Blanking distance refers to a critical concept in the operation of Nortek's current meters and profilers. This distance represents the vertical space above the transducers where echoes are ignored for a brief period after transmission.

All our instruments use the same transducers to both transmit and receive signals. When these instruments emit sound waves into the water, the transducers vibrate to transmit the signal. After transmitting, the power to the transducers is shut off to stop their vibrations and prepare them to receive echoes. However, the acoustic activity around the transducers does not instantly cease; it gradually diminishes over time. This is a phenomenon known as ringing.

To address this, Nortek's instruments are programmed to wait momentarily after transmission before listening for returned signals. This waiting period, known as blanking, allows the transducers to settle down, ensuring that only relevant echo signals are detected. Echoes that originate very close to the instrument during this time are effectively ignored. The blanking distance is the vertical distance where this occurs (see figure below).

The duration of blanking and thus the blanking distance vary based on the specific instrument and the angle of the transducer. This adjustment ensures that the instruments effectively filter out unwanted echoes caused by ringing, thereby enhancing the quality and reliability of the collected data.

The minimum blanking distance is determined by the distance sound travels during the attenuation of ringing. This distance varies depending on the frequency of the transducers used in the instruments. Transducers of different frequencies require varying amounts of time to stop vibrating, therefore affecting the necessary blanking distance. Generally, lower frequencies necessitate longer blanking distances.

Additionally, the minimum blanking distance is influenced by the strength of the returned signals. In conditions with strong echoes, such as water with lots of suspended particles, the blanking distance can be shorter compared to situations with weaker echoes, such as in very clear water.

During deployment planning, the instrument software automatically sets a default blanking distance. However, this default can be adjusted within a specified range based on the instrument's frequency. Changing the blanking distance shifts the position of measurement cells. The blanking distance, along with the number of cells, cell size, and scattering condition, directly impacts the total profiling range of the instrument.