Definitions

 

1. THERMOCOUPLE:  A Thermocouple is a device capable of measuring Temperature, as developed by Physicists, and always has the following properties:
   
1. It is a junction, of two dis-similar metals, or alloys, of known types, such that when joined, (twisted, &/or welded), a small EMF(ElectroMotive Force) is generated at the junction. This EMF is measured as a Millivoltage-DC, or rarely as a Microvoltage-DC. A laymans example of this that my be felt by anyone who has metal-fillings in teeth. If you ever had say a potato, cooked inside of "Aluminum, or Tin-Foil", where a small piece of this foil contacted a metal filling, you will receive a small electrical shock to the nerve under that tooth! This is technically known as "Galvanic Action". This is usually painful, and you probably will never forget that it happened, but did not know why! At least you will want to be careful not to have that happen again!

2. This millivoltage will vary, up, or down in value, as the temperature increases or decreases.

3. In order for this measured DC-millivoltage to be meaningful, it must be referenced against a known value of DC-millivoltage, in what is known as a "Bridge-Circuit". How this works may be described as "ratio & proportion", but must be discussed later, in another lesson. This is only to give basics. For more detailed info, go to www.pyromation.com and search through the Index, until you find what you want answered.

4. A Thermocouple is an "ACTIVE" device because it generates it's own millivoltage, (mV), independent of any other voltages in the circuit from other sources.

5. An error signal could occur, adding or subtracting from the generated mV, caused by a difference in temperature between the wires, if a "Reference Junction" is not used. A reference junction cancels the error voltage by referencing it with an ice-bottle at 32-deg.F, or 0-deg.C. Modern Reference Junctions today are not done with Ice bottles, but now are small component devices, that accomplish the same thing, where the T/C signal enters an instrument. Again, more on this may be researched elsewhere, and at www.pyromation.com.

6. To display the temperature from the T/C, an analog Pyrometer, or Digital Display Meter, of various types is used, taking the now known millivoltage, and displaying it using the correct T/C Curve Value of Temperature.

7. Physicists have developed "Calibration-Curves" that are non-linear by nature, of Millivolts vs Temperature, (using a graph having X, and Y-Axis's) These "Curves" are designated by Letters, of the Alphabet, such as "J, K, E, T" Etc. These curves are different, depending on Range of Temperatures, in their most-close to linear portion of each curve. Tables of values are shown at www.pyromation.com

 

Consult other sources for more technical info on this subject. The words above are not taken from any other copywrited material. They are words combined to phrases by Don Eberhardt, of KDE Instrumentation, strictly from his 46-years experience at explaining Temperature Measurement info, to people who often are not familiar with these details.

 

2. "RTD" (Resistance Temperature Device)
   In comparison to a T/C, An RTD, ("Resistance Temperature Device) is a passive device, that also measures Temperature. It is passive, because it must have a small DC Current passed through it from another source in it's circuit. RTD's are usually made from either a thin finely wound platinum wire, (in our Industrial Use Industry) for example, or a "Thin-Film" material described at www.pyromation.com , whereas the Resistance* of the wire or material changes* up, or down, with temperature applied to the device. Using Ohm's Law:  {"I";(DC current) = "E"(DC voltage) / "R";(Resistance in Ohms)}, where R-is the variable*, a chart of resistance vs Temperature may be made, and the result displayed on a digital meter device. Samples of these Tables may be found at the "RTD-Section" Index Tab, at www.pyromation.com