Universal User Guide, Rev L, February 2022
17
3
TCM Tilt Current Meters
(The following section applies only to TCM Current Meters. Users of the MAT-1 Data Logger for non-
current related measurements may want to skip this section.)
3.1
Theory of Operation
All TCM-x meters measure current using the drag-tilt principle. Typically, the logger is buoyant and is
anchored to the bottom via a short flexible tether. Drag from moving water tilts the logger in the
direction of flow. The MAT-
1 data logger’s accelerometer
and magnetometer channels are used to
record the amount of tilt and direction of tilt (compass bearing).
Ideally, for constant water velocity, the buoyancy, drag, gravity and anchor line tension are all in balance
and the tilt angle and direction are static. In practice, even in ideal conditions, the position of the meter
is not static.
induces oscillation in the TCM. Vortex shedding is normal and is factored
into the calibration of the meter but it must be recorded. In the field turbulence and waves add
additional motion. Fortunately, oscillations are relatively easy to filter out by taking multiple
measurements and determining the average tilt and bearing.
The frequency over time of vortex shedding varies with water speed and model number but is in the
range of 0.5 to 2 Hz. To avoid
, the meter should be configured to sample at least two complete
oscillations: a minimum of 4 samples per second for at least 4 seconds but we normally record a
minimum of 8 Hz for 10 seconds. (Additional consideration should be made for waves, turbulence and
improved accuracy.)
Domino is used to
configure the logger to record in “Burst Mode” prior to deployment and it
is used
again after the deployment to post process the data file and calculate the average speed and direction
per interval.