TP01 is the leading sensor for long-term monitoring of soil thermal conductivity. A measurement with TP01 may also be used to estimate soil thermal diffusivity and volumic heat capacity, leading to a better understanding of dynamic (variable heat flux) thermal behaviour of soils. TP01 is designed for long-term use at one measurement location. Applied in meteorological surface flux measurement systems, TP01 improves the estimates of heat transport in the soil and of the so-called storage term.

  • Leading sensor for long-term monitoring of soil thermal conductivity
  • Long-term use at one measurement location
  • Record breaking sensitivity



Measurand Thermal conductivity
Optional non-traceable measurands Thermal diffusivity and volumic heat capacity
Optional trend monitoring Soil water content
Rated operating environment Surrounded by soil
Rated measurement range 0.3 to 5 W/(m•K)
Rated operating temperature range -30 to +80 °C
Sensor foil thickness 0.15 x 10-3 m
Interval between measurements 6 hr (typical), optionally 3 to 12 hr
Duration of measurement 360 s
Heating interval duration 180 s
Heater rated power supply 1 to 2 VDC, < 0.4 A
Power consumption daily average 0.007 W
Standard cable length 5 m
Required readout and control 2 x differential voltage, 1 x switchable power
Required uncertainty (k = 2) 10 x 10-6 V at 10-3 V
5 x 10-3 V at 2 V
Order code TP01/cable length in m


TP01 is the leading sensor for long-term monitoring of soil thermal conductivity.
TP01 measures soil thermal conductivity. It is designed for long-term on-site operation, buried in the soil. Its rated operating range is 0.3 to 4 W/(m∙K), which covers most inorganic soil types. The sensor inside TP01 is a temperature difference sensor consisting of 2 thermopiles. It measures the radial temperature difference around a heating wire with a record breaking sensitivity. Both the heating wire and the sensor are incorporated in a very thin plastic foil. It can be connected directly to commonly used data logging systems. The low thermal mass of TP01 also makes it suitable for measuring the thermal diffusivity.

Suggested Use
A typical TP01 is part of a meteorological surface flux measurement system in which also wind, humidity, soil heat flux, soil temperatures at different depths and net radiation are measured. TP01 then serves to improve the estimate of the so-called storage term, which is used to model thermal heat transport in the soil. Measurements with TP01 are often combined with soil temperature profile measurements with sensor model STP01 and measurements with heat flux sensor model HFP01SC. Soil thermal properties change as a function of depth, in particular close to the soil surface. A typical measurement location is equipped with sensors at several depths. For good spatial averaging at least 2 sensors (> 5 m apart) should be installed at every depth. Hukseflux has equipped several testbeds in the electrical power industry, to monitor dryout, thermal runaway and thermal stability around  mockup high-voltage power lines. Here the capability to perform a crude measurement of  thermal diffusivity is an important feature, for modelling behavior under dynamic loads.

Areas of Application

Material characterization / thermal conductivity measurement
Meteorology / surface energy flux measurement
Scientific research / heat and heat transfer measurement

Frequently asked questions

How to measure heat flux?

Heat flux sensors measure energy flux onto or through a surface in [W/m²].
The source of the heat flux may be:

  • conduction
  • radiation
  • convection

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
  • sensitivity
  • 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.

What matters most when measuring with a heat flux sensor?

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.

Which sensor(s) to use for surface energy flux measurement?

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.

How to measure R-value and U-value of buildings?

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.

Where can I find complete heat flux measuring systems?

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.

Cannot find what you are looking for? Please contact us.