Gasahol Equation

I'm making a % alcohol meter for gasahol. It is an article for my monthly
column in Kitplanes magazine.
However, it has been a very long time since I've done a ratiometric equation, so
would somebody please check my work? Many thanks.
I start with an air variable capacitor of 365 pf (the old crystal set tuner).
I dunk it in pure gasoline with a dielectric constant of 1.94 and I should come
up with a 708 pf capacitor.
I then mix 90 ml of pure gasoline (e = 1.94) with 10 ml of pure ethyl alcohol (e
= 24.5) and come up with a composite dielectric constant of 4.2, which gives my
capacitor dunked in this mixture a value of 1533 pf ... (which then lowers the
audio frequency of the oscillator that the capacitor is controlling, which then
goes to the frequency to voltage converter, which goes out to the Rat Shack
panel meter calibrated in percentage...)
Did I do that right? If that is correct, then all the rest of the percentages I
calculated on the spreadsheet from 1 to 10 should be OK also.
Jim
Reply to
Jim Weir
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I read in sci.electronics.design that Jim Weir wrote (in ) about 'Gasahol Equation', on Sat, 17 Jul 2004:
Looks OK to me.
Reply to
John Woodgate
"Did I do that right" and "metrology" never seem to go together well. I think the more you know the less you're willing to say it must be right.
* Are you sure that the relationship is linear? It sounds like you've tested it for a couple of mixtures which is good, it may be a good idea to test a bit more (perhaps a 50% mixture).
*
What happens if you measure wet gasahol, i.e. gasahol that's sat around for a bit with that ever so hydroscopic alcahol in it -- will it get conductive and change the frequency significantly? Do you have an independent way of checking? Your capacitor will certainly have a great deal of surface area for a conductive liquid to contact.
Reply to
Tim Wescott
I was asked once to build a device for measuring the dielectric constant of ink. The ink was an oil/water emulsion and I never really fully understood the consequences of the fact this medium was conductive. To try and reduce its effect I used a capacitor in series with the test cell, but whilst that removed any dc bias, it wasn't a full solution. The laboratory that used the device told me some time later that it turned out to be very good at measuring some other property of emulsions, even if it didn't really measure dielectric constant. I'm not sure if alcohol is conductive, but unless specially prepared as anhydrous, even 'pure' alcohol has a couple of percent of water in it and its conductivity may vary greatly with concentration of dissolved impurities.
Scrim
Reply to
Scrim
Um, I believe "pure" alcohol is 180 proof, or ten percent water.
Reply to
Don Lancaster
Well, ok, to be precise, the strongest ethanol/water mix that can be produced by simple distillation is 95.6% ethanol, 4.4% water. This is an azeotropic mixture, which means that when distilled, the vapour contains the constituents in the same proportion as the liquid and so can't result in any greeater concentration.
Scrim
Reply to
Scrim
By definition, pure alcohol is 200 proof, and one laboratories can get white tax stamp (untaxed) Triple Distilled Ethanol which (if i remember correctly) has under 1% water (the main "contaminant").
Reply to
Robert Baer
Everybody seems to be concerned about the water being conductive. Water isn't a conductor. [0] alcohol isn't a conductor. gasoline isn't a conductor.
What part of "isn't a conductor" is the conductivity coming from?
As far as the calculations, I'd have to see more data points to get a better idea of the transfer function, but even with them, I'm way too lazy to do that much arithmetic. I'd calibrate it by doing measurements on known mixes, as you seem to be doing. :-) Then, who needs math when 1048576-byte EPROMS are a few bucks? Can you say look-up table?
Reply to
Rich Grise
Clean, pure, distilled water with no impurities isn't a conductor. Clean, pure, dry salt isn't a conductor, either. Water with salt in it _is_ a conductor. Clean, pure, distilled water added to clean, pure, distilled alcohol and clean, pure distilled gasoline may not be a conductor, either, but do you know? And what happens if there's some crap in whatever tank you're pulling your "gasohol" out of? Can you guarantee it won't be conductive then?
I don't know either, which is why I asked "will it get conductive". If it were me I'd check, one way or another.
Reply to
Tim Wescott
Post that to sci.physics and watch for a reply from Uncle Al.
Reply to
Boris Mohar
Tch.
Ink dots on a Rat Shack panel meter are even cheaper when you apply them by hand.
Jim
Rich Grise shared these priceless pearls of wisdom:
Then, ->who needs math when 1048576-byte EPROMS are a few bucks? Can you say ->look-up table?
Reply to
Jim Weir
no such thing as "pure" gasoline. There are 49 different mixes of Gasoline Naphtha, Ether, Octane, Heptane, benzene and many others - Mixes very from state to state, winter/summer, and regional atmospheric pressure There will be a small unknown percentage of water absorbed from the air humidity too. So your bounds or range for the dielectric constants needs to be expanded, they are not single points.
Reply to
Anamet Dorsey
Yes there is. It is called 100LL aviation fuel, and the mix is constant across the nation and across the year.
Jim
"Anamet Dorsey" shared these priceless pearls of wisdom:
->no such thing as "pure" gasoline. ->There are 49 different mixes of Gasoline ->Naphtha, Ether, Octane, Heptane, benzene and many others - ->Mixes very from state to state, winter/summer, and regional atmospheric ->pressure ->There will be a small unknown percentage of water absorbed from the air ->humidity too. ->So your bounds or range for the dielectric constants needs to be expanded, ->they are not single points.
Reply to
Jim Weir
True, pure water is not conductive but water is a polar molecule and has a relativly high dielectric constant. Even a small amount of water can greatly increase the capacitance throughing off your readings. Seems like I rember the dielectric constant of water being about 10 times greater than alchohol.
Reply to
Jimmie
The circuit he is using could be used to determine the amount of H2O present in the alcohol. The main problem with the water is not that it may be conductive but that it has a very high dielectric constant due to it being a polar molecule that can invalidate the readings.
Reply to
Jimmie
concentration
Why is its high dielectric constant a problem?
Scrim
Reply to
Scrim
Because he's deducing the alcohol to gasoline ratio by the dielectric constant of the mixture. Put a 3rd ingredient in there and you have one equation and two unknowns -- so you don't know your ratio any more.
Reply to
Tim Wescott
ratiometric
controlling,
Dielectric constant (DC)of h20 is about 80, gasoline about 2 and his mix of alcohol about 25. Doesnt take much extra water to really skew the results. This may not be problem though as I suspect the alcohol the OP is using normally has a significant amount of water already in it per the quoted DC. a little more may not make a lot of difference.
Reply to
Jimmie

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