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# Enthalpy of air from air tables differ from enthalpy from psychrometric chart

## Enthalpy of air from air tables differ from enthalpy from psychrometric chart

(OP)
Hi,
I have uploaded two images. One is a table of Low Pressure Air Properties. The other is a typical psychrometric chart.

If you look up the enthalpy of air at 60F (520 R) in the low pressure air table, you get an enthalpy of 127.27 btu/lbm.

If you look up the enthalpy of air with zero humidity (dry air) in the psychrometric chart at 60F, you get an enthalpy of roughly 13 btu/lbm.

I know I am making some basic mistake here. Can anyone spot why these values do not match or what mistake I am making?

Thanks

### RE: Enthalpy of air from air tables differ from enthalpy from psychrometric chart

enthalpy (and entropy) have arbitrary zero points. Different systems (or tables or charts) can have that "zero" point at different conditions.
your table may have zero at actual zero °Rankin or Kelvin. the HVAC chart may have it at 0°F or so.

### RE: Enthalpy of air from air tables differ from enthalpy from psychrometric chart

(OP)
Thank you, that answer sounds correct to me and also possibly explains why on some tables the enthalpy is shown as a negative number once you get below a certain temperature.

On the tables I looked up the following
520 R (60F) ==> h =124.37 btu/lbm
545 R (85F) ==> h approx 130 btu/lbm

delta h (low pressure air tables) approx 5.73 btu/lbm

On the phychrometric chart
60 F ==> h approx 14.5
85F ==> h approx 20.5

delta h (psychrometric chart) approx 6 btu/lbm

The two enthalpies are likely the same and have a different zero point. Thank you very much, that really helped me out.

### RE: Enthalpy of air from air tables differ from enthalpy from psychrometric chart

this thermodynamics book by Klein and Nellis really explains it all well

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