MEMS sensor signal conditioning for a thermal flow meter
●SenSorS for tHe flowMetering of mass,volume,or velocity use very different physical principles of measurement in order to satisfy a whole range of different requirements.One principle of flowmetering that is rapidly gaining acceptance is the thermal method,in which the transfer of heat in a liquid or gas is measured.In this procedure a heating element supplies a certain amount of heat and temperature sensors measure the heat distribution.this principle is well suited for miniaturization with MeMS sensors and for ASSP system integration.Low flow rates can also be precisely measured and controlled,as in medical applications,for example.
●the fundamental mathematical principles of thermal flowmetering were devised 99 years ago by physicist Louis Vessot King who formulated King’s law for the calculation of flow velocity.The law describes that the amount of heat extracted from a heated wire mounted vertically in a measuring cylinder per unit of time is dependent on the average flow velocity and density of the gas or liquid.King’s calculation assumes that the measuring cylinder is completely perfused and that the heated wire causes minimum disruption to the flow.As the diameter of the measuring tube is known,all that has to be defined is the flow pattern of the medium to be measured.With compressible gases it is necessary to also measure the pres-sure so that the volume(m3/s)or mass flow rate(kg/s)can be computed from the density.the heat output can be set to either CPA(constant power anemometer)or CtA(constant temperature anemometer).Typical MEMS flow sensors have one or two temperature sensors.Figure 1 represents a flow sensor with one heating element and one temperature sensor and illustrates the principle distribution of temperature of the flowing gas or liquid.
●Figure 2 shows a flow sensor with one heating element and two temperature sensors in a setup where heat is distributed with no flow.Where a second temperature sensor is used,the flow can be measured in both directions.
●Applying King’s Law and a simplified equation,the rate of flow is calculated as:
■heat output PHe=(tHe–tM)·(k1+k2·Sn),
●where tHe is the temperature of the heater and tM the temperature of the flowing medium.Specific constants k1 and k2 for each sensor are determined by calibration and S stands for the average rate of flow.Exponent N is ideally 0.5 yet and may also be in a range of 0.25 to 0.5,depending on the design of the sensor
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Please see the document for details |
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March 2013 |
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