Abstract: Disclosed is an apparatus useful in controlling the operation of ovens of the type typically associated with baking food products. Broadly, the apparatus permits the linear display of heat input to one or more zones of an oven responsive to gas flow. Heat input is indicated by fluid levels in manometers having their legs dimensioned to permit linear display of heat input despite the variance of gas flow with the square of the pressure. The manometers are positioned next to an adjustable scale which accommodates combustible gases of varying heat contents. The adjustable scale is preferably marked for the preferred heat input profile across the various zones of the oven to facilitate control of optimum heat input to the oven as a whole. Also disclosed are similar display devices for exhaust gases.