A fluidic sensing and processing system monitors the enthalpy and/or sensible heat of an outside air and of return air to select the air placing the minimum load on a mechanical refrigeration apparatus. Fluid pressure signals correctponding to the dry-bulb temperature of the air and to the relative humidity of the air are created and summated in a resistor network including individual fluidic summing resistors connected to a common summing point. The summing resistors are directly interrelated with a particular ratio producing a constant output pressure with constant enthalpy but with varying temperatures and humidity. The common psychrometric chart may be employed to indicate the area over which a particular resistance ratio will produce a predictable and reliably significant correlated output pressure. The pair of sensing networks provide output related to the relative enthalpy of the return air and the outdoor air. The output of the networks are connected to a fluidic comparator to establish a selection signal connecting the return air or outside air to the mechanical refrigeration unit. An overriding temperature control comparison is further introduced to provide an overriding control in accordance with the relative outside air temperature with respect to the temperature of the return air from the conditioned zones. If the outside air dew point is below the discharge temperature, air from either source will pass through the mechanical refrigeration without condensation of moisture and selected air should be that with the lowest sensible heat regardless of its enthalpy. The two resistor networks may be applied to a fluidic amplifying device such as a summing impact modulator to produce a proportional output which continuously adjusts the mixture of the two air sources to produce air with a constant enthalpy.