Nortek NDV receiver arm loose
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Hi,
I was working with an NDV in high flow conditions (river) and after a while noticeed that the SNR and COR values for beam 3 were 25-50% lower than beams 1 and 2. I checked the probe and found that beam 3 was slightly loose. I attempted a temporary repair with tape but this only improved SNR and COR values slightly and I suspect that beam 3 is not perfectly aligned.
What should I do?
Thanks,
Martin
Hi Martin,
Unfortunately we no longer support the NDV as it's a fairly old instrument. I'm not certain we have the ability to repair it any longer, and rapiers to the probe head on any type of velocimeter are fairly difficult.
The loose arm will have two potential effects on the instrument. One is the transformation from beam to XYZ coordinates will be affected since the probe geometry has changed. Any velocity data you obtained with the loose arm is going to be affected by this.
The loose receiver arm also will eventually permit water to leak into the head which will cause problems for the electronics. I wouldn't be surprised if the internal wiring has been slightly damaged for this receiver which could be causing elevated noise levels and lower received signal strength.
My best advice would be to purchase a newer system like the Vectrino (it really is much better than the NDV) or the Vectrino II (even better than the Vectrino since it's got a "II"). You could contact your sales rep and see if they can generate a quote for repair, but I suspect it would be fairly prohibitive compared to purchasing a new system if anything could be done at all.
P.J.
Hi P.J.,
Thanks once again for your advice. As the NDV is not ours (borrowed from another institution) I was hoping for something more optimistic!
I'll get in touch with a sales rep I know for Nortek UK and keep my fingers crossed!
Out of interest - I think the arm became loose because of high frequency vibration of the probe when using it in fairly harsh flow conditions (U 1-1.2 m/s, RMSu 10-12 m/s). Have you ever come across this sort of thing before?
M
I think the early Vectors (early 2000s) had a few problems with this as they were using a glued head. This led to a redesign where all the fittings are machined and held in place by various mechanical bits now.Out of interest - I think the arm became loose because of high frequency vibration of the probe when using it in fairly harsh flow conditions (U 1-1.2 m/s, RMSu 10-12 m/s). Have you ever come across this sort of thing before?
Dear Martin,
I can confirm from Nortek main office that the NDV is no longer serviced. You have probably been told this by our UK office already.
Regarding the old Vectors, the problem was not due to vibrations. Bonding between the molded epoxy and the titanium probe was the problem here, and only at some temperatures.
We have not experienced problems with this after the redesign. I can also assure customers that have this version - if it hasn't leaked yet, you're safe.
Best regards
Jonas
Your best bet is to ask around at other labs and see if there's anyone who also purchased an NDV and is no longer using it. I honestly think the time and expense of this is going to be more than buying or leasing a Vectrino (assuming leasing is an option in the UK) to replace the damaged NDV. There's also a field probe option for the Vectrino which is designed for more energetic environments than the standard lab probes.
P.J.
Thanks again, P.J. Nortek UKs Ramsay Lind has also been very helpful.
After attempting a repair on the arm and testing in standing water we are finding that the signal amplitude is slightly lower for the corresponding beam 3 (green on the attached screenshot). Despite this, COR and SNR values for the beam are comparable to beams 1 and 2. In your experience, is this symptomatic of damage or is it a normal consequence of beam geometry? Do you see this as a potential problem for turbulence measurements?
Martin
The main concern is the probe geometry has now changed so the transformation matrix is no longer valid. You can see one of the beams is no longer focused on the sample volume, showing an earlier decay in the signal. This means your XYZ velocities will be wrong.
For turbulence measurements, you could still use the beam velocities as a surrogate for the vertical turbulence fluctuations, but I would not trust any of the XYZ data to give you reliable measurements.
P.J.
We have now reset the repair and the NDFCheck diagnostic shows that all 3 beams have very similar peaks at around 90-95 counts and noise levels are 55-60 counts. The NDV manual suggests that these levels are normal but does the probe still require recalibration and, if so, how do we go about recalibrating if Norway no longer service this model?
Martin
I would still recommend recalibration since the receiver arm has probably moved from its original calibrated position.
Calibration involves towing at known speeds and measuring in beam coordinates. Using the known tow velocities, the coefficients needed to obtain the corresponding XYZ velocities from the measured beam velocities are determined, typically with a least squares solution.
P.J.
Thanks again P.J., your help has been invaluable.
I may be able to get access to a towing tank to carry out the recalibration but is anyone able to give me more detail on the recalibration procedure? Specifically, in the original factory procedure, what range of velocities were the probes calibrated across?
Also, when the coefficients are obtained how do I edit the calibration file (e.g. Nxxxx.PRO)? Bearing in mind that, beyond some R and MATLAB, I am not much of a programmer!
Martin
Hi Martin,
I'm not sure what the original calibration tows were done at, but probably 10, 20 and 30 cm/s would be a good guess. Unfortunately, higher tow velocities are not practical in most tow tanks used for calibration because they are so short an adequate amount of data can't be obtained.
I have no idea on editing the *.PRO file. Honestly, I would just leave that alone and do the correction in post processing. The basics of the coordinate transforms can be found on the forums by searching for "coordinate transform". It's actually pretty simple once you understand what's going on (at least between beam and XYZ).
What we do these days for the Vectrino is a least squares solution to obtain the transformation matrix. This basically falls out of the above coordinate transformation by solving the matrix equation Ax = b. If x represents the XYZ velocities (knowns) and b is the beam velocities (measured), you can solve for A. This is the matrix to go from xyz to beam, the matrix inverse of A is then what would take you from beam to XYZ.
P.J.
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