EQUIPMENT AND SUPPLIES--New sample splitter for water-quality samples
In Reply Refer To: July 3, 1980
EGS-Mail Stop 412
QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 80.17
Subject: EQUIPMENT AND SUPPLIES--New sample splitter for
water-quality samples
Bob Middelburg of the Quality of Water Branch has developed a new
sample splitter called a cone splitter. The cone splitter divides
a water sample or sample of a water-sediment mixture into ten
equal parts. The cone splitter was originally developed for the
Urban Hydrology Studies Program for use in splitting samples taken
with an automatic pumping sampler for analysis of chemical and
physical constituents. In addition, the cone splitter will be
quite useful in compositing proper proportions of several samples
taken throughout a runoff event into a single discharge-weighted
sample that represents the flow event.
Enclosed for your information is an article describing the cone
splitter; its accuracy, application, and procedure for use. The
differences in application between the cone splitter and the churn
splitter are noted in the article. Cone splitters are currently
being distributed to all projects that are part of the Urban
Hydrology Studies Program. Further information regarding the use
or availability of the cone splitter can be obtained by calling
Bob Middelburg, Quality of Water Branch, FTS 928-6834.
R. J. Pickering
Chief, Quality of Water Branch
Enclosure
Distribution: A, B, S, FO, PO
Key Words: Water quality, instrumentation, subsampling,
sample splitting
Superseded memoranda: None
The USGS Cone Splitter
Sampling methods have been developed that produce samples that are
representative of flow through a cross-section. These methods
frequently conclude with one bulk volume of water-sediment
mixture. Unfortunately, preservation techniques and analytical
methods do not always allow the submission of one sample in a
single container to the laboratory for analysis. The sample must
be subdivided, usually within a short time after collection, into
a number of subsamples each of which must be virtually equivalent
in concentration of suspended and dissolved constituents.
The USGS churn splitter can be used to subsample a very large
volume (8-12 liters) sample collected for chemical analysis. It
allows obtaining different subsample volumes from the sample while
still maintaining the same basic chemical and physical properties
of the original sample. The churn splitter has proven to be an
invaluable tool for the collection and processing of composited
cross-section samples from rivers and streams. The major
disadvantages of the churn splitter are l) sample volumes less
than about 6 liters cannot be split, and 2) inorganic sediments
coarser than 62 um cannot be split with an accuracy of less than
about +/- 10-15%.
The recent use of automatic samplers has introduced a problem that
makes the use of the churn splitter impractical. Automatic
samplers usually collect relatively fixed sample volumes, most of
which are at or below the minimum volume of water required for
proper operation of the smallest available churn splitter. Most
automatic samplers collect only between 0.5 to 3 liters in one
sampling cycle, which is not enough volume for proper use of a
churn splitter.
The new cone splitter was first developed in December 1979 as a
means to reliably subsample samples collected for the Urban
Hydrology Studies Program conducted by the U.S. Geological Survey
in cooperation with the U.S. Environmental Protection Agency.
Tests have shown that the cone splitter can split samples as small
as 250 mL volume into 10 equal subsamples, each subsample being
with +/- 3 percent of the correct volume and sediment
concentration.
Description of Cone Splitter
The cone splitter illustrated in Figure 1 is a pour-through
device. A funnel-shaped reservoir on the top receives the sample
and directs it into the splitting chamber. Located in the
reservoir funnel is a 2 mm-mesh screen which retains large debris
such as leaves that could clog or interfere with the splitting
process. The screen reduces the vortex action of the water leaving
the funnel and also helps mix the sample.
Below the funnel is a short section of stand pipe. Its function is
to direct water as a steady stream into the splitting chamber
which contains a cone-shaped splitting head.
The cone splitter housing is machined from a solid block of Lucite
or comparable material. Ten exit ports have been precisely drilled
through one common point at a 45-degree angle from the vertical
and spaced at 36-degree intervals around the circumference. The
resultant configuration in the splitting chamber is a notched cone
with 10 equally spaced exit ports about its base. There are no
flat walls, benches, or surfaces inside the splitting chamber that
can retain material or interfere with the splitting process.
The 10 exit ports direct the individual subsamples into
distributor tubes leading to the subsample containers. The tubes
are of sufficient size and alignment to prevent any back pressure
or restriction of flow from the splitting chamber. They also are
kept to a minimum length to prevent submergence of the end in the
subsample. Any restriction of flow from an exit port will
interfere with the rate of split-sample entry into that port,
causing a bias in the splitting.
Evaluating the cone splitter
Two prototype cone splitters were constructed and tested for
accuracy and bias. The tests were conducted using both clear water
and prepared samples of water and sand-size sediment. In addition,
tests were made to determine the effect of tilting the splitter
and of pouring the sample into the splitter at different rates and
orientations.
To test the accuracy and bias with respect to volume, Bruce M.
Delaney of the New Mexico District sediment laboratory prepared
six samples of deionized water placed in l-gallon plastic
containers, similar to commercially used milk, juice, and water
jugs. The volumes for all observations were determined by weighing
to the nearest 0.1 grams (essentially equivalent to 0.1 mL using
deionized water). Samples were introduced into the splitter by
inverting the sample bottle over the reservoir, allowing it to
empty as rapidly as possible. The splitter was allowed to sit for
approximately 1 minute after splitting for draindown before the
subsamples were removed and weighed for volume tests.
The results of the six volume tests are given in Table 1. After
weighing each subsample, it was determined that on the average 2
mL of water was lost during a splitting process due to droplets of
water adhering to various parts of the splitter. A small bias in
the distribution was observed from outlet to outlet. This was
probably due to slight variations during the fabrication process.
Table 1 shows that the average discharge from tube No. 8 was
consistently high by 1.5 percent, but this is considered well
within acceptable limitations. It should be noted that the outlet
numbers do not correspond to the sequence that the outlet ports
were drilled.
To check the accuracy with respect to volume splits, each
subsample was compared to the mean volume for each split. The
maximum error observed was +1.9 percent (outlet 8, test 6) and the
minimum error was -1.7 percent (outlet 2, test 3). The standard
error in percent (standard deviation divided by the mean times
100) for each test was 1.1 percent or less. These observations
indicate that the cone splitter is capable of accurately
subdividing a sample into 10 equal parts by volume within an
arbitrary acceptable error limit of +/- 3 percent.
An additional series of tests were made using a water-sediment
mixture to test the splitter capability to produce subsamples
equivalent in physical composition to the original sample. Six
samples were prepared. Each consisted of 1.0 grams of 62- to 125-
um sand, 4.5 grams of 125- to 250- um sand, and 0.5 grams of 250-
to 500- um sand plus deionized water to bring the total sample
weight of 2500.0 grams. Using a suspended-sediment mixture of
predominantly sands was considered to be a worst-case condition
test because sands will not easily stay in suspension as compared
to silts or clays. Particle sizes finer than sand (< 62 um) should
split with an accuracy comparable to the volume-test results. If
the cone splitter operates properly, the sediment concentrations
of the subsamples should be virtually equivalent for each outlet
and should not vary with the variation in volume from outlets.
The results of the water-sediment mixture tests shown in Table 2
indicate that the splitter will subsample samples containing sand-
size sediment with a precision of 2.3 percent as calculated by
averaging the standard deviations from each test. Test 2 produced
both the maximum (+5.6, outlet 10) and the minimum (-4.4, outlet
3) individual subsample errors. Figure 2 shows the plot of the
mean, maximum, and minimum volume and concentration for each
outlet. There does not appear to be any correlation between the
variation in sediment-concentration means and volume means from
outlet to outlet. Variability about the mean is greater for
concentration than for volume, which is expected because the
measurement of sand-size sediment concentration is less precise
than measurement of volume alone.
The 10 subsamples obtained from test 2 were further analyzed for
particle-size distribution. Summary results given in Table 2 show
that the percent by weight of each subsample in the size range 125
to 250 um is well distributed among the subsamples with a maximum
deviation of 3 percent from the mean.
The series of sediment-concentration tests do indicate a possible
bias in the splitter operation, although the error of the bias
appears to be acceptable. When observing the percent variation
from the mean concentration, the outlets having a positive
differences are grouped together. For example, for test 1, table
2, outlets 10, 1, 2, 3, and 4 are all greater than the
concentration average and they all represent one side of the
splitter outlet ports. This pattern was observed in the other
tests, although it was not always the same group of outlets that
contributed an above-average concentration. This phenomenon may be
attributed to a slight vortex action associated with the flow
through the stand-pipe. The sand leaving the funnel may tend to
string out into a ribbon rather than mix. Further tests and
changes of stand-pipe and screen designs will be necessary to
determine the actual cause. It is believed, however, that such
additional tests are not warranted considering that the observed
errors are well within an acceptable range and the fact that the
tests were conducted using predominantly sands, which represent
the worst-case situation.
Testing of new cone splitters
To obtain reliable results, as observed in the series of tests
previously discussed, a controlled operating procedure must be
followed. Before using a new splitter, operators should
familiarize themselves with the individual instrument by running a
series of tests to determine any bias that could result from
imperfection or operator procedures. The following test procedure
should be followed:
1. Inspect the cone splitter housing and outlet ports. They should
be smooth and symmetrical without any burrs or chips visible. Make
sure the cone splitter is clean and place on a stable platform or
bench in a level position. Visual leveling is sufficient.
2. Connect 10 discharge tubes to the outlet ports. All tubes must
be approximately the same length, and the length should be as
short as possible. The tubes need only extend into the receiving
containers sufficiently to prevent spillage. They must not extend
in so far that the end becomes submerged. Mark the outlets from
1 to 10.
3. Wet the cone splitter by pouring through several liters of
clear water. Lightly tap the system to dislodge adhering water
drops, then discard the water. Replace an empty container under
each outlet.
4. Accurately measure approximately 3 liters of clear water into a
l-gallon narrow-mouth plastic bottle.
5. Rapidly invert the gallon bottle over the reservoir, letting it
flow as fast as possible. Rest the inverted bottle on top of the
reservoir. The rising water level in the reservoir will regulate
the rate that water will leave the gallon bottle once the bottle
opening becomes submerged. For proper operation, the stand-pipe
must be discharging at its full flowing capacity.
6. After all water has passed through the splitter, tap the
assembly several times to dislodge adhering water drops. Check for
spills and leaks. If any are observed, discard the test, correct
the problem, and repeat the test.
7. Accurately measure the volumes of the 10 subsample within
+1 mL. Record the volumes for each outlet on a form similar to
Table 1.
8. Repeat the test two more times for a total of three tests. Use
approximately the same initial volume for each test.
Calculating Results
To determine the accuracy of the cone splitter tested, calculate
the mean volume of each subsample (x) and standard deviation (Sx)
for each test, by
calculate the standard deviation in percent (Ex) by the following:
Ex = Sx/x x 100
also calculate the error for each subsample (Ei) by
where Xi is the measured volume for the individual subsample.
Finally, compute the average standard error (Ex) for the three
tests and note the maximum and minimum errors (Ei) for all tests.
A cone splitter is considered acceptable for sample processing if
the average standard error (Ex) for the three tests is 3.0 percent
or less, and no individual errors (Ei) exceed +/- 5.0 percent.
Note the error patterns for individual outlets to determine which
outlets show consistent bias and mark them with their average
percent bias error.
Using cone splitters
The cone splitter works best when the following procedure is
followed. A consistent procedure such as always tapping the
assembly at the end of a split and always wetting the system
before a split should be practiced to help assure unbiased
results.
1. Set up the cone splitter on a flat open area. Check for level
and proper tubing lengths. Visually inspect the splitter for
broken parts, misalignment or debris.
2. Rinse through one or two liters of deionized water. Discard the
water.
3. Place containers under each outlet.
4. Shake the sample for 10 to 15 seconds.
5. Rapidly invert the sample container over the reservoir and rest
it on the reservoir top.
6. After the flow has stopped, tap the assembly to dislodge
adhering drops.
7. Remove desired subsamples, Repeat as necessary if any of the
subsamples need splitting starting with step 3.
8. At completion of all splits for the station being processed,
disassemble the splitter and clean before splitting another
sample.
All subsamples do not have to be collected in separate bottles.
Outlet tubes can be combined to collect various combinations of
the original sample. Care must be taken, however, when combining
outlet tubes into a single bottle to make sure there is no
backpressure resulting from restriction of the flow.
Consider for example, the following subsamples are required from a
3-1iter sample:
3-250 mL subsamples for chemical analyses (total).
1-500 mL subsample for chemical analysis (total).
2-250 mL subsamples for chemical analysis (dissolved).
1-500 mL subsample for chemical analysis (dissolved).
The sample then is split by placing a 500-mL bottle under three
outlets, two outlet tubes are combined into a l-L bottle, and the
remaining five outlet tubes can be combined into one convenient
container for later filtering. The resulting split of the 3-L
sample would provide three 500-mL bottles with 300-mL each and the
liter bottle with 600 mL. There would then be 1500 mL left for
filtering. These volumes are close enough to the desired amounts
for lab analysis.
If a more exact subsample volume is desired the following
procedure is used. For example, if 440 mL is required from a
sample of 2850 mL the first step is to compute the percentage
needed. In this case 450 mL is 16 percent of 2850 mL. The 16-
percent split is achieved by first obtaining 10 percent from one
tube from the first pass. The remaining 6 percent is obtained by
pouring one of the lO-percent splits through the splitter a second
time and drawing off 6 tubes or 60-percent. By this procedure a
subsample of +l percent of the whole sample can be obtained by two
passes through the splitter.
Care and maintenance of cone splitter
Cone splitters must be cleaned before being used for processing
any samples. It is not necessary to clean it before splitting
repetitively from one sample, but between a series of samples from
the same station and runoff event, rinse the splitter with several
liters of distilled water. Before using a previously cleaned
splitter, start by pouring several liters of deionized water
through the splitter. After using a splitter, acquiring a new
splitter, or before starting to process a sample from a different
station, clean the splitter by disassembling it and washing the
parts in soap and water using a good quality laboratory detergent.
A soft bristle test-tube brush works well for cleaning inside the
ports. Rinse thoroughly with tap water followed with deionized
water. Store cleaned cone splitters in plastic bags between
usages.
The cone splitters should be visually inspected for damage
especially the cone splitting chamber. Units that show damage or
wear should be retested to check their serviceability. Check
discharge tubing frequently for proper length and cleanliness.
Replace tubes as conditions warrant.
The cone splitter is built to very close tolerances which are
required for accurate and reliable operation. Given proper care
and handling and operated according to the approved methods, the
cone splitter should produce reliable results for a considerable
number of samples.