TP02 Non-steady-state probe for thermal conductivity measurement

sensor specifications:
Test method ASTM D 5334-00 and D 5930-97 IEEE Std 442-1981
Needle length 150 mm
Traceability NPL
Range (λ) 0.1 to 6 W/m.K
Sensitivity (ΔT) Thermocouple K, ANSI MC96.1-1982
Temperature range (including cable) -55 to +180 °C
Accuracy (@ 20 °C) ± (3% + 0.02) W/mK
Temperature dependence of the accuracy ± 0.02 %/K (additional)
Heating cycle duration 100 s (typical)
Power requirements (switched) 3V, 1 Watt (max)
Medium / sample requirements Granular materials, powders, slurries, gels, pastes. Some samples require pre-drilling. Sample size: Min 20 mm radius. Smaller samples: consult HUkseflux. Caution: please consult the product manual for more details.
Protection needle and base IP 68
Protection total sensor IP 67

Figure 1: TP02 Non-Steady-State Probe consists of a needle with 2 thermocouple junctions (3), (4) (the tip acts as a reference) and a heating wire, (2). It is inserted into the medium that is investigated. In the base, (6), a temperature sensor, (1), is mounted. Advantages of this design: an optimal accuracy independent of the medium temperature, a minimal sensitivity to thermal gradients, a high sensor stability and the possibility to use normal cables and connectors. All dimensions are in mm.

Figure 2 The signal of TP02 as a function of the natural logarithm of time (ln(t)). After a transient period the graphs show linear behaviour. In this phase the slope of the graph is inversely proportional to the thermal conductivity λ.

TP02 is a probe that offers the possibility to perform a practical and fast measurement of the thermal conductivity (or thermal resistivity) of the medium in which it is inserted at the highest accuracy level. It works in compliance with the ASTM D 5334-00, D 5930-97 and IEEE 442-1981 standards. The standard TP02 probe has proven suitability in soils, thermal backfill materials, sediments, foodstuff, powders, sludges, paints, glues and various other materials. The Non-Steady-State Probe (NSSP) measurement method (also known as transient line source, thermal needle, hot needle, heat pulse- and hot wire technique) has the fundamental advantages that it is fast and absolute while the sample size is not critical. Hukseflux is specialised in NSSP design. Special models have been developed for in-situ field experiments. For permanent installation in soils, a dedicated model, TP01, is available. TP02 has been designed and tested in collaboration with the Applied Physics Group of Wageningen University.

A nice clear print of the original product brochure.


Hukseflux is the present market leader in the design of needle type thermal conductivity sensors. Many models have been made on customer specification. These include heavy duty models for field use, small (miniature models for analysis of the thermal conductivity of small samples, like radioactive materials short and strong models for use in offshore soil thermal resistivity measurement.

TP02 is very often used with Campbell Scientific CR10X. A manual on application of TP02 with CR10X is obtainable here. More details on the science: non-steady state probe science. For a free copy of the product brochure click here for the PDF version. For a smaller version of TP02, see TP08.

The standards of the American Society for Testing and Materials (ASTM) and Institute of Electrical and Electronics Engineers (IEEE) describe proven methods for determining the thermal conductivity of materials.

ASTM D 5334-00 and D 5930-97 and IEEE Std 442-1981 "Standard Test Methods" specify the use of Non-Steady-State Probes (NSSP) in various applications. 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 in the medium that is investigated. The NSSP principle relies on a unique property of a line source: after a short transient period the temperature rise, ΔT, only depends on heater power, Q, and medium thermal conductivity, λ:

ΔT = (Q / 4 π λ) (ln t + B)

With ΔT in K, Q in W/m, λ in W/mK, t the time in s and B a constant. By measuring the heater power, and tracing the temperature in time (for TP02 typically during 2 minutes), λ can be calculated. The sample size is not critical, as long as a 20 mm radius around the needle is covered.

The measurements of Q, t and ΔT are all direct measurements of power, time, and temperature respectively. These are done without need of reference materials. The measurement with TP02 is absolute.

TP02 can be obtained as a separate probe for incorporation in the user’s measurement and control system. It is suitable for operation with the Campbell Scientific CR10X and CR1000.

TP02 Design

The Hukseflux TP02 is designed to be able to cover a wide range of applications. Design considerations are as follows:

Optimal Accuracy: TP02 incorporates 2 thermocouple junctions in the needle producing a voltage output, U, that is proportional to ΔT. The one in the tip is not heated, and serves as a reference junction.
In this configuration the voltage before starting the measurement is always small, independent of the medium temperature. In designs with only one sensor in the needle, the signal U will appear on top of a larger signal.
The two junction design has a superior accuracy when measuring at high and low temperatures.
In addition, by having both the reference- and the hot junction in the needle, the sensitivity to temperature changes of the medium is minimised.

Temperature sensor in the base: The sensor in the base (Pt1000) serves as a cold junction measurement for establishing the absolute medium temperature T. (As required by ASTM). This is used for temperature correction of the thermocouple sensitivity, again offering superior accuracy across the temperature range.

Temperature and moisture resistance: All the materials in TP02 are highly temperature resistant. It has a full stainless steel needle and base, with a welded tip (IP68) and a potted cable (IP67). The needle is 100% waterproof.

Standard cables and connectors: Putting the Pt1000 temperature sensor in the base allows the use of normal copper core cables and connectors for cable extension of TP02.

Long term stability: The fully sealed construction is essential for prevention of internal corrosion and maintaining stability of the sensor over time.

Data processing: the data obtained with TP02 can be processed in any normal spreadsheet program.

Calibration/ ISO 9000

Verification of the stability of the total probe can be done by repeated (yearly) testing in glycerol, preferably at several temperatures. Alternatively Calibration Reference Cylinders (CRC) traceable to NPL can be purchased at Hukseflux. TP02 is suitable for use by ISO certified labs.

Suggested Use

  • Studies of soils and soft rock
  • Studies of foodstuff, plastics and powders


  • Alternative designs: Hukseflux is specialised in NSSP design. Alternative models, for instance smaller, more robust or temperature resistant have been manufactured on request.
  • Permanent installation in soils: The dedicated model TP01 is specifically designed for long term monitoring. A separate brochure is available.
  • Solutions for measurement and control: See the TP02 manual or inquire at Hukseflux. A program for Campbell Scientific CR10X is available. Turn key systems are offered for sale: see TPSYS.

Similar Products

More Information

  • More details on the science: non-steady state probe science.
  • Standards: ASTM standards can be obtained from ASTM at


  • For high accuracy calibration CRC Calibration Reference Cylinders are available
  • For insertion into hard soils GT Series Guiding tubes can be applied
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