Maximum Sampling Depths and Transit Rates for Suspended Sediment and Water-Quality Samplers In Reply Refer To: January 31, 1994 Mail Stop 415 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 94.05 Subject: Maximum Sampling Depths and Transit Rates for Suspended Sediment and Water-Quality Samplers There have been several questions recently about the acceptable transit rates for sediment samplers and why depth integrating samplers are limited to sampling of water depths less than about 15 feet. The following discussion explains reasons for the restrictions as well as presenting the limits on transit rates and depths for some sampler/nozzle combinations which have not been readily available in the past for some sampler/nozzle combinations. For a sediment sampler to collect a representative volume of the surrounding medium, the water-sediment mixture must move through the nozzle undisturbed. To move into the nozzle undisturbed, the water must not change directions to enter the nozzle, which implies that it must enter the nozzle at the same velocity as it is moving in the stream at the point through which the nozzle is passing. When the velocity is undisturbed upon entering the nozzle, the condition is characterized as isokinetic. All depth- and point-integrating samplers used by the U.S. Geological Survey (USGS) that have rigid sample containers sample isokinetically only if pointing directly into the flow and if they are used within certain ranges of depths. Depth-integrating samplers also operate isokinetically only when the vertical transit rate is maintained within a given range. A sampler will not operate properly if used at too large a depth. For example, if the volume of the water-sediment mixture is to be two-thirds of the total container volume for a depth-integrated sample, the sampler should be one-third full as it reaches the bottom of the river so that it will have room for the water collected on the return trip. At the bottom of the river, therefore, the volume of air in the sampler must be at least two- thirds of the total volume. At sea level the pressure of the air in the bottle before it enters the water is about 34 feet of water (atmospheric pressure). Boyle's gas law states that the product of the pressure and volume is a constant, so equating the products at the surface and bottom gives an expression for the maximum pressure (depth) for the sampler: 34 Vs = P 2Vs/3 in which Vs = the volume of the sample container (which cancels out) and P equals the maximum pressure, in feet of water, for the sampler. Solving for the pressure gives P = 51 feet. Subtracting atmospheric pressure (34 feet) leaves 17 feet as the compression limit of the sampler. If the sampler is lowered below the compression limit it will intake too much water and not sample isokinetically. The compression depth limit varies for various depth-integrating samplers, as shown in Table 1, depending on the maximum percentage of useful volume. It also varies with sample container size and volume of the pressure compensating chamber for point-integrating samplers. The values given in Table 1 are for sea level conditions. The maximum depth decreases about 1 foot for every 1000-foot increase in elevation. There are two factors which control the maximum vertical transit rate for a depth-integrating sampler. These factors include: approach angle and the compression rate. The approach angle is determined as the ratio of vertical velocity of the sampler (rate at which it is lowered or raised) to the mean stream velocity. If the sampler is lowered or raised at a rate exceeding 0.4 times the mean flow velocity, the intake velocity will be less than the stream velocity (FISP 1952). The maximum vertical transit rate for any depth integrating sampler, therefore, should not exceed 0.4 times the mean stream velocity of the section. The compression rate, which is related to the compression limit, may restrict the vertical transit rate to less than 0.4 times the mean stream velocity. As the sampler is lowered through the water column, the increased water pressure compresses the air in the sampler. If the sampler is lowered slowly, the incoming water more than takes up the space created by the compression of the air and the excess air exits through the exhaust vent. If the sampler is lowered too rapidly, however, the incoming water does not compress the air within the sampler fast enough so the pressure on the inside of the sampler is less than the hydrostatic pressure outside the sampler. When this occurs the intake velocity increases above the stream velocity. Water may even enter the sampler through the air exhaust vent. If the sampler is raised too rapidly, the compressed air inside the bottle will not escape fast enough through the exhaust vent and the intake velocity will be less than the mean stream velocity. The compression-rate limit is a function of the size of the nozzle and sample container. For large bottles with small nozzles it can limit the vertical transit rate to less than 3 percent of the mean stream velocity. Table 1 lists the maximum transit rates that should be used with most USGS samplers for various combinations of nozzles and container sizes. Edwards and Glysson (1988) discuss the proper use of the samplers and transit rate ratios for some of the more common combinations used by the USGS. Because of small allowable vertical transit rates and difficulty of maintaining a slow transit rate, the Office of Surface Water does not recommend using the 1/8-inch or 3/16-inch nozzle on the D-77 sampler with a 3-liter container. References: Edwards, Thomas K. and Glysson, G. Douglas, 1988, Field Methods for Measurement of Fluvial Sediment, U.S. Geological Survey Open- File Report 96-531, 118 p. Federal Interagency Sedimentation Project, 1952, The design of improved types of suspended-sediment samplers - Interagency Report 6: Minneapolis, Minnesota, St. Anthony Falls Hydraulics Laboratory, 103 p. Charles W. Boning Chief, Office of Surface Water Attachment WRD DISTRIBUTION: A, B, FO, PO