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Enabling better global research outcomes in soil, plant & environmental monitoring.

Hukseflux FTN02

The FTN02 Field thermal needle system performs accurate on-site (field) measurements of the thermal resistivity and thermal conductivity of soils. Measurements with FTN02 comply with ASTM D5334 and IEEE 442 standards. The system is specifically designed for surveys that require measurements from the surface down to a depth of 1.5 m. Due to the long and thin lance to which the sensor is mounted, the insertion of the thermal needle into the soil requires just a small-diameter access hole, typically made using a ground drill. Using a lance rather than digging trenches saves time. FTN02 is operated and powered from the hand-held control and readout unit CRU02.


FTN02 is a thermal needle measuring system for on-site measurements of thermal conductivity (or the inverse value, resistivity) at depths from the surface down to a depth of 1.5 metre. Due to its robustness and length, FTN02 is the best solution for route-surveying of high-voltage electric power cables and heated pipelines (typical depth of burial of 1.5 m). The measurement method is based on the use of a “thermal needle”.  This method employs a heating wire and a temperature sensor in a needle. The FTN02 system consists of the thermal needle, model TP09, mounted on a long lance, LN02, and a control and readout unit CRU02. FTN02 is easy to use. After making a small-diameter access hole, the thermal needle TP09 is brought down to just above the point that must be investigated and then pushed into the local undisturbed soil below. The user performs control and readout of the measurement from the handheld CRU02. The measurement result is generated immediately by the CRU02 from the analysis of the time series of the temperature and the heating power during the heating interval.

FTN02 design and benefits

Suitable for on-site (field) surveys: FTN02 is sufficiently robust to survive rough handling during a typical survey. It performs measurements as a stand-alone unit. The batteries in the CRU02 can be recharged through the cigarette-lighter receptacle in an automobile, using the CA02 car adapter, or from 220/110 VAC using the WSA02 wall socket adapter.

Saving time: With FTN02 there is no need for digging a large-diameter access hole to perform measurements. Because of the lance LN02, drilling a small access hole made with a ground drill, typically 30 mm in diameter, is sufficient. With measurement systems without a lance, such as MTN02, the user has to make a large-diameter access hole or even to dig a trench. A typical measurement requires around 60 s to get to thermal equilibrium plus 300 s for the actual heating interval.

Easy data processing: CRU02 gives an immediate indication of the end result and an indication of the quality of the measurement. It can store 50 measurements. ASTM and IEEE require that the end result is visually checked and recalculated. This is typically done during later analysis of the measured data in a spreadsheet. CRU02 is connected to a PC by using USB.

Local calibration / conformity assessment: The accuracy of the measuring system can be verified (before a survey) by measurement in glycerol. For high accuracy verification against a reference material with formal traceability, CRC05 calibration reference cylinders can be separately purchased.

Suggested use

  • route surveys, on-site (field) measurements
  • with extension for laboratory analysis of soil specimens (see TNS02)


Applicable standards are IEEE 442-1981(03) Guide for Soil Thermal Resistivity Measurements and ASTM D5334-08 Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock.

System extension for laboratory use

For laboratory use, FTN02 may be combined with a shorter and thinner sensor mounted on a short insertion tool (MTN02). See system TNS02.


The measurement method is based on the so-called Non-Steady-State Probe (NSSP) technique, which uses a probe (also called thermal properties sensor or thermal needle) in which both a heating wire and a temperature sensor are incorporated. The probe is inserted into the soil. From the response to a heating step the thermal resistivity (or the inverse value, the conductivity) of the soil can be calculated. The measurement with FTN complies with the IEEE Guide for Soil Thermal Resistivity Measurements (IEEE Standard 442-1981) as well as with ASTM D 5334-00 Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock. The main application of FTN is route surveying for high voltage electric power cables and for heated pipelines.

In general a NSSP consists of a heating wire, representing a perfect line source, and a temperature sensor capable of measuring the temperature at this source. The probe is inserted into the soil that is investigated. The NSSP principle relies on a unique property of a line source: after a short transient period the temperature rise, DeltaT, only depends on heater power, Q, and medium thermal conductivity, lambda:

DeltaT = (Q / 4 * Pi * lambda) (ln t + B)

With DeltaT in K, Q in W/m, lambda in W/mK, t the time in s and B a constant. By measuring the heater power, and tracing the temperature in time (for MTN typically during 5 minutes), lambda can be calculated.

Measurands -thermal conductivity
-thermal resistivity
Measurement range (λ) 0.1 to 6 W/(m•K) (all common soils)
Rated operating temperature range from 0 to +50 °C
Measurement method absolute measurement per ASTM D 5334-14 and IEEE Standard 442-1981(03)
Data analysis initial analysis by CRU, final review of measurement on PC (required by ASTM)
Uncertainty (at 20 °C) +/- (6 % of reading + 0.04) W/(m•K)
Heating interval 300 s (typical)
Data storage capacity 50 measurements
Length LN02 lance 1.4 m
Length TP09 needle 0.17m
Diameter TP09 6.3 x 10-3 m
Data communication USB


  • Hukseflux CRC series
    Calibration reference cylinders for thermal needle probes TP02/TP08, TP07 and TP09