# How accurate are the property values?

About 98%. The real gas properties, however, are based on generalized compressibility chart and may produce significant error especially at low pressures, 1% of the critical pressure or lower. For R-134a, the h and u values from TEST do not agree with those in my textbook. Is this a bug? The absolute value of enthalpy has absolutely no meaning. The absolute values depend on what one chooses as the reference for the internal energy u. Because different people choose different reference values, you will find different values for the u or h of the same substance under the same conditions in different textbooks. But the important thing to understand is that it does not matter. It is the difference in h, u or s that is important in a solution. If you ever find a thermodynamic problem where you are asked to evaluate the absolute energy or enthalpy of a substance, you must ask what is the reference state? The same is true when you evaluate potential energy using the formula mgh.

About 98%. The real gas properties, however, are based on generalized compressibility chart and may produce significant error especially at low pressures, 1% of the critical pressure or lower. For R-134a, the h and u values from TEST do not agree with those in my textbook. Is this a bug? The absolute value of enthalpy has absolutely no meaning. The absolute values depend on what one chooses as the reference for the internal energy u. Because different people choose different reference values, you will find different values for the u or h of the same substance under the same conditions in different textbooks. But the important thing to understand is that it does not matter. It is the difference in h, u or s that is important in a solution. If you ever find a thermodynamic problem where you are asked to evaluate the absolute energy or enthalpy of a substance, you must ask what is the reference state? The same is true when you evaluate potential energy using the formula mgh. You have to k