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4-20mA電流本(ben)身就可以為(wei)變(bian)(bian)(bian)(bian)送器供電。變(bian)(bian)(bian)(bian)送器在電路(lu)中(zhong)相當于一個特殊(shu)的負載，特殊(shu)之處(chu)在于變(bian)(bian)(bian)(bian)送器的耗電電流在4~20mA之間根據傳感器輸出而變(bian)(bian)(bian)(bian)化。顯示儀表只(zhi)需要串在電路(lu)中(zhong)即可。這(zhe)種變(bian)(bian)(bian)(bian)送器只(zhi)需外接2根線，因而被稱為(wei)兩線制(zhi)變(bian)(bian)(bian)(bian)送器。工(gong)業電流環標準下限(xian)為(wei)4mA，因此(ci)只(zhi)要在量程范圍內，變(bian)(bian)(bian)(bian)送器至少有4mA供電。這(zhe)使(shi)得兩線制(zhi)傳感器的設計成為(wei)可能。
在(zai)工業應用(yong)中，測量點一(yi)般在(zai)現(xian)場，而顯示設備或者(zhe)控制(zhi)(zhi)設備一(yi)般都在(zai)控制(zhi)(zhi)室或控制(zhi)(zhi)柜(ju)上。兩者(zhe)之間距離可(ke)能數十至(zhi)數百(bai)米。按一(yi)百(bai)米距離計算，省(sheng)去2根信(xin)號傳(chuan)輸導線意味(wei)著成(cheng)本降(jiang)低近百(bai)元(yuan)！另外四線制(zhi)(zhi)變(bian)(bian)送(song)(song)(song)器和三線制(zhi)(zhi)變(bian)(bian)送(song)(song)(song)器因導線內電流不(bu)對稱必須使(shi)用(yong)昂貴的(de)屏蔽線,而兩線制(zhi)(zhi)變(bian)(bian)送(song)(song)(song)器可(ke)使(shi)用(yong)非常便宜的(de)的(de)雙絞線導線,因此(ci)在(zai)應用(yong)中兩線制(zhi)(zhi)變(bian)(bian)送(song)(song)(song)器必然是(shi)選(xuan)擇。
差(cha)壓(ya)(ya)(ya)(ya)變送器(qi)(qi)是測(ce)量(liang)變送器(qi)(qi)兩端(duan)壓(ya)(ya)(ya)(ya)力(li)(li)(li)之差(cha)的(de)變送器(qi)(qi)，輸出標準信(xin)號（如(ru) 4~20mA、0~5V）。差(cha)壓(ya)(ya)(ya)(ya)變送器(qi)(qi)與一般(ban)的(de)壓(ya)(ya)(ya)(ya)力(li)(li)(li)變送器(qi)(qi)不同的(de)是它們均有2個(ge)壓(ya)(ya)(ya)(ya)力(li)(li)(li)接口， 差(cha)壓(ya)(ya)(ya)(ya)變送器(qi)(qi)一般(ban)分為正壓(ya)(ya)(ya)(ya)端(duan)和負(fu)壓(ya)(ya)(ya)(ya)端(duan)，一般(ban)情(qing)況下， 差(cha)壓(ya)(ya)(ya)(ya)變送器(qi)(qi)正壓(ya)(ya)(ya)(ya)端(duan)的(de)壓(ya)(ya)(ya)(ya)力(li)(li)(li)應(ying)大于(yu)負(fu)壓(ya)(ya)(ya)(ya)段(duan)壓(ya)(ya)(ya)(ya)力(li)(li)(li)才能測(ce)量(liang)。
