MTN01

Multi-purpose thermal needle system for thermal resistivity / conductivity measurement

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The MTN01 Multi-purpose Thermal Needle System allows performing fast measurements of the thermal resistivity or conductivity of soils. It is specifically designed to be reasonably robust, as well as accurate. It is therefore suitable for laboratory as well as field- measurements of soil thermal properties. The sensor is a Non-Steady-State Probe (NSSP), TP07, which is mounted on the Insertion Tool, IT02. The system is operated using a hand-held Control and Readout Unit CRU01.

Introduction

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 MTN complies with the IEEE Guide for Soil Thermal Resistivity Measurements (IEEE Standard 442-1981) as well as with ASTM D 5334-92 Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock. The main applications of MTN are the analysis of soil samples in the laboratory and field experiments in relatively soft soils.

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, 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 MTN typically during 5 minutes), λ can be calculated.

MTN design

Suitability for laboratory measurements:
MTN's primary focus has been on the capability to perform laboratory measurements. The needle of TP07 is relatively thin, so that common samples can be used. The IT02 can be mounted on the levers that are commonly used in machine shops so that the needle can be vertically inserted into the sample.

Suitability for field measurements: MTN01's can, provided that soils are relatively soft, also be used for field measurements. In case of harder soils or measurements at greater depths (up to 1.5m) the use of stronger needles (like TP09) and a lance (like in the system FTN01) is recommended. MTN01 is able to perform measurements without external power source. The system is sufficiently robust to survive manual insertion into most common soils. The system runs as a stand-alone unit, powered by the batteries in the CRU. Recharging can be done by a 12VDC source or a car battery using the CA01 car adapter, or on 220/110 VAC using the WSA01 wall socket adapter.

Automatic processing: CRU01 automatically processes the measurement data, and gives both an end-result and a quality indication of the measurement. CRU01 can archive 30 measurements. In case of review, the end result is preferably checked and recalculated by analysis of the measured data in a spreadsheet (like EXCEL) or a mathematical program.

Local calibration: verification of the stability of the total system can be done by repeated testing in glycerol. This test can also be performed in the field.

Suggested use

  • Laboratory measurements
  • Studies of soil and soft rock

Options

  • Alternative designs: Hukseflux is specialised in NSSP design. Alternative models, for instance for laboratory use, are available at Hukseflux.
  • It is suggested to also consult the brochures of complementary systems FTN and TNS, as well as TPSYS, which is more accurate but has less robust needles.

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