Heat flux and temperature measurements compared

Heat flux: a neglected quantity

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 even taken into consideration.

Heat flux is not merely a different way to look at temperature, but a physical quantity of its own. It may turn out to be exactly the kind of thermal phenomenon you always wanted to measure but were not able to quantify with thermometers.

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 however will show that the amount of heat energy leaving the body increases up to 10 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 of no use. A heat flux measurement on the other hand allows you to directly quantify the heat losses through a given surface.

How do I profit form a heat flux measurement?

Below a short collection of reasons why you should consider heat flux measurements

• 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 the need for putting a probe inside. The advantages of this approach are obvious, if your system happens to be the human body.
• 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, way 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.
• Radiative measurements
  A radiator is warm when you touch it. It has a surface temperature you can measure with a thermometer. And 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. This is the reason why sunlight is not measured in degrees but in power per incident area. And yes, you can measure the power of the solar radiation simply by exposing a coated flux meter to the sun.

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