Sensors and Actuators B, 12 (1993) 91-94 91 A calorimetric combustible gas detector employing platinum film heaters Edward B. Makovos, Frederick W. Montague, Laurie Dudik and Chung-Chiun Liu* Chemical Engineering Department, Case Western Reserve Unioersiiy, Cleveland, OH 44106 (USA) (Received April 29, 1992; accepted September 7, 1992) Abstract Calorimetric sensors employing exposed metallic film heaters have been evaluated on gas mixtures containing hydrogen or carbon dioxide. The sensor output exhibits high sensitivity to the tested gases. A combustible gas detector based on the described method appears suitable for integration into a multicomponent gas analyzer. Introduction The detection of combustible gases in air and other oxygen-containing media is often performed by measur- ing the amount of heat produced from the catalytic oxidation of those gases. A typical sensing element contains two insulated wire heaters forming opposite arms of a Wheatstone bridge [l-3]. One of the heaters is coated with a catalyst suitable for the oxidation process of interest, whereas the other is coated with an oxidation inhibitor. The presence of a combustible gas around the sensing element causes a slight temperature increase of the catalyzed element, thereby raising its resistance. If the Wheatstone bridge is balanced in an inert atmosphere, the presence of a combustible compo- nent produces an out-of-balance potential (OBP) pro- portional to the concentration of the combustible gas. relation to our current research. The wire heaters em- ployed in the presently available instruments are difficult to miniaturize and integrate into a combined sensor. Recently Chen et al. [ 31 and Vauchier et al. [ 71 reported possibilities of replacing the wire heaters with a thick or thin metallic film strip. This option would introduce a considerable flexibility to the geometric configuration of the sensor. In addition, the application of a single metal deposition method for the different parts of the possible multicomponent sensor is expected to reduce its cost. This research investigates the performance of the calorimetric sensor containing the metallic film heaters under well-controlled isothermal conditions. Its quanti- tative response to carbon monoxide and hydrogen is reported. This calorimetric method is especially suitable for cases where differentiation between various combustible components is not necessary. However, an array of such sensors with catalysts of various specificities would enable the simultaneous detection of all combustible components [4]. We are actively exploring the integra- tion of a combustible gas detector and high-tempera- ture actively operated oxygen sensor [5,6] into a single unit. The use of heaters in the calorimetric method of combustible gas detection is important for its role in a possible multicomponent gas detector. Since the oxygen sensor may also have to be heated to maintain its desired temperature, the heaters will serve both the oxygen and the combustible gas sensors. Experimental However, the physical configuration of the calorimet- ric detectors of combustible gases is a disadvantage in *Author to whom correspondence should be addressed. The calorimetric sensor for combustible gases was fabricated on 25 mm x 51 mm x 0.64 mm alumina wafers (Coors Ceramics Company, Grand Junction, CO). The heaters were deposited on one side of the substrates by the thick-film printing process with a platinum ink (Composition A3443, Engelhard Corpora- tion, East Newark, NJ). The dimensions of the platinum heaters are shown on Fig. 1. The opposite sides of some substrates were coated with a uniform mixture of gold and palladium in the ratio 1.0:2.3 by weight, prepared from the inks for thick-film printing (Composition 8835 [Au], Electra-Science Laboratories, Inc., King of Prus- sia, PA and Composition A4855 [Pd], Engelhard Corpo- ration, East Newark, NJ). This coating was intended to catalyze the oxidation of combustible gases, and was formed to a thickness of 10 f 2 pm. 0925-4005/93/$6.00 @ 1993 ~ Elsevier Sequoia. All rights reserved