1.Principles of thermal mass flow measurement

1.1.Measuring gaseous substances Inlet air Volume flow

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Load In contrast to liquids, gases can becompressed. The gas density changesdepending on the pressure and temperature. Pursuant to the ideal status equation for gasesthe volume in the example in Figure 1changes from 1 m Meter 1CompressorLong pipe runMeter 2Meter 3 p
1 m 3 /h 0.172 m 3 /h 0.167 m 3 /h 1 · V
1 = p
2 · V
2 Temperature 20 °C (293 K) 80 °C (353 K) 20 °C T
1 T
2 Pressure 1 bar(a) 7 bar(a) 6 bar(a) Density 1.205 kg/m 3 7.001 kg/m 3 7.23 kg/m 3 Mass flow 1.205 kg/h 1.205 kg/h 1.205 kg/h 3 upstream of thecompressor to 0.172 m 3 downstreamof the compressor manufacturer. Since there is a flow in the example, the volume isspecified dependent on the time (volume flow). The transported quantity of substanceis independent of the pressure and temperature.The figure shows the air mass flow.This remains constant at 1.205 kg/h over the entire distance. The densitychanges from 1.205 kg/m Figure 1: Diagram of volume flow change, depending on the measurementposition p
3 1 · V
1 T
2 1 bar · 1 m 353K7 bar == 7.001
3 2 =0.172 m T ··==V
1 p
2 293K
3 upstream of the compressor to downstream of the compressor. Thegas volumes can only be compared if they relate to the same conditions. In general the mass flow is specified as a standard volume flow, in otherwords, in the form of a volume flow as defined in DIN 1343. , mV 1.205 kg0.172 m kgm
2 =
3 3 2