The D-77 bag sampler can be used in streams as shallow as 2 to 3 feet deep. Frame-type bag and bottle samplers require deeper streams in order to minimize the effect of the unsampled zone. Point samplers are known to contaminate trace-element samples and cannot be easily sterilized so that if samples are to be analyzed for trace elements or bacteria, bag samplers must be used. Smaller nozzles provide a larger difference between the slowest and fastest, total round-trip transit time. These statements may seem counter intuitive but examination of the tables for bottle transit rates and reeling rates will clarify the statement.
Smaller nozzles require slower transit rates.
Nozzle size has little effect on minimum sample volumes. No substantial increases in depth capabilities are provided by reducing the nozzle size for any other bottle. For bag samplers, smaller nozzles may be preferred because they provide isokinetic sampling in greater depth and velocity ranges and smaller minimum volumes than do larger nozzles. Smaller nozzles also provide a greater range between the slowest and fastest, total round-trip transit time. And, as opposed to bottle samplers, smaller nozzles also provide greater range between the slowest and fastest isokinetic transit rates.