Table 9222 Comparison Of Manual And Automatic Sampling Techniques

Advantages

Disadvantages

Manual Grabs

Appropriate for all pollutants Minimum equipment required

Manual Flow-Weighted Composites (multiple grabs)

Appropriate for all pollutants Minimum equipment required

Automatic Grabs

Minimizes labor requirements Low risk of human error Reduced personnel exposure to unsafe conditions Sampling may be triggered remotely or initiated according to present conditions

Automatic Flow-Weighted Composites

Minimizes labor requirements Low risk of human error Reduced personnel exposure to unsafe conditions May eliminate the need for manual compositing of aliquots Sampling may be triggered remotely or initiated according to on-site conditions

Labor-intensive

Environment possibly dangerous to field personnel May be difficult to get personnel and equipment to the storm water outfall within the 30 min requirement Possible human error

Labor-intensive

Environment possibly dangerous to field personnel Human error may have significant impact on sample representativeness Requires flow measurements taken during sampling

Samples collected for O&G may not be representative Automatic samplers cannot properly collect samples for VOC analysis

Costly, numerous sampling sites require the purchase of equipment Requires equipment installation and maintenance Requires operator training May not be appropriate for pH

and temperature May not be appropriate for parameters with short holding times (e.g., fecal streptococcus, fecal coliform, chlorine) Cross-contamination of aliquot if tubing/bottles not washed

Not acceptable for VOC sampling Costly if numerous sampling sites require the purchase of equipment Requires equipment installation and maintenance, may malfunction Requires initial operator training Requires accurate flow measurement equipment tied to sampler

Cross-contamination of aliquot if tubing/bottles not washed a depth change. In critical flow flumes, the surface profile in the constriction passes through the critical depth. The flume discharge can then be directly related to the depth immediately upstream of the throat.

Flumes are sometimes classified according to throat shape. Common types include rectangular, trapezoidal, semicircular, and composite throat flumes. Flumes with a bottom contraction (a hump) are suitable for installation in sewers. The Parshall flume, the cut-throat flume, and the Palmer-Bowlus flume are also popular.

Rating curves for critical flume geometry may be constructed from solution of the Bernouilli Equation at points upstream of and in the flume throat. While they generally exhibit excellent characteristics of self-cleaning, flumes do not share the brand flow measurement characteristics of weirs.

There are a large number of other flume designs that can be used in drainage studies. For example, the Soil Conservation Service has HS, H, and HL flumes designed to measure small, moderate, and large runoff flows, respectively. These devices combine the best features of both flumes and weirs, with wide ranges of measurement, good self-cleaning characteristics, small head loss, and relative insensitivity to submergence.

0 0

Post a comment