Maximum Sampling Depths and Transit Rates for Suspended Sediment and Water-Quality Samplers

In Reply Refer To:                                January 31, 1994
Mail Stop 415


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 

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.  


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