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28 Pages

### measuringsuction

Course: ENGR 732, Fall 2009
School: Wisconsin
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Word Count: 1427

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Suction Total Measuring Suction - The Kelvin equation indicates that total suction is directly related to vapor pressure, or relative humidity. - Use devices that measure relative humidity, and compute y, p suction. Thermocouple Psychrometers (Daniel et al 1981 STP 746) al. 1981, - a thermocouple psychrometer measures the relative humidity of soil gas and thus yields total suction gas, suction. - Psychrometrics or...

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Suction Total Measuring Suction - The Kelvin equation indicates that total suction is directly related to vapor pressure, or relative humidity. - Use devices that measure relative humidity, and compute y, p suction. Thermocouple Psychrometers (Daniel et al 1981 STP 746) al. 1981, - a thermocouple psychrometer measures the relative humidity of soil gas and thus yields total suction gas, suction. - Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. Thermocouple Psychrometer The Th psychrometer h t is operated in two steps: (1) cool to form a drop of water on tip tip. (2) l t d let drop evaporate and measure voltage output. y p y y Nearly all psychrometers today are the Peltier type, which do not require wetting prior to measurement. Seebeck Principle Seebeck (1821) discovered that the EMF (V) generated in closed circuit of two dissimilar metals when two junctions are at different temperatures. Basis of a thermocouple Seebec Seebeck Effect ect continued That i Th t is, if Tr i k is known and Tr Tm, then a voltage will be generated, which can be calibrated to Tm-Tr. Peltier Effect Peltier found th t passing a P lti f d that i current through a circuit of two dissimilar metals causes one junction to cool and the other to warm. Cooling thermocouple to dew point temperature (temperature at which pore (t t t hi h air is at saturated vapor p pressure) forms a drop of ) p condensation on the junction. T = existing temperature Td = dew point temperature Procedure Cool to dew point using Peltier effect. When dew point is reached, temperature will stabilize (thermal mass of water). water) Stop cooling and monitor change in lt i voltage. A soon as cooling As li current stops, the water drop will begin to evaporate, which reduces the junction temperature (like water evaporating from skin). A maximum voltage will be recorded. Record maximum voltage which voltage, is related to relative humidity via a calibration curve. Calibration Curve Joule Heating and Maximum Suction There is a minimum temperature that can be obtained during cooling, after which joule heating cooling compensates for Peltier cooling. The i i Th minimum t temperature is a function of t i f ti f thermocouple type, and puts a limit on lowest relative humidity and highest suction that can be measured. Joule heating effect limits conventional thermocouple psychrometer to a maximum suction of approximately 8 MPa. Calibration: Requires relationship between V & RH or V & b t t Calibrate by suspending psychrometer over salt solution with known vapor pressure (see Tables 4 3 4.3 and 4.4 in text). Range of Application Maximum ~ 200 MPa, but only possible with Richards psychrometer where water drop placed Richards on junction. 8 MPa with Peltier psychrometer. Minimum: ~ 100 kPa if very carefully done (usually 1 MPa). Accuracy: 10 kPa, if temperature maintained 0.001 C. 0 001 C y p Very temperature sensitive! Chilled Mirror Hygrometer (Water Activity Meter) aw = water activity = pv / ps as RH) = RT ln(aw) M (same Cool mirror above sample holder to dew point dew point. Use infrared thermometer to measure temperature abo e sa p e easu e e pe a u e above sample holder at dew point From dew point temperature, Td, p p , obtain pv of soil gas, which is ps (at dew point). Very accurate, measure aw 0.003 over entire range (1 -100 MPa). Reproducibility Compare water content-suction data measured by three operators on two soils. Good agreement for both soils. Comparison with Other Data Compare water content-suction data measured with CMH and Richards psychrometer. psychrometer Good agreement for both soils. Filter Paper Method Indirect method to measure soil suction. Assume moisture flow into filter p p paper will cause filter p p to paper equilibrate with water in soil. Water content of filter paper is correlated with soil suction. If filter paper is not in contact with soil, then total suction is obtained. If in direct contact with soil matric soil, suction is measured. Must paper with calibration curve (Whatman No. 42, Schleicher & Schwell #589). ASTM Calibration Curves Procedure for Undisturbed or Compacted Soils Equilibration Time Equilibration Generally requires 1 to 2 weeks. y q Longer for wetter soil, up to 30 d for < 100 kPa. See Marinhno & Oliveria (2006), GTJ, 29 (3). Biological Growth Concern that biological will growth affect weight of filter paper and therefore alter suction measurement. 3% pentachlorophenol or 0.005% mercuric chloride have been used. Recent studies suggest not necessary. Measurement Must be done quickly (3-5 s) and with accurate balance (microbalance: 0 0001g) 0.0001g) Use tweezers (moisture and oil on fingers). Oven dry in a clean, metal container with a loose fitting lid. Temperature Effects Average temperature not significant, but avoid temperature significant fluctuations. Store in an insulated b ( St i i l t d box (cooler). l ) Comparisons & Applications McKeen (1981) - suction M K ti measured with filter paper and thermocouple p y p psychrometers. Suction profiles agree well. Soil-Water Characteristic Curve Extend soil water characteristic curve beyond suctions measured using pressure plate. In pressure plate, begin with saturated soil and then dry back to desired water contents. Upper bound on matric suction in pressure plate is about 1 MPa. p p Use filter paper to extend SWCC by drying specimens to lower water contents, and then equilibrating in sealed jar. Wenatchee silty clay y y These measurements were made by compacting the specimens with filter paper inside. Matric Suction Matric suction is obtained directly by measuring uw (with or without axis translation) or indirectly using sensors that can be calibrated t matric suction. lib t d to ti ti Tensiometer Measure uw directly, ua=0. Water saturated from tip to gauge Htip=Hgage (no flow) ugage = uw - zw Therefore tensiometer will read higher suction (by zw) than actually exists in soil. High Air Entry Pressure Interfaces Interface between the soil and the gage is essential. Must allow transmission of pressure but pressure, prevent flow into soil in response to (will try to pull water into soil). il) Use material with fine uniform pores, such as sintered kaolin. Referred to as ceramic plate or tip. p Make pores in interface fine enough to so that water will not be extracted under suction. Maximum before interface desaturates is called the air air entry suction or pressure (a or AEP). Interfaces can be purchased with different a, such as 0.1, 0.5, and 1.5 MPa. Air Entry Pressure Determine the AEP by applying air pressure l i i to saturated disk, and watch when air blows through. g Without axis translation, maximum 80 kPa. At kPa higher suctions, water will cavitate. Thus, a 100 kPa air entry ti i fi f a i t tip is fine for tensiometer. Plates with higher AEP are used for testing devices that y g employ axis translation to measure higher suctions while maintaining uw in a measurable range. Osmotic Tensiometers Osmotic tensiometer works by pre-stressing the tensiometer fluid to high pressures. Then, reduction in pressure measured due to soil suction is matric suction. Temperature Effects T Eff Osmotic pressure is very sensitive to temperature ~25 kPa/C. Requires careful temperature control. OK in laboratory, laboratory but would be problematic in field. Good for suctions up to 2000 kPa. Axis Translation Recall definition of matric suction, = ua - uw ua increased so that uw stays within measureable range. System is closed, so no drainage occurs when ua is applied (no change in water content = no change in suction). Ceramic disk stays saturated provided AEP not exceeded. Key to this technique is speed. Must assemble and pressurize in about 30 s. Place specimen, apply surcharge, bolt together, and p pressurize so uw < -80 kPa. Good for suctions up to 1500 kPa. Nitrogen is gas in chamber. Why? Less soluble in water than air. Temporal Evolution in Matric Suction 80 kPa (cavitation) reached in < 3 minutes. Must apply air pressure expediently. Note how water content affects . Equilibrium < 30 minutes. Thermal Dissipation Sensor Water content of ceramic varies with suction of soil in which it is embedded. Thermal conductivity varies with water context of ceramic. More water more heat conduction Record initial temp, T. Heat needle for specified time period (30 s) with fixed current. Turn off current and record T at two times to define rate of heat dissipation (T and T). Compute T= T -T. Calibration Curve T d T T * d T s = T d T = Temperature drop at given T t d t i Td= T for dry ceramic Ts = T for saturated ceramic T*= Normalized temperature drop For given manufacturer, shape is nearly constant. Can be defined based on Td, Ts , and suction at knee (air entry suction for ( y ceramic). Calibration Curve
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