Thermopile

Table of Contents

“Thermopile is used to measure temperature difference. it convert thermal energy into electrical energy. it uses several thermocouples connected in series or parallel. Thermopiles are used for contact-less temperature sensing. The function of a thermopile is to transfer the heat radiation emitted from the object to a electrical voltage output. The output is in the range of tens or hundreds of milli-volts.”

Thermopile principle

Thermopile sensor is designed to measure temperature from a distance by detecting infrared (IR) energy of a hot object. The higher temperature of hot object emits the more IR energy. A diagram showing non-contact temperature measurement is shown as follows-

Here as a sensor to detect the temperature, Thermocouple is used in non-contact pyrometers and thermocouple generates thermo-emf (in mV) based on principle of seebeck effect. When the sensor is placed at distance not nearly hot object, the output of a single thermocouple will be quite small to detect. For this reason, instrument manufacturers offer a series-connected array of thermocouples called a thermopile to generate a stronger electrical signal (in range of 10 to 100mV). A conceptual diagram of sensor using thermocouples is given as follows-

Thermopile working principle

To understand thermopile, four E-type thermocouples (chromel and constantan) are used in this example and connected in series as such manner that all the ‘hot junctions’ voltages aid each other, as well as ‘cold junctions’ voltages. Like all thermocouple circuits, though, the each cold junction voltage opposes each the hot junction voltage. When hot junction of all thermocouples are towards hot object, sensing the same heat and cold junction of thermocouples are kept at same cold junction temperature, the final output will generate four times the potential difference than a single type E thermocouple. Thus, it act like multiplied thermocouple, generating more voltage than a single thermocouple junction could under the same temperature conditions.

Construction of a thermopile

Construction of a thermopile is showing in figure as follows-

The sensing element is composed of small thermocouples on a silicon chip which absorb the energy and produce an output signal. Here a reference sensor is designed into the package for compensation. Thermopile infrared (IR) temperature sensors measure non-contact temperature and are available with various lens, and filters allowing use in multiple applications, from industrial pyrometers, to climate controls and medical devices. There are two distinct types of thermopile detectors-

Thin Film Based Thermopile
Silicon Based Thermopile.

For Thin Film based thermopiles, the arm materials are antimony (Sb) and bismuth (Bi). In a Silicon based thermopile, the arm materials can be alternating n-type and p-type Poly-Silicon or n-type with gold (Au) or aluminum (Al). Thin Film based have lower resistance and lower noise voltage than a Silicon based; therefore Thin Film detectors provide a higher Signal-to-Noise ratio. A Thin Film type with equal output to that of a Silicon based will have a slower time constant. In general, Thin Film based are available with larger active areas.

Honeywell thermopile

Honeywell Thermopiles were older non-contact pyrometer manufacture and poplar design was radiamatic, using ten thermocouple junction pairs arrayed in a circle. In this all hot junctions of thermocouples (of thermopile) were placed at the center of this circle where the focal point of the concentrated light fell, while all the cold junctions were situated around the circumference of the circle away from the heat of the focal point.

Thermopile Application-

Hand-Held Non-Contact Temperature Measurements
Non-Contact Temperature Measurements in industrial applications and process control
Tympanic Thermometers
Infrared Radiometry
Refrigerant Leak Detection
Automotive Exhaust Gas Analysis of CO, CO2, & HC
Medical Gas Analysis including Anesthetic, incubator CO and CO2, Blood Alcohol Breathalyzers
Horizon sensors for aircraft, satellites, and hobbyist applications
Commercial building HVAC & lighting control
Thermal line scanners
Automotive HVAC control
Automotive Occupancy Sensing
Household appliance temperature measurement
Hazard detection including flame and explosion
Fire detection in transportation tunnels
Aircraft Flame and Fire Detection
Blood Glucose monitoring
Black ice detection and early warning
Security Human Presence and Detection