induced into the conductors and produce a false reading. Industrial shielding is used more often than not in most industrial settings. Nonshielded wire can be used in indoor environments that have a minimal degree of external EMF voltages (i.e. a laboratory or assembly area). Shielding generally consists of Mylar with aluminum-backed material. A solid copper drain wire is helically wound about the sensor cable. Then the cable and copper wire are wrapped with the Mylar in such a way that the aluminum contacts the copper. Extension leadwire insulation material selection is critical for environments where temperature and moisture could effect the readings. Depending on the ambient environment, you have your choice of candidates. Teflon-insulated wires are generally rated as acceptable for contact with environmental or surface temperatures of approximately 400º F-500º F. PVC insulation can be used for temperatures up to 220º F and fiberglass up to 900º F. For higher temperatures, other insulation materials are available, such as silica fiber for up to 1800º F. Teflon and PVC provide excellent moisture resistance and generally very good abrasion resistance. Fiberglass and silica provide poor moisture resistance and only fair abrasion resistance. When stripping the insulation to connect the wires to the terminals, be careful not to make cuts into the conductors. Such cuts could set up a stress point within the wire and leave it prone to failure by breakage or opening over time. The general rule of thumb is to remove approximately 1.5 to 2 in. off the outer jacket and about 1/2 in. off the individual conductors. If there is a drain wire, expose enough of it to extend to the grounding screw. The posts inside the terminal head are not usually made of thermocouple material. In most cases, thermocouple material and RTD wires are connected to nickel-plated brass posts. Although some people worry about the effects of these third metals on the tempera- ture readings, they shouldn't. The terminal head is typically in a defined environment, with temperatures at equilibrium, so no delta T exists across the terminals. Any thermoelectric effects generated by the third material cancel out. If you are concerned about the effect of third metals, instead of connecting the wires to the provided terminal posts, simply wrap the like conductors about each other and use a wire nut, avoiding the posts altogether. Selecting Extension Wire The extension wire can have a major effect on the accuracy of temperature readings transmitted from the sensor assembly to the control room. First, you need to consider the environment. For most indoor applications, the extension wire will run through conduit in an environment that is generally moisture free, so there should be minimal problems with induced heat or moisture. Outdoor applications are another story. Moisture, water droplets, or even accumulated water buildup can cause low IR-to-ground conditions with ground loops and potential oxidation (rust) of the thermocouple wire, depending on the thermocouple type used. With contact no longer solid, the reading can be erroneous. Drops of moisture creating links between wires can cause a secondary junction and produce an erroneous reading. These problems can be avoided by using Teflon- or PVC-insulated extension wire. The second key point to consider is Reference Junction/Cold Junction compensation. Industrial thermocouples function on the principle that any piece of wire exposed to a thermal gradient will produce a voltage. When you pair two dissimilar metals, you can determine a measurement in millivolts that can be related to set points to determine the temperature reading. The reference junction refers to the temperature at ice point (32º F). www.conaxbuf .conaxbuff conaxbuf@conaxbuff 716-684-4500 Fax: 716-684-7 4-7433 1-800-223-2389 · www.conaxbuf falo.com · conaxbuf@conaxbuf falo.com · 716-684-4500 · Fax: 716-68 4-7433 6