Are measurements corrected for a change in tilt?

Hei,

I have got a question regarding the internal data processing of vectors. Imagine the instrument to sit in completely still water. The measurements then should show zero movement or no flow  whatsoever (theoretically). When we then tilt the instrument, it will read a water flow until the instrument is kept in one position. Will the measured data be processed with regards to the tilt readings? In other words: will the ENU data show any flow during tilting the instrument
or will the data be processed internally to correct for the movement of the instrument?

We will use vectors with the head turned by 90°. That means, that a rotation of the instrument along it's vertical axis  will lead to a measured water flow (the effect will hence be the same as described for the tilting of a standard vector above). Will the data be internally corrected for the change of compass direction?

If there is such a correction, how does it work exactly, when the compass update rate is 1Hz, while the flow measurements happen faster?

If there is no correction for the data, does anyone have routines for data handling and post-processing regarding the tilt or compass data? I would simply reject data, when the changes of the compass direction change too fast (the error due to instrument movements would be calculated with respect to the average horizontal flow speed during the measurements), but maybe there is another solution?

Best regards,

Lars  Gansel

Hei Lars,

The person who can best answer these questions will be back in the office next week. Please bear with us until then, or maybe someone else out there have some experience with this?

- Ketil

Hello Lars,

Though I've not designed these instruments, I'm sure the velocity readings are not corrected for a change in compass reading. Corrections like that require an instantaneous and highly accurate measurement of the rate of turn of the instrument and the exact vector from the centre of rotation to the actual measuring point. Since I know the compass used in the AquaDopp or vector is not capable of measuring rate of turn, and I also know there is no information in the instrument as to the exact position of measurement it is just impossible.

You could try to do it in post processing, but I'm afraid the internal low pass filtering of the compass heading information will be a problem.

Best regards,

Cees Meijer

Qmetrix BV

Hei Cees,

thank you for your comments. I was also thinking about the frequency of the compass readings as a problem considering a correction of the data during post processing.

Anyway, it seems to me that it is best to not use data, when the change of the compass direction is "too large" from one second to the next. It is simple enough  to calculate the average error in the measurements due to rotation of the instrument, as we know the distance between the rotational axis of the instrument and the measured water volume. I will use a static approach and decide on a limit for the change of compass direction between two seconds and I will not use data within those seconds, when this limit is exceeded.

It might be an alternative to use a dynamic approach and relate the average error within one second due to the rotation of the instrument to the measured velocity in that second. 

I will be grateful for further comments or suggestions...

Best regards,

Lars Gansel