To get around or away from the shortcomings of the previous methods, composition can be computed from temperature and pressure measurements. One technique is to measure pressure deviations from flowsheet pressure, PFS, calculate temperature changes at constant composition due to these pressure deviations, and subtract these temperature changes from the actual temperature. In effect, the actual temperature is compensated back to what it would be at flowsheet pressure. A particular hardware concept (see Figure 10.2) we have used assumes that the P-T curve at constant composition is a straight line for small deviations. If, then, pressure deviations from PFS are large, there will be some error.
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