Relative humidity calc at high temps

Ivan Seeking

I am looking for equations valid at temps up to 500 degrees F, that allows one to calculate the relative humidity for air based on wet bulb and dry bulb temps. Everything that I can find is only accurate at lower temps. For that matter I can't even find a psychrometric chart that goes that high. Am I missing something here?

Any ideas?

I need to calculate the wet bulb temp based on the rh, and Tdry, over the same range as well, but I can work around this if needed.

russ_watters

Psychrometric charts don't go that high, but steam tables do. You can get RH from partial pressure and saturation pressure, though I'm not sure offhand how to get from that to wet bulb temp.

Ivan Seeking

thanks Russ.

I did find one chart that goes to 250F [dry bulb].

Have you ever looked at the psychrometric data and equations in the ASHRAE manual? I am thinking that it may be worth buying it, but then again it may not have any more information that I have now.

I know that we can get the saturation line at a given temp and press, but to dynamically determine the partial pressure of water is apparently a real problem at high temps. I had a long talk with the NIST this morning, and according to them, wet bulb temps are virtually meaningless once we get up around 100C. In fact they are working on this very problem right now for a 200C test, and they have no realible way to get a measurement as yet.

I suspect that there are going to be a few raised eyebrows when I pass along what I have found. It seems that there has been 50 years of reliance on meaningless data.

russ_watters

Well, since the saturation pressure at 500F is 680 psi, it is a little meaningless to talk about "relative humidity" - there could be 50 times more water than air in the sample (unless the air is highly pressurized in the sample too).

I remember doing calculations on this sort of thing (not RH or wet bulb temp, though, just reproducing the steam tables) in thermo, but that was one of those this-is-how-its-done-we'll-never-do-it-again type things.

May I ask what this is for?

Ivan Seeking

Still a fraction though... I expect that the rh is always very low at those temps.

Its for an industrial application. As usual I can't say much more or someone might break my legs.

russ_watters

I didn't mean about the application, just the conditions. Can you at least tell me if the pressure you are dealing with is atmospheric? I guess it is ok if you can't, it doesn't really change my point:

The point I was trying to make (and I was slightly wrong in what I said....) is that you could have a vessel that is completely filled with water vapor (no air) at 500F and atmospheric pressure. Without having any air, is there even such a thing as "relative humidity"? Or you could have a mixture of half air and half water vapor. I would think that in that situation, you don't want to say you have 1.1% relative humidity (7.35/680), but rather that you simply have a 50% volume fraction of water vapor in your mixture.

Ivan Seeking

The pressure could range from about 0.5 to 2 atmospheres. This also needs to be a dynamic input for the calculation.

It seems that the industry standard is rh, but either way the problem is to dynamically determine the moisture content present in air up to 500F, to within 0.5% accuracy. IR beam attenuation is one approach, but contaminants in the air stream make this impractical.

Also, earlier I said that web bulb is meaningless at these temps, but what I should have said that it is far too inaccurate for this application at the higher temps. Apparently it is still good to within five or ten percent, even at these temps and pressures.