Blanking distance

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Blanking distance

Posted by liu huan at June 23. 2010

     Hi, I want to know the minimum blanking distance of Nortek-ADP HR which frequency is 1MHz, when I set the distance to be 0.2m, it will get a warning. Can I set this value to measure?  How big is blanking distance to velocity?

     Thank you!

 

Re: Blanking distance

Posted by P.J. Rusello at June 23. 2010

The blanking distance allows the transducers to stop ringing from the transmit pulse before collecting velocity data. For the 1 MHz HR Profiler, we recommend a blanking distance of 0.4 m. You may set this value lower (as low as 0.2 m), but cells closer to the profiler than 0.4 m may be subject to bias and you will need to verify their measurements carefully.

 

P.J.

Re: Blanking distance

Posted by mary at May 08. 2012

Hi P.J. 

I was wondering what blanking distance you recommend for 2 MHz HR Profiler. The minimum should be 0.03m. so is 0.05 m a good number?

Thanks,

Mary

 

Re: Blanking distance

Posted by P.J. Rusello at May 09. 2012

Hi Mary,

  I believe our minimum recommended is 0.1 m, although we allow it to be set lower. Same caveat as above, if using something shorter than what we recommend the user will need to verify this data is not biased by ringing.

Re: Blanking distance

Posted by mary at May 09. 2012

Thanks P.J.

I think 0.1 m would work too. 0.03 was the minimum blanking I found in the datasheet.

Also, in data sheet it mentions Max. Sampling rate we can get is 1Hz (continous mode), 8Hz (burst mode). We would like to have 8Hz data for as much time as possible. Would you guide me on  how long of a period (maximum) we can measure at this rate? When I try to change the Meas/Burst interval in the deployment planning, it keeps giving me the message of Invalid configuration. Is there a maximum duration we can measure at 8 hz?

Thanks a lot,

Maryam

Re: Blanking distance

Posted by P.J. Rusello at May 09. 2012

Hi Mary,

  To get 8 Hz sampling there are tradeoffs. Your cell size will need to be large enough to permit sampling at this high rate. The software tries to guide you in choosing an appropriate setup. I would recommend if you want to sample at 8 Hz, enable burst mode, determine how long you want your bursts to be and how often you would like them to occur, and your desired velocity or profile range. Then you can play with the cell size to achieve a valid configuration. You will need to leave a short gap between bursts to allow processing to occur, typically a few seconds is sufficient.

  I believe there was a maximum number of samples you can collect in a  burst before the internal counters used to identify samples reach their maximum values. I believe this was somewhere in the 16000 range, but there is a thread on it in the HR Profiler forum.

P.J.

 

Re: Blanking distance

Posted by mary at May 10. 2012

Thanks! I think now I am on the right path. I program it on the burst mode and considered what you mentioned.

just some more things to double check and then I think we are ready to go:

_ We would like the Vector / ADP sit in the bottom for a few days. So we are planning to use an extra canister with Alkaline batteries and external cable to power the Vector. I was wondering if it would work this way by the same settings or do we have to do some extra set up to tell the device to get the power from the connected canister

_ We would like to retrieve data after one day to check about some velocities, we thought the retrieving data time would be much faster if we use the Recorder/ upgrade baud rate of 115200 instead of the defaul 9600. in the manual it mentioned it would cause some complications though. Do you recommend using the high baud rates?

_ We also  would like to see the velocity plots after one day in the field.  Can we have a plot of Vector / ADP velocities on the Nortek softwares or any other software so that we can check them in the field rather fast?

Thank you..

Re: Blanking distance

Posted by P.J. Rusello at May 10. 2012

Previously mary wrote:

Thanks! I think now I am on the right path. I program it on the burst mode and considered what you mentioned.

just some more things to double check and then I think we are ready to go:

_ We would like the Vector / ADP sit in the bottom for a few days. So we are planning to use an extra canister with Alkaline batteries and external cable to power the Vector. I was wondering if it would work this way by the same settings or do we have to do some extra set up to tell the device to get the power from the connected canister

Nope, no additional setup is needed.

_ We would like to retrieve data after one day to check about some velocities, we thought the retrieving data time would be much faster if we use the Recorder/ upgrade baud rate of 115200 instead of the defaul 9600. in the manual it mentioned it would cause some complications though. Do you recommend using the high baud rates?

Use a higher baud rate if possible, the data will transfer faster. If you get warnings or the connection times out, then you'll need to switch to a lower baud rate. Make sure to power from the external power supply when downloading data so you don't use up the internal batteries.

 

_ We also  would like to see the velocity plots after one day in the field.  Can we have a plot of Vector / ADP velocities on the Nortek softwares or any other software so that we can check them in the field rather fast?

For the Vector we offer the ExploreV software for data analysis. For the HR Profiler the software SeaState will let you visualize the data. Other options would be export the data and look at it quickly in Matlab or something similar. I'll ask Brandon to be in touch with you about the software.

P.J.

 

 

Thank you..

 

Re: Blanking distance

Posted by JCS at August 09. 2012
Does increasing the blanking distance actually change anything in terms of the way the instrument samples (e.g., changes the pulse lag, etc.) or does it just not report the measurements closer to the ADCP than the blanking distance? e.g., would it be just as good to keep as small a blanking distance as possible, then inspect the data close to the ADCP for interference in post-processing - as way of determining on a case-by-case basis what distance

is affected by ringing.

 

Thanks

JCS

 

Previously P.J. Rusello wrote:

The blanking distance allows the transducers to stop ringing from the transmit pulse before collecting velocity data. For the 1 MHz HR Profiler, we recommend a blanking distance of 0.4 m. You may set this value lower (as low as 0.2 m), but cells closer to the profiler than 0.4 m may be subject to bias and you will need to verify their measurements carefully.

 

P.J.

 

Re: Blanking distance

Posted by P.J. Rusello at August 10. 2012

You can generally regard the transmit and receive as two separate operations (there are some dependencies in between them, especially in a mono-static system), but specifying a different blanking distance won't affect the velocity range. It does control where the range bins are located however.

You're always free to specify a smaller blanking distance than we recommend and perform data screening. The largest tradeoff here will be in memory usage. Over a reasonably long deployment, recording 5-10 extra cells which are contaminated can add up and reduce your deployment length.

The blanking distance also moves the measurement region out of the flow disturbance region caused by a frame, piling, or other mounting structure.

 

P.J.

Re: Blanking distance

Posted by Ayal Anis at February 11. 2017
Hi Forum, 
 
As you know, the aquadopp sofware warns if blanking is set less than 
0.4 m for the 1 MHz Aquadopp or less than 0.1 m for the 2 MHz Aquadopp. 
Brochure set the minimum blanking at 0.2 and 0.05 m for the 1 and 2 Mhz 
Aquadopps, respectively. 
 
As explained above (and elsewhere) there is the issue of ringing for low 
amounts of scatterers in the water. I was wondering if anyone has perhaps 
carried out some methodical tests to explore data quality in the first 
bins under different blanking setups and environmental conditions and 
is willing share her/his experience. 
 
Best,
Ayal

Re: Blanking distance

Posted by SergioAdace at February 18. 2017
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