The Gas Tiny Flow Measurement Instrumentation

Abstract

The paper presents the design and experimental experience with the gas flow measurement instrument for the range of (5 – 25) ml/hr. The aimed application area is in a biochemical laboratory for the study of reaction kinetic of sediments decomposition in waste water. The time – of – flight type of sensor with one upstream and one downstream temperature sensor has been chosen for the study. We explain the basic operation principles of the tiny flow measurement and the sensor structure. In the numerical model paragraph, we are describing the basic configuration model and the modelling results. As the three – dimensional simulation would be very time consuming process, we have simplified the simulation for only two – dimensional task. The presented diagrams are derived for different gases (air, nitrogen, carbon oxide and chlorine) and sensor tube materials, namely steel, copper, and plexi-glass. We present also the experimental set – up including the design and sensor parameters. The paragraph with experimental results and discussion on them illustrates the good correspondence with expected values. The paper concludes with the employment of designed gas flowmeter in the biochemical laboratory. INTRODUCTION The accurate measurement and control of tiny liquid flows in the amount of nanoliters up to milliliters per minute is becoming more and more important for a lot of applications in the life science. In some applications, such as process control in precise semiconductor manufacturing, chemical and pharmaceutical industries and biochemical engineering, miniaturized liquid flow sensors are more and more encountered. Most of them operate on the method of thermal transport and are fabricated from a silicon crystal by using micromachining technology. The thermal devices for flow measurement may be grouped in two different classes. The first class groups thermal mass flowmeters that are measuring the effect of the flowing fluid on a hot body (the increase of heating power with constant heater temperature, the decrease of heater temperature with constant heating power). They are usually called hot – wire, hot – film sensors or hot – element sensors. The resistive element is used both as heater and as sensor simultaneously. The temperature can be obtained from its electrical resistance. The second class group thermal mass flowmeters that measure the displacement of temperature profile around the heater which is modulated by the fluid flow. These sensors are called calorimetric sensors. The special type of thermal mass flowmeters in the class mentioned above is thermal mass flowmeter that measure the heat pulse passage time over a known distance. They are usually called time – of – flight sensors [1]. Many of the microflow sensors use a thermopile as a temperature sensor; however, the thermoelectric coefficient of the standard elements used in the integrated circuit is smaller than that of conventional thermocouples. Thus, a resulting output signal may be very small which requires amplifiers integrated directly into the sensor [2]. Up to now, very few of nowadays commercially offered flow sensors are equipped with the features mentioned above. One of possible method of liquid flow measuring is presented in this paper. The properties of a time – of – flight sensor are studied. The FEMLAB program was used for the study of flow sensor properties. SENSOR STRUCTURE AND BASIC OPERATING PRINCIPLE The flow sensor consists of a heater and one or more downstream temperature sensors, as shown in the Figure 1a. The heater is activated by current pulses. The transport of the generated heat is a combination of diffusion and forced convection. The resulting temperature field can be detected by temperature sensors located downstream. The detected temperature output signal of the temperature sensor is a function of time and flow velocity. The sensor output is the time difference between the starting point of the generated heat pulse and the point in time at which a maximum temperature at the downstream sensor is reached, Figure 1b. This type of sensor has the same constraints as the intrusive type of calorimetric sensors: corrosion, Proceedings 29th European Conference on Modelling and Simulation ©ECMS Valeri M. Mladenov, Petia Georgieva, Grisha Spasov, Galidiya Petrova (Editors) ISBN: 978-0-9932440-0-1 / ISBN: 978-0-9932440-1-8 (CD) erosion and leakage [3]. Since the signal processing needs some time to measure the time difference, this sensor type is not suitable for dynamic measurement. On the other side, the advantage of this type of flow sensor is the independence on the fluid temperature in the wider flow range. The influence of fluid properties on the mass flow sensor output is described in [4]. Flow T Q Temperature sensor Heater Temperature