Introduction
There are many reasons for wanting to measure rainfall, and not just the quantity of rain but its intensity. A highly intense storm, lasting just a few minutes, can cause massive damage in agriculture, eroding the topsoil of unplanted fields and dumping millions of tonnes of silt in rivers. Flash floods cause damage everywhere in the world to infrastructure and buildings, and can disrupt electricity and gas supplies. A good knowledge of average rainfall is essential for proper management of rivers, waterways, marshes and reservoirs.
For an excellent introduction to measuring precipitation see The Weather Observer’s Handbook by Stephen Burt (ISBN 978-1-107-66228-5), where a wide variety of raingauges (pluviometers) is discussed, though not including the Pluvimate. These instruments range from graduated bottles and tipping buckets to very expensive professional weighing instruments and laser systems. The Pluvimate attempts to combine affordability with reliability, high precision and high resolution, and achieves this through a process of transforming randomly spatially distributed raindrops of random size into well-controlled drops of constant volume. Hence the rate at which drops fall is directly proportional to rainfall intensity. The lid of the logger is sensitive to mechanical disturbance, and each drop corresponds to an electrical pulse which drives a logger. The logging interval is easily set using the Gemini Explorer software, which is also used for data visualisation and retrieval.
For an excellent introduction to measuring precipitation see The Weather Observer’s Handbook by Stephen Burt (ISBN 978-1-107-66228-5), where a wide variety of raingauges (pluviometers) is discussed, though not including the Pluvimate. These instruments range from graduated bottles and tipping buckets to very expensive professional weighing instruments and laser systems. The Pluvimate attempts to combine affordability with reliability, high precision and high resolution, and achieves this through a process of transforming randomly spatially distributed raindrops of random size into well-controlled drops of constant volume. Hence the rate at which drops fall is directly proportional to rainfall intensity. The lid of the logger is sensitive to mechanical disturbance, and each drop corresponds to an electrical pulse which drives a logger. The logging interval is easily set using the Gemini Explorer software, which is also used for data visualisation and retrieval.
Overview
Logging rain gauges have existed for many years based around the venerable 400 year-old tipping bucket, and much useful data has been gathered this way. However, this technology has certain disadvantages, of which the three most important are: a) volume resolution depends on the volume of the bucket; b) they are susceptible to mechanical failure and jamming due to oxidation, carbonate accretion, insects and other fauna and flora; c) they do not use a regular timebase. The Pluvimate logging pluviometer addresses all these issues.
How it works
The Pluvimate is not a cumulative pluviometer but a rainfall intensity logger, and is built around the well-established Stalagmate drip-logger – each drop of water falling onto the lid of the Stalagmate is recorded as an "event". There is no requirement to empty any container, though with minor modifications it is possible to collect the water for calibration purposes of later chemical analysis. Cumulative rainfall is simply obtained by integrating (summing) the intensity readings. Each sampling interval of the logger has a 16k (214) event capacity, with 32k (215) sampling intervals, so an absolute maximum of 229 events can be recorded. The sampling interval can be set by the user from minutes to hours or days, but a typical temporal resolution for a rain monitor might be 10 minutes, giving 227 days (i.e. more than 7 months) of continuous measurement.
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Tate's Law
Tate's law predicts that, at least for diameters that are not too large, the volume of a drop of fluid at constant surface tension forming on the end of a tube is proportional to the diameter of the tube. It is known that, provided the drip rate is not too fast (< 1 drop/5 sec), the volume of the drop remains constant, all other things being equal (Collister C, Mattey D, 2008, J. Hydrol, 358, 259-267). Therefore, if the diameter of the collecting funnel is known, along with the volume of the drop and the interval of time over which a given volume is collected, then the precipitation rate, or rainfall intensity, is also known.
Deviations from the assumption of constant drop size can be due to variations in temperature or surface tension – sometimes measurements are made, quite intentionally in forests under the tree canopy where the secondary droplets can acquire surfactants picked up from the leaves. These reduce the surface tension of the water, and hence reduce the size of the drop.
Deviations from the assumption of constant drop size can be due to variations in temperature or surface tension – sometimes measurements are made, quite intentionally in forests under the tree canopy where the secondary droplets can acquire surfactants picked up from the leaves. These reduce the surface tension of the water, and hence reduce the size of the drop.
The Funnel - wettability
Rain falls into a moderately high-sided (50mm high) anodised aluminium funnel 5" (127 mm) in diameter; this UK Meteorological Office standard diameter has the peculiarity that its area in cm2 is the same as its diameter in mm. No surface treatment or colouring has been, or should be, applied to the anodised surface, and care should be taken to avoid any contaminant such as grease, leaving the surface easily wettable to allow raindrops to migrate smoothly towards the neck of the funnel. A highly polished or hydrophobic surface is undesirable since drops tend to accrete and then suddenly run into the neck, triggering premature release of a drop which may be in the process of formation.
The funnel is held in place above the drip logger in a 110mm diameter rigid plastic tube approximately 400 mm in length. The distance through which the drop falls onto the lid of the logger is about 300 mm. The logger itself sits centrally on the base of the tube on a quasi-conical feature. Four drain holes are provided to allow the water to disperse (these can be blocked off if it is necessary to collect the water via a pipe or rubber tube). In the unlikely event of all four holes becoming blocked (spiders, beetles, slugs, etc), two additional drainage holes are provided further up the tube. However, it has been found in some cases that these additional holes are attractive to wasps, which then build their nest inside the tube!
The funnel is held in place above the drip logger in a 110mm diameter rigid plastic tube approximately 400 mm in length. The distance through which the drop falls onto the lid of the logger is about 300 mm. The logger itself sits centrally on the base of the tube on a quasi-conical feature. Four drain holes are provided to allow the water to disperse (these can be blocked off if it is necessary to collect the water via a pipe or rubber tube). In the unlikely event of all four holes becoming blocked (spiders, beetles, slugs, etc), two additional drainage holes are provided further up the tube. However, it has been found in some cases that these additional holes are attractive to wasps, which then build their nest inside the tube!
Nozzle plug (“stubb”)
The funnel of the Pluvimate terminates in a special, easily wettable, hollow plug called a "stubb". Wettability of the stubb is very important, and currently the material chosen for this is aluminium that has been aggressively etched, though other materials can also be used. The stubb is grooved along its length to allow water to drain down the sides as well as down the centre. The inner diameter of the stubb is a little less than twice the capillary radius for water, so a drop will tend to form across the whole diameter (including the funnel nozzle) rather than localising itself on a small part of the perimeter. The diameter of the nozzle therefore determines the volume of the drop. CAUTION: the accuracy and reliability of the Pluvimate depends on the formation and subsequent detachment of a droplet of water of known, constant volume. Any grease (in particular silicone greases, oils or rubbers) or other contaminant on the surface of the stubb may prevent the correct sized drop from forming. When removing or replacing the stubb, handle with care! However, the design of the stubb is not cast in stone, and according to their own particular circumstances, users can feel free to experiment with different materials such as copper or brass, ceramic or even wood. Bamboo works particularly well. Porosity in the original ceramic stubbs led to their disintegration in sub-zero temperatures, while wood can expand and contract and eventually rot. Wettability of both funnel and stubb improves with time and exposure to the atmosphere.
Resolution
Experimental studies have shown that Tate's law for drop volume is linear with radius over the range of tube radii from 1 to 5 mm (i.e. diameters from 2 to 10 mm), and is given with good accuracy by: V = +0.02550r +0.00981, where V is in ml and r is in mm. The overall diameter of the Pluvimate nozzle is 9.2 mm, so the corresponding drop volume is 0.127 ml. The funnel area is 127cm², so this drop volume corresponds to a rainfall height of precisely 0.010 mm, i.e. this is the resolution of the logger. However, due to manufacturing and other tolerances, we recommend that the user conduct a series of calibration tests to determine the exact volume of a drop. Such a test simply might involve very slowly dripping or spraying a known volume of water into the Pluvimate funnel and comparing the recorded drip count with the known volume. This can also be done by collecting the rainfall in the base and comparing this volume with the number of counts. Example: Using the nominal volume given above, an intense burst of rain equivalent to 50mm/hr corresponds to 5000 drops/hr (a volume of 635ml) or 833 drops per 10 minute interval. For this intensity, a 6 minute interval will equate to 500 drops, and a 12 minute interval will equate to 1000 drops. The maximum recordable number of drops per interval is 16384.
Filter
The Pluvimate is fitted with a coarse filter to keep out leaves, insects and other rubbish which might clog the stubb. The stainless steel mesh has a water retention factor of about 0.04 ml/ml (water/mesh), so the ab initio offset error introduced by the absorbency of a completely dry filter is just under 0.1 mm of recordable rainfall. The filter is easily removable for cleaning. Sometimes there is a risk of wind-blown leaves blocking the funnel, and a layer of small stones (1 to 2 cm mean diameter) provides sufficient roughness for water to pass under the leaves; these stones can also reduce small errors due to splashing during periods of very intense rainfall, though they will, of course, add to the (small) overall water retention.
Setting Up
The drip logger relies on the impact of a drop falling onto its lid, and a drop falling anywhere within a radius of 10mm from the centre will reliably record an event. Current Pluvimates are shipped with a flat-bottomed stubb, so any slight tilt means that the drop is no longer perfectly symmetrical, though future Pluvimate versions will feature a stubb with a spherical surface to address this problem. Hence the Pluvimate must not deviate from the vertical by more than 2° for reliable operation, and some care must be taken in setting up using a plumb-line, circular spirit level or linear spirit level applied in two planes. Note that the base of the Pluvimate is slightly concave, so a small quantity of water (or a marble or ball-bearing) in the base can act as an effective stand-in for a spirit level. The Pluvimate should be sited away from trees if possible to avoid wind turbulence and problems due to leaves or pollen. If the Pluvimate is to be installed in the ground, it should be firmly embedded in sand or gravel with adequate drainage. The inner diameter of the tube is sufficiently wide (104mm) to enable all but the most muscular arms to withdraw the logger from inside, though it will probably be necessary to remove heavy jackets and thick woolly jumpers first. A stiff wire hook can also work, as can a permanently attached string cradle.
It has been reported in the literature that rain-gauge funnels producing droplets require "conditioning" before they become fully consistent, i.e. predictably wettable. The reasons are obscure, but appear to be related to the build-up of a chemically stable layer of oxides, nitrates and sulphates in equilibrium with the atmosphere for optimum wettability. However, the Pluvimate funnels are raw anodised aluminium with no colorants or treatments, and wettability is generally good, increasing with time and exposure.
If the Pluvimate is to be left in a location where no rain is expected for many days, it is a good idea to add some water first, making sure the entire end of the stubb is wetted and clean; observe the drop forming across the whole diameter. Any water in the concave base will ensure that the humidity of the interior remains high for several days until actual rainfall allows the stubb to become fully conditioned.
It has been reported in the literature that rain-gauge funnels producing droplets require "conditioning" before they become fully consistent, i.e. predictably wettable. The reasons are obscure, but appear to be related to the build-up of a chemically stable layer of oxides, nitrates and sulphates in equilibrium with the atmosphere for optimum wettability. However, the Pluvimate funnels are raw anodised aluminium with no colorants or treatments, and wettability is generally good, increasing with time and exposure.
If the Pluvimate is to be left in a location where no rain is expected for many days, it is a good idea to add some water first, making sure the entire end of the stubb is wetted and clean; observe the drop forming across the whole diameter. Any water in the concave base will ensure that the humidity of the interior remains high for several days until actual rainfall allows the stubb to become fully conditioned.
The wick
Each Pluvimate is provided with a fabric “wick” which should remain in place for reliable operation. The purpose of this wick is to prevent water from “pooling” on the surface, creating a cushioning effect that can lead to a failure to record all drips.
Deployment
Figure 1 shows three Pluvimates deployed in different ways in SW France. Figure 2 shows the reverse side of one of them, with four holes drilled through the plastic pipe to accommodate a couple of heavy-duty nylon TyRaps. As a "belt & braces" precaution, holes can also be drilled in the angle-iron or the TyRaps can be replaced by wire, and/or the Pluvimate can be attached using duct tape. A stainless steel bracket is optionally available for fixing to vertical surfaces or posts. Finally, figure 3 shows the Pluvimate logger with a length of viscose fibre fabric around the box acting as a wick (see below)
The wick
Each Pluvimate is provided with a fabric “wick” which should remain in place for reliable operation. The purpose of this wick is to prevent water from “pooling” on the surface, creating a cushioning effect that can lead to a failure to record all drips.
Precautions
The surface tension of water shows some variation with temperature:
γ(T) = – 0.1500T + 75.73 (where T is in degrees Celsius and γ is in mNm⁻¹), or relative to (20°), -0.21% /degC. Ideally, a temperature record of the site should also be kept, and used to compensate for drop volume if very accurate records are required.
Make sure the stubb (and funnel) is very clean! If necessary, use a volatile solvent such as acetone for degreasing, and/or clean the end lightly using medium grit wet abrasive paper.
There is some, but not much, tolerance on verticality. To ensure that this is so, use the dished base of the Pluvimate as a crude levelling device, or use a circular spirit level.
If possible, check the filter and the drain holes from time to time for leaves, insects and other debris.
When very new, the lid of the logger can be rather hydrophobic, leading to "pooling" on the surface, with a small probability of “cushioning” a drop to the extent that it may not be recorded. This problem is generally avoided by the fabric band (supplied with all new Pluvimates) around the box to wick the water away; for reliable operation, this band should be kept in place.
γ(T) = – 0.1500T + 75.73 (where T is in degrees Celsius and γ is in mNm⁻¹), or relative to (20°), -0.21% /degC. Ideally, a temperature record of the site should also be kept, and used to compensate for drop volume if very accurate records are required.
Make sure the stubb (and funnel) is very clean! If necessary, use a volatile solvent such as acetone for degreasing, and/or clean the end lightly using medium grit wet abrasive paper.
There is some, but not much, tolerance on verticality. To ensure that this is so, use the dished base of the Pluvimate as a crude levelling device, or use a circular spirit level.
If possible, check the filter and the drain holes from time to time for leaves, insects and other debris.
When very new, the lid of the logger can be rather hydrophobic, leading to "pooling" on the surface, with a small probability of “cushioning” a drop to the extent that it may not be recorded. This problem is generally avoided by the fabric band (supplied with all new Pluvimates) around the box to wick the water away; for reliable operation, this band should be kept in place.
Data Retrieval
The datalogger board is a proprietary product of Gemini Loggers Ltd, and is programmable using Gemini Explorer for Windows 10, Windows 7, Vista, XP, 2000 and ME. A time-limited version of the software is available free from the Gemini website with frequent downloadable updates. Power for the detector board is derived from the logger which provides current only when the logger is activated after “Launch”. Since the logger draws only a few μA when in “sleep mode”, battery life is preserved for over 10 years. Note that if the Stalagmate is activated for testing purposes, it must then be deactivated by pressing the “Stop” button in Gemini Explorer to preserve the battery in storage.
Note that the USB cable supplied with each logger requires a driver (generally found automatically if your computer is connected to the internet). In most cases the data port is also found automatically, but if not, go to "Options - Communications Options - Use fully automatic com port detection". When the green "Launch" button is pressed, the logger serial number should be found, and the logger is ready to be programmed. Very occasionally a brand new logger can show a "low battery" warning - this is a minor bug and can be ignored. Software functions are fairly self-explanatory.
The logger can be set up to start counting before setting off into the field, but this does mean that any “data” collected will be due to knocks and bumps in transit, and will have to be edited out later. Alternatively, the logger can be set at "Launch" to start after a programmed time delay of one minute to several months.
Note that the USB cable supplied with each logger requires a driver (generally found automatically if your computer is connected to the internet). In most cases the data port is also found automatically, but if not, go to "Options - Communications Options - Use fully automatic com port detection". When the green "Launch" button is pressed, the logger serial number should be found, and the logger is ready to be programmed. Very occasionally a brand new logger can show a "low battery" warning - this is a minor bug and can be ignored. Software functions are fairly self-explanatory.
The logger can be set up to start counting before setting off into the field, but this does mean that any “data” collected will be due to knocks and bumps in transit, and will have to be edited out later. Alternatively, the logger can be set at "Launch" to start after a programmed time delay of one minute to several months.
Waterproofing
The logger is rated to IP67 but only when the supplied protective cap is fitted to the data cable socket. Make sure that the threads on the socket and/or blanking plug are clean and free of grit. The cap has a soft rubber seal. CAUTION: DO NOT OVER-TIGHTEN – it is not necessary. Over-tightening may put excessive stress on the adhesive bond between the connector and the box.
BATTERY
The battery is a 1/2 AA Lithium Thionyl Chloride type, which produces a nearly constant 3.6 V up to the point of exhaustion. The 1200mA hour capacity is sufficient to power the logger for at least 4 years. They are readily obtainable from suppliers such as Farnell, RS Components and Amazon, and should NOT be confused with 3.0V Lithium/Manganese dioxide camera batteries, which are also (confusingly) available in 1/2AA sizes.