23/35 9.3 Humidity -Dew-point Temperature Sensor Principle of operation Optical sensing Microprocessor Outputs bridge system Photo- transistor LED'S Power Dew point supply Sample temperature ● gas TE cooler Power in Heat sink Fig.9.5(b) Principle:Dew-point temperature can be determined by noting the temperature of a polished(打磨过的，发光的)metal surface(mirror）when the first traces of condensation (appear.(TE cooler thermo-electrical cooler) In Fig.9.5b,a microprocessor-controlled version of this instrument which uses a dual- mirror system to correct continuously for mirror contamination()which causes errors and requires periodic shutdown and/or cleaning in systems without this feature. 24/35 9.4 Chemical Composition In years past:chemical tests of samples in laboratories. Today:relatively continuous and automatic measurements without the need for a human operator. Most of the analyzers used for chemical composition and related measurements are rather complex systems,rather than simple sensors. However,some simple sensors which do perform useful "chemical analysis,"one such example is from the important area of combustion control. ●The oxygen sensors of the zirconium oxide ceramic type(锆氧化物陶瓷类)， consists of a closed-end hollow tube made of zirconium oxide ceramic with porous platinum electrodes coated on both the inner and outer surfaces. Oxygen sensor m 02 Thermocouple Fig.9.6 Zirconium-Type Oxygen Sensor9.3 Humidity – Dew-point Temperature Sensor Principle: Dew-point temperature can be determined by noting the temperature of a polished(打磨过的，发光的) metal surface (mirror) when the first traces of condensation (凝露) appear. (TE cooler = thermo-electrical cooler) In Fig. 9.5b, a microprocessor-controlled version of this instrument which uses a dual￾mirror system to correct continuously for mirror contamination(玷污), which causes errors and requires periodic shutdown and/or cleaning in systems without this feature. Fig. 9.5(b) 23/35 z In years past: chemical tests of samples in laboratories. z Today: relatively continuous and automatic measurements without the need for a human operator. z Most of the analyzers used for chemical composition and related measurements are rather complex systems, rather than simple sensors. z However, some simple sensors which do perform useful “chemical analysis,” one such example is from the important area of combustion control. z The oxygen sensors of the zirconium oxide ceramic type(锆氧化物陶瓷类), consists of a closed-end hollow tube made of zirconium oxide ceramic with porous platinum electrodes coated on both the inner and outer surfaces. 9.4 Chemical Composition Fig. 9.6 Zirconium-Type Oxygen Sensor 24/35