Author Topic: How do i calculate rate of evaporation of gas?  (Read 9988 times)

Offline boatdetective

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How do i calculate rate of evaporation of gas?
« on: February 22, 2011, 11:13:10 AM »
Long story, but i need to calculate the rate at which gasoline evaporates in the open. I'm talking normal temperatures- 70-90F.  Any engimaneers out there who know a quick and dirty formula?
Jonathan K
Marblehead, MA

1981 V50III "Gina"
2007 Griso 1100 "Bluto" (departed but not forgotten)
2003 EV "Lola" gone to the "Ridin' Realtor" in Peoria
2007 1200 Sport "Ginger"

"Who's the cat who won't cop out, when there's danger all about?"  -Isaac Hayes

Offline kitze2

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Re: How do i calculate rate of evaporation of gas?
« Reply #1 on: February 22, 2011, 12:23:10 PM »
How about this...
Place a small amount in an open container (in a safe and temperature stable location)
Place the container on a reloading scale and weight the whole set up.
Then reweigh it and plot the change in weight.
Not super scientific but might get you what you need.
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Offline egschade

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Re: How do i calculate rate of evaporation of gas?
« Reply #2 on: February 22, 2011, 01:37:26 PM »
All I know is that lower octane = greater volitillity, hence regular causing detonatiion in higher compression engines.

Ethanol is lower volititlity so greater E percentage has an impact.
The oldest Eric in NJ
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Offline jreagan

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Re: How do i calculate rate of evaporation of gas?
« Reply #3 on: February 22, 2011, 01:43:42 PM »
It is going to depend on temperature of the gas, the temp of the air, altitude, vapor pressure, surface area of gas, even wind speed across the gas, etc. 
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Offline NOLAGuzzi

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Re: How do i calculate rate of evaporation of gas?
« Reply #4 on: February 22, 2011, 02:08:36 PM »
You could do it empirically like this:

put a volume of gas in a graduated cylinder and record volume.

wait some period of time and record time

measure volume left in cylinder.

calculate rate of evap by (initial volume - final volume / time in min)
surface area would be an important factor too. so you could calculate the cross sectional surface area in cm2 of the cylinder (pi*r2) and then divide the rate by that to give units that show change in volume per time per surface area (ml evaporated/min)/cm2.

this would not be generalizable to all situations but it would give you an idea of what goes on at atmospheric conditions.


« Last Edit: February 22, 2011, 02:13:55 PM by NOLAGuzzi »
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Offline Sasquatch Jim

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Re: How do i calculate rate of evaporation of gas?
« Reply #5 on: February 22, 2011, 02:17:38 PM »
You could do it empirically like this:

put a volume of gas in a graduated cylinder and record volume.

wait some period of time and record time

measure volume left in cylinder.

calculate rate of evap by (initial volume - final volume / time in min)
surface area would be an important factor too. so you could calculate the cross sectional surface area in cm2 of the cylinder (pi*r2) and then divide the rate by that to give units that show change in volume per time per surface area (ml evaporated/min)/cm2.

this would not be generalizable to all situations but it would give you an idea of what goes on at atmospheric conditions.



The graduated cylinder would not be applicable to all situations as the tube shape of the cylinder would inhibit evaporation. I would go with the open dish and the accurate scale weight.

Sasquatch Jim
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Offline NOLAGuzzi

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Re: How do i calculate rate of evaporation of gas?
« Reply #6 on: February 22, 2011, 03:31:54 PM »
That is true Jim.  The cylinder is not applicable to all conditions.  Fill the cylinder all the way up, or most of the way, and measure for a short time.  Then it should not matter. 

About weighing gas, how can one back calculate to a volume by weighing a volatile liquid of unknown density with any kind of accuracy?  I am not nit picking, I'm just trying to understand how that would work.

I don't think of gasoline in terms of weight but in volume so that confuses me. 
04 Breva 750
NOLA = New Orleans LA

Offline charlie b

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Re: How do i calculate rate of evaporation of gas?
« Reply #7 on: February 22, 2011, 03:39:41 PM »
(mass loss rate)/(unit area) = (vapor pressure - ambient partial pressure)*sqrt( (molecular weight)/(2*pi*R*T) )

vapor pressure chart

           WINTER   WINTER    SUMMER  SUMMER  AVIATION  JET FUEL
         GASOLINE  GASOLIN GASOLIN GASOLIN GASOLINE  & KEROSENE
TEMP    REID       REID        REIDVP    REID
F        14-VP      12-VP      10-VP       8-VP  

40   4.8   4   3.4   2.8   2.2     0.9
50   5.9   4.9   4.1   3.4   2.8     1
60   7.4   6   5   4.1   3.5     1
70   8.9   7.4   6   5   4.2     1.2
80   10.7   9   7.1   5.9   5.1     1.4
90   12.8   10.6   8.6   7   6.2     1.7
100   14   12   10   8   7.4       2

And,for stagnant conditions you have to take into account the change in local ambient partial pressures as it vaporizes, ie, near the surface you'll get a layer of high partial pressures which will slow the evaporization.

If you have any wind blowing then I'd estimate it with a zero ambient or maybe 50% (ie, half the gas vapor pressure).

charlie
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Offline charlie b

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Re: How do i calculate rate of evaporation of gas?
« Reply #8 on: February 22, 2011, 03:40:52 PM »
That is true Jim.  The cylinder is not applicable to all conditions.  Fill the cylinder all the way up, or most of the way, and measure for a short time.  Then it should not matter. 

About weighing gas, how can one back calculate to a volume by weighing a volatile liquid of unknown density with any kind of accuracy?  I am not nit picking, I'm just trying to understand how that would work.

I don't think of gasoline in terms of weight but in volume so that confuses me. 

Measure it first in a closed container.
1984 850 T5
2009 Dodge Cummins 2500
2011 Honda Element

Offline radan2

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Re: How do i calculate rate of evaporation of gas?
« Reply #9 on: February 22, 2011, 03:44:55 PM »
At this site is the procedures and formulas used by the industry:

www.epa.gov/ttn/chief/ap42/ch07/final/c07s01.pdf
AP-42, CH 7.1: Organic Liquid Storage Tanks

It is NOT quick and dirty.
Jim Wayne
Jacksonville, NC, USA
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Offline NOLAGuzzi

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Re: How do i calculate rate of evaporation of gas?
« Reply #10 on: February 22, 2011, 03:49:12 PM »
That is true Jim.  The cylinder is not applicable to all conditions.  Fill the cylinder all the way up, or most of the way, and measure for a short time.  Then it should not matter. 

About weighing gas, how can one back calculate to a volume by weighing a volatile liquid of unknown density with any kind of accuracy?  I am not nit picking, I'm just trying to understand how that would work.

I don't think of gasoline in terms of weight but in volume so that confuses me. 

Measure it first in a closed container.

Ahh that makes sense.
04 Breva 750
NOLA = New Orleans LA

Offline RayB

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Re: How do i calculate rate of evaporation of gas?
« Reply #11 on: February 22, 2011, 06:36:44 PM »
I guess the precision you need depends on the problem you are trying to solve. If you are just curious, I would use the weight method rather than volume because of expansion, etc. I would, however take the temp of the liquid at the start time and end time.

If you have a real engineering problem that has a large $ impact based on the decision you make with the info, precision is justified.

If you tell us why you want to know this we could probably give you a better method.
01 EV
82 BMW R100

Offline Sasquatch Jim

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Re: How do i calculate rate of evaporation of gas?
« Reply #12 on: February 22, 2011, 07:41:18 PM »
And this knowledge of gas evaporation rates affects our riding by?

Sasquatch Jim
Sasquatch Jim        Humanoid, sort of.

Offline boatdetective

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Re: How do i calculate rate of evaporation of gas?
« Reply #13 on: February 22, 2011, 08:13:05 PM »
Sorry Jim, definitely NGC.  I have a boat fire investigation. One theory is that the supply hose (low pressure) from the tank was pulled off-line which wad the source of gas to start the fire. The hose was 24" x 3/8" ID. Volume of hose was only 1.5 oz.  There's a check valve on the tank- so that would have to limit leakage to the hose volume. So, we have 1.5 oz of fuel- but even that needs to vaporize.  Approx. Volume of the engine compartment is 50 cu. Ft.  Lower explosive limit of gasoline is 1.4%.  Ifwe grant that every drop of the gas varied (no way), the vapor would only amount to just under .5%.  Even if we could bump it up to the lel, there would be no additional fuel left to support the fire (these cases result in a big Ka-woof-but snuff out).  The final bit was that the engine compartment was open.  I think I'm pretty safe in finding this theory ca-rap.
Jonathan K
Marblehead, MA

1981 V50III "Gina"
2007 Griso 1100 "Bluto" (departed but not forgotten)
2003 EV "Lola" gone to the "Ridin' Realtor" in Peoria
2007 1200 Sport "Ginger"

"Who's the cat who won't cop out, when there's danger all about?"  -Isaac Hayes

Offline cloudbase

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Re: How do i calculate rate of evaporation of gas?
« Reply #14 on: February 23, 2011, 10:59:16 AM »
Assuming the check valve didn't leak. 


Offline Two Checks

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Re: How do i calculate rate of evaporation of gas?
« Reply #15 on: February 23, 2011, 12:42:49 PM »
I have found gas evaporates from my tank faster when I twist the grip further. :-)
BD good luck with your investigation. Reading the latest issue of Good Old Boat.
1990 Cal III f/f  "Il Duce' III"
1987 1000 SPII "Il Duce' II"

Offline boatdetective

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Re: How do i calculate rate of evaporation of gas?
« Reply #16 on: February 23, 2011, 02:51:44 PM »
TwoC, I'd rather be reading about good old boats, too. I'm living it now- chasing down 230 linear feet of @#$%! gloat stress cracks on my 1970 Betram in prep for dipping the whole thing in Awlgrip. Oy vay!
Jonathan K
Marblehead, MA

1981 V50III "Gina"
2007 Griso 1100 "Bluto" (departed but not forgotten)
2003 EV "Lola" gone to the "Ridin' Realtor" in Peoria
2007 1200 Sport "Ginger"

"Who's the cat who won't cop out, when there's danger all about?"  -Isaac Hayes

Offline Offcamber1

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Re: How do i calculate rate of evaporation of gas?
« Reply #17 on: February 23, 2011, 09:52:03 PM »
Gasoline evaporates instantly.  Shortly after someone consumes way too many beers and says, "Hey y'all, watch this!"
Its a fair cop, but society is to blame.

 


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