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Fire resistance furnaces need to be harmonized so that differences in thermal severity are minimized. An economical option is to retain existing furnaces and change the combustion gas temperature sensors used to control the furnace from conventional thermocouples to plate thermometers, so that a more realistic ratio of convective to radiative heat transfer is measured. Results are presented for nine tests involving two gas-fired furnaces in the United Kingdom and one oil-fire furnace in Germany. The tests were made in full size floor furnaces using a purpose-made rig which formed a well insulated roof to the furnace and from which a number of thermocoupled uninsulated stainless steel calibration plate and rod elements were suspended in the combustion gases. Comparisons of the temperature-time profiles for these elements were made when controlling the furnace with conventional thermocouples as standard practice or when controlling using plate thermometers. The tests showed that plate thermometer control markedly reduced differences in the thermal severity of the three furnaces near their centre but showed little advantage when the calibration element temperature-time profiles were averaged over the whole plan area of the furnace.