Velocity calculations

Up to Current profilers and current meters

Velocity calculations

Posted by jonas-rA-stad at March 14. 2005
The change in velocity (dv) is given by
dv=-(1500/2)*df/f

df - change in frequency
f   - transmitted frequency
the soundspeed is divided by 2 because of transmission both ways in the water.

Quote
(1) How much is the allowance of transmitting frequency?
How many kHz do you allow in transmitting frequency for 1MHz Aqd Profiler ?
And for 2MHz instrument, for 600kHz instrument as well ?

(2) How do you convert received frequency to current velocity ?
(when sound speed is assumed to be 1500m/s)
In case of 1MHz Aqd Profiler, to how many frequency change does correspond
current speed of 1m/s ? And in case of 2MHz and of 600kHz ?


(1) The accuracy of the Aquadopp Profiler is 1% of measured value +/-0,5cm/s.
The +/- 0,5cm/s part is caused by the transmitting pulse related to drift of the clock.
Using the formula above this leads to an allowance in frequency of

600kHZ :  +/- 40 Hz
1MHz    :  +/- 66.67 Hz
2MHz    :  +/- 133,33 Hz

(2) Again, using the above formula 1m/s correponds to a frequency of

600kHZ :  (600000-800)Hz       = 599200 Hz
1MHz    :  (1000000-133,33)Hz = 999866,67 Hz
2MHz    :  (2000000-266,67)Hz = 1999733,33Hz

Re: Velocity calculations

Posted by Steffen Morvan at March 27. 2018

Hello,

 

I'm sorry to come back on this very old post, but I'm not sure if the answers are still correct.

 

First of all, it is said that the +/- 0.5 cm/s accuracy is due to  "the transmitting pulse related to drift of the clock". But in a more recent post from 2011, it is said : "A slight change in center frequency will, in other words, not affect the measured frequency shift, or the Doppler shift, which is entirely depending on the velocity of particles in the water". 
(http://www.nortek.no/en/knowledge-center/forum/current-profilers-and-current-meters/488032715)

So, is the Aqd central frequency shift the reason of this
+/- 0.5 cm/s accuracy ?

Furthermore, are the frequency values for the frequency shift good ? Because with +/- 0.5 cm/s I find a frequency of 13.33 Hz and not 133.33Hz for the 2MHz Aqd...

Thank you for your time.

Re: Velocity calculations

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