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SWT4 User Manual v3.1

Service and maintenance

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33

4.2 Testing

4.2.1 Calibration

When delivered Tensiometers are calibrated with an offset of 0 hPa
(when lying horizontally) and a linear response. The offset of the
pressure transducer has a minimal drift over the years. Therefore, we
recommend you check sensors once a year and re-calibrate them
every two years.
Return the Tensiometers to Delta-T for recalibration, or contact us for
details about available calibration accessories.

4.2.2 Check the Offset

There are two ways to check the offset.

1. Connect the Tensiometer to a readout device. Fill the beaker with
distilled or de-ionized water and immerse the ceramic cup to a depth of
7.5 cm. Wait until the reading is stable. If there are bubbles inside the
cup this might take a while. The reading now should be
0 kPa.

2. To check the zero-point more precisely unscrew the cup. Shake the
pressure sensor to remove water from the pressure transducer hole.
The offset is acceptable when the reading is between -0.5 and +0.5
kPa.

The pressure sensor diaphragm is inside the small hole on the
pressure sensor body. It is very sensitive and must never be
touched! It can be destroyed even by slightest contact! No
contamination should get on the sealing and gasket.

Before reassembling cup and sensor body carry out the degassing
procedure (see chapter 4.1.2 "Refilling in the lab" on page 27).

For testing the gain a calibration kit is required. Contact Delta-T for
details

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Содержание SWT4

Страница 1: ...User Manual for the Tensiometer types SWT4 SWT4R Delta T Devices Ltd SWT4 UM 3 1 ...

Страница 2: ...d by the manufacturer Design changes Delta T Devices Ltd reserves the right to change the designs and specifications of its products at any time without prior notice SWT4 User Manual Version 3 1 Oct 2009 Delta T Devices Ltd Tel 44 1638 742922 130 Low Road Burwell Fax 44 1638 743155 CAMBRIDGE CB25 0EJ e mail sales delta t co uk U K tech support delta t co uk web http www delta t co uk Strictly obse...

Страница 3: ... 3 1 1 Selecting the measuring site 15 3 1 2 Number of Tensiometers per level 15 3 1 3 Extent of the site 15 3 1 4 Protection of refilling tubes SWT4R only 16 3 1 5 Use of Tube Jackets and Conduit 16 3 1 6 Ideal conditions for installation 17 3 1 7 Documentation 17 3 2 Selecting the installation angle 18 3 2 1 Vertical tilting downwards SWT4R only 18 3 2 2 Horizontal tilting upwards SWT4R only 18 ...

Страница 4: ...ing 35 6 Useful notes 37 6 1 Maximum measuring range and data interpretation 37 6 2 Temperature influences 39 6 3 Vapor pressure influence 39 6 4 Osmotic effect 39 6 5 Using Tensiometers as a piezometer 40 7 Troubleshooting 40 8 Appendix 41 8 1 Technical specifications 41 8 2 Wiring configuration 42 8 3 Accessories 43 8 3 1 Connecting and extension cables 43 8 3 2 Handheld measuring device 44 9 Te...

Страница 5: ...ht damage sensors and instruments Frost Tensiometers are filled with water and therefore are sensitive to frost Protect Tensiometers from frost at any time Never leave Tensiometers over night inside a cabin or car when freezing temperatures might occur Tensiometers are not usually damaged when the cup is installed in a frost free soil horizon Excess pressure The maximum non destructive pressure is...

Страница 6: ... directly depends on it being used correctly At the beginning of a measuring task or research project the target all effective values and the surrounding conditions should be defined This then leads to the requirements for the scientific and technical project management which describes all quality related processes and decides the methods to be used the technical and measurement tools the verifica...

Страница 7: ...nsiometers measure soil water tension a measure of the soil matrix potential which is the work the plant needs to do in order to extract water from a unit volume of the soil These Tensiometers work from 100 kPa water pressure to 85 kPa suction or soil water tension If the soil gets drier than 85 kPa the Tensiometer runs dry and must be refilled as soon as the soil is sufficiently moist again see F...

Страница 8: ...d or pebbly soils a fine pored slurry might create a water reservoir which slows down the response With the special Tensiometer gouge auger type SWT4 AUG slurrying is unnecessary because of the accurate fit of the Tensiometer into the hole 3 Take off the protective plastic bottle from the Tensiometer cup Tilt and pull the bottle off carefully If necessary carefully turn it counter clockwise only a...

Страница 9: ...rk must exactly face down The optimal installation angle is about 5 Fig 1a Downwards installation Fig 1b Upwards installation 5 Push down the shaft water retaining disk to a position directly on the soil surface 6 For SWT4R Slide a thermal insulation tube over the capillary filling tubes 7 If the plug is not connected right away leave the protective cover on the plug Dirt will influence the imperm...

Страница 10: ...nfield7 Manual Readout Unit or a data logger Keep it less than 200 kPa i e 2000 hPa The less air left inside the cup and the better the soil s conductivity is the faster the Tensiometer will respond to tension changes If the soil is dryer than 90 kPa 900 hPa it does not make sense to refill the Tensiometer immediately Wait until any Tensiometer that is installed in the next lower level again reach...

Страница 11: ... into water Cable gland IP67 SWT4 and SWT4R can be completely buried if required If buried cables and tubes should be protected Special cable glands are available for making tight seal to with plastic protection tube for the cable see Accessories on page 43 Transparent acrylic shaft One piece shafts from 10 cm to 200 cm are available Shafts over 200 cm are divided with threaded adapter and are ava...

Страница 12: ...e of the pressure sensor The maximum permissible i e non destructive pressure is 3 bar 300 kPa 3000 hPa Higher pressure will damage the sensor and absolutely must be avoided High pressures can appear for example when cup and sensor are reassembled when inserted in wet clayey soils or in tri axial vessels used for measuring shear forces 2 1 3 Reference air pressure The reference atmospheric air pre...

Страница 13: ...as outstanding suitability to work as the semi permeable diaphragm for Tensiometers The cup has a lifetime guarantee against breakage Ceramic cup Do not touch the cup with your fingers Grease sweat or soap residues will influence the ceramic s hydrophilic performance 2 2 External refilling SWT4R only The SWT4R is equipped with 2 stainless steel capillary tubes which are led through the cable gland...

Страница 14: ...ut signal directly depends on the supply voltage the supply voltage needs to be constant and stabilized As the pressure transducer is a Wheatstone full bridge it has to be connected in a certain way See also Connecting SWT4 and SWT4R on page 23 and the user manual for your display unit or data logger before connection Black mark on tube ...

Страница 15: ...d or taken into account when interpreting the data 3 1 2 Number of Tensiometers per level The deeper the level the less the variation is in water potential In lower sandy or pebbly horizons just one Tensiometer per depth is sufficient Close to the surface about 3 Tensiometers per level are recommended Guiding principle More heterogeneous sites and soil structures require a higher number of Tensiom...

Страница 16: ...parts of the tube will expand when heated up by solar energy This causes a drop of the water tension and some water will flow from the cup into the ground Thus readings will fluctuate around the actual reading during solar radiation specially with low water potentials Furthermore under permanent solar exposure the tubes get sticky and brownish Slide the supplied thermal insulation tube over the sh...

Страница 17: ...g has to be repeated to reach the required depth 3 1 7 Documentation For every measuring spot you should Measure out the position where the pressure sensor will be placed A must for installations below the ground surface Take documentary photos before during and after installation Save a soil sample Write down installation depth and angle with each sensor identification serial number Mark all conn...

Страница 18: ...erting the Tensiometer orient the shaft so the black mark near the shaft end points upwards Do not rotate the shaft after it is inserted into the ground as this might loosen the cup 3 2 2 Horizontal tilting upwards SWT4R only When installed horizontally from inside a well or pit hole the Tensiometer must point upwards This means the cup is in a higher position than the end of the shaft An upward a...

Страница 19: ... the hydrophilic performance Do not leave the cup in air for more than 5 minutes as Tensiometer water will evaporate and the Tensiometer will need to be refilled Procedure 1 Mark the required drilling depth on auger and Tensiometer shaft The reference point is the center of the cup Drill a hole with the desired depth on the chosen measuring spot Auger in steps Take care when drilling the last 20 c...

Страница 20: ...e any force Do not hit the Tensiometer this may damage cup and pressure sensor Especially in clayey soils the pressure reading must be monitored as high pressures might build up The pressure should not exceed 200 kPa 2000 hPa because 300 kPa is fatal Note for SWT4R Tensiometers Important Pay attention to the engraved black spot on the shaft s top end that marks the position of the exit opening of ...

Страница 21: ... signal cable and lead it back through the thermal tube 12 Protect the cables against rodent bites Lead the cables through plastic pipes or use the plastic protection tubes conduit or trunking Contact Delta T for details 3 4 Offset correction for non horizontal installations The pressure transducer is calibrated without a cup Thus no compensation is required for horizontal installations If a SWT4 ...

Страница 22: ...45 0 43 Angle to vertical line 45 60 70 75 80 90 Offset correction in kPa 0 35 0 25 1 7 1 3 0 9 0 The offset is entered as in your logger if you regard the soil water tension to be negative 0 85 kPa 3 4 1 How to measure the zero offset of a tilted tensiometer As an alternative to using the table above if the tensiometer is to be installed pointing downwards submerge it up to one half the height of...

Страница 23: ...calibrated at 10 6 VDC It requires a stabilized power supply Some logger types can measure bridge circuits directly other loggers require additional measures because the signal minus and the supply minus do not have the same ground If supplied with exactly 10 6 V i e supply minus 0 V and supply plus 10 6 V the Tensiometer output signal range is between 3 2 V min and 6 8 V max relative to the power...

Страница 24: ...le GP1 Logger A GP1 logger can take two SWT Tensiometers Each tensiometer requires a GP1 PBA1 adapter and connection cables type SWTCC XX In addition 2 SM200 soil moisture sensors or 10K thermistors and a WET sensor may also be connected at the same time along with two digital sensors such as rain gauges When fitting 5 or more sensors use the GP1 LID2 expansion lid which has additional cable gland...

Страница 25: ...onnected to a DL2e logger It provides a stabilised 10 6 V power supply but with the supply minus at 5 V and supply plus at 5 6 V This arrangement ensures the Tensiometer signals are less than 1 V The TV Batt itself requires power from a 12V battery or a mains to 12 V power converter Each TVB M module has screw terminals to power up to 15 Tensiometers See also Voltage regulators on page 46 ...

Страница 26: ... finger Now draw up the syringe as far as possible to create a vacuum inside Rotate the syringe to create one big bubble Remove your finger or open the valve and squeeze out the bubble Repeat this procedure a few times 4 1 1 When do Tensiometers need to be refilled Tensiometers need to be refilled if the curve of the readings apparently gets flatter for example a rain event has no sharp peak but i...

Страница 27: ...t necessarily have to be degassed when a pump is used Fig 4 1 Laboratory refill kit SWT4 RK2 2 Unscrew the cup in clockwise direction and empty it Do not touch the ceramic cup with your fingers Wrap a clean towel or paper towels around the cup The pressure sensor diaphragm is inside the small hole on the pressure sensor body It is very sensitive and must never be touched It can be destroyed even b...

Страница 28: ... and body and the cup is completely filled with water 7 Before screwing together cup and sensor body connect the sensor to a measuring device to observe the pressure signal Warning Do not exceed 2 bar as 3 bar will destroy the sensor Hold the cup in an upright position fill it completely and with an overlapping meniscus of water Carefully and slowly screw the cup on the sensor body Allow the exces...

Страница 29: ... from the soil If the refilling tubes have a total length of 5 meters or more use the hand operated vacuum pump see section 4 1 4 With this method a range of at least 80 kPa can be measured Fig 4 2 Downward tilted installation the marked tube is the refilling tube the unmarked tube the de airing tube Fig 4 3 Upwards tilted installation the marked tube is the de airing tube the unmarked tube the re...

Страница 30: ...water into the refilling tube until no bubbles come out of the de airing tube Inject at least 25 ml Check the pressure all the time 6 Remove the syringe Put a drop of water on the ends of both the rubber and steel tubes and then connect them together 4 1 4 Refilling with a vacuum pump SWT4R only To achieve the maximum possible measuring range Tensiometers can be completely degassed using a vacuum ...

Страница 31: ...and keep an eye on the pressure signal all the time 2 In a downwards tilting installation the marked tube is the refilling tube In an upwards installation the unmarked tube is the refilling tube Pull off the rubber tube from the refilling tube 3 Degas the water inside the syringe as described above Connect the syringe to the refilling tube and close the valve 4 Connect vacuum bottle and de airing ...

Страница 32: ...e at the same time the air bubble is sucked into the vacuum bottle Repeat this 2 or 3 times until no more bubbles come out 6 Close the valve of the vacuum bottle and remove the bottle Inject 5 ml of water from the syringe into the refilling tube Remove the syringe Put a drop of water on the end of both the rubber and steel tubes and connect them together ...

Страница 33: ...ionized water and immerse the ceramic cup to a depth of 7 5 cm Wait until the reading is stable If there are bubbles inside the cup this might take a while The reading now should be 0 kPa 2 To check the zero point more precisely unscrew the cup Shake the pressure sensor to remove water from the pressure transducer hole The offset is acceptable when the reading is between 0 5 and 0 5 kPa The pressu...

Страница 34: ... below 5 C Do not leave filled Tensiometers overnight in your car in a measuring hut etc Do not fill the Tensiometers with Ethanol as this is corrosive to some materials i e PMMA and will destroy these Also it is not recommended to fill the Tensiometers with Decalin mono ethylene glycol di ethylene glycol etc These could harm any of the materials destroy the ceramic cup or leak into the soil SWT4 ...

Страница 35: ...usceptible to electrical surge Our equipment is protected against over voltage and wrong polarity as far as this is technically achievable There never can be total lightning protection Lighting strikes are unpredictable and vary significantly with region voltage and destructiveness Proper lightning protection has to be considered whenever a system with several sensors and loggers is installed Pass...

Страница 36: ...or lightning protection of enclosures Install surge protection devices in one corner of the measuring enclosure All lines to and from the surge protection devices should not run parallel System protection of stations with enclosure and mast Install a separate earth grid 50 cm below the soil surface to equalize variations in the ground potential between the sensors and the mast grounding rod Lightn...

Страница 37: ... with 20 C and an atmospheric pressure of 950 hPa the Tensiometer cannot measure a tension below 927 hPa even if the soils gets drier than that The readings remain at a constant value Fig 6 1 between day 10 and 16 Fig 6 1 Tensiometer readings with tensions beyond 15 000 hPa If the soil dries to 15 000 hPa the ceramic s bubble point is reached The water in the ceramic cup will run out quickly and t...

Страница 38: ...ctual soil water tension Depending on the size of the developed bubble readings will get less close to the maximum Fig 6 2 after day 20 Fig 6 2 Tensiometer readings with tensions to 10 000 hPa Other problems that can be recognized by checking the data Soil water tension normally change only slowly An erratic signal curve with many discontinuities could indicate for example loose contacts moisture ...

Страница 39: ...e at soil temperature from Scheffler Straub Grigull 6 3 Vapor pressure influence If the temperature of a soil with a constant water content rises from 20 C to 25 C the soil water tension is reduced for about 0 85 kPa due to the increased vapor pressure which opposes the water tension Temperature in C 4 10 16 20 25 30 50 70 Pressure change per Kelvin in hPa 0 6 0 9 1 2 1 5 1 9 2 5 7 2 14 6 4 Osmoti...

Страница 40: ...H2O 998205 kg dm3 at 4 C 1 0 kg dm3 Pa N m2 N kg m s2 Pa kg s2 m A water column of 100 cm causes the following pressure p Pa N m2 998 205 kg m3 x 9 81 m s2 x 1m p 9792 39 kg m3 x m s2 x m 9 792 kPa Accordingly 10 kPa at 20 C indicate a water column of 102 15 cm 7 Troubleshooting Please refer to the Delta T website where you will find a regularly up dated list of FAQs http www delta t co uk product...

Страница 41: ...length 1 5 m minus shaft length Length 2 3 m from sensor body effective length 2 3 m minus shaft length Plug Male 4 pin thread M12 IP67 Measuring range Pressure transducer Water tension Water level 100 kPa 100 kPa electronically 85 kPa 100 kPa physically 85 kPa 0 kPa Tensiometer 0 kPa 100 kPa Piezometer Output signal Pressure 85 mV 85 kPa Tensiometer 0 mV 0 kPa 100 mV 100 kPa Piezometer Accuracy P...

Страница 42: ...ser Manual v3 1 8 2 Wiring configuration Signal Wire Pin Function Vin brown 1 Supply plus V blue 3 Supply minus A OUT white 2 Signal plus A OUT black 4 Signal minus Pin and wire configuration for connecting cables SWTCC ...

Страница 43: ...WT EXT xx have one each male and female plug M12 IP67 Plugs are supplied with protective covers Item Delta T no 4 pin connection cables Length 1 5 m SWTCC 01 Length 5 m SWTCC 05 Length 10 m SWTCC 10 Length 20 m SWTCC 20 4 pin extension cable Length 5 m SWTEXT 05 Length 10 m SWTEXT 10 Length 20 m SWTEXT 20 Additional items Clip on cable markers Please contact Delta T Plastic protection tubes Please...

Страница 44: ...atus Includes an automatic offset correction for water column height and installation angle Suitable for all SWT Tensiometers The unit comes with a small carrying case Item Delta T no SWT MR manual readout SWT MR A USB converter is available for data collection from the SWT MR to a PC via PC or laptop It includes Windows PC software TensioVIEW Item Delta T no USB PC adapter for SWT MR SWT MR USB ...

Страница 45: ...gger A GP1 logger can take two SWT Tensiometers Each tensiometer requires a GP1 PBA1 adapter and connection cables type SWTCC XX In addition 2 SM200 soil moisture sensors or 10K thermistors and a WET sensor may also be connected at the same time along with two digital sensors such as rain gauges When fitting 5 or more sensors use the GP1 LID2 expansion lid which has additional cable glands DL2e Lo...

Страница 46: ...ish to fit more than 15 Tensiometers to one DL2e logger Item Delta T no TV batt for DL2e logger TVB1 TV batt module only TVB M 9 1 2 Refill kits SWT4 FRK2 Refill kit for externally refillable Tensiometers SWT4R incl hand operated vacuum pump 250 ml bottle refill syringe tubes valves SWT4 LRK2 Laboratory refill kit for Tensiometers SWT4 SWTT4R incl stand clamps adapter for SWT4 sensor body 500 ml b...

Страница 47: ...ge auger with specially shaped blade The tip of the blade has the same shape and diameter as the Tensiometer cup so the Tensiometer fits tightly into the borehole Thus no slurrying of the cup is necessary Set includes gouge auger and handle with hammering head Item Delta T no Auger with handle SWT4 AUG Extension rod length 100 cm Please ask ...

Страница 48: ...0 01 0 1 0 33 0 85 1 10 15 100 1 000 10 000 MPa 0 001 0 01 0 033 0 085 0 1 1 0 1 5 10 100 1 000 kPa J kg 1 10 33 85 1 100 1 000 1 500 10 000 100 000 1 000 000 cm H 2 0 9 8 98 1 323 6 834 5 980 7 9 806 6 14 709 9 98 066 5 980 665 9 806 650 hPa 10 100 330 851 1 000 10 000 15 000 100 000 1 000 000 10 000 000 pF 1 2 01 2 53 2 93 3 4 4 18 5 6 7 Field capacity Standard Tensiometer range Permanent wiltin...

Страница 49: ...ge or by repairing the goods free of charge at Delta T s premises in the UK during the warranty period If Delta T requires that goods under warranty be returned to them from overseas for repair Delta T shall not be liable for the cost of carriage or for customs clearance in respect of such goods However Delta T requires that such returns are discussed with them in advance and may at their discreti...

Страница 50: ...e normally expect to complete repairs within one or two weeks of receiving the equipment However if the equipment has to be forwarded to our original supplier for specialist repairs or recalibration additional delays of a few weeks may be expected For contact details see below Technical Support Users in countries that have a Delta T distributor or technical representative should contact them in th...

Страница 51: ...ic 13 Ceramic 41 clay 17 Conditions of sale 49 Conduit 16 connection cables 43 Contact Details 50 Copyright 2 Current consumption 41 D data logger 23 Description 12 DL2e Logger 24 DL6 tens 24 45 Durability 6 E Emptying 35 F Frost 34 G GP1 Logger 24 gravel 17 grounding 35 I Infield7 44 Installation 15 procedure 19 installation angle 18 J Jacket Tubes 16 L Lightning protectio 35 loess 17 loggers 45 ...

Страница 52: ... RK2 18 Refill kits 46 refilling 13 Refilling 26 S Service 26 50 soil sample 17 soils 7 Spares 50 specifications 41 SWT MR 44 T Technical Support 49 50 Temperature 39 Terms 49 Testing 33 Thermal effect 16 Troubleshooting 40 tubes insulation 43 protection 43 U Units 48 Unpacking 6 USB converter 44 V Vapor pressure 39 Voltage regulator 25 Voltage regulators 46 voltage Vin 41 W warnings 5 Wiring 42 Z...

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