Thermal resistivity: Real RHO values for the power engineer ® Popular Terms Food Applications QA & QM Production R & D Applied industries Cured meats Pet foods Baked snack foods Functional foods Products Product overview Water activity meter Moisture content analyzer Vapor sorption analyzer Platform SKALA MAT Knowledge base Case studies Education guides Expertise library Market insights Webinars Podcast Support Services Consumables FAQs Instrument training Distributors Downloads Environment Applications Soil science Weather monitoring Hydrology Plant science Irrigation management Soil thermal resistivity Geotechnical engineering Applied industries Plant breeding Ag weather networks Soil infiltration testing Sensor integrators Products Product overview Data logger & telemetry Weather Soil moisture Hydrology Plant & canopy Thermal properties Laboratory instruments Platform ZENTRA Cloud Knowledge base Case studies Education guides Measurement insights Webinars Podcast Support FAQ Videos Expertise library Downloads Services Grants Distributors Horticulture Applications Cultivation Processing Applied cultivation Indoor Greenhouse Products Product overview Telemetry Substrate sensors Climate sensors Lab processing moisture Platform AROYA Knowledge base Case studies Education guides Office hours Company Corporate History Philosophy Awards News Contact us Impact Corporate stories Foundation Sports Career Create the future Working at METER Jobs at METER Group Jobs at METER Group AG Jobs at ADDIUM Request a quoteRequest a quoteThermal resistivity: Real RHO values for the professional power engineer Understanding a soil’s thermal stability can help power engineers more accurately design power distribution systems to prevent thermal runaway. CONTRIBUTORS Thermal stability 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. Accurate ampacity calculations depend on measured rho values Appendix B of the National Electrical Code (B.310.15(B)(2)) states, “Typical values of thermal resistivity (rho) are as follows: Average soil (90 percent of USA) = 90 Damp soil (coastal areas, high water table) = 60 Very dry soil (rocky or sandy) = 120.” 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. 90 is not the magic number Forty years of soil thermal research shows that: Soil and rock rho values actually vary from 10 to 1000 ℃ cm/W. There is no “typical” value for 90% of soil types. Thermal resistivity of porous materials like soil, rock, and concrete are not constants. Resistivity changes with density, water content, and temperature of the soil or concrete. Measure, don’t guess 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. A rho value reported in a void can be misleading 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 (see example). 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 generated automatically with the VARIOS lab instrument. TEMPOS compliance with ASTM and IEEE standards See how the TEMPOS field thermal properties analyzer complies with ASTM and IEEE standards here. Methods of soil analysis: part 4 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. Thermal properties testing The TEMPOS is a fully-portable field thermal properties analyzer. The VARIOS lab instrument measures thermal resistivity as a function of water content and generates easy, automated thermal dryout curves. Both use the transient line heat source method, which reduces water movement for higher accuracy and speeds up measurement time. Sophisticated data analysis is based on 40+ years of research experience on heat and mass transfer in soils and other porous materials. Equipment rental Want to do thermal properties testing but not quite ready to make the full investment? Consider renting the TEMPOS to get the data you need. Contact METER for pricing, availability, and rent-to-own details. Lab services Accurately measuring material thermal properties is easy with the TEMPOS, 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. Rho resources Questions? Talk to an expert—> Read thermal resistivity FAQs Learn about thermal stability TEMPOS operator's manual See the VARIOS lab instrument for automated thermal dryout curves Underground power cable installations: soil thermal resistivity Thermal resistivity app guide Understand how rho changes with changing density, temperature, composition, and water content of backfill Producing thermal dryout curves for buried cable applications TEMPOS compliance to ASTM and IEEE standards Lab-Ferrer's Spanish rho blog Questions? Our scientists have decades of experience helping researchers and growers measure the soil-plant-atmosphere continuum. Talk to an expert—> Request a quote—> Measurement InsightsSee all articles Why underground power cable installations need soil thermal resistivity measurementsReadSoil physics is increasingly critical in the design and implementation of underground power transmission and distribution systems.Measurement Insights7 min read Thermal properties: Why the transient line heat source method outperforms other techniquesReadThere’s no way to measure the properties of moist, porous materials with the steady state method (guarded hot plate). The transient line heat source method, however, is able to measure the thermal properties of moist, porous materials, and it can even measure thermal conductivity and thermal resistivity in fluids.Measurement Insights9 min read Soil moisture sensing—evolvedReadTEROS sensors are more durable, accurate, easier and faster to install, more consistent, and linked to a powerful, intuitive near-real-time data logging and visualization system.Measurement Insights3 min read Case studies, webinars, and articles you’ll love Receive the latest content on a regular basis. ® Contact USA 2365 NE Hopkins Ct. 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