Mike wrote:
Dear all,

I'm new to the group and would like to benefit from your combined
knowledge on humidity... In fact my question does not relate to
weather, but does relates to calculations of conditions in the
atmosphere of a test rig. It is a very long time since I was doing
these calculations and I am very rusty.

A new test method has been developed for ageing some samples, the test
conditions are as follows;

Pressure 2.1MPa, temperature 85 deg C, liquid water in the bottom of
the pressure vessel.

The people who delveloped the test believe that the samples inside the
pressure vessel will be saturated with water and exposed to the effects
of water and oxygen thereby simulating an accelerated ageing process -
I am unconvinced. They hypothesise that the atmosphere in the test rig
exposes the samples (above the liquid level) to liquid and water
vapour. According to the calculations I have done there is actually
very little water in the atmosphere of the rig...

can anyone assist me?

Under the above conditions as far as I can determine the vapour
pressure of water in the rig is 57.8kPa. Using the Ideal gas law
(pV=nRT) I calculate that there is approx 25ml water in the rig
atmosphere. At this stage I reach the boundary of my capabilities!!

What is the humidity in the chamber? is the air saturated with water
vapour - is it close to the dew point (i.e. will water deposit upon the
samples?)

Apologies in advance for my ignorance in this area and grateful in
advance for your assistance.

Mike

Your calculations are correct as far as they go. But the
humidity in the vessel is 100%. (ie saturated). It must be - there is
excess liquid water in it regardless of the temperature and pressure.
Your samples will be exposed to as much water as they can be, short of
dripping hot water over them. They will not *quite* have water
condensing on them but I would imagine when first put into the vessel
they will be at room temperature and water will condense onto them as
the rig warms up. But in the steady state they will be *just* not
actually wet. It seems, alas, that your colleagues are right. The
small quantity of water present as vapour is merely a reflection of the
saturation vapour pressure of water relative to the total pressure. At
atmospheric pressure the water temperature that would give the same
proportion is 23°C. BTW as a check, I calculate from your figure of
25 ml water that the volume of the vessel is 41 litres. Is that right?

Tudor Hughes (retired industrial chemist), Warlingham, Surrey.