What is heat flux?

Heat flux is the thermal energy flow that occurs whenever there is a difference in temperature between two objects. A heat flux sensor measures the flow of thermal energy. It is important to realise that a heat flux sensor is not a temperature sensor. A heat flux sensor is measuring energy (in Watts over an area), a temperature sensor is measuring the thermal motion of the object (in Kelvin, Celsius or Fahrenheit). The temperature is a result of the thermal energy. So, by using a heat flux sensor to measure the energy flow, it is possible to determine if an object is getting hotter or colder, and at what rate. So, a heat flux sensor can predict where the temperature of a system will end up.

Example of heat flux measurement

Consider an unpowered electric fire in a room. The electric fire will consume power when turned on. However, the room temperature remains unchanged at the instant that power is applied. The electric fire will continue to consume power at a fixed rate while switched on, and the room temperature will rise. Eventually, the room temperature will reach a maximum. At this point, the heat energy output by the electric fire is balanced by the heat energy loss through the walls of the room.

A heat flux sensor placed in front of the electric fire will measure the heat energy flow from the fire. Zero heat flux will be measured while the electric fire is switched off. When the electric fire is switched on, the heat flux sensor will indicate the heat flux flow from the electric fire to the room. Similarly, a heat flux sensor placed on the wall will indicate the energy flow wall. By measuring the thermal energy going into and out of the room, the direction and speed of the temperature change in the room can be estimate. This is something that a temperature sensor cannot do.

Types of heat transfer

There are 3 types of heat transfer:

Radiation: Net radiative heat exchange between two surfaces
Convection: Convective transfer from a solid surface to a moving fluid or gas
Conduction: Conducted heat transfer through a solid, or a stationary liquid

The sensors that Siretta offer, and their sensitivity to the different modes of heat flux are listed below:

 Model  Technology  Radiation  Convection  Conduction
Heat flux sensor models
 TG1000  Thermogage  Yes  Yes  No
 TG2000  Thermogage  Yes  Yes  No
 TG9000  Thermogage  Yes  No  No
 TGH2000  Thermogage  Yes  Yes  No
 HFM-6  Microsensor  Yes  Yes  Yes
 HFM-8  Microsensor  Yes  Yes  Yes

In addition, there are a number of amplifiers available to provide signal conditioning to these heat flux sensors. These amplify the output voltages to increase the sensitivity of the measurement system.