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4-20mA電流(liu)本身就(jiu)可(ke)以(yi)為(wei)變(bian)(bian)(bian)(bian)(bian)送器供電。變(bian)(bian)(bian)(bian)(bian)送器在電路中相當于一個特(te)殊的負載，特(te)殊之(zhi)處在于變(bian)(bian)(bian)(bian)(bian)送器的耗電電流(liu)在4~20mA之(zhi)間根據傳(chuan)(chuan)感器輸出而變(bian)(bian)(bian)(bian)(bian)化(hua)。顯示儀表只需要(yao)串在電路中即可(ke)。這種變(bian)(bian)(bian)(bian)(bian)送器只需外接2根線，因而被稱為(wei)兩(liang)線制變(bian)(bian)(bian)(bian)(bian)送器。工業(ye)電流(liu)環標(biao)準(zhun)下(xia)限(xian)為(wei)4mA，因此只要(yao)在量程范(fan)圍內，變(bian)(bian)(bian)(bian)(bian)送器至少有4mA供電。這使得兩(liang)線制傳(chuan)(chuan)感器的設計成(cheng)為(wei)可(ke)能。
在(zai)工業應用中(zhong)，測量點一(yi)(yi)般在(zai)現(xian)場，而顯(xian)示設備或者控(kong)制(zhi)(zhi)(zhi)設備一(yi)(yi)般都在(zai)控(kong)制(zhi)(zhi)(zhi)室(shi)或控(kong)制(zhi)(zhi)(zhi)柜(ju)上(shang)。兩(liang)者之間(jian)距離(li)可(ke)能數(shu)十(shi)至數(shu)百米。按一(yi)(yi)百米距離(li)計算(suan)，省去2根(gen)信號傳輸導線(xian)(xian)(xian)(xian)意味著成本降低(di)近百元！另外四線(xian)(xian)(xian)(xian)制(zhi)(zhi)(zhi)變(bian)送器(qi)和三線(xian)(xian)(xian)(xian)制(zhi)(zhi)(zhi)變(bian)送器(qi)因導線(xian)(xian)(xian)(xian)內電流不對稱必(bi)(bi)須(xu)使用昂貴的(de)屏蔽線(xian)(xian)(xian)(xian),而兩(liang)線(xian)(xian)(xian)(xian)制(zhi)(zhi)(zhi)變(bian)送器(qi)可(ke)使用非常便宜(yi)的(de)的(de)雙絞線(xian)(xian)(xian)(xian)導線(xian)(xian)(xian)(xian),因此在(zai)應用中(zhong)兩(liang)線(xian)(xian)(xian)(xian)制(zhi)(zhi)(zhi)變(bian)送器(qi)必(bi)(bi)然是選擇。
差(cha)壓(ya)變送(song)(song)(song)器(qi)是(shi)測量變送(song)(song)(song)器(qi)兩端(duan)壓(ya)力(li)之差(cha)的變送(song)(song)(song)器(qi)，輸出標準信號（如 4~20mA、0~5V）。差(cha)壓(ya)變送(song)(song)(song)器(qi)與(yu)一般(ban)的壓(ya)力(li)變送(song)(song)(song)器(qi)不同的是(shi)它們均有2個壓(ya)力(li)接口， 差(cha)壓(ya)變送(song)(song)(song)器(qi)一般(ban)分為(wei)正壓(ya)端(duan)和負壓(ya)端(duan)，一般(ban)情(qing)況下(xia)， 差(cha)壓(ya)變送(song)(song)(song)器(qi)正壓(ya)端(duan)的壓(ya)力(li)應大(da)于負壓(ya)段壓(ya)力(li)才(cai)能測量。
