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It measures the time that it takes to detect the echo from each pulse and from this it can measure how far away the water surface is, and hence calculate the depth. Ultrasonic - this aims ultrasonic sound pulses at the water surface. As they don't require infrastructure such as stilling wells, installation is relatively inexpensive. They can be easily shifted when necessary. These are often mounted on a bridge and used for measuring the water level in braided rivers that often change their course. Radar - this aims microwave pulses at the water surface. From the echo it measures the 'time of flight' of each pulse and from this it can calculate how far away the water surface is, and hence the depth. The gas is usually air or nitrogen supplied from a gas bottle or may be air compressed by a small electrically-powered pump. The lower end of the tube is securely fastened underwater at a reference depth. The weight of the water is coupled to the pressure measuring element through a gas-filled tube, rather than being in direct contact with the water. Gas bubbler pressure transducer - this is like the submersible but the pressure transducer is located above the water, safe from floating debris and other threats. Because they are vulnerable in a sometimes hostile underwater environment, the gas bubbler option is usually preferred in longer term deployments. Submersible pressure transducer - this measures the 'weight of the water' above it and outputs an electrical signal, or a number proportional to the relative depth.

As they need to be installed in a stilling well, installation costs are higher than they are with other types of water level instrument. The encoder outputs a number that relates to the position of the water surface. NIWA commonly uses five different types of water level instrument:Įncoder - an instrument that mechanically couples the position of the water surface to an electrical rotary encoder, using a wire with a water surface float connected at one end and a counterweight at the other. It's important to select an instrument that's right for your application to ensure you'll get reliable data. There are different types of instrument available for measuring water level.