Calibration mass-flow sensors

It provides a more direct approach for temperature and pressnre compensation than other presently-available mass flow sensors requiring measurement of temperature and pressure. For some gas mixtures of varying composition, mass flow is indicated accurately (e.g. CO2 and He) without calibration corrections. Because it can be fabricated by conventional thin film deposition and silicon processing techniques. It offers the possibility of lower cost and broader applications than present conunercially available gas flow sensors. 

This is necessary both for process control as well as the reliabihty of the system. The integration of sensors into the microreactor or building a multisensor module for the four functions of state is easy for a microreactor made of sUicon. For the process pressure, the piezoresistive principle is used often. With diEFerential pressure measurements, the flow rate can be determined. Alternatively, calorimetric principles are used widely. These are easy to implement technically, but a calibration is needed for eatii new medium. The most robust sensors are the Coriolis mass flow sensors. In process engineering, they are very common, but in terms of micro process engineering, there is still a need for research. In Ref. [26], sensors of this type are described. Ref [25] is a good summary of other microflow sensors. For measurement of temperature, there are many equivalent principles but will not be discussed here. Substantially, it is more difficult to measure the concentration in the reactor. In addition to optical principles, the impedance spectroscopy is often used. See Ref [27-31] for more details. 

Sensors were exposed to between 500 pL/L and 6500 pL/L hydrogen in air, as determined by the mixing of a calibrated bottle of 2 % hydrogen in air with additional air via the mass flow controllers. The dimensions and flowrates used resulted in gas velocities from 15 cm/sec to 25 cm/sec. 

Flow. Nonintrusive sensors that can be maintained at the process temperature are ideally suited to measure the flow rate of feed and product streams. Magnetic flow meters are suitable and inexpensive choice for aqueous streams. Organic streams with low dielectric constants require a vibrating tube mass flow meter to satisfy these criteria. Although commonly installed, flow meters that operate by inducing a pressure drop proportional to the flow rate present restrictions for solids accumulation that may alter the calibration. An alternative approach is to monitor the rotational speed of a positive displacement pump. Accuracy of this method is subject to wear and tolerances in the pump. 

In the case of the latter alternative, the set volumetric flow rate is attained via Mass Flow Controllers (MFC). Using this method, complete chains of analyzers can be automatically calibrated with ease. The MFCs receive a signal between 0 and 5 V and transform this into the corresponding flow rate. A sensor then measures the flow rate and a controller aligns the actual value with the set point via a metering valve, thereby controlling the dosing process. 

All three of these PhD theses contain sections on the theory of operation of each component of the mass/heat-flow sensor and experimental details such as block diagrams of the apparatus, sample preparation, data acquisition and control, calibration, and data analysis. 

The local temperature field depends on the quality and velocity of fluid medium, integrates on a chip of the sensor to measure temperature distribution through calibration. The specially designed signal processing circuit will be converted to mass velocity and mass flow rate of medium into a linear relationship between output voltage value (Durst F 2003). 

Flow measurement using thermoelectric devices and sensors implies the use of heat transfer and temperature measurements in microchatmels to determine the near-wall velocity. With appropriate calibration procedures, the mean fluid flow velocity or mass flow rate can be determined by measurements of the local wall temperature. Thermoelectric temperatrue probes and sensors, also known as thermocouples, rely on the Seebeck effect, where a temperatrue difference between two different metal contacts induces a voltage drop which can be electrically mea-strred. An electrical resistance heater introduces a heat flux into the fluid flow. The temperature is measured directly either at the heater, in its vicinity, or at the wall downstream of the heater. Often, the upstream mean temperature of the fluid flow is also measured to provide a comparison. Thermoelectric flow rate measurement is a very common measuring technique and for laminar flow one of the most accurate, reliable, and cost effective. 

Preparations, conducted at 40°C, take place in a stirred glass vessel equipped with sensors (pH, temperature, torque motor measurement of the stirrer shaft) allowing the control of the diffrrent steps of the preparation. Reactants pretreated at Ae preparation temperature, are introduced in the reactor at defined mass flow rates by means of calibrated peristaltic pumps. Reactants can be introduced in the reactor, successively (batch process) or simultaneously (semi-continuous process). In the later case reactants are mixed in the feeding pipe. 

The calibration of in-line mass flow meters is an important issue. In one paper the authors argue that as direct measurement of instantaneous mass flow rate is measured it is absolutely essential that a standard mass flow rate sensor is used - rather than the weighing of accumulated product [11]. 

For a voltammetric sensor, the current or potential peak shift that may relate to the concentration of the sensing species is an important measurement. In a dynamic situation in which polarization characteristics are obtained, it is essential that the mass transfer characteristics are reproducible for both calibration and actual measurements. In the case of a stationary planar sensor, stagnant solution or steady flow conditions in a flow cell provides good reproducibility. Or in another case, a sufficiently high concentration of an electrolyte is used to maintain a constant ohmic drop in the cell, regardless of the concentration of the pertinent sensing component. Under these conditions, the mass transfer can be purely diffusional and adequately described by Pick s law of diffusion. 

Finally, a note on the calibration of sensors for carbon dioxide. This is traditionally achieved using commercially available gas mixtures or producing mixtures using flow meters. In some laboratories sensors may be calibrated against a mass spectrometer although the expense of such Instruments limits their use considerably. In a recent communication Calabrese and Maillet describe a photochemical method of producing reproducible... 

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