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I am a little confused about what the differences are between heat dissipation sensors and thermal probes. I am currently using Campbell Scientifics 229 heat dissipation sensors and briefly this is what I understand as their function: Heat dissipation sensors indirectly measure the soil matric suction by measuring the thermal conductivity of the sensor. The sensor, a thermocouple and heating element inside a porous ceramic matrix, is placed into the soil. Water moves into and out of the sensor (altering the sensors degree of saturation and hence its thermal conductivity) until the sensor comes into equilibrium with the surrounding matrix - that is if the soil is at 40 kPa suction, the sensor will be at 40 kPa suction. The sensor is then heated and the temperature rise measured (the wetter the sensor, the greater the thermal conductivity, the more heat will be dissipated, and the lower the temperature rise). The heating pulse is entirely contained within the sensor body so the thermal properties of the soil do not come into consideration. This fact makes calibration much easier since it is completely independant of soil type (infact I may install sensors in a sulphur block to measure infiltration). This may also be the diffence between heat dissipation and thermal conductivity sensors. A laboratory calibration curve between sensor output (temp rise) and matrix suction must be developed in order to obtain the matrix suctions for the soil. This is probably the biggest con to this type of sensor. But there are few sensors that don't need some sort of lab calibration. As a hydrogeologist, these suction values are of great importance as they are the data required for estimating unsaturated flow, recharge, infiltration, etc. If water contents are necessary, they can, given the suction, be estimated from soil water characteristic curves (SWCC, suction vs. volumetric water content) for the soil in question. The SWCCs can be developped by estimation based on soil properties (e.g., grain size), field determinations (e.g. guelph permeameter, infiltrometers, etc), or by laboratory testing (essentially a "fancy permeameter"). The estimation will only be as good as the SWCC, but if the soils are homogeneous, this may not be a problem. I'm not quite sure why water contents are emphasized so much anyways as it is suction/head (or their respective gradients) which causes water to flow and suction which prevents water from being available to or sucked up by plants (non-botanist comment). So here are a couple questions: Is the emphasis on water contents and not suctions appropriate? (just trying to ruffle feathers with this comment) Do the thermal conductivity sensors being discussed measure the thermal properties of the sensor of directly measure the thermal properties of the soil itself. A comment was made that heat dissipation sensors do not measure high enough suctions. What in this case is considered to be a high suction? Regards, David ---- David Thomas thomas@geo.ucalgary.ca Geology & Geophysics ph (403)220-6596 University of Calgary fax (403)284-0074