How to Measure a Current

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A clod card made of alabaster, adhered to an acrylic plate and strapped to a brick.

One of the simplest and cheapest ways to measure how much water moves by an area is to take advantage of materials that dissolve in water.  The faster water moves past the object, the more of the material will dissolve away, proportional to the exposed surface area of the object.  Materials that have been used for this purpose include plaster of paris, alabaster, and even Lifesavers.  These are typically called “clod cards” because the first ones consisted of clods of plaster attached to paper cards.

Our lab measures current flow with a combination of clod cards made of alabaster (inexpensive to produce, easy to deploy at many sites at once) with an Acoustic Doppler Velocimeter (very expensive, can only be deployed at one site at a time).

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Derek Smith building clod cards.

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Derek Smith and Cori Kane hauling a ton of bricks.

One of the disadvantages of clod cards is that they can only tell you about relative current flow.  That is, we can tell if Site A has faster flow than Site B, but not the actual average current speed at either site.  ADVs are able to measure three-dimensional water movement (“velocimeter”) by sending sound waves (“acoustic”) to bounce off of particles in the water.  When the echos from the sound waves return to the sensor, the ADV measures the doppler shift in the waves (“doppler”).  We pair clod cards with the ADV so we can approximately calibrate dissolution of the clod cards to an average current speed.

ryan installing adv

Ryan Knowles installing the ADV (Photo by Megan Cook).

The white instrument at the top right is the actual measurement probe.  Sound waves emanate from the three prongs, bounce off of particles in the water, and are received at the midpoint of the instrument head.  The probe is elevated above the bottom so that we measure free-stream flow instead of the reduced current in the boundary layer.  The large yellow canister strapped to the cement base is the battery pack so it can record for weeks at a time.  One of the calibrating clod cards is off to the left.