Sounding weights are used in streams where discharge measurements are made by suspension techniques from bridges, cableways or boats. The sounding weight is suspended below the current meter to keep it stationary in the water. The USGS Columbus-type or “C”-type weights are streamlined to minimize resistance to flowing water and are longer than the current meter, thereby offering some protection from bridge piers and flowing debris. The added length also produces better directional and stabilizing characteristics than that obtained with other weights.
The hanger slot is shaped to allow the hanger to tilt forward 15 degrees and backward 5 degrees from the vertical. This prevents the meter from coming into contact with, and being damaged by, the weight. These weights come in 15, 30, 50, 75, 100, 150, 200 and 300-pound (7, 14, 23, 34, 45, 68, 91 and 136 kg) sizes. The 15-pound (7 kg) weight is cast from brass, but all others are cast from lead and have aluminum tailfins. All are properly balanced and tested by an approved method before shipment. The weights can be epoxy coated if required.
Required weight can be estimated by the following formula:
Depth x Velocity = Required Weight (all units in English)
Example: 10 feet deep x 5 feet per second = 50 lbs weight required.