No insulation is placed around the ceramic heaters to achieve better control of... 

If very thin wires are required, use larger-diameter extension wires for regions where there is almost no temperature gradient (typically the room temperature leads that go to the voltage-measuring device). 

Thermocouple Components and Fabrication. A thermocouple measurement assembly includes a sensing element assembly, extension wires (when used), reference junction, connecting wires, an EMF-measuring device (possibly with signal-processing equipment), and other hardware needed for applications in adverse environments such as protection tubes, connectors, adapters, and so on. Each of the above components will be discussed in the following paragraphs. 

Two pressure transducers are calibrated at a common source and the relative error between the transducers is automatically eliminated by the AP cell. Next, both transducers are connected across the top and bottom taps of the distillation tower or reactor. The pressure transducers convert pressure to an electronic signal. The signals from both pressure transducers are brought back to the AP cell by an extension wire. The AP ceil subtracts the low pressure from the high pressure and digitally displays the difference. 

One possible means of constmction is as follows A scouting aircraft (helicopter) confirms a satellite-surveyed ice field as suitable and delivers to it a small tractor with extensible wire-saw. The tractor saws up the ice platform to hew there from a platform of a specified size (including allowance for melting before insulation for example, 500 x 500 x 10 m) (Figs. 55.6 and 55.7a) and an ice-braker ship tows this platform to open water. Here the platform is equipped with air-film covers, protected by from warm water on all sides. The platform is towed to a place where it will be provided with final protection and other improvements a suitable location... 

When using an RTD or a TC the time constant considerations are similar, but the actual response times of the devices will vary. The FTS and RTD will have response times of nearly the same magnitude, whereas the TC is somewhat faster. For a thermocouple, tb varies with the device s constmction and length of the extension wires. Hence, a TC made of small wire with short extension wires will give a fast response. A typical TC response is around 0.5 s. 

The installation of extensive thermocouple equipment requires the services of qualified instrument technicians, and special attention should be given to extension wires, reference junctions, switches, and terminal assemblies. 

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