Printable Format - AN: Deploying the TCM-1 Tilt Current Meter in an Inverted (Hanging) Orientation

See how our meters were deployed under sea ice in Greenland.

1       Introduction

The TCM-1 Tilt Current Meter (TCM) is typically mounted in a vertical orientation where the meter is anchored on (or near) the sea bed and the meter floats above the anchor. But for some deployments, it is advantageous to measure water velocity near the top of the water column in an inverted orientation with the meter hanging below the attachment point. This application note describes how to use the TCM in an inverted orientation and includes recommendations on site selection, data logger configuration, anchoring techniques and data post-processing.

2       Requirements

The following items are required to use the TCM-1 in an inverted orientation:

• TCM-1 Current Meter consisting of:
  • MAT-1 Data Logger
  • 46 cm (18”) float
  • Nose cone with lanyard
  • 1 x Ballast Washer
• TCM-1 Inversion Kit consisting of:
  • 2 x Silicon Bronze TCM Ballast Washer Range Weights (part # TCMINVK)
  • 1 x 5/16-18 by 1-1/4 inch long silicon bronze hex head cap screw
• Windows PC with Domino Software
• USB A to micro-B cable (not shown)

3       TCM Theory

Tilt Current Meters operate based on the drag-tilt principle. The meter is tethered to a fixed point and the instrument floats (or sinks) vertically when there is no flow and tilts as water velocity increases. The direction of flow is determined by the direction of tilt.

Modern TCMs use three-axis accelerometers to determine the angle of tilt and 3-axis magnetometers to determine the direction of tilt. An important assumption is that the tilt measurement is determined in quasi-static conditions (i.e. the meter is not accelerating). TCMs typically oscillate between 0.25 and 0.5 Hz due to vortex shedding. This relatively low-frequency oscillation is averaged out of the raw data by sampling above the Nyquist frequency (twice the oscillation frequency). Likewise, the effect of swells can be filtered by increasing the averaging period to at least double the swell period. But movement and vibration of the anchor point, especially above the sampling frequency, adds error that is difficult to eliminate. Therefore, care must be taken when selecting a site for a top-mounted TCM to reduce or eliminate motion/oscillations/vibrations at the anchor point.

4       Site Selection

There are several factors that should be considered when selecting a site for a TCM.

Water Velocity: The TCM-1 is recommended for sites with water speeds of 0-80 cm/s. The meter saturates at 120 cm/s. Accuracy between 80 and 120 cm/s is not specified.

Waves: The TCM is designed for sites with low wave energy. Avoid sites with breaking waves as the rapid acceleration of the TCM will break the quasi-static assumption and will add error to the measurements. Waves are even more of a concern for meters deployed near the surface. Even when oversampling and filtering are used, the meter will be more accurate in relatively protected environments, such as protected harbors and coves, or deployed at least twice the wave height below the surface.

Anchor Point: The ideal anchor point is a fixed structure because it will eliminate the possibility of movement of the anchor point. But fixed structures are often not available, so the next best attachment point is a large floating dock, fish pen or other anchored floating structure. Small floats and buoys should be avoided because they are much more likely to move/oscillate/sway due to wind, waves and currents.

5       Meter Assembly

To assemble the TCM-1, screw the MAT-1 Data Logger into the threaded end of the float until tight. A firm grip is sufficient. Attach the three ballast washers to the end of the float using the bronze cap screw. Do not over-tighten.