The key factors controlling an acoustic Doppler current profiler’s maximum range include fixed and variable parameters. Fixed parameters are constant over time and do not change with a particular configuration or instrument setup. These include: transducer frequency, beam directivity and output power (but see note below on output power). The variable parameters include the reflective characteristics of the particles in the water, and this can change both with time and space.
The lower an instrument’s operational frequency, the longer it can profile. This is analogous to sound in air, where for example, the bass sound of a piano will travel farther than the higher pitched notes. For Nortek instruments, for example, the Signature55, which operates at 55 kHz, can profile up to 1000 m of water, while the 2000 kHz Aquadopp Profiler will normally reach up to about 10 m.
Beam directivity is also another factor, although not as easily explained. Beam directivity relates to how much energy from a transducer is actually directed along a narrow cone-shape area in front of the transducer. The more that this energy escapes to outside of this central cone (called main lobe), the less energy will be present in the farthest reaches of the current profile and therefore the shorter the range. Nortek profilers have beam spreading angles in the order of 1-4°, approximately, and every effort is made in manufacturing to keep each beam as focused as possible. Beam directivity does not vary with the life of the instrument.
Output power also impacts the profiling range and is directly proportional to it—the greater the power, the longer the range. However, power cannot be increased indefinitely as the supply (battery) is limited and too much power can damage the transducers. Nortek instruments have variable power which is set by the user upon instrument programming. However, once set, it remains the same for the deployment until it is reset by the user.
The impact that frequency, beam directivity, and power have on the instrument’s profiling range is well understood and can be fairly well defined and modelled. However, the impact that the particles have on the acoustic pulse is not. This is because the acoustic reflectivity of the particles in the water (called scatterers) varies with a number of factors such as the concentration of the particles, how acoustically transparent they are, density gradients in the water column, the size distribution of the particles, the shape of the particles, etc. This also varies with both space (where in the water column you are measuring) and with time, and therefore it is very difficult to model and predict.
Because of this, the maximum ranges published for all acoustic Doppler profilers refers to a nominal value, with the possibility that the actual values obtained could be longer or shorter than the desired range. This is something that impacts any type of acoustic instrument from any type of manufacturer, and it is not unique to Nortek. Even a cursory review of any ADCP literature will reveal clarifying statements on range such as “maximum range depends on the acoustic scattering conditions” or “maximum range is a nominal value based on typical ocean backscatter” or “actual range will vary depending on environmental conditions.”
However, Nortek has made its reputation on the quality of its products and its support for its clients. Specifications given are based on our long experience, and also on empirical data. So although it so not possible to guarantee a particular instrument’s maximum range will always be achieved, we feel strongly that the nominal ranges given in our documentation are conservative values and may be achieved under most conditions.