Samples representative

for the purpose of laboratory analysis, it is necessary to obtain a small quantity of powder from a larger batch. For maximum accuracy and reproducibility it is necessary that the sample chosen be representative of the larger initial quantity. 

The rifflers shown in Figs 13.1 and 13.2 operate by loading the powder sample into a vibrating hopper which delivers the sample down a chute into eight rotating collectors. Both the delivery and rotational rates can be controlled. 

The sample, when loaded in the hopper, will be segregated. Therefore, 

Recognizing that Al/At is proportional to the feed rate F yields 

Equation (13.5) asserts that the change in particle diameter entering each collector can be made as small as necessary by decreasing the feed rate or 

The discussion may be opened with a problem. Temperature measurements conducted at a spring revealed 16° C at the edge of the water body, 18° C at the center at a depth of 5 cm, and 21° C at the center at the maximal depth, which was 1.3 m. Has the spring three temperatures Is one value more representative than another Or should we apply the average measured temperature The measured temperatures differed substantially, and the cause of these differences has to be understood in order to select the right value. In this example it is clear that the spring water emerged at a certain temperature, but reequilibrium with the ambient air temperature occurred 

Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 

Wells also have many faces —a nonpumped well contains a column of water that has possibly interacted with its surroundings and reveals certain temperature values and a certain composition. Upon pumping, water that was shielded in the aquifer enters the well and the properties of the water in the well change. For this reason, water from pumped wells is in many cases regarded to be most representative of its aquifer water. Pumping should be continued until temperature, conductivity, and other parameters reach a constant value. 

Overpumping of a well may change the local pressure distribution in the water system to the extent that water from an adjacent aquifer may breach in and change the water properties. Thus overintensive pumping can introduce new complications. An obvious example is encroachment of seawater into coastal wells. 

Standard latexes, and other reference materials, are available from various organizations [18—24]. One of the calibrations standards available to fineparticle scientists are latex spheres which were made on board the space shuttle in 1985. Because these spheres were formed in the absence of gravity they are perfectly spherical. The National Bureau of Standard makes available standard reference material in the form of ten micron microspheres mounted on glass slides. In the first type of slide a few thousand microspheres are deposited as a regular array on a glass microscope slide. In the other type, the fineparticles are randomly distributed [18]. A series of standard non-spheri-cal fineparticles have been prepared by the Community Bureau of Reference Commission of the European Community for use in comparing the performance of size methods. These reference powders are known as BCR standards and several publications are available describing the use of such reference materials [19]. 

Holes closed while probe is inserted or removed 

Another disadvantage of the riffler system is that a small sample size (as required by some modern size characterization methods) requires many successive rififlings and the cleaning of the apparatus can be tedious and time consuming. 

With the jar closed and the carrier cube assembled, the cube Is placed into the drum where the dimpled lining helps to carry the cube to a sufficient height before It tumbles chaotically, again aided by the dimples, to the bottom of the drum. 

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Fluid samples may be collected downhole at near-reservoir conditions, or at surface. Subsurface samples are more expensive to collect, since they require downhole sampling tools, but are more likely to capture a representative sample, since they are targeted at collecting a single phase fluid. A surface sample is inevitably a two phase sample which requires recombining to recreate the reservoir fluid. Both sampling techniques face the same problem of trying to capture a representative sample (i.e. the correct proportion of gas to oil) when the pressure falls below the bubble point. 

Sampling saturated reservoirs with this technique requires special care to attempt to obtain a representative sample, and in any case when the flowing bottom hole pressure is lower than the bubble point, the validity of the sample remains doubtful. Multiple subsurface samples are usually taken by running sample bombs in tandem or performing repeat runs. The samples are checked for consistency by measuring their bubble point pressure at surface temperature. Samples whose bubble point lie within 2% of each other may be sent to the laboratory for PVT analysis. 

Manufacturer verification" describes the measurement method of flaw charaeteristics. These characteristies are reported on the data sheet delivered with the equipment. Measurements are performed on a representative sample of flaw deteetors produeed. 

A separator is fed with a condensate/gas mixture. The condensate leaves the bottom of the separator, passes a flowmeter and is followed by a choke valve, after which the condensate is boiling. The flow can not be measured using the transit time method, due to the combination of short piping, the absence of a suitable injection point and the flow properties of the condensate, which is non-newtonian due to a high contents of wax particles The condensate can not be representatively sampled, as it boils upon depressuratioh... 

One of the most important uses of specific surface determination is for the estimation of the particles size of finely divided solids the inverse relationship between these two properties has already been dealt with at some length. The adsorption method is particularly relevant to powders having particle sizes below about 1 pm, where methods based on the optical microscope are inapplicable. If, as is usually the case, the powder has a raiige of particle sizes, the specific surface will lead to a mean particle size directly, whereas in any microscopic method, whether optical or electron-optical, a large number of particles, constituting a representative sample, would have to be examined and the mean size then calculated. 

Examine a procedure from Standard Methods for the Analysis of Waters and Wastewaters (or another manual of standard analytical methods), and identify the steps taken to compensate for interferences, to calibrate equipment and instruments, to standardize the method, and to acquire a representative sample. 

A plan that ensures that a representative sample is collected. 

More attention to selecting and obtaining a representative sample. The design of a statistically based sampling plan and its implementation are discussed earlier, and in more detail than in other textbooks. Topics that are covered include how to obtain a representative sample, how much sample to collect, how many samples to collect, how to minimize the overall variance for an analytical method, tools for collecting samples, and sample preservation. 

The choice of the best method for answering this question is governed by the specific nature of the system under investigation. Few general principles exist beyond the importance of analyzing a representative sample of suitable purity. Our approach is to consider some specific examples. In view of the diversity of physical methods available and the number of copolymer combinations which exist, a few examples barely touch the subject. They will suffice to illustrate the concepts involved, however. 

There are many sources of errors in the plant. The principal ones are related to sampling (qv), mass flow rates, assaying, and deviations from steady state. Collecting representative samples at every stage of the flow sheet constitutes a significant task. Numerous methods and equipment are available (10,16,17). 

Much effort is made to obtain a representative sample from bulk dry soHds. This can be a difficult task for very coarse material and from large... 

Sampling Plan. The first step to assuring accuracy, ie, the conformity of the measured value to the tme or expected value (23), is to obtain a representative sample. Sampling may be the responsibiUty of either production or of QC. A written sampling plan approved by QC should be followed. It is the responsibihty of the QC unit to ensure that samples represent all the material under evaluation. 

In the case of l,4-ben2oquinone, the product is steam-distilled, chilled, and obtained in high yield and purity. Direct oxidation of the appropriate unoxygenated hydrocarbon has been described for a large number of ring systems, but is generally utilized only for the polynuclear quinones without side chains. A representative sample of quinone uses is given in Table 5. 

Stored Free-Flowing Material. It is practicahy impossible to representatively sample stationary free-flowing powder because of the severe segregation that has almost certainly occurred. If there is no alternative but to sample this material, several samples should be taken and analyzed separately, so that an estimate can be made of the rehabhity of the measured parameter. For free-flowing materials stored in small hoppers, dmms, cans, boxes, and... 

When a sample of ca 100 g has been obtained, a representative sample for use in size characterization equipment must then be taken. Some of the more modem methods of size characterization require as Htde as 1 mg of powder, thus obtaining a representative sample can be quite difficult. If the powder flows weU and does not contain too many fines, a device known as the spinning riffler (Fig. 4c) can be used. A spinning riffler consists of a series of cups that rotate under the powder supply. The time of one rotation divided into the time of total powder flow should be as large a number as possible. Although this device has been shown to be very efficient, problems can be encountered on very small (1 mg) samples, and the powder must be processed several times. Moreover, in order to avoid cross-contamination, cleanup after each of the sampling processes can be quite difficult. Furthermore, if the powder is cohesive and does not flow weU, the equipment is not easy to use. A siUca flow agent can be added to the powder to enable the powder to flow... 

Sampling. The procedures for taking a sample, reducing the particle size of the sample, and separation of a smaller portion for later analysis are given in ASTM D2234 and D2013 (18) and BS1017. The procedures describe the minimum amount of sample needed to maintain a representative sample for analysis. 

In terms of the tensile-strength table previously given, the respective chi-square sample values for the daily, weekly, and monthly figures couldbe computed. The corresponding df woiJdbe 4, 24, and 99 respec tively. These numbers would represent sample values from the respec tive distributions which are summarized in Table 3-6. 

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