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Updated : 16/10/2016

GHowSAW Weather Rain

Introduction

The amount of rainfall appears to be as significant to a lot of people as the actual temperature. When measuring rainfall there are several things that need to be recorded:- How much has it rained this year/month/week/today and how hard is it raining now?

Reporting

Rainfall is reported as mm of rainfall over a specified period of time year, month, week, day, or even hourly. When measuring to report the rate of rainfall that the sampling period be increased to at least 5 minutes. This is because under very heavy rainfall conditions the drips entering the bucket cause waves which disrupt the tipping level

Sensors

The simplest and most direct way of measuring rainfall is to use a tipping bucket rain gauge. The mechanism of this device (shown left) collects rain in the collector and directs it to drip through a small hole into the "top" bucket. As the bucket fills, it gets heavier. At a certain point, the weight causes the bucket to fall. As the bucket assembly moves, the magnet mounted near the bottom swings past the reed switch, which momentarily closes,triggering the counter circuit.

After the bucket falls, the water spills out and the other bucket is now in the "top" position to catch the dripping water. When it gets full, it falls, triggering the reed switch as the tipping bucket assembly swings back. This process repeats as long as water is dripping from the collector. Many of the tipping bucket rain gauges are designed to tip at 0.01 inches of rain. The first tip usually takes a little more than this, because some of the first few raindrops "stick" to the collector surface.

A pair of adjustment screws usually provides calibration. By shortening the screws so that the bucket assembly is tipped further, it takes more weight to tip it. This has the effect of reducing the counts for the same amount of water. Conversely, by lengthening the screws, there is less travel and buckets tip more often.

The reed switch is connected to a DS2423 l-Wire counter. For each switch closure the counter increments by one. Looking at the schematic below, the counter input is pulled low via resistor Rl. When the reed switch closes, the counter input is pulled high momentarily, producing a count. The DS2423 incorporates internal filtering on the counter inputs to minimize contact "bounce."

In order to maintain the rain count during one-Wire bus power outages, battery BATl provides power to the counter via D3. Using a CR2035, the battery should last at least 5 years. With the counter always powered, there is no way to reset the counts and so you must zero the counter in your software.

General Problems with tipping bucket rain gauges

A brief summary of the problems that TBRGs may experience in attempting to measure rainfall.

  1. Blockages
    The principal difficulties are due to partial or total blockages. Partial blockages lead to misleading estimates of rain rates and to inaccurate descriptions of the lengths of wet and dry periods. Complete blockages lead to underestimation of total rainfall.

  2. Wetting and evaporation
    A problem common to all gauges using collecting funnels is that of ‘wetting’. This refers to water that, remaining on the funnel surface without passing into the collecting chamber, subsequently evaporates. Losses due to wetting are typically around 0.05 mm per rainfall event. Losses due to evaporation may be of the order of 0.004 mm/h.

  3. High rain rates
    The basic TBRG consists of a pair of adjacent buckets. When a bucket fills with water, its centre of gravity is moved such that it overbalances. This results in the second bucket moving into position while the first bucket empties. The tipping process may take as much as half a second during which rain continues to fall. The result is an under-estimation of the rain rate. Although the under-recording of rainfall is usually slight, the under-stating of extreme rain rates can be considerable. The non-linearity of the relation between apparent and real rain rates depends both on the age of the equipment and the gauge type, so that each individual gauge requires its own calibration.

  4. Wind effects, position and shelter
    Gauges are typically installed at about a metre above ground level, though the standard heights vary from country to country and also within a country. However, since the local windfield varies with height, gauges placed close to one another, but at very different heights may record quite widely different rainfalls.

  5. Hail Damage
    Whilst the 'RainWise' TBRG is of a fairly rugged construction it is not sufficiently robust to withstand the impacts of a South African Highveld summer hail storm. Hail frequently accompanies the summer thunder storms but once or twice a year the hail is fairly large (from golf ball to tennis ball) and on two occasions have destroyed the collecting funnel. I did consider a metal version but what not sure that I could stand the noise!

Installation

The rain gauge must be mounted on a flat horizontal surface clear of any overhangs. The gauge should be installed suffiently distant from any other surface to avoid splatter or wind blown spray entering the collection bucket. Because the mechanism relys on the tipping of a balance bucket it is imperative that the device be rigidly fixed and absolutely horizontal

Adjustments

  1. Use a spirit level to make sure that the device is horizontal.

  2. Use a vernier calliper to make sure that the bucket stop screws are exactly the same height. This is to ensure that the same volume of water is measured in each tipping direction.

  3. Ensure that the buckets are clean and free of algae.

  4. Ensure that the collector and the drip tube are clear of any debris.

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