Hi Friends, first and foremost sorry for my absent for few weeks as I was busy with my office work. Last week I was working on Thermocouple interfacing with Cortex M4, so I have collected a lot of data regarding it. I am sharing some of the basic information with you.
A thermocouple is a temperature sensor that is used to gauge temperatures in manufacturing, machining and scientific applications, as well as everyday appliances. A thermocouple is formed when two dissimilar metals are joined together. It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges.
Thermocouple wire is available in several different metallurgical formulations per type, typically, in decreasing levels of accuracy and cost: “special limits of error”, “standard”, and “extension” grades.
The worst part of thermocouple in using them is that the voltage to be measured is very small, with changes of about 50 uV per °C (a uV is 1/1000000 Volts). While it is possible to read these voltages using a clean power supply and nice op-amps, there are other complications such as a non-linear response (its not always 50uV/°C) and cold-temperature compensation (the effect measured is only a differential and there must be a reference, just as ground is a reference for voltage).
I suggest using an interface chip (thermocouple amplifiers) that will do the heavy lifting for you, allow you to easily integrate the sensor without as much pain. In next part of this tutorial we will use a MAX6675 and MAX31855 (Original version was MAX6675) thermocouple interface chip which doesn’t even require an ADC, spitting out a nice digital data signal of the temperature. The only issue with these thermocouple amplifiers is that they are not compatible with grounded thermocouples.
Connecting a thermocouple to circuit is not a difficult task, as they are properly color coded. The color codings are different from country to country, so it is best to look up the different color coding based on the country from which the material is sourced.
Types of Thermocouple
Calibration types have been established by the American Society for Testing and Materials (ASTM) with reference to their temperature versus EMF characteristics in accordance with ITS-90, in standard or special tolerances.
These are four different classifications of thermocouple pairings, most distinguished by a capital letter heading. These are the home body class, the upper crust class, the rarified class and the exotic class. The home body class consists of “standard” or commonly used metals, while the upper crust class represents all platinum combinations. The rarified class consists of refractory metals and the exotic class is much more specific in nature, usually special combinations of rare metals used for specified applications. Little description about types is a below:
Next part of this tutorial we would be interfacing thermocouple with Cortex M3 ARM controller…don’t forget click like on our facebook page ….thanks!
Few Reference Web links: