The sampling of single-phase systems

Fluid sampling systems generally consist of four main sections, viz. the probe, the sample line, sample treatment and sample disposal. 

Type of Gas Detector Used to Detect Bulk Gas (or Carrier Gas where Appropriate) Principle of Operation Relative Sensitivity Application 

Ultrasonic Mainly gases of low molar mass Hj, N2. COj, He. Ar. A quartz crystal transducer transmits a sound wave through a sample of gas with a similar crystal used as the receiver. The velocity of the sound wave is proportional to the square root of the molar mass of the sample. The phase shift of the sound signal is measured by comparison with a reference signal. Precise temperature control is required. 1-10 Gas chromatography 

Catalytic (Pellistor) Flammable gases Air Measures the heat output due to the catalytic oxidation of flammable gas molecules. A stream of the sample is passed over the sensor which is usually a ceramic bead impregnated with Pt or Pd. The temperature variations in the sensor due to reaction are monitored. Dependent on individual design. Flammable gas detector. Usually portable 

Semiconductor Flammable and other gases Air Two small coda of platinum wire are fixed side by side within a bead of semiconductor material of diameter 2-3 mm. The sample gas flows over the bead and molecules of gas are adsorbed on the surface of the semiconductor—thereby changing the resistance of the semiconductor as measured between the two coib. One coil is also employed as a heater to ensure that the adsorption is reversible. The device is sensitive but has poor selectivity. Dependent on individual design. Flammable gas detector. Inexpensive 

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