Abstract: An instrument for measuring the air/fuel mixture strength supplied to e.g. an I.C. engine comprises a primary catalytic reactor having a matrix of annular cross-section in whose bore is housed a secondary catalytic reactor and a zirconia oxygen sensor. Propane gas or other hydrocarbon fuel is mixed with air and the mixture passed through the primary reactor and catalytically combusted. A flow of the combustion products of the catalytic reaction is withdrawn and mixed with an equal flow of the engine exhaust gases, and the mixture is passed over the secondary reactor (heated by the primary catalytic reaction) to oxidize its oxidizable constituents, and the products of this secondary reaction are passed over the zirconia sensor (heated by the primary catalytic reaction) for measurement of the oxygen content. The output signal of the zirconia sensor is processed electronically and supplied to a digital indicator giving a direct reading of the equivalence ratio of the mixture supplied to the engine intake.