LIMNIMETRY : Limnimeter sensors

Limnimeter sensors, or level probes, can be classified in 2 categories:

Limnimeter sensors, or level probes, with contact and those without contact with water.

From limnimetric scales to radar the limnimeters present a wide variety of sensors.

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Limit sensors, or level probes, with contact :

Direct or indirect reading, travel time measurement, pressure measurement, are the methods of level measurement with contact.

- Direct reading is carried out on limnimetric scales. These are enamelled metal plates graduated in centimetres and placed vertically or flat on sloping floors. The determination of the water level is carried out by a direct-reading observer. The investment is very small, the installation often requires civil engineering work, maintenance is limited to the periodic cleaning of the ladders. Their resistance over time depends on the shocks they are subjected to. The operating cost is high if the observer is permanent. The accuracy of the measurement is random and subject to human error. Equipment historically implanted on watercourses, ladders are often the only means of measurement and always the historical reference of the ratings.

- The indirect measurement concerns the float. Connected to a tracer on graduated paper, it is the limnigraph which is nowadays abandoned, because it is too restrictive in operation and not very safe. Connected to an encoder it avoided the inconveniences linked to the use of paper, pen, ink, the float which jams or the cable which slides on the pulley remain major inconveniences which generate false measurements and many displacements. The investment is important, the installation requires a big civil engineering work often mishandled by important floods. On-site monitoring must be constant and involves a significant operating cost. The accuracy of the measurement is good but the durability is not guaranteed. The collection of measurements for the limnigrapher is manual, it can be ensured by acentral acquisitioncentreif an encoder is added to it.

- Travel time measurement is used by the Limnimeter sensors, or level probes, at ultrasonic submerged. This is a piezoelectric cell attached to the bottom of the watercourse and connected by cable to acentralacquisitionunit which is also designed to excite the cell. When excited, the cell emits a frequency which is returned to it when it reaches the air/water interface Thecentral measures the time it takes for the frequency wave to travel the distance and derives the measurement of the water level This method can be expensive in terms of investment and requires civil engineering at the bottom of the watercourse to fix the cell and bury the cable. Maintenance consists of cleaning the surface of the cell when it is covered with silt. Accuracy is good when the level sensor is clean and the water temperature varies little. This level sensor system is not simple to install and requires some maintenance.

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- The pressure measurement is carried out by means of limnimeter sensors, or level probes, known as "bubble to bubble" or by immersed probes. The method, of the limnimeter sensor or level probe, "bubble to bubble" consists in immersing and maintaining a tube in the watercourse. At the other end, a compressor cyclically sends pressure into the tube. An acquisition system records the pressure value that is necessary for a bubble to escape from the tube at the other end. This method requires little infrastructure and investment. However, the presence of a compressor is essential and limits the interest of this method. Either the compressor is large and therefore requires a mains supply, or it is miniature and becomes the mechanical weak point of the system. Immersed probes (pressure sensors) are immersed metal cylinders containing a silicon or ceramic cell and miniature electronics. The pressure exerted by the water column above the pressure sensor, or level probe, produces a deformation of the cell which generates a voltage proportional to the pressure exerted and therefore to the water level. The cable that connects the probe to the acquisition system carries the signal to this system. Investment and installation costs are low. This is the most economical way of supplying low current and maintenance-free power. The resolution for this type of level sensors is of the order of a millimetre, the accuracy depends on linearity and is inversely proportional to the full scale. It is only influenced by variations in water temperature.

The common disadvantage of all submerged level sensors is that in case of flooding the level sensors are often damaged or even destroyed. The use of submerged water level sensors in hydrology can be justified, but in case of floods, the risk of not having any more measurements during a crisis period should lead to the use of submerged water level sensors or level probes in combination with them or to their replacement by other means.

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Non-contact level sensors or level probes :

There are two means: ultrasound and hyper frequency(radar).

To put it in a nutshell, both use travel time measurement. That is to say that the sensitive cell of the limnimeter sensor at ultrasound, or the hyper frequency antenna of the level probe radar, emits a frequency and measures the time it takes to return to it after bouncing on the water surface. Knowing, in the case of the level sensor at ultrasound, the speed of sound in the air and, in the other case, the speed of light, the electronics calculate the distance between the level sensor and the water. The acquisition system receives this information from the level sensor and deduces the water height from it. The measurement given by the water level sensor at ultrasonic is strongly influenced by temperature and wind. The inaccuracy of the water level sensor at ultrasonic is directly related to the air distance between the sensor and the water surface. This means that it is not well suited for measuring watercourses, it is ideal for measuring in tanks. The investment is reasonable.

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The measurement given by the limnimeter sensor radar is not influenced by anything and its accuracy is the same regardless of the distance between the sensor and the water. It is the ideal means of measuring watercourses. The investment is 3 to 4 times higher than for an ultrasonic water level sensor. In both cases, the installation can be very simple (fixed on a bridge) or require significant civil engineering depending on the location. In all cases, except for the "bubble to bubble" water level sensor and the immersed ultrasonic water level sensor, it is possible to considerably limit the installation work by making the water level sensors, or level probes, autonomous in energy and transmission. To this end, the level sensors are supplied with low voltage by solar panels and the signals are transmitted by short range radio to the power station of acquisition.