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Branch Enclosure Distribution: A, B, S, FO, PO Key Words: Water quality, instrumentation, subsampling, sample splitting Superseded memoranda: None The USGS Cone Splitter Sampling methods have been developed that produce samples that are representative of flow through a cross-section.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.