thermopile

A thermopile is a device made by connecting multiple thermocouples in series, so that the small voltages they generate add together into a measurable signal. Each thermocouple junction produces only millivolts when exposed to a temperature difference, but stacking 50, 100, or more junctions in series can yield output in the range of 5 to 50 millivolts, making the signal practical to amplify and measure with standard electronics.

The two dissimilar metals in each thermocouple (typically bismuth and antimony for infrared sensors) are arranged so that one set of junctions sits at the hot source and the other at a reference temperature, often a heat sink or the device housing. As temperature rises at the hot junction, the Seebeck effect drives electrons differently through each metal, and the voltage difference accumulates across all junctions in series. This stacking is the key difference from a single thermocouple; where one junction might produce 40 microvolts per degree Celsius, 100 junctions produce 4 millivolts per degree.

Infrared thermopiles are the most common industrial variant. The hot junctions are coated black and sit behind a window lens or dome; infrared radiation from a distant surface heats them without physical contact. The device measures object temperature from a distance, making it useful in furnace monitoring, food processing, and nondestructive testing. Some models incorporate a thermistor nearby to measure ambient temperature and compensate for it automatically.

Thermopiles appear in pyrometers, thermal imagers, and gas analysers. Specifications typically list the thermopile resistance (hundreds of ohms to a few kilohms), the sensitivity in microvolts per kelvin, and the time constant (how fast the device responds to temperature changes). The signal is small enough that thermal noise and lead resistance matter; shielded cable and low-noise amplifiers are standard practice to avoid false readings.

The main failure mode is loss of vacuum or partial pressure inside the sealed envelope. Most commercial thermopiles are evacuated to around 0.01 Pa to minimize convective heat loss from the hot junctions; if the seal degrades, the device becomes sluggish and unreliable. Mechanical shock and thermal shock can also crack the substrate, and extended exposure to very high temperatures can alter the metal properties and drift the calibration.