Thermocouples, devices used to measure temperature, are composed of two dissimilar metals that produce a small voltage when joined together—one end of a thermocouple joins each metal. A thermocouple thermometer then reads voltage produced. Thermocouples can be manufactured from a range of metals and typically register temperatures between 200 and 2,600 degrees Celsius (C). Depending on the types of metals in the thermocouple, the specific temperature range will vary.
Types of Thermocouples
Considering the temperature range, the chemical resistance of the thermocouple metals, and overall vibration and abrasion resistance can help when selecting a thermocouple. There are four common calibrations of thermocouples, J, K, T and E, each of which is suitable for a different temperature range. Sheathed thermocouples are available in three different junctions: grounded, ungrounded and exposed. Grounded junctions feature wire junctions that are attached to the inner probe wall, enabling effective heat transfer from the outside of the probe wall to the junction. Ungrounded probes feature unattached wire junctions, which promotes electrical isolation. Exposed junctions feature a junction that extends beyond the sheath, enabling a quick response time but limiting their use to non-corrosive and non-pressurized environments.
In order to achieve accurate readings from a thermocouple, it’s essential to calibrate the device accordingly. Typically, thermocouples are standardized by using 0 degrees C as a reference point, and many devices can adjust to compensate for the varying temperatures at thermocouple junctions.
To calibrate a thermocouple, various types of measuring equipment, standards, and procedures must be in place. First, a control temperature must be established that is stable and provides a constant temperature; it must be uniform and cover a large enough area that the thermocouple can adequately be inserted into it (such as an ice bath). Sources of controlled temperatures are called fixed points. A fixed point cell is composed of a metal sample within a graphite crucible, with a graphite thermometer submerged in the metal sample. When this metal sample reaches the freezing point, it maintains a very stable temperature. The freezing point occurs when a material reaches the point between the solid and liquid phase. A reference junction temperature must also be established; typically, 0 degrees C is used. A measuring instrument, such as Fluke 702 calibrator, can be used to measure thermocouple output.
A simple calibration process can be done by following a few basic instructions. (For do-it yourself enthusiasts, be sure to check with a professional before calibrating a thermocouple.) A basic calibration process involves heating water to 30 degrees C in a thermo bath. Next, the thermocouple is turned on and each of two multimeter leads are attached to one end of the thermocouple—at this point, the multimeter should register one microvolt. One junction of the thermocouple is then placed into the thermo bath. The voltage can be recorded once the multimeter reading becomes stable. The water temperature is increased to 35 degrees C, and again the voltage is recorded. This process is repeated by increasing the temperature by five degree increments and recording the voltage, until 60 degrees C is reached.
Once all the measurements have been taken, the voltage for the thermocouple type at the room’s temperature is determined. (This figure can usually be looked up.) Then the given figure is added to each of the recorded voltage values gathered previously. Next, a curve-fitting method is used to fit a line to the recorded data—the slope of the line will be the voltage increase per each degree of temperature increase.