| Summary: | An on-line multi-frequency electrical resistance tomography (<i>mf</i>ERT) device with a melt-resistive sensor and noise reduction hardware has been proposed for crystalline phase imaging in high-temperature molten oxide. The melt-resistive sensor consists of eight electrodes made of platinum-rhodium (Pt-20mass%Rh) alloy covered by non-conductive aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) to prevent an electrical short. The noise reduction hardware has been designed by two approaches: (1) total harmonic distortion (THD) for the robust multiplexer, and (2) a current injection frequency pair: low <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mi>f</mi><mi>L</mi></msup></mrow></semantics></math></inline-formula> and high <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mi>f</mi><mi>H</mi></msup></mrow></semantics></math></inline-formula>, for thermal noise compensation. THD is determined by a percentage evaluation of <i>k-</i>th harmonic distortions of ZnO at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mrow><mi>f</mi><mo>=</mo><mn>0</mn></mrow></mrow></semantics></math></inline-formula>.1~10,000 Hz. The <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mi>f</mi><mi>L</mi></msup></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mi>f</mi><mi>H</mi></msup></mrow></semantics></math></inline-formula> are determined by the thermal noise behavior estimation at different temperatures. At <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo> </mo><mrow><mi>f</mi><mo> </mo></mrow><mo><</mo><mn>1</mn></mrow></semantics></math></inline-formula>00 Hz, the THD percentage is relatively high and fluctuates; otherwise, THD dramatically declines, nearly reaching zero. At the determined <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mi>f</mi><mi>L</mi></msup><mo>≥</mo></mrow></semantics></math></inline-formula> 10,000 Hz and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mi>f</mi><mi>H</mi></msup><mo>≈</mo></mrow></semantics></math></inline-formula> 1,000,000 Hz, thermal noise is significantly compensated. The on-line <i>mf</i>ERT was tested in the experiments of a non-conductive Al<sub>2</sub>O<sub>3</sub> rod dipped into conductive molten zinc-borate (60ZnO-40B<sub>2</sub>O<sub>3</sub>) at 1000~1200 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mo>°</mo></semantics></math></inline-formula>C. As a result, the on-line <i>mf</i>ERT is able to reconstruct the Al<sub>2</sub>O<sub>3</sub> rod inclusion images in the high-temperature fields with low error, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi mathvariant="sans-serif">ς</mi><mrow><msup><mi>f</mi><mi>L</mi></msup><mrow><mo>,</mo><mo> </mo><mi>T</mi></mrow></mrow></msub></mrow></semantics></math></inline-formula> = 5.99%, at 1000 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mo>°</mo></semantics></math></inline-formula>C, and an average error<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo> </mo><mo stretchy="false">⟨</mo><msub><mi mathvariant="sans-serif">ς</mi><mrow><msup><mi>f</mi><mi>L</mi></msup></mrow></msub><mo stretchy="false">⟩</mo></mrow></semantics></math></inline-formula> = 9.2%.
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