Research And Development Of An Automatic Testing System For Pumping Speed Of Diffusion Pump

The diffusion pump plays an important role in vacuum industry. The principle, device and procedure for measurement of diffusion pump performance characteristics are defined in detail in the Chinese standard GB/T19955. However the standard testing process is built on the basis of manual operation, as a result, there is no automatic device for diffusion pump performances measurement appearing in China.A newly developed automatic testing system used in laboratory for measurement of pumping speed of diffusion pump is introduced in this Master thesis. The system can automatically test the ultimate pressure and pumping speed of a diffusion pump. The key techniques for the automation are as follow:The liquid level heights in gage glass tube are automatically detected by two optical fiber liquid level sensors instead of by the testing operators’ naked eyes. The liquid level rising time is recorded by the computer instead of the manual stopwatch. We try to regulate the pressure in the test dome and the gas throughout with the needle adjusting valve droved by a stepper motor, instead of by manual turning along with naked eye observation of pressure meters.The measurement principle and key mechanical components of the system meet the Chinese national standard, thus the test results have good credibility. The whole testing system and measuring process are automatically controlled by a two-stage control system with an upper PC and a lower PLC. The reliability of the control system is studied by fault tree analysis. The visualization monitoring software in the upper PC is written by Visual Basic6.0under the Windows environment, and it can monitor the whole testing process, draw the pumping speed curve. By using Access database, monitor software can manage the data of tester, pump information and testing results.In this project, we need to precisely indicate the specified height for transparent or semitransparent thin liquid column in the gage glass tube, which can not be stably achieved with present optical fiber sensor. A novel fiber-optic liquid level sensor is reported which remarkably enhances the sensitivity by regulating the external light path based on the principle of light refraction. The structure and working principle of this sensor is introduced. A light path model is developed on the condition that the incident light ray obliquely penetrates the medium in the liquid level indicator. The formulae of deflection angle are deduced. Factors affecting the precision of the sensor are analyzed by means of an example. Finally the feasibility of this novel design is verified by experiments.The uncertainty analysis theory is applied to the performance analysis of this newly developed automatic testing system. Based on the test principle and system structure, it is studied how much influence each component and test step contributes to the uncertainty of results. According to differential method, the mathematical model for systematic uncertainty transfer function is established. Finally, by example analysis, the measurement accuracies of manual system and automatic system are compared with each other. The reasonableness and practicality of this newly developed automatic testing system is proved. And the uncertainty analysis method and accuracy calculation results in this paper also provide the theory basis and an instance to the uncertainty analysis for other types of vacuum pump performance testing systems.Demonstrative Experiment of this system was shown to the students of the Northeastern University7th Vacuum Training Class from various vacuum companies of China. In addition, this project successfully applied for two Chinese patents:New method and relevant device for measuring liquid level by using optical fiber photoelectric switch (No:200910011361.5); An Automatic Testing system and test method for performances of vapor stream pump (No:200910012545.3).