PURPOSE: To improve the accuracy in approximation and to detect exactly the descending trend of the charge in a furnace by approximating curvilinearly the relation between the measuring depth and time by sounding of the charge in the furnace by a cubic or polynominal expression.

CONSTITUTION: The depth at which the charge material in a furnace descends from the time Tsi when the depth measurement of the i-th batch starts until the time TD(i+l) of the (i+l)th batch to be made of a particular problem in the depth data of the i-th and (i+1)th batch of said charge is approximated by a cubic curve as shown by the extension of a curve (e). The amt. Zi of embedment is added in a constant term and if a slip arises in the i-th batch, the descending amt. thereof is added in a constant term ZSL and is expressed generally by the cubic expression like the equation in which Yij: the depth (the j-th point of the i-th batch), tij(=Tij-Tsi): the time (the time with the time Tsi at the j-th point of the i-th batch as a reference), di: a constant, ZSLi: the amt. of slip. The ci and di are determined from the curve of the (i-1)th batch on assumption that the inclination of the curve for the relation between the depth and time in the stage of each charging is equal. The (a), (b), Zs and Zi are determined from the data for the i-th and (i+1)th data.