Goff-Gratch Equation

[David]

I've scripted in Perl the Goff-Gratch, using 373.16 K and 1013.246 hPa,
then I used 373.15 K and 1013.25 (the ITS-90 numbers), and there's a
slight difference in the output, obviously. The ITS-90 numbers agree
with the table in the chem text that I use.

This got me to thinking, is the vapor pressure of water also dependent
on atmospheric pressure? For instance, at 20.0 C the vapor pressure
given in the text's table is 2.34 kPa, which is what the Goff-Gratch
calculates, however this assumes atmospheric pressure of 101.325 kPa.
If the atmospheric pressure were different, would that affect the vapor
pressure of water at 20.0 C?

My hunch is that the pressure exerted by water vapor is independent of
the surrounding pressure because it's being exerted by the kinetic
motion of the evaporating particles, which seems to me only dependent on
temperature, and not on whether or not there's a greater or lesser
opposing force from the atmosphere.

Thanks for any clarifications, and best regards,
David

[Uncle Al]

Post by David
I've scripted in Perl the Goff-Gratch, using 373.16 K and 1013.246 hPa,
then I used 373.15 K and 1013.25 (the ITS-90 numbers), and there's a
slight difference in the output, obviously. The ITS-90 numbers agree
with the table in the chem text that I use.
This got me to thinking, is the vapor pressure of water also dependent
on atmospheric pressure? For instance, at 20.0 C the vapor pressure
given in the text's table is 2.34 kPa, which is what the Goff-Gratch
calculates, however this assumes atmospheric pressure of 101.325 kPa.
If the atmospheric pressure were different, would that affect the vapor
pressure of water at 20.0 C?
My hunch is that the pressure exerted by water vapor is independent of
the surrounding pressure because it's being exerted by the kinetic
motion of the evaporating particles, which seems to me only dependent on
temperature, and not on whether or not there's a greater or lesser
opposing force from the atmosphere.
Thanks for any clarifications, and best regards,

Applying inert gas pressure to a liquid is tantamout to applying a
permeable piston. Look at your physical chemistry. Vapor pressure
will rise with inert gas pressure at constant temperature, but the
effect is very small.

[David]

Post by Uncle Al
Applying inert gas pressure to a liquid is tantamout to applying a
permeable piston. Look at your physical chemistry. Vapor pressure
will rise with inert gas pressure at constant temperature, but the
effect is very small.

My thinking's backward on this. When I try to visualize some kinetic
model, I come up short. The "permeable pistion" would be the inert gas
particles slamming the water molecules into the vapor phase?

Anyway, thanks for the info. I appreciate the help.
David

[Uncle Al]

Post by David

Post by Uncle Al
Applying inert gas pressure to a liquid is tantamout to applying a
permeable piston. Look at your physical chemistry. Vapor pressure
will rise with inert gas pressure at constant temperature, but the
effect is very small.

My thinking's backward on this. When I try to visualize some kinetic
model, I come up short. The "permeable pistion" would be the inert gas
particles slamming the water molecules into the vapor phase?

The piston compresses the water thereby increasing its chemical
activity or fugacity. The increased vapor pressure diffuses through
the piston.

http://www.hvacindia.org.in/journals/1999apr/article06.html
middle

[A.H.]

Post by David
I've scripted in Perl the Goff-Gratch, using 373.16 K and 1013.246 hPa,
then I used 373.15 K and 1013.25 (the ITS-90 numbers), and there's a
slight difference in the output, obviously. The ITS-90 numbers agree
with the table in the chem text that I use.
This got me to thinking, is the vapor pressure of water also

dependent

Post by David
on atmospheric pressure? For instance, at 20.0 C the vapor pressure
given in the text's table is 2.34 kPa, which is what the Goff-Gratch
calculates, however this assumes atmospheric pressure of 101.325 kPa.
If the atmospheric pressure were different, would that affect the vapor
pressure of water at 20.0 C?
My hunch is that the pressure exerted by water vapor is independent of
the surrounding pressure because it's being exerted by the kinetic
motion of the evaporating particles, which seems to me only dependent on
temperature, and not on whether or not there's a greater or lesser
opposing force from the atmosphere.

Look up "Poynting effect" in a thermo textbook. It's pretty nearly
negligible around standard atmospheric pressure, but can get big if you
go up to 50 or 100 atmospheres.

Of course there is also an effect (again, tiny at normal pressures)
from the air dissolving in the water, making the mole fraction of H2O
less than one and reducing the effective vapor pressure accordingly.
Then you have the effect of nonideality in the vapor-phase mixture,
which is usually of some significance and gets worried about a lot by
people who do humidity standards.

By the way, Goff-Gratch is rather out of date, although I gather it is
pretty close to current standards for typical conditions. It was
superseded by:
R.W. Hyland and A. Wexler, ASHRAE Trans., vol. 89, pp. 520-535 (1983).
An extrapolation to higher temperatures and pressures has recently been
presented by:
H.F. Nelson and H.J. Sauer, HVAC&R Research, vol. 8, pp. 311-334 (2002)

Dr. Allan H. Harvey, Boulder, CO, ***@aol.com
(insert typical disclaimers here)

[David]

Thanks for the below info. Much better than I had hoped to get!
Best regards,
David

Post by A.H.
Look up "Poynting effect" in a thermo textbook. It's pretty nearly
negligible around standard atmospheric pressure, but can get big if you
go up to 50 or 100 atmospheres.
Of course there is also an effect (again, tiny at normal pressures)
from the air dissolving in the water, making the mole fraction of H2O
less than one and reducing the effective vapor pressure accordingly.
Then you have the effect of nonideality in the vapor-phase mixture,
which is usually of some significance and gets worried about a lot by
people who do humidity standards.
By the way, Goff-Gratch is rather out of date, although I gather it is
pretty close to current standards for typical conditions. It was
R.W. Hyland and A. Wexler, ASHRAE Trans., vol. 89, pp. 520-535 (1983).
An extrapolation to higher temperatures and pressures has recently been
H.F. Nelson and H.J. Sauer, HVAC&R Research, vol. 8, pp. 311-334 (2002)
(insert typical disclaimers here)