Thermocouple Tidbits

Fig. 1 A schematic drawing of a thermocouple.
Thermocouples are the most common type of temperature sensor used and nearly 16% of all process instrumentation measures, indicates, or controls temperature. Thomas Johann Seebeck is credited for inventing the thermocouple in 1821. His experiment consisted of two dissimilar metal wires joined at the ends to form a loop with each end held at a different temperature (see Fig. 1). Seebeck detected the induced current by the displacement of a compass needle that was near one of the wires. Further study, revealed that the temperature gradient induced an electric current and when this circuit was broken at the center, an open circuit voltage was measured, i.e. the Seebeck electromotive force. The thermocouple is based upon the concept that for small changes in temperature (T(hot)-T(cold)), the voltage is proportional to the temperature difference.

Most modern thermocouple readers are temperature compensated and do not require an ice-water cold junction. A simple test to determine if your instrument is functioning correctly, is to short the instrument terminals with an electrical conductor, e.g. copper. The thermocouple reader should read ambient temperature. Deviations from ambient temperature indicate that the instrument needs repair or calibration.

Operating environment and temperature are important considerations for picking the correct thermocouple. Table I provides some practical guidelines for selection. Chromium-containing nickel alloys should not be used in reducing atmospheres since a condition of "green rot" will remove the chromium from the alloy and will cause temperature drift. Green rot occurs in atmospheres that are oxidizing to chromium, but reducing to nickel, or in hydrogen atmospheres with high dew points. Also, platinum alloys should not be used in reducing atmospheres since excessive grain growth will occur and change the thermocouple calibration. Although chromel-alumel (K-type) thermocouples can be used up to 1260°C, they are subject to aging and temperature drift when cycled through 980°C (1800°F).

When using a thermocouple, it is very important to understand that the measured voltage is developed along the entire length of the thermocouple. Steep temperature gradients should be avoided since any defect in the wire within the gradient will contribute a large error. Steep gradients may also induce recrystallization and grain growth, thus changing the calibration. In this regard, feeding thermocouples through insulation is critically important since deformation of the wires may produce recrystallization during operation.

Fig. 2 Radiation error in the measured temperature associated with varying wire gauge diameters. Gauge numbers are shown next to the data. Measurements conducted at 1093¯C (2000¯F), 1 atmosphere, and mach 0.3 gas velocity.
Perhaps the most serious error for thermocouples is from radiation loss at high temperatures. Fig. 2 shows the radiation error associated with bare metal thermocouples at 1093°C (2000°F). For example, a 16 gauge wire will have an error of 127°C (260°F). Larger radiation losses occur when the thermocouples are sheathed. Smaller gauge thermocouples have the advantage that less radiation loss occurs; however, environmental degradation may require a heavier gauge. Large thermocouples used at elevated temperature should always be calibrated by comparison methods, e.g. optical pyrometer. IH