Plaster standards to measure water motion

Plaster of paris blocks (clod cards) were re‐evaluated as devices to measure water motion in freshwater and marine environments. Clod cards, developed in 1971, have only been used as indicators of relative water motion. We generated standard curves relating the dissolution rates of clod cards to water speed, temperature, and salinity by attaching the cards to a mechanical arm rotating in a tank of water. Free convection experiments were also conducted by suspending clod cards in quiescent solutions held in ice chests. We describe calibration methods that use either forced convection or free convection to correct field results for the effects of temperature and salinity on dissolution rates of clod cards. Although dissolution rates were lower in freshwater than brackish or seawater, salinity in the range of 20–40‰ did not greatly affect dissolution. Properly calibrated, clod cards now offer a simple, practical method for measuring integrated water motion, expressed in readily visualized rate units, over a wide range of temperatures, salinities, and water speeds.