Absrnet Aimingat the fact that the directon -linein strumentme asuring the degrees Baumeo fbl ackli quoris ve rye xpensive,of soft~ -ingte chniquean in directwa yto fi gure out Baumeis proposed in this paper detectingd erivativep ressurea ndte mperatureo fth eb lackli quoron -line.The soft measuring model and the methods for its parameter identification and adjustment, ilustrated in detail. The corresponding soft sensor implemerited Siemens PLC is also introduced which has been used in a plant with good results. Keywords Softm easuringte chnique Blackl iquor DegreesB aume PLC