描述

1X00416H01

70年代，一機(jī)部組織了DDZ－Ill型儀表的統(tǒng)一設(shè)計(jì)工作。DDZ-III型是在線(xiàn)性集成電路（運(yùn)算放大器）技術(shù)、國(guó)際統(tǒng)一信號(hào)、本質(zhì)安全防爆等技術(shù)的基礎(chǔ)上發(fā)展起來(lái)的，與此同時(shí)，組裝式儀表裝置也在興起。
60年代末70年代初，國(guó)際電工委員會(huì)IEC規(guī)定了國(guó)際統(tǒng)一信號(hào)為直流4～20mA，直流l～5V為輔助信號(hào)。即以4～2（ADC為傳輸信號(hào)，以1～5VDC為接收信號(hào)。這個(gè)國(guó)際統(tǒng)一信號(hào)在當(dāng)時(shí)執(zhí)行起來(lái)，基本沒(méi)有多大阻力，原因是其技術(shù)先進(jìn)、切實(shí)可行，而且原來(lái)的制造廠和用戶(hù)都迫切希望有統(tǒng)一的信號(hào)制。
DDZ－III型儀表總體設(shè)計(jì)時(shí)，為了順應(yīng)500萬(wàn)噸／年原油處理量的大型煉油廠的自控系統(tǒng)的要求，一是系統(tǒng)要求本質(zhì)安全防爆，二是要考慮現(xiàn)場(chǎng)儀表與中控室距離較遠(yuǎn)（約500米以?xún)?nèi)）。在符合國(guó)際統(tǒng)一信號(hào)的基礎(chǔ)上，選用24VDC統(tǒng)一供電和兩線(xiàn)制變送器及安全柵的方案。當(dāng)時(shí)線(xiàn)性集成電路和晶體管的水平能做到在24VDC供電條件下，控制室儀表外接導(dǎo)線(xiàn)電阻與兩線(xiàn)制變送器內(nèi)阻或執(zhí)行器內(nèi)阻串聯(lián)，可具有恒流性能。4mA是為了保證兩線(xiàn)制變送器放大器工作點(diǎn)必要的供電而設(shè)的。在要求防爆的場(chǎng)所，也能夠承受24VDC、20mADC或比它稍高的電量，所以能滿(mǎn)足上述兩項(xiàng)要求。

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