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Heat Flux Measurement

A heat flux sensor is a commonly used name for a transducer generating a signal that is proportional to the local heat flux. This heat flux can have different origins; in principle convective-, radiative- as well as conductive heat can be measured. Some heat flux sensors are plates meant to be mounted on a surface with the obvious purpose of measuring the local conductive heat flux. Other heat flux sensors are actually radiometers having the purpose to measure radiative heat flux. Of those radiometers some are called heat flux sensors because the radiation is the dominant flux. (convection is negligible).

Heat flux sensors are known under different names, such as heat flux transducers, heat flux gauges, heat flux plates. In addition there are several instruments that actually are single-purpose heat flux sensors like pyranometers (for solar radiation measurement) and Schmidt Boelter gauges and Gardon Gauges (for measurement of heat flux from fire). In SI units heat flux is measured in Watts per square meter.

Usage

Heat flux sensors are used for a variety of applications. Common applications are studies of building envelope thermal resistance, studies of the effect of fire and flames or laser power measurements. More exotic applications include estimation of fouling on boiler surfaces, temperature measurement of moving foil material, oven profiling etc.

The total heat flux is composed of a conductive, convective and radiative part. Depending on the application, one might want to measure all three of these quantities or single one out. An example of measurement of conductive heat flux is a heat flux plate incorporated into a wall.

An example of measurement of radiative heat flux is a pyranometer for measurement of solar radiation.

An example of a sensor sensitive to radiative as well as convective heat flux is a Gardon or Schmidt Boelter gauge, used for studies of fire and flames.

There are various examples of sensors that internally use heat flux sensors examples are laser power meters, pyranometers etc.

We will discuss three large fields of application in what follows.

Applications in meteorology and agriculture

Soil heat flux is a most important parameter in agro-meteorological studies, since it allows one to study the amount of energy stored in the soil as a function of time.

Typically two or three sensors are buried in the ground around a meteorological station at a depth of around 4 cm below the surface. The problems that are encountered in soil are threefold:

• First is the fact that the thermal properties of the soil are constantly changing by absorption and subsequent evaporation of water.
• Secondly the flow of water through the soil also represents a flow of energy, going together with a thermal shock, which often is misinterpreted by conventional sensors.
• The third aspect of soil is that by the constant process of wetting and drying and by the animals living on the soil, the quality of the contact between sensor and soil is not known.

The result of all this is the quality of the data in soil heat flux measurement is not under control; the measurement of soil heat flux is considered to be extremely difficult.

Applications in building physics

In a world ever more concerned with saving energy, studying the thermal properties of our buildings has become a growing field of interest. One of the starting points in these studies is the mounting of heat flux sensors on walls in existing buildings or structures built especially for this type of research.

The measurement of heat flux in walls is comparable to that in soil in many respects. Two major differences however are the fact that the thermal properties of a wall generally do not change and that it is not always possible to insert the heat flux sensor in the wall, so that it has to be mounted on top of the wall. When the heat flux sensor has to be mounted on top of the wall, one has to take care that the added thermal resistance is not too large. Also the spectral properties should be matching those of the wall as closely as possible. If the sensor is exposed to solar radiation, this is especially important. In this case one should consider painting the sensor in the same color as the wall. Also in walls the use of self-calibrating heat flux sensors like model HFP01SC should be considered.

Applications in fire testing and oven profiling

In fire testing a heat flux sensor is used to verify the intensity of flames, so that the exposure of test specimens is standardised. Heat flux sensors like SBG01 are in fact unshielded broadband radiometers. They are typically water-cooled because the heat fluxes are so high that the sensors would overheat if not cooled.

In oven heat flux profiling the purpose is to get a fingerprint of the heat flux, air temperature and local air speed. These can all be characterised by using a combination of radiative and total heat flux as well as local air temperature with a sensor like model RC01.

Applications solar- , inra red- and net radiation measurement

Heat flux sensors can be combined with window / filter materials like glas and silicon to construct broadband radiometers for measurement of solar and infra red radiation. The resulting sensors are called pyranometers, for solar radiation, pyrgeometers for infra-red radiation and net radiometers (combined instrument, also looking two directions).