Understanding P-h chart

[Mattman83]

I know this is pretty basic, but I always use tables and am trying to get in the habit of plotting out a vapor compression cycle on a P-h graph. Anyway, I am working with ammonia and the enthalpy results don't seem to match between chart and graph (-200 vs 700 kJ/kg?!). How am I misreading it or what basic principal am I forgetting (my thermo is rusty)?

Looking at 100C saturated liquid:

 

[Latexman]

The graph may have a different reference/basis than the table, so the absolute enthalpy at a point may be different, but the change in enthalpy from State 1 to State 2 should be about the same. Yep, it is. Check it out on the heat of evaporation at 100 C.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[Mattman83]

Yep, that's a great answer. You would think the basis would be standardized...at least in the same publication. In any case, I appreciate the explanation - makes sense now!