Abstract: A performance map is provided comprising a plurality of performance curves showing head/flow conditions at various compressor speeds, terminating at one end at surge points. The performance curves are converted into deviation curves representing deviations from the surge point at the head/flow conditions. The curves are merged into a universal compressor curve with a merge-function and a pre-defined tolerance. Imaginary map head/flow values are calculated using the universal compressor curve, merge-function, deviation curves, and performance curves at the compressor speed. A ratio of the imaginary map head/flow values is compared to a ratio of the actual head/flow values at the compressor speed to determine if the ratios are within a pre-defined tolerance. The process repeats until the ratios are within the tolerance. The molecular weight of the compressor gas is calculated using a function of the actual and imaginary flow values and/or head values.