FHF04 is a sensor for general-purpose heat flux measurement. It is thin, flexible and versatile. FHF04 measures heat flux through the object in which it is incorporated or on which it is mounted, in W/m2. The sensor in FHF04 is a thermopile. This thermopile measures the temperature difference across FHF04’s flexible body. A type T thermocouple is integrated as well. The thermopile and thermocouple are passive sensors; they do not require power.
Multiple small thermal spreaders, which form a conductive layer covering the sensor, help reduce the thermal conductivity dependence of the measurement. With its incorporated spreaders, the sensitivity of FHF04 is independent of its environment. Many competing sensors do not have thermal spreaders. The passive guard area around the sensor reduces measurement errors due to edge effects and is also used for mounting.
Using FHF04 is easy. It can be connected directly to commonly used data logging systems. The heat flux in W/m² is calculated by dividing the FHF04 output, a small voltage, by the sensitivity. The sensitivity is provided with FHF04 on its certificate. Equipped with a metal connection block, which may serve as strain relief, and with potted protective covers on both sides so that moisture does not penetrate, FHF04 has proven to be very robust and stable.
FHF04 calibration is traceable to international standards. The factory calibration method follows the recommended practice of ASTM C1130 – 17. When used under conditions that differ from the calibration reference conditions, the FHF04 sensitivity to heat flux may be different than stated on its certificate. See the user manual for suggested solutions. See also our application note how to install a heat flux sensor.
Would you like to to study energy transport / heat flux in detail? Hukseflux helps taking your measurement to the next level: order FHF04 with radiation-absorbing black and radiation-reflecting gold stickers. You can then measure convective + radiative flux with one, and convective flux only with the other. Subtract the 2 measurements and you have radiative flux. BLK – GLD stickers can be applied by the user to the sensor. Optionally, they can be ordered pre-applied. See the BLK – GLD sticker series user manual and installation video for instructions.
FHF04 is a sensor for general-purpose heat flux measurement, often applied as part of a larger test- or measuring system. FHF04SC is a self-calibrating version of the standard model FHF04, combined with a heater. FHF04SC is used when the highest level of quality assurance is required and for long-term heat flux measurement. See the insightful video “FHF series explained” on our YouTube channel.
Building physics / insulation, thermal comfort and energy budget measurement
Heat flux measurement to validate CFD thermal simulations
Industrial monitoring and control / heat flux and heat transfer measurement
Meteorology / surface energy flux measurement
Scientific research / heat and heat transfer measurement
BLK – GLD stickers
Black and gold stickers for measuring convective and radiative heat flux separately
convective and / or radiative heat flux
|Measurand||heat flux, temperature|
|Measurement range||(-10 to +10) x 10³ W/m²|
|Sensitivity (nominal)||11 x 10⁻⁶ V/(W/m²)|
|Temperature sensor||type T thermocouple*|
|Rated bending radius||≥ 7.5 x 10⁻³ m|
|Rated load on cable||≤ 1.6 kg|
|Outer dimensions foil with guard||(50 x 50) x 10⁻³ m|
|Sensor thermal resistance||11 x 10⁻⁴ K/(W/m²)|
|Sensor thickness||0.4 x 10⁻³ m|
|Uncertainty of calibration||± 5 % (k = 2)|
|Rated temperature range – continuous use||-70 to +120 °C|
|Rated temperature range – short intervals||-160 to +150 °C**|
|IP protection class||IP67***|
|Standard wire length||2 m|
|Options||with 5 m wire length; without wiring, without metal connection block; BLK-5050 black sticker; GLD-5050 gold sticker
* temperature measurement uncertainty: 2 % of value in ˚C. For details, refer to user manual
** when measuring at temperatures of -160 °C , contact Hukseflux.
*** sensor is not suitable for continuous exposure to water
Frequently asked questions
Heat flux sensors measure energy flux onto or through a surface in [W/m²].
The source of the heat flux may be:
Convective and conductive heat transfer are associated with a temperature difference. Heat always flows from a source to a sink, from a hot to a cold environment. Convective and conductive heat flux is measured by letting this heat flow through the sensor. Radiative flux is measured using heat flux sensors with black absorbers. The absorbers converts radiative to conductive energy. Hukseflux started in 1993 with sensors for measurement of heat flux in soils and on walls. In the course of the years, we have added specialised sensors and systems for many other applications.
Heat flux sensors manufactured by Hukseflux are optimised for the demands of different applications:
- rated temperature range
- rated heat flux range
- response time
- chemical resistance, safety requirements
- size, shape and spectral properties
Hukseflux is the world market leader in heat flux measurement. We have prepared a white paper briefly explaining the fundamentals of measuring with heat flux sensors. It also offers general directions what to watch out for and some, perhaps surprising, applications of heat flux sensors. Take a look at our white papers.
There are quite a few general considerations when starting a heat flux measurement.
- Representativeness in time and space; average!
A heat flux sensor measures at a certain location. Is this location representative of what you need to measure? If possible, use a relatively large sensor, rather than a small one, and consider use of multiple sensors. Thermal processes often have large time constants; instantaneous measurements may be misleading. Average to get the full picture.
- Optical properties
When heat flux sensors also measure radiation, pay attention to the surface color. If needed paint the sensor surface. Please mind that shiny metallic surfaces reflect both infra-red and visible radiation. Paints may have different colors in the visible range, but are usually “black” absorbers in the far-infra-red.
- Sensor thermal resistance
A heat flux sensor distorts the local heat flux. In order to minimize this effect, use the sensor with the lowest possible thermal resistance.
- Edge effects
A heat flux sensor locally distorts the heat flow pattern, in particular around the edges of the sensor. A passive guard, i.e. a non-sensitive part around the sensor is essential to avoid errors due to edge effects.
There are more characteristics that matter. Please find them in our white paper on heat flux fundamentals and applications.
Hukseflux manufactures a range of sensors for surface energy flux measurements. All have proven reliability.
These state-of-the-art sensors are made for the global fluxnet community:
- NR01 is a market leading 4-component net radiometer.
- HFP01 and HFP01SC measure soil heat flux.
- STP01 offers an accurate temperature profile measurement.
- TP01 is the leading sensor for soil thermal conductivity.
Sensors made by Hukseflux are designed for compatibility with most common datalogger models. For many models we have example programs and wiring diagrams available.
On-site measurements of thermal resistance, R, are often applied in studies of buildings. Alternatives are to measure its inverse value, the thermal conductance which is called the Λ-value, or the thermal transmittance which includes ambient air boundary layer thermal resistance, the U-value. The measurements of R are based on simultaneous time averaged measurement of heat flux Φ and differential temperature, ΔT, (using two temperature sensors on each on a different side of the wall).
R = ΔT / Φ
Hukseflux provides a range of sensors and measuring systems for use in measurement of the energy budget of buildings and characterization of construction materials.
HFP01 heat flux sensor and TRSYS01 measuring system are widely used for on-site measurements on walls, windows and other construction elements in building physics.
- HFP01 can be used for in-situ measurement of building envelope thermal resistance (R-value) and thermal transmittance (H-value) according to ISO 9869, ASTM C1046 and ASTM 1155 standards. HFP01 is the world’s most popular sensor for heat flux measurement in the soil as well as through walls and building envelopes. HFP01 measures heat flux through the object in which it is incorporated or on which it is mounted, in W/m². More information? Visit the HFP01 product page.
- TRSYS01 is a high-accuracy system for on-site measurement of thermal resistance, R, thermal conductance, the Λ-value, and thermal transmittance, the U-value, of building envelopes. TRSYS01 is mostly used for measurements according to standard practices of ISO 9869 and ASTM C1155 / C1046. The system is equipped with high-accuracy electronics, two heat flux sensors of model HFP01 as well as two pairs of matched thermocouples. The two measurement locations provide redundancy, leading to a high level of confidence in the measurement result. The high accuracy of the heat flux sensors and temperature difference measurements ensures that TRSYS01 continues measuring when other systems no longer perform; in particular at very low temperature differences across the wall.
Hukseflux, market leader in heat flux measurement, offers both sensors and systems.
These measuring systems typically include a Measurement and Control Unit and one or more sensors for measuring heat flux as well as other measurands, such as temperature and humidity. Examples are the TCOMSYS01 Hot Cube thermal comfort measuring system, including a TCOM01 sensor, and the TRSYS01 measuring system, incorporating two HFP01 heat flux sensors and two pairs of matched thermocouples.
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