兩種煙霧傳感器(原理及對(duì)比).doc

There are two main types of smoke detectors: ionization detectors and photoelectric detectors. A smoke alarm uses one or both methods, sometimes plus a heat detector, to warn of a fire. The devices may be powered by a 9-volt battery, lithium battery, or 120-volt house wiring.Ionization DetectorsIonization detectors have an ionization chamber and a source of ionizing radiation. The source of ionizing radiation is a minute quantity of americium-241 (perhaps 1/5000th of a gram), which is a source of alpha particles (helium nuclei). The ionization chamber consists of two plates separated by about a centimeter. The battery applies a voltage to the plates, charging one plate positive and the other plate negative. Alpha particles constantly released by the americium knock electrons off of the atoms in the air, ionizing the oxygen and nitrogen atoms in the chamber. The positively-charged oxygen and nitrogen atoms are attracted to the negative plate and the electrons are attracted to the positive plate, generating a small, continuous electric current. When smoke enters the ionization chamber, the smoke particles attach to the ions and neutralize them, so they do not reach the plate. The drop in current between the plates triggers the alarm.Photoelectric DetectorsIn one type of photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm. In the most common type of photoelectric unit, however, light is scattered by smoke particles onto a photocell, initiating an alarm. In this type of detector there is a T-shaped chamber with a light-emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A photocell, positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light. Under smoke-free conditions, the light beam crosses the top of the T in an uninterrupted straight line, not striking the photocell positioned at a right angle below the beam. When smoke is present, the light is scattered by smoke particles, and some of the light is directed down the vertical part of the T to strike the photocell. When sufficient light hits the cell, the current triggers the alarm.Which Method is Better?Both ionization and photoelectric detectors are effective smoke sensors. Both types of smoke detectors must pass the same test to be certified as UL smoke detectors. Ionization detectors respond more quickly to flaming fires with smaller combustion particles; photoelectric detectors respond more quickly to smoldering fires. In either type of detector, steam or high humidity can lead to condensation on the circuit board and sensor, causing the alarm to sound. Ionization detectors are less expensive than photoelectric detectors, but some users purposely disable them because they are more likely to sound an alarm from normal cooking due to their sensitivity to minute smoke particles. However, ionization detectors have a degree of built-in security not inherent to photoelectric detectors. When the battery starts to fail in an ionization detector, the ion current falls and the alarm sounds, warning that it is time to change the battery before the detector becomes ineffective. Back-up batteries may be used for photoelectric detectors.主要有兩種類型的感煙探測(cè)器：電離檢測(cè)器和光電探測(cè)器。煙霧報(bào)警器使用一種或兩種方法，有時(shí)也加了熱探測(cè)器，警告火災(zāi)。這些設(shè)備可以通過(guò)一個(gè)9伏電池，鋰電池，或120伏的室內(nèi)布線供電。 離子化檢測(cè)器 離子化檢測(cè)器有一個(gè)電離室和電離輻射的來(lái)源。電離輻射的源是镅-241（一克的也許1 /第5000），它是粒子（氦核）的源極的微小量。該電離室包括由約一厘米分開(kāi)的兩個(gè)板組成。電池施加電壓于板，充電一個(gè)板正面和另一板負(fù)。 粒子不斷地釋放由镅敲電子空氣中的原子，離子化室中的氧和氮原子。帶正電荷的氧和氮原子被吸引到陰極板和電子被吸引到正極板，產(chǎn)生一個(gè)小的，連續(xù)的電流。當(dāng)煙霧進(jìn)入電離室，該煙顆粒附著到離子和中和它們，所以它們不到達(dá)板。板之間的電流的下降將觸發(fā)報(bào)警。 光電探測(cè)器 在一種類型的光電器件，煙霧可以阻擋光束。在這種情況下，光的減少到達(dá)光電池設(shè)置關(guān)閉報(bào)警。在光電單元中最常見(jiàn)的類型，但是，光被散射的煙霧顆粒到一個(gè)光電池，啟動(dòng)警報(bào)。在這種類型的探測(cè)器有一個(gè)T形的腔室用的發(fā)光二極管（LED），該拍攝橫跨T的水平桿光電管，定位在T的垂直底座的底部的光束，產(chǎn)生電流，當(dāng)它暴露于光。下無(wú)煙條件，光束相交于一個(gè)不間斷直線T的頂部，而不是撞擊位于所述光束下面直角的光電管。當(dāng)煙存在時(shí)，光被散射的煙霧顆粒，并且一些光被引導(dǎo)向下的T求取光電管的垂直部分。當(dāng)有足夠的光照射到電池，目前觸發(fā)報(bào)警。 哪種方法更好？ 這兩種電離和光電探測(cè)器是有效的煙霧傳感器。這兩種類型的感煙探測(cè)器必須通過(guò)相同的測(cè)試被認(rèn)證為UL煙霧探測(cè)器。離子化檢測(cè)器更快速地燃燒的大火與小顆粒的燃燒反應(yīng);光電探測(cè)器更迅速地