Research On Liquid Micro-flow Calibration Facility

At present,the flow range of micro-flow meter is continually expanding and its accuracy is improving,which puts forward new requirements for the calibration of micro-flow.The field of micro-flow measurement is particular.On the basis of highly accurate flow detection,it is also necessary to maintain the stability of flow output.At the same time,it needs to face some new problems such as evaporation,degassing,surface tension,temperature variation and so on.Therefore,the method of uncertainty analysis was used in this subject to evaluate the sources and effects of the problems and solve them in the micro-flow facility.This thesis starts with the overall design and solutions for key parts of the flow facility,works are carried out as the following:(1)In view of the requirements of calibration,a micro-flow calibration facility was designed and built up.The facility can accomplish the compatibility of the flying-start-and-finish method and the standing-start-and-finish method.The flow range of the calibration facility can reach 0.1ml/min-80ml/min,and the combined uncertainty of the flow calibration is 0.10%(k=2).(2)In view of the particularity of the flow diverting problem under the micro-flow condition,a closed diverter structure was proposed as a solution,which changed the situation that the diverter can only flow in one direction in the previous calibration.The circuit about time acquisition and logical control was designed.The overall time uncertainty of the facility is 0.0021%,and the diverter disturbance error is about 0.8mg.(3)Aiming at the evaporation problem in the process of micro-flow calibration,the analysis process and solution were presented.Three different methods were used to solve the evaporation problem,and the results of theoretical correction for the evaporation are not good enough.The evaporation was reduced to 0.083mg/min by a semi closed container.In addition,aiming at the problem of gas-liquid mixing in the process of operating the syringe pump,a degasser which can passively separate bubbles and liquid was developed.(4)The calibration experiment about the syringe pump was carried out,taking the micro-flow calibration facility as a platform.The factors producing a stable flow were analyzed.The influence of the motor speed,calibration duration and syringe cross-sectional area was analyzed respectively,then the improvement was made.Experiments show that the flow stability in the micro-flow calibration facility was better than 0.2%.The calibration of a Coriolis mass flowmeter with diameter of1 mm and accuracy of 0.5% was also carried out on the platform.The uncertainty and flow stability of the calibration facility can meet the calibration requirements.