Why measure heat flux in addition to temperature: advantages and techniques

greenTEG AG: Many developers and engineers are not familiar with heat flux as a physical measurand. Unlike temperature measurements, the measurement of heat flows is therefore often neglected or not taken into consideration. This is unfortunate since heat flux is not merely a different way to look at temperature, but it is a physical quantity of its own. In fact, heat flux may turn out to be exactly the kind of thermal phenomenon you always wanted to measure but were not able to quantify with thermometers. Below you will learn how heat flux compares to temperature, what the advantages of heat flux measurements are, and how they are conducted.

Heat flux is about energy

To better understand heat flux, let’s first take a look at its unit: Watt per square meter (W/m2). This is a basic illustration of what heat flux is: thermal power crossing a surface. Heat flux measurements allow you to quantify the energetic flows in your system. These may be completely unrelated to temperature. Let’s assume you want to estimate the calories consumption of an athlete at rest and during training. Since the body regulates temperature, a temperature measurement is pointless. A heat flux measurement will however show that the amount of heat energy leaving the body increases up to ten fold under intense physical activity. If you want to quantify the heat energy that gets lost through the window glasses or through a wall in your house, a temperature measurement alone is not sufficient. A heat flux measurement on the other hand allows you to directly quantify the heat losses through a given surface.

Advantages of heat flux measurements

Heat flux measurements, especially when combined with temperature measurements, allow you to gather precious additional information about your system. For instance, heat flux measurements allow the measurement of all kind of radiative energies as well as the dynamic and non-invasive prediction of internal temperatures.

Non-invasive

By measuring the heat flows leaving your system, you can deduce the thermal power released inside the system. This allows you to draw conclusions about what is happening inside your system without invading it with a probe.

Predictive measurements

Heat flux measurements tell you whether energy is entering or leaving your system. In other words, they tell you whether you may expect a rise or fall in temperature. This allows you to react to outer thermal perturbations almost instantaneously, before a temperature probe would detect a change. This is a big advantage for the thermostatic regulation of sensitive measurement equipment or to compensate for thermal expansion effects in micromechanical machinery.

Measure thermal radiation

A radiator is warm when you touch it. It has a surface temperature you can measure with a thermometer. But what about sunlight? It is warm, too. But what is the “temperature” of a sun beam? This is something you cannot measure with a thermometer and why sunlight is measured in power per incident area instead of degrees. And yes, you can measure the power of the solar radiation simply by exposing a coated heat flux meter to the sun.

How to measure heat flux: measurement techniques

You can measure heat flux indirectly with two temperature sensors or directly with a heat flux sensor. Let’s have a look at these techniques.

The hard way: indirect measurement with temperature sensors

The indirect heat flux measurement with temperature sensors uses the Indirecttemperature difference across a thermal resistance to derive the heat flux. This technique has several disadvantages: it is either inaccurate or very thermally invasive. Furthermore, it is very difficult to define the thermal resistance between A and B, since it depends on the component’s material, shape, size, and distance between A and B.

The easy way: direct measurement with heat flux sensors

Direct heat flux measurements rely on one heat flux sensor (HFS). The heat flux direct hfssensor is mounted on the position of interest where it measures the heat passing through the sensor surface. The main advantage of this technique is that it is simple and has a very high heat flux resolution and low invasiveness. With a gSKIN® Heat Flux Sensor you will typically resolve heat fluxes of < 0.1 W/m2.

Contact us if you need further information or if you would like to discuss your application.