17/35 9.2 Liquid Level -Using Radioisotopes Detector (ionization chamber) I loe-upx (9.4) where I intensity of radiation falling on detector; lo intensity at detector with absorning material not present; Radioisotope source e base of natural logarithms; u mass absoption coefficient Fig.9.4(h)Liquid Level (constant for given sorce and absorbing material),cm2/g; Measurement Strategies p≌mass Using Radioisotopes density of absorbing material,g/cm3; (放射性同位素) x thinckness of absorbing materialm,cm. -The absorption (of B-ray or y-ray radiation varies with the thickness of absorbing material between the source and the detector. 1835 9.2 Liquid Level Using Hot-wire or Carbon Resistor Elements -The heat-transfer coefficient at the surface of the resistance element Hot-wire element changes radically(急剧地)when the liquid or carbon resistor surface passes it. -This changes its equilibrium temperature and thus its resistance, causing a change in bridge output voltage. -By locating resistance elements at known height intervals,the tank level may be measured in discrete increments. Fig.9.4(i)Liquid Level Measurement Using Hot-wire or Carbon Resistor Elements9.2 Liquid Level - Using Radioisotopes ▬ The absorption (吸收) of β-ray or γ-ray radiation varies with the thickness of absorbing material between the source and the detector. Fig. 9.4(h) Liquid Level Measurement Strategies Using Radioisotopes (放射性同位素) ܫ=ܫି݁଴ఓఘ௫ (9.4) where ܫ ≜ intensity of radiation falling on detector; ܫ ≜ ଴intensity at detector with absorning material not present; ݁ ≜ base of natural logarithms; ߤ ≜ mass absoption coefficient constant for given sorce and absorbing material ,ܿ݉ଶ⁄݃; mass ≜ ߩ density of absorbing material,݃ ܿ݉ଷ ⁄ ; ݔ ≜ thinckness of absorbing materialm, ܿ݉. 17/35 ▬ The heat-transfer coefficient at the surface of the resistance element changes radically(急剧地) when the liquid surface passes it. ▬ This changes its equilibrium temperature and thus its resistance, causing a change in bridge output voltage. ▬ By locating resistance elements at known height intervals, the tank level may be measured in discrete increments. Fig. 9.4 (i) Liquid Level Measurement Using Hot-wire or Carbon Resistor Elements 9.2 Liquid Level - Using Hot-wire or Carbon Resistor Elements 18/35