Sampling thief

Large volumes of stationary liquids are normally sampled with a thief sampled, i.e., a device for collecting aliquots at different levels. 

Samples are best drawn (with a thief sampled) at various depths diagonally instead of vertically down so as to have a better cross-section of the bulk liquid. 

Pavlushenko et al. (P2) studied the suspension of screened fractions of sand and iron ore in a variety of liquids, with a 1-ft. diameter unbaffled vessel filled to a depth of one foot. Square-pitch three-blade propellers of 3-, 4-, and 5-in. diameter were used, and most of the observations were made with a 1 to 4 weight ratio of solids to liquid. Thief samples were taken at various levels in the vessel. In some cases, the contents did not become uniform at any impeller speed in other cases the contents became uniform at some impeller speed and remained so at higher speeds in a third type of behavior, the upper part of the vessel reached the over-all vessel average and then exceeded it as impeller speed was increased. Using the observations from the second and third types of behavior, a critical speed was defined as the lowest impeller speed at which the solids concentration at each level, or in the upper layers of the liquid, was equal to the over-all average solids concentration. This critical speed Nc in revolutions per second had the following relation to the operating variables ... 

Outer tube case of thief sampling device... 

This method is no more accurate than scoop or thief sampling, that are simpler to carry out, but gross errors are to be expected (Figure 1.5). Coning and quartering should never be used with free-flowing powders. 

It eliminates blend sampling error issues related to thief sampling. 

Relationships between three extraction parameters—solvent composition, temperature, and particle size—can be determined from simple beaker tests. Raw material ground to the desired particle size is contacted with the chosen solvent at the chosen temperature in a beaker and mixed over time. Thief samples are removed at various time intervals and analyzed for the marker. At the completion of the test, the marc is separated from the extract, dried, and analyzed. (In instances when the phytochemical is heat sensitive, the marc can be analyzed wet and the assay calculated on a dry basis). Based on this information, the amount of marker remaining in the dry marc can be determined after correcting for the marker in the absorbed extract in the wet marc. From these data, the equilibrium relationship defined in Equation 11.1 can be determined, as well as the time it takes to reach maximum extraction. Conditions are changed to give a high equilibrium constant and a short extraction time to reach equilibrium. Based on this information, one can select the appropriate extraction equipment as well as the operating conditions to operate the equipment. 

Another method, which is most suitable for finely divided powders, is thief sampling. The thief is a long, hollow tube which is thrust to the full depth of a container to fill it with material. After withdrawal the core sample may be conveniently discharged. 

Where possible, liquids should be thoroughly stirred before the sample is taken. Dip sampling is carried out by lowering into the liquid a small container fastened to a long pole. The container is often fitted with a tight-fitting stopper which may be opened at the desired depth by means of a rod attached to it. Thief sampling may be used in a similar manner to that previously described the lower end fitted with a valve. 

Bottom or thief sampling of liquids of 13.8 kPa (2 psia) RVP or less tank cars, storage tanks thief samping... 

Application—The core thief sampling procedure may be used for sampling liquids of 101 kPa (14.7 psia) RVP or less in storage tanks, tank cars, tank trucks, ship, and barge tanks. 

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