Raingauge - Ekotechnika
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1. User Manual for the Raingauge type RG1 RG1 UM 3 A E DELTA T DEVICES ABOUT THIS MANUAL COPYRIGHT This manual describes how to install and use the raingauge type RG1 l Copyright O 1996 Delta T Devices Ltd 128 Low Road Burwell Cambridge CB5 OEJ England All rights reserved Under the copyright laws this book may not be copied in whole or in part without the written consent of Delta T Devices Ltd Under the law copying includes translation into another language Revised Jan 1996 In this Manual all references to loggers and weather stations relate to the Delta T Logger DL2 its software LS2 the WS01 Weather Station and the sensor codes used with them The references also apply to the DL2E and its derivatives For use with the DL3000 logger you must refer to the DL3000 documentation It contains the specific connection details and sensor type codes used by the DL3000 General information concerning the performance and installation of the sensor contained in this manual remains relevant Page ii Raingauge Delta T Serial Number RGI Sensor type code RG1 Date Calibration Correction of last Factor Factor calibration SSSSSSSSCSSSSSASSSASHSSSSSHHEHHSESHSHSESEESHSSSHEHSSESCSESCOHCHHBSOCHEH SCUESHFSOSHCHOEHHSSHREHSHKRAHSSAOKROSHSHEHOHESETSEAEHEEE SSCSSSSSSSSSSSSSSSSSSSSSSSSSSSSseseeeseeseseseseeeeees TOT UTETETTITTETTTETETITET TTT TTT 1 Oopgs proRnnnectrn peon sn re2. parts and accessories for sensors not manufactured by Delta T but supplied as part of the weather station may be obtained from the original manufacturer We will endeavour to obtain parts if requested but a certain amount of additional delay is inevitable Should it prove necessary instruments may be returned to our works for servicing We normally expect to complete repairs of our own instruments within 2 days of receiving the equipment Other manufacturers sensors supplied by us and returned for servicing will take longer They will have to be returned to the original manufacturer for servicing and may be subject to additional delays of two to four weeks Users in countries which have a Delta T Agent or Technical Representative should contact them in the first instance Jan 96
3. practical it should be done using a good quality graduated measuring cylinder Allow the water to drip slowly into the gauge taking at least 30 minutes to empty the container At the end of this period there should have been 80 tips It is unlikely however that there will be exactly this number Note how many tips actually occurred and also estimate as precise ly as possible what fraction of a tip is left in the bucket Only if the adjustment has been very precise indeed will the bucket tip at the moment the last drop passes into it The Calibration Factor can now be calculated Page 9 RGI FIGURE 5 1 DIAGRAM OF RAINGAUGE DYNAMIC CALIBRATION SET UP DYNAMIC CALIBRATION METHOD om CONTAINER seit es HOLDING 1000cc WATER TAP FOR ADJUSTING U RATE OF DRIPPING ENSURE BUCKET IS LEVELLED PRECISELY RAINGAUGE FITTED WITH COUNTER LOGGER SUPPORT STATIC CALIBRATION METHOD PIPETTE WM MMM LL VM MMM Page 10 RGl How to Calculate the Calibration Factor Let N equal the number of tips This should include an estimate of the fraction of a tip left in the bucket eg 0 5 of a tip The Calibration Factor is then 0 2 x 80 N For example if N 78 8 then 0 2 x 80 78 8 0 203 mm per tip Provided the Conversion Factor lies between 0 195 and 0 205 mm or 78 0 and 82 1 tips respectively then it is acceptable for most purposes The gauge is calibrated at manufacture such that each
4. tip will lie within the above limits There is no reason however why users should not if they so wish aim for closer agreement with the nominal tip of 0 2 mm by re peated adjustments of the calibration screws followed by further dynamic tests Only time limitations will set a limit to this Please refer to the appropriate section of the Delta Logger User Manual for general instructions Page 11 RGI REFERENCES 1 Rodda J C 1967 The rainfall measurement problem Proc IAHS Gen Ass Bern IAHS Pub No 78 215 231 2 HMSO 1956 Handbook of Meteorological Instruments Part 1 Met O 577 published by Her Majesty s Stationery Office U K 3 HMSO 1982 Observers Handbook Met O 933 published by Her Majesty s Stationery Office U K 4 Hughes C Strangeways I amp Roberts A M Field Evaluation of two aerodynamic raingauges Weather 1993 Vol 48 No 3 66 71 GLOSSARY OF TERMS Calibration Factor Conversion Factor Defined by the raingauge manufacturer as the number of mm of rain per tip eg 0 198 mm tip The number of tips per mm of rain This is the reciprocal of the calibration factor and is the figure the logger software will ask for when defining a sensor type library The logger will round this to the nearest integer 5 Correction Factor The number by which the output of the logger must be multiplied in order to compensate for the integer conversion factor and so calculate t
5. dcoon nono cnn nop pg n CONTENTS INTRODUCTION SITING THE RAINGAUGE WIRING CONNECTIONS CONFIGURING THE LOGGER USE OF CONVERSION amp CORRECTION FACTORS SPECIFICATION CALIBRATION 9 NO NWN wm A REFERENCES 12 GLOSSARY 12 GUARANTEE REPAIRS amp SPARES Page 3 RG1 INTRODUCTION Conventionally shaped raingauges interfere with the airflow so that the catch is reduced This raingauge was aerodynamically designed by the Institute of Hydrology and is very similar to that derived theoretically and independently by the UK a ie Office as an ideal shape The raingauge is fabricated in UV resistant vacuum formed plas tic It operates in the same way as any other tipping bucket rain gauge The water collected by the funnel is passed into a double sided container which tips when a set amount of water has been collected in one side the process repeating indefinitely At each tip the bucket causes a magnet to pass a magnetic reed switch closing its contact for a fraction of a second This contact closure is counted by the Delta T Logger The gauge is calibrated to tip once for each 0 2 mm of rain An individual calibration factor is given for each raingauge Page 4 RG1 SITING THE RAINGAUGE The gauge should be installed in accordance with the standard rules applicable to any raingauge such as ensuring that it is not closer to an object by more than twice the object s height This avoids any shiel
6. ding effect The weather station mast has quite a low wind profile but we provide 5m of cable for mounting the raingauge at a distance from the mast Further information on raingauge exposure is given in the publications listed at the end of this section Research has shown that since a raingauge acts as an obstruction to the flow of wind the flow of air speeds over the top causing less rain to be collected by the funnel than would have fallen on the ground if the raingauge had not been there In most cases this is ignored but it may be corrected for arithmetically or overcome physically by placing the gauge in a pit with the tip of its funnel level with the ground The pit is covered by a grating to simulate the ground aerodynamically while preventing splash in to the funnel While excellent this method may not always be prac tical To minimise the effect of wind turbulence around the fun nel the profile of the raingauge has been designed to reduce wind effects so that the catch of the gauge is increased Such a gauge can therefore be deployed successfully in more exposed sites An anodised aluminium base plate is provided for levelling the raingauge and to attach it firmly to the ground This plate can either be pinned to the ground with the stakes provided or it can be bolted to a block of concrete or wood Three threaded studs attached to the base plate are bolted to the plastic base of the raingauge It can be levelled by adjusting t
7. eply mm It will then ask you the number of counts per mm The accurate Conversion Factor to use is in this example 5 051 counts per mm However the logger will not accept a non integer conversion factor for a counter input thus losing the potential high accuracy which can be achieved with this type of calibration This is a limitation in the software of the logger If you tell it 5 051 it will round the conversion factor to the nearest integer value ie 5 The only way to regain the accuracy of the measured rainfall is to multiply the logged data by a correction factor corresponding to the difference between the true conversion factor and the rounded integer value used by the logger For example Given 1000 tips during a recording interval The calibration for a particular raingauge is 0 198 mm per tip ie 5 051 tips per mm The logger calculates 1000 5 200 mm of rain The true value is 1000 5 051 or 1000 x 0 198 198 mm of rain The correction factor to be applied is 198 200 0 99 This procedure acknowledges two things Firstly that the user probably does not have the time to adjust and test adjust and test many times over to get the setting exactly on 0 2 mm per tip nor to repeat such a laborious procedure on a regular say yearly basis Secondly the logger software cannot handle non integer conversion factors for the counter input channels The processed data printed out by the logger or collected by your compute
8. g the calibration Only by means of a dynamic test can the exact calibration of a tipping bucket of any type be established This test is described below Many manu facturers and users of tipping bucket raingauges aim to adjust the bucket settings until EXACTLY the correct calibration is obtained To achieve this 1t would be necessary to repeat the dynamic test many times over and this is very time consuming In any case it 1s almost impossible to get the adjustment absolutely correct Therefore it is considered better practice by the Institute of Hy drology to adjust the settings as closely as is reasonably practical and then to derive a Calibration Factor for each gauge The Calibration Factor can be found using the dynamic calibration procedure described below a Dynamic Calibration When the static burette test described above has been done the calibration of the gauge should be checked as follows most users will start from here This procedure was carried out prior to dispatch and the resulting Calibration Factor written inside the raingauge and at the front of this chapter Set up the calibration test as shown in Figure 5 1 Ensure that the gauge is level Connect the output wires to channel 61 or 62 of the logger or any convenient electronic or electromechanical counter Fill the container with 810 4 cc of water This 1s done most precisely by weighing the water on scales capable of measur ing to 0 1 gm If this is not
9. he heights of the nuts on the studs and by observing the built in spirit level It is important to ensure that the raingauge is level Failure to do this will result in a systematic error Many users pay only scant attention to this yet it is one of the simplest means of improving the accuracy of rainfall measurements Page 5 RG1 WIRING CONNECTIONS RAINGAUGE LOGGER earth T a Ga ieee a tp l J Pe l CS CHANNEL 61 l Digital L aes m The screen may be connected either to the Logger digital earth or to the raingauge frame It should not be connected to both _ Otherwise there is a possibility of errors from earth loops CONFIGURING THE LOGGER Instructions on configuring the logger using the PC software are given in the Delta T Logger User Manual The example wiring diagram assumes that the raingauge is con nected to channel 61 For highest accuracy the results should be corrected as described below see Use of Conversion and Correction Factors Page 6 RG1 USE OF CONVERSION AND CORRECTION FACTORS Consider this example For a particular raingauge it is found that each tip corresponds to 0 198 mm of rain This corresponds to a Conversion Factor of 1 0 198 5 051 tips per mm of rain When you programme the logger and are defining a raingauge sensor type the logger will ask you what engineering units to use to which you r
10. he true rainfall Calibration Factor x 5 eg 0 198 x 5 0 99 Page 12 GUARANTEE REPAIRS AND SPARES Our Conditions of Sale ref COND 91 11 set out Delta T s legal obligations on these matters For your information the following paragraphs summarise Delta T s position but reference should always be made to our Conditions of Sale which prevail over the following explanation Instruments manufactured by Delta T are guaranteed for one year against defects in manufacture or materials used The guarantee does not cover damage through misuse or inexpert servicing or other circumstances beyond our control For the U K this means that no charges are made for labour materials or return carriage for guarantee repairs For other countries the guarantee covers free exchange of faulty parts during the guarantee period Alternatively if the equipment is returned to us for guarantee repair we make no charge for labour or materials but we do charge for carriage and U K customs clearance We strongly prefer to have such repairs discussed with us first and if we agree that the equipment does need to be returned we may at our discretion waive these charges SERVICE AND SPARES We recognise that some users of our instruments may not have easy access to technically specialised backup Spare parts for our own instruments can be supplied from our works These can normally be despatched within 1 working day of receiving an order Spare
11. r should be multiplied by the appropriate correction factor Page 7 RGI SPECIFICATIONS Sensitivity Calibration Calibration limits Accuracy Funnel diameter Overall height Switch Output Weight Levelling Mounting Cable 0 2 mm per tip nominal Individual calibration factor provided 0 195 to 0 205 mm per tip No accepted standard for comparison 234 mm 10 inches 350 mm without baseplate and tripod magnetic reed switch 300 mA max current allowable Contact closure at tip 1 0 kg without base plate and tripod Tripod levelling mechanism with spirit level Anodised aluminium base plate pinned to the ground with 4 zinc plated steel stakes Alternatively the base plate can be bolted to a block of concrete or to a heavy block of wood 6 m PVC screened twisted pair Page 8 RGl CALIBRATION PROCEDURES Static Calibration The raingauge is calibrated during manufacture as described below This may be repeated by the user if desired Install the raingauge as illustrated in Figure 5 1 over a sink ensur ing that itis level Using a burette or pipette adjust the two cali bration screws under the tipping bucket until 1t tips with the correct amount of water 10 13 cc for a 0 2 mm sensitivity It will not be possible to set the screws with very high precision in this way but 1t should be done with as much care as possible This stage may be omitted if recheckin
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