Thermopile Sensor Basics: Working Principle, Pros & Cons

This page provides a comprehensive overview of thermopile sensors, including their basic principles, working operation, advantages, and disadvantages.

What is a Thermopile Sensor?

Definition: Due to the very low voltage output of a single thermoelectric cell, multiple cells are often arranged in series or parallel to achieve a larger signal output. This arrangement of a “pile” of thermocouples is known as a thermopile.

Beyond just a pile of thermocouples, a thermopile sensor also utilizes an infrared absorber plate, as illustrated in Figure 1. Thermopiles are generally larger and thicker compared to individual thermocouples. They produce a higher voltage output, typically in the order of tens or hundreds of mV.

In essence, a thermopile sensor is a device that converts thermal energy into electrical energy.

Figure 1: Thermopile sensor structure

The voltage output of a thermopile in response to a temperature difference across the material is known as the thermoelectric coefficient or Seebeck coefficient. It’s measured in volts per kelvin (V/K) or mV/K.

Thermopile sensors rely on infrared radiation for heat transfer, making them suitable for non-contact temperature measurement. As a result, they are used in various applications such as infrared thermometers, industrial pyrometers, life-care devices, and temperature monitoring of moving objects.

Thermopile Sensor: Working Operation

Figure 2: Thermopile sensor working principle

Let’s delve into the working principle of a thermopile infrared sensor:

• A thermopile infrared (IR) sensor comprises multiple thermocouples connected in series. These thermocouples have “hot” junctions attached to a very thin IR absorber. This absorber is typically a micro-machined membrane on a silicon chip.

• The temperature of the absorber rises or falls based on the temperature difference between the absorber and the target object. This temperature change depends on the exchange of infrared radiation between the object and the absorber.

• To accurately measure the object’s temperature, it’s crucial to ensure that the object completely fills the sensor’s field of view. This ensures that the infrared radiation originates solely from the object of interest and not from its background.

• The performance of thermopile sensors can be significantly enhanced by using filters and lens components, as depicted in Figure 2. Silicon is commonly employed as a micro-machined lens due to its opacity to visible light but transparency to wavelengths longer than 2µm. This range is crucial because most emissions occur within it for temperatures below 200°C (or 450°F).

• Therefore, thermopile sensors are designed to measure the temperature of objects (moving or fixed) from a distance by detecting the infrared energy emitted by the object.