tailieunhanh - A Practical Guide to Particle Counting for Drinking Water Treatment - Chapter 5

Grab lấy mẫu Grab lấy mẫu là một hình thức quen thuộc của mẫu đo. Trong số các lợi thế của nó là giảm chi phí thiết bị và khả năng để đo các mẫu từ bất kỳ nơi nào có thể truy cập trong quá trình dòng. Một số trong những nhược điểm là lao động trực tiếp hơn là tham gia, và mẫu có thể được thay đổi trong quá trình thu thập và thử nghiệm. Rõ ràng, một hình ảnh hoàn toàn thay đổi quá trình có thể được sản xuất so với một phép đo trực tuyến. | CHAPTER 5 Grab Sampling Grab sampling is a familiar form of sample measurement. Among its advantages are lowered equipment costs and the ability to measure samples from any accessible place in the process stream. Some of the disadvantages are that more direct labor is involved and the sample can be altered during collection and testing. Obviously a much less complete picture of process changes can be produced in comparison to a continuous online measurement. While this is true for all forms of grab-sample data collection the high sensitivity of particle counters makes careful consideration of these factors imperative. This chapter begins with a brief description of how grab-sample particle counters operate then as with 4 to 20 mA current loops tries to talk the reader out of using them. After the reader has run this gauntlet and remained unconvinced the discussion will turn to the practical aspects of grab-sampler operation. A. PARTICLE COUNTER GRAB-SAMPLER OPERATING PRINCIPLES A grab-sample particle counter is similar to an online particle counter in basic operation. The main difference is that an automated method of propelling the sample through the particle counter sensor has been incorporated usually in the form of a pump. Unlike a turbidity grab sample which can be measured directly from a stationary sample the particle counter sensor operates at a fixed flow rate. This is because particle counts are measured per unit volume. The astute reader may point out that a fixed volume can be passed through the particle counter regardless of the consistency of the flow rate. This is correct and some of the older pressurized particle counter batch samplers operated under this principle. They were designed to dispense a fixed volume of liquid for each test run. Some samplers of this type are still in use in water plants. These units were designed to handle viscous fluids which require a good deal of pressure to force the sample through the particle counter flow cell. .