9.2 Liquid Level Using Pressure Tx 15/35 Differential Pressure pressure pickup pickup (d) -Fig.9.4(c)&(d)allow measurement of the liquid level in open and pressure vessels,since hydrostatic pressure (is related directly to liquid level. 个 Air Regulator Pressure slightly higher than greatest liquid head Fig.9.4 Liquid Level Measurement Strategies 一Fig.9.4e:a"bubbler'"or purge(净化；清诜)system,the gas pressure downstream of the flow restriction is the same as the hydrostatic head(静水压头)above the bubble-tube end. 16/35 9.2 Liquid Level -Using Capacitance Tx Probe (one "plate Insulotor of capacitor) 00 Insulator- Tank (other "plate" ot capacitor 0 lg) Figure 9.4 Liquid Level Measurement Strategies -Fig.9.4(f):for essentially nonconducting liquids(conductivity <0.1 uS/cm3),the bare- probe arrangement may be satisfactory since the liquid resistance R is sufficiently high. -Fig.9.4(g):For conductive liquids the probe must be insulated to prevent short- circuiting of capacitance by the liquid resistance.▬ Fig. 9.4(c) & (d) allow measurement of the liquid level in open and pressure vessels, since hydrostatic pressure (静液压) is related directly to liquid level. 9.2 Liquid Level - Using Pressure Tx ▬ Fig. 9.4(e): a “bubbler” or purge(净化;清洗)system, the gas pressure downstream of the flow restriction is the same as the hydrostatic head (静水压头) above the bubble-tube end. Fig. 9.4 Liquid Level Measurement Strategies 15/35 9.2 Liquid Level - Using Capacitance Tx Figure 9.4 Liquid Level Measurement Strategies ▬ Fig. 9.4(f): for essentially nonconducting liquids(conductivity < 0.1 uS/cm3), the bare￾probe arrangement may be satisfactory since the liquid resistance R is sufficiently high. ▬ Fig. 9.4(g): For conductive liquids the probe must be insulated to prevent short￾circuiting of capacitance by the liquid resistance. 16/35