Measurement of soil moisture using gypsum blocks
Contents
1. 13 G DISCUSSION et et er epe eta 15 Y Conclusions Te teer in asin deen iW unge EL rand 17 8 Acknowledgement ke 18 9 c doi EH RORIS E ERE 19 Appendix A Test Method 20 Calibration of gypsum blocks for soil moisture measurements 20 disi EA 20 2 Field of 20 3 References acted dee e d e der dede oie deu eda 20 4 Definitions ne mee De ee EORR esta 20 5 Samplirig eerie e td 20 G Test method ii eue lois M LU ett Lean dg 20 AppendbcB z us octo m IDEE AEE FAR E 25 Sammentfatnirig 28 Foreword Moisture conditions in the soil surrounding a building are of great importance to the moisture conditions inside the building and some cases of moisture problems in buildings may be due to high moisture content in the surrounding soil In order to detect the source of water in the building measurements of soil moisture are needed In soil science gypsum blocks have been used to determine soil moisture content for the past 50 years and several types of commercial gypsum blocks are available on the market The gypsum blocks are provided with a generalized calibration curve There is no individual calibration curve for the gypsum blocks and there is no informatio2. d 5 Y 0 064 X P4 200 P m 4 Ae 100 rd 0 0 2000 4000 6000 8000 R T 23 C Ohm Figure 9 Plot showing the slope of resistance relative to temperature against the resistance at 23 C In Figure 9 a straight line through origin represents a proposal for describing the influence of temperature on the resistance of gypsum blocks Using this proposal the following relation between resistance and temperature can be found R T 0 064 36 To T R To Using this expression it is possible to estimate the resistance at T 23 C when the resistance at a given temperature is known 6 Discussion The results obtained in the calibration procedure are all based on an experimentally determined retention curve on kaolin Therefore the uncertainty of the calibrations is correlated to the uncertainty of the retention curve The retention curve for kaolin presented in Figure 6 has the same shape as the retention curve for clay presented in Figure 3 However it has not been possible to check the accuracy of the measurements made with the thermocouple psychrometer In order to compare the results with other principles of measurements the Whatman No 42 filter paper method has been applied In Figure 10 the results from the filter paper test are shown together with the results using the thermocouple psychrometer Also some results obtained as a by product of measurements using the pressure plate
3. 10000 1000 100 10 100000 10000 1000 100 10 x X No 1 In Y 1 897110006 In X 5 881492414 FL TTTTTI pee Ime Ell 0 1 10 100 Suction bar Figure 11 Results from calibration of gypsum block No 1 shown together with curve fit Pd X X No2 In Y 2 169254397 In X 6 057218363 PL TTTTTH ls oed td le xp whee hols iua 0 1 10 100 Figure 12 Results from calibration of gypsum block No 2 shown together with curve fit Suction bar 25 26 10000 x T 1000 E s E 2 100 Eg x No 3 In Y 1 490756234 In X 6 368459876 10 IT 0 1 1 10 100 Suction bar Figure 13 Results from calibration of gypsum block No 3 shown together with curve fit 10000 x 1000 E 8 E 9 2 N 100 x No 4 In Y 1 825053063 In X 5 798195426 10 TT TT 0 1 1 10 100 Suction bar Figure 14 Results from calibration of gypsum block No 4
4. apparatus Str mdahl 1997 are shown in this figure 0 6 7 2 0 5 7 3 A 7 gt t 5 K 0 4 gt lt Xt 8 VS A 2 E Y ov A 0 3 d FT Thermocouple psychrometer At A Pressure plate apparatus 25 E e Whatmann No 42 filter paper 0 2 Vetta ttt TTTTT TTT 10 1 0 1 0 01 Suction bar Figure 10 Retention curves for kaolin according to different methods of measurements Measurements of suction pressure in the high moisture range are prone to large uncertainties This is indicated in Figure 10 by the large differences in the results from different test methods The thermocouple psychrometer is considered the most accurate method for this type of measurement Nevertheless the results from the calibration of the gypsum blocks are related to the uncertainty of the retention curve Therefore it is preferable to make an inter laboratory test to ensure that the retention curve for kaolin is correct The results from the calibration are shown in Figure 7 assuming that the retention curve for kaolin obtained by use of the thermocouple psychrometer is correct In the same figure the standard curve proposed by the manufacturer of the gypsum blocks is shown Unfortunately there is a significant difference between the experimental results and the standard curve The difference is up to a factor of 5 and thus the results from the calibration should be seen
5. every 10 minutes until a new equilibrium was obtained within 1 2 days The gypsum block was removed from container B and left to dry at room conditions A kaolin sample was taken from each container immediately after removal of the gypsum blocks The sample was to be used for determining the water content of the mix and should preferably be taken from areas that had been in direct contact with the gypsum block The water content was determined from the mass of the fresh sample Mwet and the mass Mary of the sample after it was oven dried at 105 C A high resolution balance was needed for small samples In the present tests a balance having a resolution of 0 001 g was used and the samples had an approximate weight of 3 g prior to drying The retention curve for kaolin was needed to be able to determine the suction pressure of the kaolin samples by their water content Unfortunately the retention curve for kaolin was difficult to find in the literature Therefore in this project the retention curve has been determined experimentally In Section 4 a short description is given of the procedure for determining the retention curve by using a thermocouple psychrometer 4 Determination of the retention curve for kaolin The retention curve for kaolin was determined by using a Decagon Devices SC 10A Thermocouple Psychrometer The psychrometer is especially suited for measurements of suction pressure in soil It was assumed that the thermocou
6. in the light of the lack of knowledge regarding the correct retention curve In any case however the calibration will reveal if a gypsum block is malfunctioning Two such defective gypsum blocks were found in this study The influence of temperature on the measured resistance of the gypsum blocks is presented in Figure 8 The relations should be seen as rules of thumb for the interpretation of results obtained at temperatures other than 23 C Soil measurements on site will most likely be made at temperatures at 5 15 16 10 C Therefore calibrations carried out at temperatures in this range are to be preferred However it is important to note that the retention curve in Figure 6 is based on measurements made at 23 C If the calibration of gypsum blocks has been carried out at temperature field measurements can then be given the following interpretation using the curve fit in Figure 9 R T 1 0 064 Ty T R T where R is the resistance and T the temperature The resistance at the temperature 7 23 C can then be used directly in the expression from the calibration In R T E 5 ex 1 where s is the suction pressure and R the resistance at calibration temperature 7 The factors 4 and B are constants determined in the calibration 7 Conclusions A calibration procedure for gypsum blocks for soil moisture measurement has been proposed The calibration method is based on the use of kaolin as a
7. shown together with curve fit Resistance Ohm Resistance Ohm 10000 x 1000 x Ja x No 5 In Y 1 687807465 In X 6 115224892 10 fee fall bod lel 0 1 1 10 100 Suction bar Figure 15 Results from calibration of gypsum block No 5 shown together with curve fit 10000 1000 x 100 x x No 6 In Y 1 422366283 In X 5 982964972 10 IT ETT L 1 1 T FTT sie IS EE 0 1 1 10 100 Suction bar Figure 16 Results from calibration of gypsum block No 6 shown together with curve fit 27 28 Sammenfatning By og Byg Dokumentation 004 Kalibrering af gipsblokke til bestemmelse af jords fugtighed Denne rapport beskriver resultater fra Nordtest projekt 1424 98 som havde til form l at udvikle en kalibreringsprocedure for gipsblokke der anvendes til bestemmelse af fugtindhold i jord Kalibreringsproceduren er blevet unders gt for repet rbarhed og for temperaturafh ngighed Den kalibreringsmetode der foresl s for gipsblokke er baseret p anvendelse af en kunstig jordpr ve der best r af en blanding af ren kaolin og vand Metoden foruds tter kendskab til retentionskurven for den anvendte kaolin For the past 50 years gypsum bloc
8. 56 0 706 22 8 u 0 500 8 exp where u is the gravimetric water content and p the suction in bar 0 6 al A 0 5 D J t A 04 E 2 J x 0 3 a 0 2 Flats Pettit a SER 10 1 0 1 0 01 Suction bar Figure 6 Results from measurements using a SC 10A Thermocouple Psychrometer to determine the relation between water content and suction pressure in kaolin 5 Results from calibration of gypsum blocks A total of 6 gypsum blocks have been calibrated in kaolin mixes Every gypsum block has been calibrated at 4 different moisture contents The results are shown in Figure 7 by the suction pressure as a function of the measured AC resistance The suction pressure at room temperature was determined by the measured water content and by use of the retention curve shown in Figure 6 100000 Gypsum blocks No 1 No 2 10000 No 3 No 4 No 5 No 6 1000 Standard curve Resistance Dhm 100 0 1 1 10 100 Suction bar Figure 7 Results from calibration of gypsum blocks The full line represents the standard curve as obtained by use of the data given in Figure 1 The results from the calibration are shown for each of the 6 gypsum blocks in Appendix B In these figures a curve fit is shown as well The curve fits have the following expression In R T A In s B where R is the resistance s the suction pressure and 7 the calibration temperatu
9. By og Byg Documentation 004 Measurement of soil Er moisture using gypsum RE blocks Measurement of soil moisture using gypsum blocks Birgitte Friis Dela By og Byg Documentation 004 Danish Building and Urban Research 2001 Title Measurement of soil moisture using gypsum blocks Subtitle Serial title By og Byg Documentation 004 Edition 1 edition Year 2001 Author Birgitte Friis Dela Language English Pages 28 References Page 19 Danish summary Page 28 Key words Building materials moisture soil calibration gypsum blocks ISBN 87 563 1075 7 ISSN 1600 8022 Price DKK 62 50 incl 25 per cent VAT Publisher By og Byg Danish Building and Urban Research P O Box 119 DK 2970 H rsholm E mail by og byg by og byg dk www by og byg dk Extracts may be reproduced but only with reference to source By og Byg Documentation 004 Measurement of soil moisture using gypsum blocks 2001 Content FORE WON E tma se A ELE bette 4 Sumiialby eo e E e EU e e t UREN ee Me dug 5 Ti Introduction eit ERR e nen eal 6 Measuring soil moisture ssssee enne 6 2 Material Jarred ea et ee t de tao et tage td ctos 8 3 Calibration procedure for gypsum 10 4 Determination of the retention curve for 11 5 Results from calibration of gypsum
10. e gypsum block The sample should weigh more than 3 g when the balance used for weighing has a resolution of at least 0 01 g The mass mMer of the sample is determined shortly after removal from the container less than a minute The sample is oven dried at 105 C until the weight loss is less than 0 01 g over a period of 6 hours The mass mz of the sample is determined after it has cooled to room temperature in a dry container with silica gel The gypsum block is dried at room condition until the AC resistance is above 100 kQ Steps 6 4 2 to 6 4 6 is repeated for at least 3 kaolin mixes having water contents ranging from 20 to 60 96 6 5 Expression of results 6 5 1 The moisture content by mass is determined by TE Mwet Mary where Mwe is the mass of the sample just after removal from the container and man is the mass of the sample after it has been dried in the oven The suction pressure s is determined by 0 202 bar 0 500 8 1 323 _ 0 706 8 where u is the moisture content by mass This expression is valid for the proposed retention curve for kaolin of the type Fluka Chemie No 60609 distributed by Sigma Aldrich Chemie If other types of kaolin or soil or material are used the retention curve has to be determined for that specific material The retention curve can be determined by using for example a thermocouple psychrometer Plotting the suction pressure s of the kaolin against t
11. e of standard curve provided by the manufacturer of Delmhorst gypsum blocks The curve shows the relation between measured resistance and suction pressure moisture tension 2 Materials The gypsum blocks used in the experiments presented in this report are made by Delmhorst Instrument Co The blocks are cylindrical having a diameter of 25 mm and a length of 35 mm Two concentric rings of wire mesh are embedded in the gypsum block and make up the electrodes In this way flow of electric current is confined to the interior of the block eliminating the effects of soil conductivity The resistance between the electrodes varies with the moisture content of the gypsum block which in turn varies with the suction pressure in the surrounding soil Thus measuring the AC resistance of the gypsum blocks provides information of the suction pressure in the surrounding soil The gypsum acts as a buffer against the effect that salts might have on the electrical conductivity In Figure 2 a picture of a Delmhorst gypsum block is shown together with an illustration of the placement of electrodes Figure 2 On the left is shown a Delmhorst gypsum block The drawing on the right hand side illustrates the shape and the placement of the embedded electrodes Calibrations of the gypsum blocks are carried out using a Campbell Scientific CR10 data logger that measures the AC resistance of the gypsum block every 10 minutes A multiplexer is preferably used when connecti
12. e test report c Name and address of the organisation or the person who ordered the test d Purpose of the test Choice of points of measurements and other circumstances date and person responsible for sampling f Method of sampling and specimen preparation 9 Address and detailed description of the place where the samples have been taken out i Date when the measurement was ordered j Date of measurement k Test method Conditioning of the sample type and probe ambient conditions during measurement temperature RH etc m Identification of the equipment used for measurement product model and laboratory equipment identification number Any deviations from method of measurement Test results according to clause 6 5 Inaccuracy or uncertainty of the measurement Date and signature n o p q YS reo 23 24 0 8 0 7 m 06 3 I S t 05 o o t 04 0 3 0 2 10 1 Suction bar 0 1 0 01 Retention curve for kaolin of the type Fluka Chemie No 60609 distributed by Sigma Aldrich Chemie 10000 1000 Resistance Ohm 100 10 x No 6 In Y 1 422366283 In X 5 982964972 0 1 Suction bar Example of calibration curve for a gypsum block 10 100 Appendix B Resistance Ohm Resistance Ohm
13. ely 35 96 were in a grainy phase whereas kaolin mixes containing more water were in a plastic phase Mixes having a water content around 35 were very heavy and a powerful mixer was needed The mixes were put in containers diameter 60 mm height 45 mm and sealed with a screw cap At least two containers A and B were needed for every mix To reduce drying the containers were stored in a closed box with a high relative humidity RH gt 95 Prior to utilization the gypsum blocks were soaked and dried at room conditions This wet dry cycle is recommended by the manufacturer in order to improve uniformity of the blocks Also the gypsum blocks were dried at room conditions RH 50 T 24 C before every calibration cycle In this way adsorption was ensured in every cycle and misleading results caused by hysteresis were prevented A gypsum block was placed in the center of a container A filled with kaolin mix The lead wire was led through a hole in the screw cap The hole was sealed with wax sealant AC resistance was measured every 10 minutes until a constant level was observed asymptotic behavior A constant level would indicate moisture equilibrium between gypsum block and kaolin mix Then the gypsum block was moved from container to container B Moving the screw cap along with the gypsum block from container A to container B facilitated the sealing of the containers AC resistance was measured
14. epared for gypsum blocks A hole is made at the centre of the kaolin in container A using a piece of pipe The pipe has an outer diameter equal to the diameter of the gypsum block 644 A dry gypsum block is placed in the hole in container A and the wire is led through an opening in the cap of the container The container is sealed and stored at high relative humidity RH gt 95 94 Measurements of the AC resistance are carried out until equilibrium is observed Equilibrium should be reached within 2 3 days 645 When equilibrium is obtained the gypsum block is removed from container A and placed in the corresponding container B The kaolin mix in container B is the same as the kaolin mix in container A The procedure in steps 6 4 3 and 6 4 4 is repeated for container B except that the gypsum block is not dry but in moisture equilibrium with the kaolin in container A Note The reason for placing the gypsum blocks in two containers with identical kaolin mixes is to reduce an error that might occur if there is a significant suction gradient from the surrounding kaolin mix to the center of the gypsum block By equilibrating the moisture content in the gypsum block before 21 22 placing it in container B the suction gradient will be reduced When equilibrium is obtained in container B the AC resistance is registered The gypsum block is removed and a sample of kaolin is taken from the area next to the hole for th
15. h However gypsum will disintegrate if exposed to frost Gypsum blocks have been used in soil science for the past 50 years Bouyoucos 1965 and several types of commercial gypsum blocks are available on the market Gypsum blocks are delivered with a standard curve showing the relation between measured resistance and soil moisture tension suction pressure There is no calibration curve for the actual gypsum block and there is no information on temperature dependency In Figure 1 an example of such a standard calibration curve is shown The curve shown in Figure 1 is also available as data listed in a table but there is no information about scatter in resistance between different gypsum blocks Hence interpretations of resistance measurements are rather uncertain To obtain better information about the relation between measured resistance and suction pressure it is necessary to calibrate each gypsum block prior to testing It is the scope of this project to devise a suitable method for calibrating gypsum blocks for soil moisture measurements Meter Readings vs Soil Moisture Tension and Electrical Resistance 100 2 o o o LM 4 4 SERENE 87 0 L L LLE BLOCK RESISTANCE IN OHMS ED REM 01 990 0 AE a ae jeg METER READING Figure 1 Exampl
16. he resistance R of the gypsum block in a double log scale a curve fit having the following expression can be found In R 1 4 In s where and B are fit constants The calibration temperature is equal to 23 C in this case For resistance measurements made at temperatures deviating from the calibration temperature the following expression may be applied R T R T 09 1 0 064 Ty T where is the calibration temperature 23 C T is the t emperature at time of measurement This expression is only valid for T 23 C If the calibration is carried out at another temperature the corresponding relation must be found 65 5 Having determined the calibration factors A and the gypsum block can be used to obtain the suction pressure in soil The suction pressure is then determined from the following expression ms ew 02 where is the resistance of the gypsum block according to Section 6 5 4 6 6 Accuracy The accuracy of the calibration method depends on the handling of the samples Samples for determining the water content of kaolin mixes should be handled in such a way that drying of the wet samples is reduced until mwer is determined Required minimum size of samples is related to the resolution of the balance 6 7 Test report The test report shall include the following information a Name and address of the testing laboratory b Identification number of th
17. ks have been used to determine soil moisture content This report describes a method for calibrating gypsum blocks for soil moisture measurements Moisture conditions inside a building are strongly influen ced by the moisture conditions in the soil surrounding the building Consequently measuring the moisture of the surrounding soil is of great importance for detecting the source of moisture in a building Up till now information has been needed to carry out in dividual calibrations for the different types of gypsum blocks available on the market and to account for indivi dual temperature dependencies But with this report the information is now available 1 edition 2001 ISBN 87 563 1075 7 ISSN 1600 8022
18. n on temperature dependency This report describes a method for calibrating gypsum blocks for soil moisture measurements By og Byg Danish Building and Urban Research Building Technology and Productivity Division January 2001 J rgen Nielsen Head of Division Summary This report describes the results of Nordtest project 1424 98 with the purpose of developing a calibration procedure for determination of soil moisture by means of gypsum blocks The calibration procedure has been tested for repeatability and for temperature dependency The method suggested for calibration of gypsum blocks is based on artificial soil made by mixing neat kaolin and water The method requires knowledge of the retention curve for kaolin 1 Introduction Moisture conditions in the soil surrounding a building are of great importance to the moisture conditions inside the building and some cases of moisture problems in buildings may be due to high moisture content in the surrounding soil In order to detect the source of water in the building measurements of soil moisture are necessary Measuring soil moisture Soil moisture can be measured by use of gypsum blocks basically by using the method for wooden dowels described in NT Build 420 1993 Gypsum is an inorganic material and therefore gypsum blocks are not sensitive to rot contrary to wooden dowels This fact is crucial when dealing with soil measurements since the moisture content of soil can be very hig
19. n used in soil science for the past 50 years and several types of commercial gypsum blocks are available on the market The gypsum blocks are delivered with a standard curve showing the relation between measured resistance and suction pressure There is no calibration curve for the actual gypsum block and there is no information on temperature dependency This calibration procedure serves to calibrate gypsum blocks prior to application 3 References 4 Definitions Moisture content The gravimetric moisture content u kg kg is the ratio between the mass of evaporable water and the oven dry mass of material Relative humidity Relative humidity RH of air is the ratio between the actual partial water vapor pressure and the saturation water vapor pressure at the temperature of the air Suction pressure The suction pressure s bar is the pressure difference between the pore water pressure and the ambient total pressure Retention curve or soil moisture retention curve Relationship between soil moisture content and suction pressure 5 Sampling Generally the number of samples decides the accuracy of the calibration At least 3 samples are needed The sample size should be at least 2x150 ml of kaolin mix 6 Test method 6 1 Principle Gypsum blocks are brought in suction pressure equilibrium with kaolin at given moisture contents AC resistance of a gypsum block is determined at time of equilibrium The water content of the s
20. ng two or more gypsum blocks to the data logger This eliminates interference between the channels The measuring range of the gypsum blocks is from suction pressure of 0 1 bar to 15 bar This range corresponds to very high relative humidity about 99 to 99 99 Artificial soil samples have been used in the calibrations in order to obtain well defined suction pressure Kaolin is a very fine grained powder with a behavior close to that of clay The retention curve describes the relation between water content and suction pressure of a porous material In Figure 3 typical retention curves are shown for different types of soil Water content vol vol 0 6 0 5 0 4 0 3 0 2 0 1 Clay loam m muU Ir ps P uM d V vd Clay P d Fine sand 2226 E P d s _ Coarse sand 0 1 Suction bar 0 01 Figure 3 Typical retention curves for sand and clay van Es and Ogden 1997 Here the water content is the volumetric water content The retention curve for clay covers a relatively wide range of water contents ranging from 0 1 to 15 bar Considering the same behavior for kaolin would facilitate control of suction pressure by water content of the kaolin mixes 10 3 Calibration procedure for gypsum blocks Water kaolin mixes of water contents between 20 and 60 96 of the dry weight of kaolin were prepared by use of an electrical mixer Kaolin mixes having a water content below approximat
21. niversity His help and kindness are highly appreciated 9 References Bouyoucos G J 1965 Plaster of paris block electrical measuring unit for making a continuous measurement of soil moisture under field conditions In A Wexler Ed Humidity and moisture Measurement and control in science and industry Vol 4 pp 105 111 New York Reinhold van Es H and Ogden C 1997 Water in soil Ithaca NY Cornell University Available 23trd August 1999 at http wwwscas cit cornell edu hmv1 watrsoil charcurv htm Nordtest 1993 Building materials wood Moisture content with electrical resistance in built in wooden probes NT Build 420 Espoo Finland Stramdahl 1997 Fugt i byggematerialer med fokus vandbinding i det overhygroskopiske omrade in Danish Moisture in building materials with focus on water retention in the superhygroscopic region MSc thesis Lyngby Danmarks Tekniske Universitet Institut for Baerende Konstruktioner og Materialer 19 20 Appendix A Test Method Calibration of gypsum blocks for soil moisture measurements Key words Building materials moisture soil calibration gypsum block electrical resistance suction 1 Scope This test method covers the determination of calibration curves of gypsum blocks used to determine the suction pressure in soil 2 Field of application Gypsum blocks can be used to determine the suction pressure in soil indirectly Gypsum blocks have bee
22. ns The psychrometer constant varies with atmospheric pressure and is relatively insensitive to temperature In Figure 5 the psychrometer factors from 3 series of calibrations are shown 11 12 0 1 V 0 08 v V V m M 5 V z V 5 006 v o S E 004 o dd gt e a 0 02 0 HT I A 0 05 0 2 0 3 0 05 0 2 0 3 0 05 0 2 0 3 NaCl concentration molality Figure 5 Results from calibration of SC 10A Thermocouple Psychrometer The calibration is based on NaCl solutions The straight line indicates the average psychrometer factor used in the tests In Figure 5 a straight line indicates the average psychrometer constant 0 0763 kPa C Measurements carried out with the thermocouple psychrometer were carried out as specified in the user s manual Small sample cups were filled with kaolin samples and placed in the set up for equilibrium After measuring the cup and samples were weighed on a high resolution balance 0 001 g and the samples were oven dried at 105 C and re weighed Also the empty cups were weighed for determination of water content of the sample The retention curve was obtained by plotting the measured suction pressure against the measured water content In Figure 6 the results from the measurements are shown and a curve fit based on the results is given The curve fit for the retention curve is presented by the following equation 0 202 bar 0 7
23. ple psychrometer could provide the most precise results on suction pressure in the range from 0 1 bar to 15 bar Alternative methods could have been among others the pressure plate pressure membrane method the Whatmann No 42 filterpaper method determining the moisture uptake from a soil sample and the tensiometer method In Figure 4 an illustration is shown of the measuring range of different types of measuring equipment for determination of suction pressure Pressure plate pressure membrane Whatman No 42 Tensiometer Decagon Thermocouple Psychrometer 2 Gypsum blocks Water content g g Retention curve for kaolin 1000 100 10 1 0 1 0 01 Suction bar Figure 4 Illustration of measuring range for different types of measuring equipment for determination of suction pressure According to the operator s manual the Decagon SC 10A psychrometer should be able to measure suction pressure down to 0 bar 0 uV However the resolution of the nanovoltmeter measuring the voltage over the thermocouple is approximately 100 nV leading to a resolution of 0 2 bar The thermocouple psychrometer had been calibrated on NaCl solutions prior to testing on kaolin samples Well known levels of suction pressure were obtained using NaCl solutions at prescribed concentrations of 0 05 molal 0 2 molal and 0 3 molal The psychrometer constant is a calibration factor determined from these calibratio
24. re The constants 4 and B are fit parameters The results from the curve fitting are given in Table 1 Table 1 Results from calibration of 6 gypsum blocks Gypsum block A B No 1 1 90 5 88 No 2 2 17 6 06 No 3 1 49 6 37 No 4 1 83 5 80 No 5 1 69 6 12 No 6 1 42 5 98 Results from the investigation of temperature dependency on the resistance of the gypsum blocks are shown in Figure 8 The results present the influence of temperature on resistance of 6 gypsum blocks stored in different kaolin mixes 13 14 16000 Gypsum block n No 1 R 23 C 656 Ohm FL MEN Paano No 2 R 23 C 133 Ohm NEIGE D OCS No 3 R 23 C 109 Ohm 12000 Ss No 6 R 23 C 1001 Ohm E 96 No 5 R 23 C 3141 Ohm t TTT No4 R 23 C 6153 Ohm ew 905 9 8000 22 g STRA URS y el e Nes 9 o WA ARR BEN ABA AC 4000 xr e E DTI ne anman pugag WY Y Y Y MARS 22 EL e p 0 T IE DE E GG G CH ORE S 4C T T T 0 5 10 15 20 25 Temperature C Figure 8 Influence of temperature on resistance of gypsum blocks As indicated by the linear curve fits in Figure 8 the resistance of gypsum blocks may be considered linearly related to the temperature In Figure 9a plot of the slope of the curves in Figure 8 is shown against the resistance of the gypsum block measured at 23 C 400 300
25. rtificial soil Knowledge of the retention curve for kaolin is required An experimental investigation of the relation between moisture content and suction pressure for kaolin has been carried out Based on the results from this investigation a retention curve has been suggested An investigation has been carried out concerning the influence of temperature on the resistance of the gypsum blocks The results have led to a proposal for a rule of thumb for the relation between resistance measured at temperatures other than 23 C The calibration procedure has been performed on 6 gypsum blocks at 4 different moisture levels The results from the calibration show only small differences between the gypsum blocks However the results from the calibration are differs significantly from the standard curve given by the manufacturer of the gypsum blocks This may be due to the fact that the retention curve is prone to large uncertainties Due to the inadequate information available on calibrations and temperature dependency of gypsum blocks today it is recommended that the calibration method is published as a Nordtest Method Furthermore it is suggested that an inter laboratory test is carried out in order to improve the accuracy of the retention curve 17 18 8 Acknowledgement The SC 10A Thermocouple Psychrometer for determination of the retention curve of kaolin was kindly lent by Dr Christian Jensen at The Royal Veterinary and Agricultural U
26. urrounding kaolin is determined at time of equilibrium The retention curve for kaolin is used to determine the kaolin suction pressure at equilibrium from the water content The measured AC resistance at equilibrium versus the kaolin suction pressure constitutes the calibration curve for the gypsum block 6 2 Apparatus Electric mixer i g mixer for cement paste and mortar for mixing kaolin and water Ventilated oven capable of maintaining a desired temperature of 105 C with an accuracy of 2 C Containers provided with a cap Gypsum block for measurement of suction pressure High resolution AC resistance measurement apparatus Kaolin powder i g Fluka Chemie No 60609 distributed by Sigma Aldrich Chemie Balance with a resolution of at least 0 01 g preferably 0 001 g Thermometer with a resolution of 0 5 C Thin walled pipe with an outer diameter equal to the diameter of a gypsum block The length should be 10 15 cm 6 3 Preparation of test specimens The gypsum blocks are soaked in water for at least 1 hour and then dried at room condition This procedure is carried out twice in order to improve the performance of the gypsum blocks 6 4 Procedure 6 4 1 The test shall be carried out at a constant temperature of 23 1 C 6 4 2 Kaolin and water are mixed thoroughly The content of water should range from 20 to 60 by mass of the kaolin 643 Sets of two containers A and B with the same kaolin mixes are pr
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