may include but are not limited to a thermocouple a chemical sensor an optical sensor or a pressure sensor inserted in a hole that is oriented downward from the outside of a vessel wall to the inside of a vessel wall reduces slag flow into the hole by the force of gravity the vessel lining grows upwards and outwards radially during thermal expansion because the angled hole is oriented in the same direction as the lining growth the angled hole moves with the lining as it expands and contracts therefore the angled probe requires less additional space to allow for the expansion and contraction of the refractory lining as the size of the hole in the lining is reduced the amount of slag that penetrates the hole is also reduced an angle of about 15 to about 45 degrees from the plane orthogonal to the vessel centerline may be implemented for the angled thermocouple probe in some embodiments the reduction in the size of the hole is dependent on the penetration angle of the probe a hole oriented perpendicular to the vessel wall may have a diameter of about 50 mm 2 inches for an example hole at an angle of 45 degrees the hole size may be reduced.