7 replies to this topic

hello,
how do you convert dew point to ppm , on the hydrogen plant we have it has a moisture transmitter which is reading -99.9 degrees c dp and 0.019 ppm on the mmi, off the pipeline there is a panametrics moisture monitor which can be switched to tanker filling for certification requirements
when on the pipeline it is reading -108.16 degrees c dp and -1.69 ppm. Both are vented to atmosphere.
Do you need the pressure to work out ppm?

[djack77494]

I can't comment on the accuracy of your analyzers, but I can explain the relationship between pressure, ppm, and dew point.

As the concentration of water vapor increases (your ppm) or as the total pressure increase for a fixed water vapor concentration, the water vapor's partial pressure is increasing. For each temperature, even in the cryogenic range, there is a corresponding pressure where water vapor will be in equilibrium with condensed water (either liquid or solid ice). This combination of a temperature and a (water vapor) pressure are the saturation curve. If you cross the curve by getting colder and/or getting to a higher water vapor partial pressure, then you will generate the condensed phase.

[Art Montemayor]

You convert the moisture dewpoint of hydrogen to parts per million (Vol.) of moisture by use of a constructed table that relates both properties.

The moisture Dewpoint of a gas (which I think is what you are referring to) IS NOT a quantitative analysis result. The ppm(v) or ppm(m) IS a quantitative analysis. The two are not analytically related and must be converted by using empirical information.

The Dewpoint is the temperature at which the first drop on moisture forms within the gas - and this is usually taken at (or referred to) atmospheric pressure. The Dewpoint at actual process pressure is another value. You must identify which one you mean.

There are both Dewpont analyzers and ppm analyzers. Depending on which one you have or bought, you should have been furnished with a conversion table.

thankyou for your replies.
yes i am talking bout hydrogen gas. I am just and electrician at this plant but i don't like not knowing how things are suppose to work.
The pipeline moisture anaylser has the ability to read either dew point or ppm(v). If reading ppm(v) then a constant pressure parameter (of anaylser i/p pressure) is set. From your reply i can only assume that from empirical data the manufacturer has developed an algorithm. For some reason the anaylser has been set up to output dew point to the PLC then wonderware software spits out the dew point and ppm(v) on the mmi. I think something must be wrong with the software as the mmi sometimes (depending on the dew point reading) shows a negative ppm(v) reading. I am going to program the analyser to read ppm(v) on its screen and see if this is anywhere near the mmi reading.
Thanks for your help.

[siretb]

For hydrogen / water we could simplify a bit.

To calculate the dew point, given the ppm(volume) water
1) calculate the partial pressure of water, multiplying the fraction of water in the gas by the total pressure
P1=(ppm water * 1.e-6) * (absolute pressure)

2) find a suitable relation giving the water pressure over ice (or water) for a given température.
let P2=f(T) this reelation
be sure to select a suitable relation, that covers the range -100° -0°C; the Goff Gratch equation should do. See: http://cires.colorad...~voemel/vp.html

3) solve P1=P2 for T, the temperature

Hope this helps. Bernard

[Art Montemayor]

Bernard:

Thanks for the useful and instructive information. As usual, your response is very helpful. Your reference URL wasn't working for some reason, so I fixed it. I believe it will take you directly to the indicated webpage now.

mbardfast:
You can find a lot of useful information on the subject of Saturation Vapor Pressure formulations - which is the underlying theory for the moisture dewpoint of a gas - on the Internet. Bernard pinpointed the classic formulation (Goff-Gratch Equation) and gives you a source for the background as well as the historical trials and tribulations that have occurred in arriving at this stage in trying to identify the vapor pressure of water in gases at low temperatures with accuracy.

For tabulations of ppmV versus Dewpoint go to:

www.iceweb.com.au/Analyzer/MoistMeas/Advantica%20Moisture%20Measurement%

http://www.airliquide.com.au/specgases/specconv.php#10

and for a means to calculate the specific value on-line, go to:

http://www.dew-point.com/calculate.html

I hope this addresses your specific problem and aids you in identifying the problem you are facing with your instrument.

[hygrometry]

You can download a freeware calculator from www.phymetrix.com that will perform all of these conversions.

Hope this helps!

By the way PPMv can not be a negative number, so your analyzer may need servicing.

[Arvind Iyer]

Dear Friends,

The moisture ppm and moisture dewpoint are related from first principles.
I will explain the general relationship.

Observe the folowing:-

The first drop of water condenses at a temperature when
Partial pressure of water=Vapor pressure of water at that temperature

When a mixture of non-condensible gas and water vapor is progressively cooled,
there will be a temperature at which the partial pressure of water equals the vapor pressure
of water at that temperature. This is because vapor pressure is a function of temperature.
As you decrease temperature, the vapor pressure decreases.

For an ideal mixture,
Volume of water vapor/ Total Volume = Partial pressure of water vapor/ Total pressure
= Moles of water vapor / Total Moles

If you know the dew point, then read the vapor pressure of water vapor
from a standard vapor pressure chart or use a correlation as suggested earlier
by Bernard.

At condensation, partial presure of water vapor can be replaced by the vapor pressure of water vapor
So, from above, you can calculate the Moles of water vapor / Total Moles.

Observe that you need to know the total pressure to calculate this ratio.
(Partial pressure of water vapor/Total pressure)
This is why you set the total pressure in the instrument.

ppm is parts per million. So, ppm = Moles of water vapor / Total Moles times 10 ⁶

The empirical portion comes from the fact that vapor pressure of water vapor is
empirically related to temperature.

And this general relationship is valid for any system of non-condensible gas and a (condensible) vapor.
Gas: Air, Nitrogen Hydrogen etc..... Vapor: Ammonia, water etc.....