The equation for calculating %O2 or %C requires knowledge of the temperature at which the measurement is made. This temperature can be obtained by a thermocouple inside the probe or by a thermocouple elsewhere on the furnace.
Important: If your kiln is not homogeneous from a temperature point of view, it is important that the temperature measurement is made as close as possible to the oxygen probe to avoid errors in the calculation of %C or %O2.
An internal thermocouple in the probe is therefore ideal for accurate measurement.