Measurement of the pig iron and slag level in the blast furnace is essential to operate the furnace stably because an increase of these liquids level can cause fluctuations of the pressure in the furnace or the temperature of the furnace which are related to the furnace condition and may finally result in reduced productivity or serious trouble.The conventional method is the EMF (electromotive force) measurement correlated with the liquid level. It can be thought that a voltage generated electrochemically (as in a battery) by the reduction reaction of iron oxides. However some reports said that EMF sometimes loses correlation with the liquid level because of thermal influence.In this paper a new method is proposed using a quite small electrical resistance measurement of the hearth of a blast furnace which contains pig iron and molten slag. Accordingly the four point contact method was adopted and a pseudo random signal as a current pattern was fed to improve the S/N ratio.Measurements were done at plural positions along the circumference of the blast furnace. However when the current signal is independently fed at each position, spike noises are generated. These noises are originated in the induced voltage which is generated at neighbor positions when the polarity of the current changes. But finally this noise problem can be solved by feeding synchronized current and measuring the signal on the noiseless timing.The measurement results indicated that the level in the hearth is not necessary identical at all positions.KEY WORDS: blast furnace; pig iron; molten slag; level measurement; electrical resistance; pseudo random signal. BackgroundStability is extremely important in blast furnace operation, as the blast furnace is the first upstream process in steel production. Nevertheless, increase of the pig iron and molten slag level prevents stable operation. Pig iron descends, and then accumulates in the blast furnace hearth. And also, molten slag descends to the hearth. However, the molten slag accumulates over the pig iron because its density is lower than that of iron.Pig iron and molten slag are drained from one of plural tap holes (in the case of large blast furnaces) by turns at regular intervals. However, molten slag has poor drainage due to its relatively higher viscosity, and under special conditions when the temperature in the furnace is lower than usual, the viscosity of the molten slag increases, making tapping even more difficult. In the worst case, an excessive rise of the pig iron and molten slag level can cause serious trouble in which the molten slag melts the tuyeres. This type of trouble does not have only an adverse influence on operation over an extended time, but is also a problem for safety. Therefore, when the pig iron and molten slag level rises, pig iron and molten slag are drained from plural tap holes at the same time, enabling control of the productivity of pig iron and molten slag in order to keep the appropriate level and realize stable operation.The conventional method of measuring the...