Higher pressure ratios are also required to limit turbine exhaust temperature and thermal stresses at the roots of the last-stage turbine blades, which tend to be long in gas turbines for large-scale combined-cycle applications and are uncooled. The pressure ratio is easily determined by taking ambient pressure, which is 14.7 lbs at sea level, adding it to your boost pressure, and then dividing it by ambient pressure. Ambient Pressure, P1 = Pressure Ratio. Compression ratio and overall pressure ratio are interrelated as follows (what follows is valid only for static pressures, not overall pressure ratios, as compression while slowing down flow without doing work is not changing stagnation pressure at all, and in supersonic flow with shock waves stagnation pressure will be less than 1 with > 1 compression ratio): Good calculators have additional features that help you maximize the potential of your turbo setup including improving the spool characteristic and maintaining reliable control over your boost pressure. Turbo calculators are useful software applications that help you choose the right turbocharger for your build up. One way to write this is: Ψ = 2 Δ h t U 2 2 (5) This gives you the absolute pressure ratio. Then it's off to the compressor maps to check airflow v. pressure ratio. Boost Pressure + Ambient Pressure, P2. According to Carnot’s principle higher efficiencies can be attained by increasing the temperature of the gas.. In choosing a particular turbo you need to know how many Pounds Per Minute or CFM the turbo compressor map indicates for a particular pressure ratio (boost pressure). From: Advanced Power Plant Materials, Design and Technology, 2010. High Pressure Ratio. But there are also limits on the pressure ratios that can be used in the cycle. Figure 14 shows how pressure ratio can vary with impeller tip speed, U 2 : Figure 14. Related terms: In general, increasing the pressure ratio is the most direct way to increase the overall thermal efficiency of a Brayton cycle, because the cycle approaches the Carnot cycle. Optimised turbo-charging system, including a larger intercooler assembly. Effect of impeller tip speed on pressure ratio A parameter that is common in fan engineering and occasionally used for turbocharger compressors  is the dimensionless pressure number, Ψ. 10 + 14.7 / 14.7 = 1.68 PR.