Water Level Instruments
There are different types of instrument available for measuring water level. Each has its strengths and weaknesses. It's important to select an instrument that's right for your application to ensure you'll get reliable data.
NIWA commonly uses five different types of water level instrument:
Encoder - 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. The encoder outputs a number that relates to the position of the water surface. As they need to be installed in a stilling well, installation costs are higher than they are with other types of water level instrument.
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. Because they are vulnerable in a sometimes hostile underwater environment, the gas bubbler option is usually preferred in longer term deployments.
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. 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. The lower end of the tube is securely fastened underwater at a reference depth. The gas is usually air or nitrogen supplied from a gas bottle or may be air compressed by a small electrically-powered pump. As they don't require infrastructure such as stilling wells, installation is relatively inexpensive.
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. These are often mounted on a bridge and used for measuring the water level in braided rivers that often change their course. They can be easily shifted when necessary. As they don't require infrastructure such as stilling wells, installation is relatively inexpensive.
Ultrasonic - this aims ultrasonic sound pulses at the water surface. 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. These are often mounted on a bridge and used for measuring the water level in braided rivers that often change their course. They can be easily shifted when necessary.
Here's an article about a major project that uses gas bubbler and radar systems to measure water level on the Rees River.
The pages below will take you to information about specific instruments that we have experience with, recommend and can supply.