Fire Detectors – Thermal Detectors

Fire Detectors - Thermal Detectors

Thermal flame detectors are high tech devices that can recognize flame and trigger an alarm within seconds. They can be used in coal conveyors, recycling plants, bunkers, rubbish dumps, refineries and so on.

In this article, we are going to be taking a deep dive into thermal flame detectors with a view to learning how they work, the various types that are available, how thermal cameras are presently being used in monitoring flames and a comparison between thermal and Quantum detectors.

What are thermal detectors

Thermal flame detectors are sensory devices that have an absorption layer that can convert light into heat and provide a signal output that shows the rapid or gradual change in absorption layer temperature. Since thermal flame detectors don’t need wavelength to work, they can serve as infrared detectors and can be used with special window materials that can transmit infrared light.

The working principle of a thermal flame detector

The thermal flame detector can be programmed to operate at a rate of rising temperature (R-O-R) or a fixed temperature. When there is a rapid increase in temperature, the special bimetal that is embedded in thermal detectors begins to curve. This, in turn, triggers an alarm or a fire suppression system.

Thermal flame detectors are likely the least sensitive detectors, especially when compared to others. They have a very high thermal inertia. This means that a fire has to be large or produce a lot of heat before the detector will trigger an alarm. Because of this, they are used in spaces like galleries, laundries, drying rooms and other areas where other flame detectors are susceptible to false alarms from smoke or water vapor.

The bimetallic strip in some new models of thermal flame detectors has been replaced by a thermostat. Thermostats are solid state devices like cobalt and manganese whose electrical resistance changes when there is an increase or a decrease in temperature. The working principles of the older and new models are quite similar.

Types of thermal flame detectors

The two main types of thermal detectors are:

  • Thermopile detectors
  • Bolometer detectors

Thermopile thermal detectors

Thermopile thermal detectors use the See beck effect to detect flames. Thermopile detectors do not depend on wavelength to work. So, they can be used with various types of different window materials for diverse applications.


  • They are designed to operate at 25-27 degree Celsius or room temperature.
  • They have zero dependence on wavelength.
  • They are long lasting and are fairly expensive.

Bolometer thermal detectors

Bolometers are small infrared sensors that do not need cooling to operate. When a beam of infrared light enters a bolometer detector the resistance gradually heats up, causing a change in resistance which when converted can trigger an alarm or a fire suppression system.

Operating principle of bolometer flame detectors

Bolometer thermal detectors have a special membrane that absorbs infrared light. The structure has two long beams that are called “legs” that provide support to the membrane. The membrane is created by a layer of floats and etching that is 2 micrometers from the substrate. When infrared from a light source or flame enters the sensitive membrane of the device its temperature increases. This, in turn, leads to a decrease in the resistance of the bolometer.

How thermal cameras are being used in monitoring flames

Thermal imaging detection devices are used for recognition of spontaneous fires inside rubbish dumps, bunkers, cardboard recycling and so on. Thermal detection devices are essential in these areas because there is a high possibility of spontaneous combustion occurring in these areas.

Thermal imaging systems use high tech infrared cameras and a software package to analyze images to detect hot spot. When a flame is detected, the device will trigger an alarm or an automatic foam or water monitor control.

Thermal vs. Quantum detectors

Thermal energy detection equipment comes in various levels of sophistication and forms. They can be categorized into quantum and thermal detectors. The distinction between them is that the thermal detectors depend on a two-stage process. The absorption of thermal energy in these detectors leads to a rapid rise in the temperature of the device, which in turn alters the temperature-dependent parameters like the electrical conductivity of the device. Quantum or photon, on the other hand, is much more sensitive but they require constant cooling to operate properly.

Thermal detectors

Thermal fire detectors included pyroelectric devices, heat sensitive devices, and thermoelectric devices. Heat sensitive coating ranges from substances that are blended to melt at a fixed temperature to special paints and that change color when there is a rapid or gradual change in temperature. While heat sensitive coatings are inexpensive or cheap, they cannot provide reliable qualitative data.

Thermoelectric devices include bolometer, thermopiles, and thermocouples. These devices provide a powerful electrical response based on the rate of change of ambient temperature. They are commonly used for point measurement in a near contact or contact mode. One amazing thing about thermal sensors is that they can be miniaturized.

For example, micro-bolometer play an active role in some advanced imaging system like those used by the fire department. The obvious benefit of thermal detectors is that they don’t require cooling to function well and they are budget-friendly especially when compared to other detectors.

Application of thermal detectors

Thermal detectors are used to accurately measure the temperature of everything from industrial furnaces to home appliance and fire detection devices to rockets.

Final note

Thermal flame detectors are high tech device that can trigger an alarm when the temperature in an environment reaches a critical level. Before buying and installing them in your facility, it is wise that you learn about the various types that are available and compare them with Quantum detectors. Doing this will put you in a better position to narrow down the right one that will suit your needs.

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