Sunday, August 1, 2010

Infared -vs- Thermal Junction Temperature Sensing Part 2

Thermal Junction/Thermocouple Sensing

                Typical semiconductor based temperature sensing is based of current flowing through some sort of thermal junction. This junction is ideally two dissimilar metals, where the voltage will be proportional to the temperature of the environment, after taking into account time lag of heat absorption.

Voltage/Temperature Relationship

                Typically there is a nonlinear relationship, and the main limitation is accuracy. “System errors of less than one degree Celisius can be difficult to achieve.” The relationship between the temperature difference and output voltage of a thermocouple is derived from a comples summation of coeffeicients based on metal type, and results in a typically non-linear relationship.

K Type Thermocouples

Type K Thermocouples are the most common general purpose TCs used. They are made of a chromel-alumel junction with a sensitivity of approx. 41uV/ C. According to the Omega NIST reference, K type thermocouples have a maximum error of 2.2C with 0.5C being more typical.

Cold Junction Compensation (CJC)

Usually, to calibrate a thermocouple, a method similar to sound noise cancellation is used. An independent junction is maintained at a fixed temperature. More commonly a thermistor, or diode (like a PN where the current varied minutely with temperature) is used. Frequently as well, temperature sensors and a look up table can be used to extract the CJC temperature indirectly.
Some examples, and a thorough treatment of circuits for this purpose are included in the Maxim Application Note cited below “Implementing cold-Junction Compensation in Thermocouple Applications.”


                Thermocouples have a very non-linear relationship to temperature. they are versatile, and usable for measurements which will frequently cover a large range. They are more commonly used in industrial applications where such temperature variation is more common.
                 For more accurate measurements, it is common to use a resistance thermometer, which are commonly referred to as RTDs, and made of platinum. For applications under 600 degrees C they are slowly eroding the use of thermocouples due to dramatically improved accuracy and repeatability.

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