Some setting parameters question

Hi,

In the frame of my PhD, I’m working with the Vectrino+ in a laboratory water thank to study the velocity behaviour of a gravity current (see the attached file for details). I’m actually realizing preliminary tests and I’d  have some questions about setting parameters…like:

•  What exactly is the Transmit Length of the instrument??
•  How is it related to the Sampling Volume and
•  How can it influence data collecting and, overall, CORRELATION?? (generally I found that small Transmit Length and small Sampling Volume = more noisy and dispersed velocity signals but a bit better correlation...why?)

I hope you’ll be kindly able to help me,

Best regards

Paolo Caccamo

Hi Paolo,

Transmit length is the length of the acoustic pulses transmitted by the instrument. Sample volume size in theory is entirely independent and set by the amount of time spent listening. In reality, for the Vectrino, the SV size has been optimized for a given transmit length for the best performance.

In theory, a shorter transmit length is better because it results in a higher signal bandwidth. In reality, as you noted, the SNR will decrease simply because there is less energy in the return signal. There are tradeoffs to be made, but in my work I have never felt a need to change the transmit length to try and further optimizes performance. After doing comparisons, I vastly prefer the performance with the default setting, with the measurements being statistically the same but with far better looking data (fewer spikes, fewer outliers) with the default setting.

P.J.

Hello P. J.

Do you have similar experience with a "regular" Vector, i.e. are the default 

sampling volume (SV) and transmit length (TL) better than e.g. larger SV 

and longer TL?

Best, 

Ayal

Hi Ayal,

I don't think I've ever needed to change to the SV size on the Vector. I imagine you might see a little benefit since the SV is larger than the Vectrino to start out and the decrease in SNR would be a little less. Honestly though, setting the nominal velocity range correctly is far more important in my mind to achieve good data than almost anything else.

P.J.

Peter,

Thank you very much for your quick answer.

Paolo

Previously Peter J. Rusello wrote:

Hi Paolo,

Transmit length is the length of the acoustic pulses transmitted by the instrument. Sample volume size in theory is entirely independent and set by the amount of time spent listening. In reality, for the Vectrino, the SV size has been optimized for a given transmit length for the best performance.

In theory, a shorter transmit length is better because it results in a higher signal bandwidth. In reality, as you noted, the SNR will decrease simply because there is less energy in the return signal. There are tradeoffs to be made, but in my work I have never felt a need to change the transmit length to try and further optimizes performance. After doing comparisons, I vastly prefer the performance with the default setting, with the measurements being statistically the same but with far better looking data (fewer spikes, fewer outliers) with the default setting.

P.J.

Hi again,

I'm trying to use the Vectrino+ (always in the same frame and test conditions of my first message) with and external trigger. When opening the door trap of the reservoir, a switch should trigger the "start recording data" but I feel like I'm only able to trigger the "start data acquisition".

I normally start the data acquisition first, and then I start the data recording when releasing the door trap. I tried to do the way round but the recorded data have a time t0 different from the door opening and equal to...I don't know!!!

Do you think is possible to start the data acquisition manually first  and then the data recording using the external trigger??

And another question rised as well during tests: which is the principle behind the measurement of the distance from the sensor to the bottom? is it the same as for the velocity measurements (doppler effect and phase shift measure) or what else?

Thank you for your help

Paolo

Previously Peter J. Rusello wrote:

Hi Paolo,

Transmit length is the length of the acoustic pulses transmitted by the instrument. Sample volume size in theory is entirely independent and set by the amount of time spent listening. In reality, for the Vectrino, the SV size has been optimized for a given transmit length for the best performance.

In theory, a shorter transmit length is better because it results in a higher signal bandwidth. In reality, as you noted, the SNR will decrease simply because there is less energy in the return signal. There are tradeoffs to be made, but in my work I have never felt a need to change the transmit length to try and further optimizes performance. After doing comparisons, I vastly prefer the performance with the default setting, with the measurements being statistically the same but with far better looking data (fewer spikes, fewer outliers) with the default setting.

P.J.

Hi Paolo,

The Vectrino only supports start or sample on sync data acquisition. If you want to record the data, please set the file details first and start recording data before the instrument receives the first or start trigger. The "Time of first measurement" field in the header file will list the time (accurate to seconds) when the first sample from the instrument was received.

The distance measurements is just an active sonar based on travel time and identification of the boundary peak position. It does not rely on the Doppler shift.

P.J.

Ok,

Thank you very much again

Best Regards

Paolo

Previously Peter J. Rusello wrote:

Hi Paolo,

The Vectrino only supports start or sample on sync data acquisition. If you want to record the data, please set the file details first and start recording data before the instrument receives the first or start trigger. The "Time of first measurement" field in the header file will list the time (accurate to seconds) when the first sample from the instrument was received.

The distance measurements is just an active sonar based on travel time and identification of the boundary peak position. It does not rely on the Doppler shift.

P.J.