Brief Information on Equipments used for Real Time Data Acquisition System-RTDAS
A Real Time Data Acquisition System (RTDAS) consists of a Telemetry Network of Automated Rainfall Stations, automated weather station and water levels stations along rivers/reservoirs which will be installed to provide inputs to the RTDAS.
Brief information on different kinds of equipments utilized for Collection of Data in RTDAS
1. Automatic Rain Gauge Stations (ARS)
Tipping Bucket Raingauge is used for measuring rainfall intensity and volume. Tipping buckets are comprised of two buckets of known volume that alternate in filling as shown in figure 1. Once one side is filled, the system tips to empty the bucket and the tipping action is recorded. Following this, the other side starts to fill up and the entire process is repeated. The tipping action of the bucket activates circuitry that produces a switch-closure that can be measured by interfaced data loggers. It come with different orifice sizes and different bucket volumes. A larger catch orifice is desired to improve catch efficiency. The India Meteorological Department (IMD) recommends a minimum catch can diameter of 20 centimeter (cm) or 8 inches.
The size of the bucket determines the amount of rainfall that must be collected before a tip takes place. A smaller bucket is needed if the prevailing rainfall intensities are lower in the region. For regions experiencing high rainfall intensities, a smaller size bucket would tip too often, resulting in missing the signal and measurement errors. Tipping bucket rain gauges, typically available in the market, can measure precipitation in 0.2, 0.5, or 1 millimeter (mm) increments. A larger bucket with 1 mm capacity is recommended for high rainfall intensity areas such as the Western Ghats and north-eastern states whereas a 0.2 mm bucket is recommended for low rainfall areas like western Punjab, Rajasthan and Gujarat. For the rest of India, rain gauges with 0.5 or 0.2 mm capacity are suitable.
Tipping bucket rain gauges need maintenance and calibration. The funnel must be kept clear of debris, and insects need to be kept out of the gauge body. Calibration is easily performed, and should be done on at least an annual basis, as indicated by the measurement environment. Tipping bucket rain gauges are relatively inexpensive, usually costing less than US$1,000/INR 65,000.
Factors affecting Site selection for ARS
The main purpose of establishing a precipitation station is to obtain representative samples of the precipitation over a basin. Particularly, wind affects rainfall measurements. There are further losses due to evaporation and splashing. To eliminate or reduce wind effects, the site should be chosen such that the wind speed at the level of the rain gauge is as low as possible, but in such a way that the surrounding does not affect the rain catch.
2. Automatic Weathering Stations (AWS)
• General Information:
Automatic weather stations (AWS) are replacing old manual stations known as Full Climatic Stations (FCS). The parameters measured in an AWS are:
• Rainfall (mm)
• Temperature (o C)
• Relative humidity (%)
• Wind Speed (m/s)
• Atmospheric Pressure (hpa)
• Wind direction
• Solar radiation (w/m2)
Brief information on equipments for measuring above parameters:
A. Temperature and humidity sensor
B. Wind velocity and wind direction
C. Atmospheric Pressure (Barometric pressure sensors)
D. Solar Radiation
3. Automated Pan Evaporimeter System
4. Instrumentation for Water-level Measurement
The water-level measurement plays an important role in water resources management. In case of reservoirs, lakes and other water bodies, the water level is used for determining available storage. In case of flowing rivers and canals, the water level represents flow rate. The methods to measure water level includes non-contact methods where the water level is measured from a height as distance between instrument and water surface; and contact solutions where instrument is in direct contact with water and the water level is estimated as thickness of water column above a reference level by measuring pressure exerted by water column on the instrument sensor.
A. Radar Type Water Level Sensor (30M / 50M / 75M )
A radar sensor provides ease to measure larger distance (large rivers) and provide greater accuracy desired (dams) The radar sensor offers accuracy to approximately 0.03 per cent of full scale and has a maximum range of up to 70 m to the target. the major advantage of using a radar is the high accuracy along with the extended range of measurement over the ultrasonic. The radar is also relatively easy to install.The disadvantages include the high cost of radar, which can easily exceed US$3,000/INR 195,000 along with the need for something to mount the radar to, such as a bridge structure.
B. Shaft Encoder
The most common method of measuring water level is a stilling well equipped with a float and shaft encoder. The components of this type of gauge include a stilling well, inlet pipes from the water, float, tape, wheel, and shaft encoder which electronically sends signals to the data collection platform.
The civil works for this type of station is among the most expensive, while the sensor and associated equipment is among the least expensive sensor solutions. The stilling well requires occasional flushing to remove sediments that may have collected at the bottom of the stilling well. If left unchecked, the sediments could eventually block the inlet/outlet pipes. This sensor never needs to be calibrated, but only checked and reset to an outside staff gauge.
5. Gate Sensors
This category describes the requirement for sensors placed on gates such as radial type spillway gates and vertical lift type irrigation and/or power outlet to measure gate openings. The measured gate opening will be used along with water elevation to determine accurate discharge past the gates. Shaft Encoder based Rotary optical/magnetic Absolute encoder type sensors are provided for exact measurement & indication of position of spillway radial gates, intake gates & silt flushing gates. These sensors shall be equipped with suitable shaft couplings and electronic circuits to transmit the signals to the data logger for indication & for further processing. All sensors are to be mounted in the outdoor locations. Hence, suitable protection class of the enclosures shall be ensured. Minimum IP65 protection class shall be provided. Suitable safe & reliable arrangements of coupling with the lifting motors of gates shall be provided. It shall be ensured that there is no slippage between the motor shaft & the transducers.