TABLE OF CONTENTS Any water analysis method is only as good as the "sample" used to represent the effluent stream. Sampling of a continuously flowing stream containing two or more phases (e.g., oil and water) is difficult unless the mixture is completely emulsified or is a very fine stable dispersion. Since the sampling techniques for oil concentration measurement and particle size distribution differ in some aspects, they are described separately here.
Generally, the larger the sample the more likely it is to be representative. However, for practical reasons, the sample size varies from 15 ml to about 11. Typically, the smaller samples are used for daily analysis, whereas the larger samples are used for monthly regulatory compliance purposes. The smaller the residual oil droplets, the more evenly dispersed they are likely to be. Care should be exercised to avoid sampling the surface of a liquid (since this is not truly representative). "Isokinetic" (which means equal linear velocity) sampling in midstream is the best, but is rarely possible. The sample probe must be inserted so that the velocity profile remains undisturbed, thereby getting a realistic particle distribution and, thus, a higher accuracy. The general guidelines are
Flush the sample line thoroughly and take the sample quickly.
Sample after a pump or a similar turbulent area where the stream is well mixed.
Obtain the sample from a liquid-full vertical pipe, if possible.
Sample bottles should be scrupulously clean and preferably of glass. Oil or other organic...
