Automatic rain gauges

 The rain gauge is a container designed to receive precipitation in order to determine the quantities of water received on the ground per unit area. The amount of water received by a rain gauge depends on the size of its free surface. Professional rain gauges have free surfaces of 200, 400, or 1000 cm². The larger the surface area of the rain gauge, the more accurate the rain gauge gives an idea of the rainfall. The measurement is obtained, either by direct reading on the rain gauge or indirectly, or by counting or weighing small volumes received, or finally by disturbing a magnetic field. In order to be representative of the rainfall falling on the ground, the rain gauge must be installed under conditions specified by the national meteorological services. The surface of the rain gauge should be horizontal and located in a clear area.

Direct reading rain gauge : 

This is the ancestral rain gauge, we receive the rain in a graduated container. The observer reads directly the quantities received by the rain gauge. If the container is not graduated, the water collected is poured into a graduated cylinder for indirect reading with the risk of losing information. The space between 2 graduations of the test tube depends on the free surface of the rain gauge. A test tube for a 200 cm² rain gauge cannot therefore give the measurement for the rainfall received in other types of rain gauges. The investment is negligible the simple installation. The maintenance of the rain gauge is limited to a regular cleaning, the operating costs of this type of rain gauge can be important and linked to the need of an observer on site or the regular movement of an agent. Without monitoring this method is very random and the accuracy is questionable.

Back to the site

Automatic counting and weighing rain gauge

In order to enable automatic recording of measurements, volume-counting rain gauges are used. The method of this automatic rain gauge is to guide the rainfall into small containers that are emptied automatically when they are full and to count the number of times they are emptied. Once their volume is known, a local data acquisition unit connected to the automatic rain gauge can total the quantities that have fallen. These are the automatic tipping bucket rain gauges. This is currently the most widely used automatic equipment. The investment is more important than for the traditional rain gauge but remains reasonable in view of the savings made in operation. A local observer is no longer essential and the movements of an agent can be spaced out at the same time as the automatic rain gauge and its troughs are cleaned. Maintenance also includes a periodic check of the horizontality of the automatic rain gauge and the calibration of the troughs. The accuracy of the measurement by the automatic bucket rain gauge depends essentially on the quality and seriousness of the maintenance. It is also influenced by the amount and speed of rainfall. The choice of the free area and volume of the tilting buckets according to the type of rainfall at the measuring location also has a great influence on the accuracy of the measurement of this type of automatic rain gauge. An automatic rain gauge with tilting troughs can be inaccurate if it is poorly chosen, poorly maintained or incorrectly used.

To limit these disadvantages, automatic weighing rain gauges can be used. The rain in these rain gauges is collected in a small container placed on a strain gauge whose role is to weigh the container. When the limit weight is reached the container is automatically emptied and the electronics in the rain gauge tare the empty container each time. This technique limits errors due to clogging of the rain gauge's troughs. Each measurement involves a larger quantity of water than with buckets and therefore unit errors are reduced. The investment in this type of rain gauge becomes important. The reliability of the strain gauge is a crucial point for this type of rain gauge and this component is often not stable over time. Maintenance is less expensive than with trough-type rain gauges, and operation can be expensive because of the strain gauge and electronics built into the rain gauge. Accuracy is significantly improved compared to tilting bucket rain gauges.

Back to the site

Automatic rain gauge with electromagnetic field

This very recent method consists of counting and evaluating the volume of the drops that pass through an electromagnetic field generated by the rain gauge. Still being tested by some potential users, these rain gauges have to prove their worth. The first results are promising in terms of the accuracy of the measurement of this type of rain gauge. This technique also makes it possible to know the nature of the precipitation: size of the drops, intensity, speed, nature of the precipitation. The investment for this type of rain gauge is very expensive, the installation is problem-free. The lack of hindsight does not allow to appreciate the other criteria of choice.

In the present state of affairs the good compromise for flood warning seems to be an automatic tipping bucket rain gauge with local or remote memory.

Documentation: here