Heat transfer in a porous medium like soil can be a complex process. Heat is conducted through soil solids and water but is also transported as latent heat in the soil pores. This makes the modeling of heat flow in soil both interesting and complex since it involves thermal and hydraulic processes.
Vapor movement across pores can carry substantial amounts of latent heat, but if the soil around the heat source isn’t wet enough for the water to move back and evaporate again, the soil at the heat source will dry out. Soil drying around a heat source like a power cable can create the potential for thermal runaway which can lead to cable failure. Understanding a soil’s thermal stability can help power engineers more accurately design power distribution systems to prevent thermal runaway.
Appendix B of the National Electrical Code (B.310.15(B)(2)) states, “Typical values of thermal resistivity (rho) are as follows:
However, as many engineers who have used “90” as a safe and typical rho value have discovered, the NEC is simply wrong. These numbers are essentially meaningless because there is no “average soil”, wet or dry.
Forty years of soil thermal research shows that:
Even in a well-designed underground cable system, the soil may account for half or more of the total thermal resistance. Soil and backfill thermal properties should not be assumed. These properties are relatively easy to measure in the field and in the laboratory. A safe, professional installation requires actual measurement and evaluation of thermal rho.
If a soil thermal resistivity report only reads “Soil X has a thermal resistivity of XXX °C-cm/W”, seek clarification. What was the moisture content? How densely was it packed? Are there organics in the soil? Soil moisture, density, and soil makeup are critical factors in determining a soil’s thermal resistivity. Any reporting of thermal resistivity for the purpose of design should include moisture content and density data. A physical description of the soil should also be included. The thermal dryout curve is the most comprehensive way to report soil thermal resistivity. Thermal dryout curves can be automated with the VARIOS lab instrument.
See how the TEMPOS thermal properties analyzer complies with ASTM and IEEE standards here.
Chapter five of the Soil Science Society of America (SSSA) Methods of Soil Analysis Part 4 addresses soil heat. The TEMPOS and VARIOS probe needle sizes, heating times, accuracy specifications, and internal data analysis meet or exceed recommendations outlined in the SSSA methods.
The TEMPOS is a fully-portable field thermal properties analyzer. The VARIOS is a lab instrument that measure thermal resistivity/conductivity in the lab as a function of water content. It automatically generates thermal dryout curves with hundreds of data points. Both instruments use the transient line heat source method which reduces water movement, speeding up the measurement and making it more accurate. Sophisticated data analysis is based on 40+ years of research experience on heat and mass transfer in soils and other porous materials.
Want to do thermal properties testing but not quite ready to make the full investment? Consider renting the TEMPOS or VARIOS to get the data you need. Contact METER for pricing, availability, and rent-to-own details.
Accurately measuring material thermal properties is easy with the TEMPOS or VARIOS, but establishing an effective measurement protocol and carefully controlling important factors that affect thermal properties can be challenging and time consuming.
METER scientists have over 40 years of experience making high-quality thermal properties measurements. We offer convenient thermal properties lab services. If you don’t have time or aren’t completely comfortable making the thermal properties measurements, our services could be perfect for you.
Contact METER for information on lab services.