Contactless Measurement of Low Liquid Flow Rates

Abstract

A sensor for precise measurements of low flow rates of specialized liquids, including medical liquids, is developed. Through the use of the sensor, it is possible to automate the process of precision dosing of liquids in real time. Keywords: flow sensor, contactless method, drip chamber of infusion system, translucent element.Special attention is now being paid to the problem of increasing the efficiency and precision of measurements of the rate of flow of liquids [1][2][3]. Precision in the measurement of low flow rates of liquids is of great importance in the chemical and biochemical branches of industry and in medicine, for example, for measurements of the rates of flow of solutions of medicines in infusion systems, liquid chemical substances, including catalysts and titers when dosing medicinal compounds, solutions, etc. [4].Existing methods of measurements and devices for implementing these methods [3, 5-7] suffer from a number of drawbacks, for example, large overall dimensions of the devices and the complexity of their construction, low precision of measurements of low flow rates of liquid, and low sensitivity. In addition, with the use of the contact method of measurement, which is employed in practically all devices, a sensor cannot be employed in corrosive and toxic media. These drawbacks reduce the precision with which low liquid flow rates are measured and limit the range of application of the sensors.A measurement instrument by means of which high precision may be achieved and, at the same time, an output signal from the sensing element transformed for subsequent processing, analysis, and generation of control actions is needed for automatic control of the rate of flow.A sensor [8] of low liquid flow rates ( Fig. 1) has been developed to realize precise measurements of low flow rates of specialized liquids. The sensor consists of a case 3 with opaque covers 1, 14 within which mounting screws 2, 13, lens mounts 4, 12, and plates with slit-like horizontal slots 5, 11 are installed. A transparent element 8 is situated in an opening 22 perpendicular to the axis of the case 3. A source of radiation (laser light-emitting diode) 20 with leads 21 is placed in mounting screw 2 while a photodetector (optoelectronic transistor or photodiode) 16 with leads 15 is placed in mounting screw 4. A scattering lens 19 designed in the form of a glass cylinder is mounted in lens mount 4 and a collecting lens 17 in lens mount 12. The translucent element 8 is supplied with an inlet 9 and outlet 18 for liquid and a drop former 10. The two lens 17 and 19 and the two plates with slit-like horizontal slots 5 and 11 form an optical system 6 that transforms the light flux from the radiation source 20 and creates a measurement plane 7 within the transparent element 8.The diameter of the transparent element 8 must be at least three times the diameter of a drop of the liquid being measured and must be greater than the length of the slots in plates 5 and 11. The length of the slot in plate 11 is slightly greater