Soil thermal conductivity selection guide

Choose the right measuring system or sensor for your application

Hukseflux is a leading manufacturer of thermal conductivity measuring systems and sensors for use in soils. This brochure offers general guidelines for choosing the right system or sensor for your soil thermal conductivity measurement application.

The right instrument for your application
Hukseflux offers a wide range of products for measurement of soil thermal conductivity, or its inverse value thermal resistivity.

Figure 1 Soil thermal route survey with FTN01 including CRU01 Control and Readout unit.

Hukseflux sells a range of measuring systems which include “thermal needles”. All systems comply with ASTM D 5334-00, D 5930-97 and IEEE 442-1981 standards. Thermal needles are suitable for repeated insertion into various specimens or into undisturbed soil.
These sensors and systems can be divided in:
• high-accuracy measuring systems, using models TP02 and TP08 thermal needles, and measuring system TPSYS02
• systems more focused at robustness during field use, compromising on accuracy (FTN01 / MTN01 and TNS01 systems) with thermal needles TP09 and TP07
• a special model for long term monitoring, foil type TP01

	Soil Thermal Conductivity Measurement
	Measurand	• Thermal Conductivity • Thermal Resistivity
	Products	• Single Sensors • Measuring Systems
	Material Types	Soils,concrete, thermal backfill material.
	Standardization	ASTM, IEEE
Figure 2 Guiding tubes for hard soils and dryout experiments	Commercial Options	• Purchase • Rent • Outsource to our thermal properties laboratory • Expert Training

Measuring systems using thermal needles: what we offer
Hukseflux offers the following measuring systems:
•  FTN01: a 100 % field instrument
•  MTN01: for mixed field / laboratory use
•  TPSYS02: a scientific measuring system typically for the laboratory, but also useful in case you are only working with soft sediments and dryout experiments

For thermal needle types TP07 (used in MTN01), TP08 and TP02 Hukseflux offers stainless steel “guiding tubes”. Using these tubes it is possible to measure in hard materials such as cements and thermal backfill (heavy clay) and dried-out soil by casting them into specimens.
For all models there are Calibration Reference Cylinders available, providing a traceable reference material. Hukseflux is also capable of supplying custom designed models (heavy duty, pressure resistant) for field or offshore application.
High-accuracy measuring systems
TP02 and TP08 are primarily intended for use in the lab providing the highest possible measurement accuracy.
Although intended for indoor measurement, TP02 and TP08 may be taken outdoors for on-site testing, provided that care is taken during operation. Operation in soft soils, such as sediments is easy. In harder soil holes must be pre-drilled, or guiding tubes may be used. TP02 and the smaller TP08 may be purchased as a single needle or a as part of a complete turnkey system: TPSYS02. Models TP02 or TP08 can be operated using “standard” electronics, found in most measurement laboratories.
The user should carefully consider which probe to use. The standard model is TP02. This offers the highest accuracy, the lowest sensitivity to external influences and the best performance at higher or lower temperatures.
In case the specimen size for TP02 (typically 150 x 10-3 m height) is not available, use of TP08 should be considered. TP08 is shorter than TP02, but its general operation is identical to that of TP02. A special sample container (TP08-CO) is available for use with TP08. TP08 is also often used in analyses of sediment cores in offshore experiments. Often, insertion of TP08 from the side of the core is possible.
Robust measuring systems for field use
For field experiments, Hukseflux offers measuring systems FTN01, MTN01 and the combined TNS01. These “heavy duty” systems are equipped with robust thermal needles suitable for penetrating soft as well as harder soils.

The FTN01 system is designed for field use, in particular surveys for high voltage electric cables (not during operation); typically measuring at a depth of 1.5 metres below the soil surface.
FTN01 includes a 1.5 metre long lance (model LN01) at the tip of which a very robust (6 x 10^-3 m diameter) needle (model TP09) is mounted. Using a lance rather than digging trenches saves time.
The MTN01 system is suitable for laboratory experiments as well as field experiments and includes the TP07 3 x 10^-3 m diameter needle mounted on a small insertion tool (IT01).

Models TP07 and TP09 are more robust than TP02 or TP08; however, the attainable measurement accuracy is considerably lower.

Figure 3 Accurate thermal needle TP08	Figure 4 The complete and robust FTN01 measuring system including the 1.5 m long lance LN01

Table 1 Comparison of systems that employ thermal needles.

1. TP02 and TP08 are also available as separate probes for use with the user’s own electronics, for instance Campbell Scientific CR1000.
2. Systems may be used outside their rated measurement range, but will then offer a lower accuracy than specified.
3. Temperature ranges may be extended upon request.
4. Guiding tubes are supplied in sets of 5 pieces. They essentially create a needle with a larger diameter. The heating time when using needles with guiding tubes is 50% longer than without. Guiding tubes are often used in cements and soil dryout experiments. Guiding tubes can also be used cast in inside plastics, glues, etc. This may be easier than specimen preparation for THASYS, which often is too costly or impossible.
5. A low-cost approach to calibration is to use glycerol with plastic fibres as calibration reference material. However, Calibration Reference Cylinders offer a formal traceability to international standards which glycerol does not offer.

Specials: TP01 for long-term monitoring
A special category is model TP01, which is a foil type sensor (so not a needle type). It is generally part of a larger meteorological system, used for long term use at one location. Contrary to other needle sensors it is waterproof. TP01 is not a good choice for repeated insertion into various specimens. Advantages of TP01 are pricing, its easy (steady state) signal analysis and the fact that it makes a crude measurement of thermal diffusivity, which is proportional to soil water content. A disadvantage might be that the measurement range is smaller than that of thermal needles; in practice this only leads to errors in very dry sand and organic soils.
Table 2 Example of thermal needles (TP01 is a modification of the thermal needle in a flat foil) developed at the request of our customers

Equipment rental
In case you only need a system like FTN01, MTN01 or TRSYS01 for one project, consider renting a system. Rental costs for one month are in the order of 20 % of purchasing costs of a system. Please ask for our rental agreements.

Expert training for system operators
Although in general the systems can be operated by following manuals and standards, we can provide you with expert training. Training vastly improves the level of service to the third party, the efficiency of working with the equipment and reduces the uncertainty of the end result.

Figure 5 Foil type sensor model TP01, for long term monitoring, IT01 insertion tool with needle type TP07 (both part of MTN01) and CRC Calibration Reference Cylinder, offering a traceable reference material

See also
Besides the possibility to purchase and rent thermal conductivity instruments, Hukseflux offers custom-made products, testing services and thermal consultancy.

About Hukseflux
Hukseflux Thermal Sensors, founded in 1993, aims to advance thermal measurement. We offer a complete range of sensors and systems for measuring heat flux, solar radiation and thermal conductivity. We also provide consultancy and services such as performing measurements and designing instrumentation according to customer requirements. Customers are served through the main office in Delft in the Netherlands, and locally owned representations in the USA, China and Japan.

Figure 6 Laboratory measurements with thermal needle model TP02 and CRC Calibration Reference Cylinders