The different variations of thermocouples available today are distinguished with the help of charts and color coding. The four most common calibrations are J, K, T and E. Each calibration has a different temperature range in order to suit specific requirements. Type J thermocouple are ideally used for legacy temperature recording and control equipment.
The most common usages for the J thermocouple range from general purpose to immersion in liquid and exposure to gas and air. Others can be customized to be used for soft solid penetration as well as surface and air temperature measurement. As most thermocouples, this type can be fitted into standard or miniature connectors.
Because its constricted temperature range is one measuring from -40 to 750 C it is regarded to be fewer in demand than the K type. In addition, due to the lower range offered the iron element can cause it to be sensitive to oxidization. Typically it is made up of copper-nickel and iron alloy, the positive wire being iron and the negative wire being copper-nickel alloy. The advantage being that it is reasonably priced and readily accessible.
On the subject of wiring, it is of vital important to utilize the correct wiring for the specified junction and the right requirement. To avoid any complications, do not mix J & K wires. Use of incorrect wires, other than the matching type, will cause serious errors, keeping in mind that these are polarity receptive wires. Taking into consideration that for both the J and K designs, red is negative, emphasizes the importance of matching similar colors when joining wires together.
The output signals typically are in the mill volt range, generally having a rather low temperature to voltage sensitiveness, which means that careful attention must be paid to the many error sources that might impact the overall accuracy of measurement. The main sources of most errors accurate measurements need to keep in mind are gain, noise and offset errors, cold-junction compensation accuracy, as well as thermocouple errors.
These small temperature gauges are manufactured using two different metals which are welded together at one end. Electromotive forces are generated as a result of the variation in temperature between its cold and hot point of fusion, generally known as the thermoelectric or Seebeck effect, thus igniting the theory behind thermocouple instrumentation. This way of measuring miniscule temperatures is the most extensively used method and remains to this day.
Manufacturers have a ready to use configuration table or code guide for every possible application in their catalogs, as well as and technical support of expert engineers should there be a need. This can ultimately eliminate errors. Guidelines tend to be uniform but may differ slightly from one manufacturer to the next.
It is important to program the controller for the specific makes to ensure correct temperature reading. The type J thermocouple is not recommended for use in applications beyond 760 C. Mainly because of the radical magnetic shift which can occur due to exposure to high operating temperature that may result in the alteration of the makeup, an occurrence referred to as recalibration. If you are unsure on how to correctly use the equipment, ask a professional to assist you.
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Author: Lenore BoltonThis author has published 2 articles so far.