Relation between absolute & relative humidity 1

[skuntz]

We are having a debate at work. We have a process where the absolute humidity is measured. They want to predict the relative humidity. Some are claiming you don't need to know the system pressure to find RH. I say no - absolute humidity is a mass ratio only, but RH depends on system pressure, because the partial pressure of the water vapor must be known to calculate RH.

Does anyone disagree? Is it possible to predict RH from AH directly without any other conditions known?

 

[DRWeig]

Skuntz,

Don't forget temperature. And yes, you're right about pressure being a factor.

Good on ya!

Goober Dave

 

[zdas04]

The amount of water vapor that represents 100% RH is a function of both temperature and pressure. At any time you can measure an AH, but to determine what that number means relative to saturation conditions you must know pressure and temperature.

One of the most widespread examples is the GPSA Field Data Book Figure 20-3. This shows a definite relationship between the mass of water vapor that can be absorbed into methane and the temperature/pressure of the gas. The lines on this graph represent the 100% RH line. You can also use this graph to determine a dew point at elevated pressure (i.e., measure the water content in the field, enter the graph on the left axis with that number, go over to the pressure line and down to the dew point). One of the most common pipeline specifications in the US is that gas should have a maximum water content at 1000 psig of 7 lbm/MMCF. Using GPSA Figure 20-3, you see that 7 lbm/MMCF is a dew point of 36F (100% RH at 1000 psig and 80F is 24 lbm/MMCF so 7 lbm/MMCF is 28% RH, at 100F and 1000 psig it is 16% RH, and at 80F and 100 psig 7 lbm/MMCF is 3% RH).

So the answer to your final question is Absolutely Not. If someone says "I have 7 lbm/MMCF gas" without pressure and temperature information you can't know if you're saturated or sub-saturated (e.g., at 1000 psig and 36F, 7 lbm/MMCF is 100% RH).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[skuntz]

After further discussions here it was revealed that the absolute humidity is being measured as gm/cubic foot, so it is a mass to volume ratio rather than mass/mass. In fact I have discovered (in books and the internet) that AH is being defined as both mass vapor/mass dry air and mass vapor/total volume. If the number is expressed as mass/volume then it turns out that the volume dependency on pressure cancels out the system pressure term in the RH. When expressed this way, the only other conditions that must be known is the temperature (to get the vapor pressure).

So once again, ambiguity in definitions was the culprit!

 

[zdas04]

I think you may want to look again. The mass-vapor/volume is not a real volume it is a "standard" volume which has more in common with mass than it does with volume. The mass of a gas that can be crammed into a fixed volume is totally dependent on the pressure and temperature of the gas. RH is pressure and temperature dependent.

David

 

[skuntz]

zdas04:

Yes that's correct. So if we define Absolute Humidity (AH) = mass water/volume gas and substitute mass water = moles water (nw) * 18, and volume (V) = nRT/P you will get AH = nw*18*P/n*R*T. But nw/n *P is the partial pressure which is also Relative Humidity (RH)* Pvap. From that you can work out an expression relating RH and AH that eliminates P.

Meanwhile we have settled our in-house dispute.

 

[zdas04]

I'm glad you've settled your in-house dispute.

I've worked through your arithemetic and I can solve for RH in terms of AH, Temperature, Gas Make-up, and water vapor partial pressure. Problem is that I can't determine water vapor partial pressure without knowing total pressure.

David