Speed well assemblies enclose the sensor element in a stainless steel protection tube with the tip sealed with a homogenous weld. This design permits a very fast response rate, as the stainless steel protection tube provides low mass and low thermal inertia. The thermocouple is sealed inside the speed well and protected from oxidation or corrosion. The thermocouple wire is insulated with silicone impregnated fiberglass for temperatures up to 900° F (482° C). Ceramic insulation can be provided inside the well for temperatures above 900° F. Speed wells are suitable for use at pressures from vacuum to 3000 psi. Temperatures range from -300° F to 2000° F (-184° C to 1090° C), depending on the pressure.