MIDAS ECM - Operating Manual
MANUAL-738407771-1 | issue: 1.3
Page | 19
©
2022
– Valeport Ltd
Appendix 1: FAQ’s
Can I make current profile measurements with the MIDAS ECM?
Yes, you can, but not in the way that an ADCP will. The M
IDAS
ECM is a point current measure-
ment device, i.e. it tells you the current speed and direction at its present location only. You can
create a profile by lowering the instrument through the water column, but you have to do this slowly
since the instrument will measure the water moving out of its way as it is lowered/raised.
I've fixed it to my ROV and I'm getting strange readings - why?
There are three possible causes of this. Firstly, check that nothing else on the ROV is physically
interfering with the current sensor, for example a cable lying across it. Anything that intrudes on
the electrical fields (which extend about 6cm from the face of the sensor) may affect the readings.
Secondly, check that the instrument is not positioned adjacent to a source of electro-magnetic in-
terference such as a thruster or other motor. These can generate large electrical fields that may
disrupt the EM sensor. Whilst it is designed to reject a certain amount of signal noise, this problem
usually manifests as an intermittent fault, coinciding with the thrusters being turned on and off.
Finally, consider the possibility that a thruster from the ROV is actually ejecting water over the de-
vice, disrupting its measurement of the ambient current.
How close can I mount it to a solid structure?
There are two issues here. Firstly, the solid structure may actually adjust the local flow conditions,
so it is a good idea to fix the instrument where it will be in the prevailing current as far as possible.
Secondly, if the structure is metallic, it may have some effect on the compass. The flux gate com-
pass shouldn’t be drawn towards the metal as a hand-held compass might be, but large metal ob-
jects or electrical signal cables may actually alter the local magnetic field of the earth slightly. The
best solution is to slowly move the instrument towards the structure and see how close it can be
positioned before the readings are affected.
Is the EM sensor affected by marine growth?
Yes, growth will affect the sensor. There is a “measuring volume” of water that the sensor is look-
ing at; the value it gives you is the speed of water flowing through that volume, which is best envis-
aged as small cylinder on the face of the sensor, about the size of half a can of soft drink. If there
is growth on the sensor that intrudes on this “measuring volume”, then there will be less water flow-
ing through that volume, and the output from the sensor will decrease. The effect will be propor-
tional; a single barnacle on the sensor face will have a virtually undetectable effect, but a collection
of mussels would be significant.
It is also important that the electrodes on the sensor face are kept in contact with the water, alt-
hough we have noticed that marine growth normally avoids these.
How often does it need calibrating?
It is our experience that in the majority of cases, performance can be maintained by recalibrating at
2-yearly interval
s. However, we are aware that many operators’ own QA requirements state an-
nual recalibration, and it is true that most instruments are returned to us on a yearly basis.
