The MTN02 - Multi-purpose Thermal Needle System allows fast measurements of the thermal resistivity or conductivity of soils. It is specifically designed to be robust, as well as accurate. It is therefore, 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, IT03. The system is operated using a hand-held Control and Readout Unit CRU02.
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 IT03 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:
MTN02’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. MTN02 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.
CRU02 automatically processes the measurement data, and gives both an end-result and a quality indication of the measurement. CRU02 can archive 50 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.
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.
|Test method||ASTM D 5334-08 and IEEE Standard 442-1981 (03)|
|Data analysis||first analysis by CRU, second review of stored data on PC (as required by ASTM)|
|Range (lambda)||0.1 – 6 W/m.K (all known soils)|
|Temperature range TP||-30 to +80°C|
|Temperature range CRU||0 to +80°C|
|Accuracy (@ 20°C)||± (6% + 0.04) W/mK|
|Measurement cycle duration||300s (typical)|
|Power requirements||recharging: 12V, 2 Watt (max) normally from a car battery|
|Data storage||50 measurements|
|CE certification||complies with CE directives|
|Software||included; new software can be downloaded through RS232 port|
|Data communication||RS232 serial port|
|ISO requirements||suitable for use by ISO certified labs|
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 (03)) as well as with ASTM D 5334-08 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, 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.