Heat Pulse 2011 - manuals.decagon.com
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1. Best Method Simply put it involves taking measurements at just two points on the dryout curve and then uses a model to interpolate the curve Figure 2 shows the agreement of the model with actual collected data As you can see the model is consistent with data gathered using the single sample method and shows the inaccuracies inherent in the multi sample method m Fire amp Ice Heat Flow in Snow But this isn t just a story about explosions and extreme sports At its heart it s a story about the microstructure of a very fascinating and difficult material Rich Shertzer who is just finishing a PhD at Montana State thinks snow may be unique among natural materials because the thermal environment it S exposed to every day can cause pretty remarkable changes in its microstructure A cold sunny day in the mountains can cause significant changes in snow crystals It can change their size and shape but more significantly it can cause a directional orientation in snow layers It s long been empirically understood that avalanches tend to form above weak layers of snow Shertzer and his colleagues are studying how the orientation of www thermalresitivity com snow crystals correlates with weak layers Most models of granular mechanics assume that the material s microstructure is randomly arranged However snow layers seem to show a regular arrangement As Shertzer explains qualitatively people hav2. Effect of Latent Heat repeats the cycle Figure 1 shows dryout curves for several different soil types at 25 C The dramatic jump in the curves at intermediate water contents indicates the start of an important but often overlooked component of heat movement in soils latent heat transfer Under certain conditions soil transports latent heat by acting as a natural heat pipe Soil A Natural Heat Pipe At intermediate water contents each soil pore acts as a natural heat pipe Water evaporates at the hotter side diffuses across the pore and then condenses on the cooler side In order for the process to work properly water has to be conducted back in the liquid phase to the hotter side of the pore to re How Heat Pipes Work evaporate A heat pipe uses liquid in an evaporation condensation cycle to continued on page 4 gt FOR A COPY OF THE DETAILED APPLICATION NOTE VISIT WWW DECAGON COM DRYOUT OR CALL 1 800 755 2751 IS THERE A DIFFERENCE the term thermal resistivity is typically used instead of thermal conductivity Thermal resistivity is the inverse of thermal conductivity and in fact most people see the difference between the two as purely semantic But there are some potential pitfalls for the uninitiated and serious mathematical errors can be made by using thermal resistivity values in models without thinking Thermal conductivity has a normal statistical distribution Resistivity doesn t So if we re comp
3. EXPLORING THERMAL PROPERTIES MEASUREMENT OF NATURAL amp ENGINEERED MATERIALS Thermal Dryout Curves How and Why r Gaylon Campbell has been measuring the thermal properties of soil for quite a few years Occasionally he ll get a call from someone asking him to tell them the thermal conductivity or resistivity of their particular soil I m always a little flummoxed by that question he says It s like calling to ask me what the temperature is outside your house I m going to guess that it s somewhere between 25 C and 38 C If you give me additional information might get closer but Nw thermalresitivity com there s no magic number for the temperature of the air No Golden Number Similarly there is no golden thermal conductivity resistivity number for soil Four things significantly affect a specific soil s thermal conductivity resistivity its composition its temperature its bulk density and its water content There are only two ways to know the thermal conductivity resistivity of a specific soil sample you can compute it using a tested model or you can measure it directly News from Decagon Devices about thermal properties amp Ice meet Flow in Snow ll hen Wired Magazine wrote Mi l up Dr Ed Adams and his colleagues in February 2011 they didn t refer to them as a team of civil engineers studying granular mechanics Instead they named them one of seven teams of Mad
4. Scientists and called them Snow Bombers It s not hard to find articles about Montana State University s avalanche studies program Just describing a typical field study makes for a good story to investigate real world avalanche conditions MSU researchers sit in an outhouse sized shack bolted to the side of a mountain while colleagues trigger an avalanche up slope Thermal Dryout Curves How and Why continued from cover Conductivity Varies 98 with Water Content It s important to understand how thermal conductivity resistivity measurements are affected by changing conditions Temperature density often relatively 0 0 10 0 20 0 30 0 40 constant in a specific water content m m soil but water content is highly variable That variability can have a significant effect on thermal conductivity To understand these effects in a specific soil it s often useful to develop a dryout graph showing how thermal conductivity varies with changes in water content in that particular soil Figure 1 Thermal Dryout curve 0 6 0 4 0 2 G Liguid Return Flow conduct heat between two solid interfaces At the hot interface the liquid absorbs heat and evaporates It moves as a vapor from the hot interface to the cold interface where it condenses back into a liquid and releases latent heat The liquid then returns to the hot interface where it evaporates and
5. strength and stiffness which will hopefully bring us back to failure and avalanches m www thermalresitivity com NEW KD2 PRO POWER MODE FUNCTION Challenges in measuring the thermal properties of snow is that the contact resistance between the probe and the snow strongly influences the reading Last year s firmware changes for the TR 1 needle created opportunities to try the KD2 Pro in snow but the TR 1 needle was originally intended for rock and soil so it had too much heat for snow applications In order to help Dr Adams s research team Decagon scientists created a new low power option for the TR 1 needle It has a very low temperature rise to avoid melting and vapor transport in the snow while still accessing the firmware changes that minimize contact resistance errors Originally this change was programmed into a single unit but Decagon has now made Power Mode a standard feature on all units A few points to consider KS 1 in low power the default mode is the best needle for liquids DON T use high power mode with liquids It will cause free convection and compromise your measurements See Chapter 6 of the KD2 Pro Manual for more information on measuring thermal conductivity in liquids DON T make measurements in low viscosity fluids with the SH 1 dual needle sensor in either high or low power mode Measuring thermal properties in frozen materials In this case we do recommend using the TR 1 set to low power n
6. EASUREMENT OF NATURAL amp ENGINEERED MATERIALS
7. e known for a while that when you look at certain snow layers chains of these ice grains seem to be forming What was trying to mathematically model is how that might affect the material properties of snow including thermal properties In order to study the thermal properties of snow samples the research team wanted a way to measure thermal conductivity in three directions That ruled out flux plates Thermal probes were an READ MORE ABOUT SNOW RESEARCH AT THE UNIVERSITY OF MONTANA VISIT WWW DECAGON COM MONTANASTATE THE ORIENTATION OF THE CHAINS FOLLOWS A DIRECTION OF INCREASED CONDUCTIVITY AND THE DIRECTIONS THAT ARE PERPENDICULAR TO THE CHAINS TEND TO DECREASE IN CONDUCTIVITY continued from page 7 obvious alternative but they brought a different set of challenges Snow has a very low thermal conductivity and as Shertzer explains if you add a lot of thermal energy to snow since it s very insulative you ll tend to raise the temperature Not only do we want to avoid melting the snow in the neighborhood of the probe but we want to prevent the probe from artificially inducing the same thermal processes we re measuring the ones that cause the crystals to change size and shape and orientation Shertzer read an article about measuring thermal conductivity in liquids where if you add too much B TR 1 Sensor heat you induce convection Our situation is similar to that he expla
8. hods that can be used to construct dryout curves Single Sample Method One The most accurate method involves collecting a single soil sample and measuring its thermal conductivity and water content as it dries down from saturation to oven dry This method is generally considered the best but is time and effort intensive Multi Sample Method Two The alternative to this method involves taking a large soil sample and mixing it to different moisture contents The samples are packed Interpolating the Thermal Conductivity of Sand Figure 2 single sample measured o multi sample measured x E gt gt m 6 2 9 de e model Volumetric Water Content m m to the appropriate bulk density and analyzed for thermal conductivity and moisture content This method is frequently used because it s fast and convenient but it lacks the accuracy of the single sample method It s difficult to get good readings on dry samples because you can t pack them to a high enough bulk density Better Approach Over the past year we ve worked to develop a method that has the best of both worlds the accuracy of a single sample method with the speed and convenience of the multi sample method The method we propose is detailed in an application note available at www decagon com Producing Thermal Dryout Curves for Buried Cable Applications Interpolated Curve
9. ins Heating the needle induces local phase change The article gave him some ideas about delivering low levels of heat for a relatively long period of time and he contacted Decagon to see if that option was a possibility 10cm needle length Thermal conductivity or thermal resistivity of soil or porous materials ASTM and IEEE compliant Unbeknownst to him Decagon s research scientists had just completed a year long project focused on reducing the contact resistance errors that occur when using the large TR1 needle to measure thermal conductivity in large grained samples This made the TR 1 needle a good candidate for measuring thermal conductivity in snow The scientists were excited about modifying KD2 Pro firmware to produce a low power version that would work in snow The resulting modification has given Shertzer some good data can definitely say that the anisotropy is there in the snow samples It S measureable and it s significant As the crystals reorient in these dept hoar like chains the ice network is more conductive than the air in between The increased conductivity follows the direction of the oriented chains and the directions that are perpendicular to the chains tend to decrease in conductivity Qualitatively it s always made sense and we were just looking for a way to actually relate it to measurable properties like conductivity We expect that this orientation also affects other properties like
10. on default mode The TR 1 needle will reduce contact resistance errors and the decreased heat input is less likely to cause phase change melting in the sample The Power Mode is accessed in the Config menu Find the latest firmware and updated User s Manual at decagon com kd2pro the KD2 Pro used to measure soils with a lot of larger aggregate ecause the TR 1 needle approximates an infinitely small diameter and infinitely long heat source in theory the sensor can calculate thermal conductivity resistivity on most solid materials However there are limitations as to the particle size of the aggregate that it can accurately measure The KD2 Pro with the TR 1 needle can accurately measure the thermal conductivity resistivity of particles as large as coarse sand 1 mm diameter Larger materials can cause problems for the TR 1 needle especially if they are dry Problems include the air gaps between the aggregate which cause contact resistance errors and the presence of a single object not representative of the overall material e g stones located next to the thermistor in the middle of the needle Adhere to the following suggestions to improve accuracy in large aggregate materials B Reduce contact resistance errors air gaps unrepresentative objects next to needle B Ensure thermal equilibrium between needle material and air before a measurement B Verify that the needle inse
11. rtion site is representative of the overall material B Conduct multiple measurements to check for sample variability B Establish and adhere to standardized testing procedures We recommend a written protocol with a minimum of three readings in three different areas of the sample Large aggregates may require a different method e g a steady state device If the sample has a rogue stone in the material carefully analyze the data to see if it is influencing the readings There is also a danger of layering in the sample and thus the thermistor may read one layer of the material that again may not be representative of the whole When testing for thermal conductivity resistivity we should always be mindful of what could be impacting our readings Following a standard testing protocol knowing the limitations of our instrumentation and adhering to a few measurement principles can greatly improve the accuracy of the data we report m A standard testing protocol improves the accuracy of the KD2 Pro o Cl a nd C iu VY O p www thermalresitivity com 11 HeatPulse 2011 2365 NE Hopkins Court Pullman Washington 99163 www decagon com www thermalresitivity com sales amp decagon com 509 332 2756 9 RENT Ask about our KD2 Pro rental program Email mower decagon com HeatPulse 2011 EXPLORING THERMAL PROPERTIES M
12. uting an average an average conductivity would be a meaningful number An average resistivity might not When interpolating over water contents conductivity is more linear with water content than resistivity is Conductivity gives a more correct perception of the significance of the numbers because it s directly proportional to heat flow which is the thing we re most interested in assume two materials of equal area one with a conductivity of 1 and one with a conductivity of O Averaging the conductivities would give 1 2 Averaging the resistivities would give infinity When we calculate the heat flow thermal conductivity would correctly predict an average heat flow of 2 Thermal resistivity predicts zero heat flow Principles discussed in this article apply to resistivity but care should be taken in working with resistivity numbers continued from page 2 Hydraulic Conductivity The hydraulic conductivity of the soil affects the liquid return flow that s required for the water to evaporate again and carry the latent heat with it As you can see in Figure 1 liquid return flow cuts off at a lower water content in sandy soil For a clay with more particle surface area the cutoff is at a higher water content Methods for Constructing Curves Because soil composition varies significantly a dryout curve should be made for each different soil type you are evaluating There are www thermalresitivity com several met
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