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Author Topic: Fuel mix how is weight vs volume different  (Read 365 times)

Offline RandySmith

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Fuel mix how is weight vs volume different
« on: December 05, 2018, 02:47:34 PM »
 " Randy would you be good enough to explain the difference between weight and volume, and which is the better?
  Thanks a lot.. "

Fuel has normally 3 ingredients, Methanol   Nitro and  oil. When you mix fuel by liquid volume, the parts are all measured by fluid ounces,  When you mix fuel by weight, you are able to save money because the expensive parts, nitro and oil, are heavier than methanol , which is the cheapest ingredient. So you are using LESS of the two expensive parts, and more of the much less expensive  methanol .
The specific gravity of nitromethane is way higher than methanol and oil is higher also.
Bottom line example of this is a gallon of whaat the fuel company says is 10% nitro is more like  7% , and  the oil they say is 18%, when a boil down test is done,, it is much closer to 14.7 % oil by volume.
This is not a huge problem, as long as you know how it is mixed, knowing that you can adjust as needed, and buy the product that most fits your needs.

Randy

PS  at leaast 1 of the fuel MFG product that was to be 18% oil , was actually 12.5%  , however most  were  14 to 15% oil
« Last Edit: December 05, 2018, 04:35:37 PM by RandySmith »


Offline Dave Hull

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Re: Fuel mix how is weight vs volume different
« Reply #1 on: December 05, 2018, 03:18:30 PM »
I see that Randy has started a new thread to pull out the fuel mix issue from the FP 40 Mods thread. So at the risk of offending someone by dropping my prior post into this one.... Randy has obviously provided the answer. But perhaps my example will help illustrate what we are both saying.

This came up (again) when Gil asked for a tutorial on why measuring by weight was different than measuring by volume. Here was my answer from the other thread.

Here is the refresher course--

The density of the three ingredients are all different. If they were the same, then there would be no difference between a percent by volume mix and a percent by weight mix.

The densities are as follows:  Nitromethane 1.14 gm/cc;  Castor 0.961 gm/cc; methanol  0.79 gm/cc

For reference, the density of water at 25C is 1.00 gm/cc

We assume that the ingredients are not miscible, thereby screwing up volume-after-mix observations or calculations. (edit--I meant to say that the three constituents, when combined, do not sum to a lesser [or greater] volume than the original parts.  I can't remember the correct term for that. Chemistry was a long time ago....)


Example

For a 10N, 22C, 68Al fuel mix by volume, the mix by weight would be 13.22N, 24.51C, 62.28Al. So if you are accustomed to thinking in volume measurements what has been working well for you, you can see that you would have to specify a higher oil content percentage if it were communicated on the basis of weight. Else, you would be a few percent short.

Conversely, if you used your "normal" percentages (volume) but your fuel mixer assumes you meant by weight, because that may be how he does it, then...

A 10N, 22C, 68Al fuel mix by weight, results in a 7.45N, 19.44C, 73.11Al mix by volume

Your engine would not run exactly the same. As you run a lower and lower oil and nitro percentage, this unintended "reduction" might be critical and cause damage.

Note that the alcohol constituent is by far the least expensive of the three. A cynic might say that a fuel mixer would make his product based on weight, but be vague about the basis, because it lets him put a percent for nitro and oil on the bottle that make you feel good, but reduces his costs significantly.

Note that it does not matter how the manufacturer is blending; in other words whether he uses a flowmeter or a scale or a whatever (as long as it is accurate) but we need to know the basis for the percentage given. I think all of us using the fuel assume that the percentage is by volume. But if the peerson blending does not follow this convention, and it isn't stated on the bottle, then we are not going to be happy with the results.

I thought that I could get to the bottom of this by looking up the Manufacturer's Safety Data Sheets (MSDS) which are required for chemical products. Unfortunately, the ones I have looked up did not give the basis of the percentage which surprised me, and further that most are also written giving a huge range in percentage of the ingredients. So they are not useful for our purpose here.

That leaves you with the same evaporative measurement testing that has been described in other posts. Unless you are doing large samples, or have very accurate measuring capabilities, you probably are not learning the true composition of your fuel.

Hope this helps,

Dave
« Last Edit: December 05, 2018, 06:33:50 PM by Dave Hull »

Online Tim Wescott

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Re: Fuel mix how is weight vs volume different
« Reply #2 on: December 05, 2018, 03:32:05 PM »
Randy (or Brett, if you've got your ears on) -- could you describe how to do the boil down test properly?  I've got some fuel that I may or may not have doctored up with extra castor, and I want to make sure.  (It was written on the side of the bottle.  In felt pen...)

Can I weigh a test tube, put in some fuel, weigh, then boil it with a heat gun until it don't boil no more, then weigh, then do some math?  If I'm at the point where it won't boil any more from a typical Monocoat gun, will that have boiled off the nitro?  Should I use my propane torch instead, or oxy-acetylene?   ~^

Brett's described this using measuring cups, but I'm a cheap skate and would like to do this with five or ten grams of fuel (my scale is good to 1/10th of a gram).
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline RandySmith

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Re: Fuel mix how is weight vs volume different
« Reply #3 on: December 05, 2018, 04:30:32 PM »
I see that Randy has started a new thread to pull out the fuel mix issue from the FP 40 Mods thread. So at the risk of offending someone by dropping my prior post into this one.... Randy has obviously provided the answer. But perhaps my example will help illustrate what we are both saying.

This came up (again) when Gil asked for a tutorial on why measuring by weight was different than measuring by volume. Here was my answer from the other thread.

Here is the refresher course--

The density of the three ingredients are all different. If they were the same, then there would be no difference between a percent by volume mix and a percent by weight mix.

The densities are as follows:  Nitromethane 1.14 gm/cc;  Castor 0.961 gm/cc; methanol  0.79 gm/cc

For reference, the density of water at 25C is 1.00 gm/cc

We assume that the ingredients are not miscible, thereby screwing up volume-after-mix observations or calculations


Example

For a 10N, 22C, 68Al fuel mix by volume, the mix by weight would be 13.22N, 24.51C, 62.28Al. So if you are accustomed to thinking in volume measurements what has been working well for you, you can see that you would have to specify a higher oil content percentage if it were communicated on the basis of weight. Else, you would be a few percent short.

Conversely, if you used your "normal" percentages (volume) but your fuel mixer assumes you meant by weight, because that may be how he does it, then...

A 10N, 22C, 68Al fuel mix by weight, results in a 7.45N, 19.44C, 73.11Al mix by volume

Your engine would not run exactly the same. As you run a lower and lower oil percentage, this unintended "reduction" might be critical and cause damage.

Note that the alcohol constituent is by far the least expensive of the three. A cynic might say that a fuel mixer would make his product based on weight, but be vague about the basis, because it lets him put a percent for nitro and oil on the bottle that make you feel good, but reduces his costs significantly.

Note that it does not matter how the manufacturer is blending; in other words whether he uses a flowmeter or a scale or a whatever (as long as it is accurate) but we need to know the basis for the percentage given. I think all of us using the fuel assume that the percentage is by volume. But if the peerson blending does not follow this convention, and it isn't stated on the bottle, then we are not going to be happy with the results.

I thought that I could get to the bottom of this by looking up the Manufacturer's Safety Data Sheets (MSDS) which are required for chemical products. Unfortunately, the ones I have looked up did not give the basis of the percentage which surprised me, and further that most are also written giving a huge range in percentage of the ingredients. So they are not useful for our purpose here.

That leaves you with the same evaporative measurement testing that has been described in other posts. Unless you are doing large samples, or have very accurate measuring capabilities, you probably are not learning the true composition of your fuel.

Hope this helps,

Dave

Hi Dave

Excellent post, thank you,  I did not  pull it from the 40 post, I had a member send to me, a PM asking this question, I chose to answer it in the  engine column , Glad you posted yours here too. 

and  Tim  This is how I do the test, I have a 100ml  glass Pyrex container, I put in 100ml of the  fuel, then I take it outside, OUTSIDE .. and put it in a pot, that sets on a hot plate, I boil the water in the pot, then put in the  100ml beaker, stir as it boils untill all that will  boil out is  gone, then see what is left, 15ml would be 15% oil  20 ml left  would be  20% oil

I assume if you have a tremendously accurate  scale that measures parts of a gram, would maybe be able to tell you what amount of oil is in your fuel, You would need to know  exactly what the Nitro percent is, and you need to know the  specific gravity weight of the oil in your batch, then do the  math of the other components, you should be able to get an answer
However I think the boil down test  would be much easier, and  you do not  have to use a  full 100ml

Regards
Randy

PS  TIM  do not  use  a torch, open flame...not good..  Use the  heat gun It will  boil off  all  the nitro and methanol

Online Tim Wescott

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Re: Fuel mix how is weight vs volume different
« Reply #4 on: December 05, 2018, 04:55:31 PM »
The part about the open flame was a joke.  Really.  Honest.

And I was thinking I could do it by weight, but was forgetting that pesky bit about the nitro being denser.  I suppose it's better to be half a cup shy of fuel and actually know what you're running through your engine than to be running Mystery Fuel.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Dave Hull

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Re: Fuel mix how is weight vs volume different
« Reply #5 on: December 05, 2018, 06:28:21 PM »
The method you use has to acknowledge the things that you don't know, or don't know accurately, or the limits of your ability to measure. An evaporation test has advantages since it is a differential measurement, potentially reducing errors. That said, the issue of accurate knowledge of the ingredients and their specific gravity is troublesome, especially as regards the types of oils. Fuel that has absorbed some water, or was made with "less than anhydrous" alcohol will over-report the oil content as well. I don't think this would be significant.

I took a look at simply using a specific gravity kind of test just to see how sensitive that would be. It turns out that the difference in weight of one gallon of 10N-22C-68AL vs. 10N-28C-62AL is about 35 grams. For a pint sample, it would only be 4.9 grams. (A pint is 473 cc, or nearly a full 500ml graduate).  This works out to about 1.2%, so you would need to be working with accuracies of less than 1% for this to be meaningful. Not sure how accurately one can read the meniscus on a fat graduate like the 500. Also need to verify that your gram scale has the accuracy claimed over a large range, since this type of measuring is at both ends--as much as most scales can take, and as little as it will measure.

Dave

Online Tim Wescott

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Re: Fuel mix how is weight vs volume different
« Reply #6 on: December 05, 2018, 06:31:17 PM »
Dave, for my purposes I just need to know if I'm working with fuel that's 20%, 25%, or 29% oil.  If I'm in that ballpark, I'm good.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Dave Hull

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Re: Fuel mix how is weight vs volume different
« Reply #7 on: December 05, 2018, 06:36:46 PM »
Tim,
What is your oil type and mix?

Dave

Online Tim Wescott

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Re: Fuel mix how is weight vs volume different
« Reply #8 on: December 05, 2018, 07:30:36 PM »
Tim,
What is your oil type and mix?

Dave

I've got some all-castor jugs, and a couple of jugs that are all synthetic RC fuel + castor.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline RandySmith

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Re: Fuel mix how is weight vs volume different
« Reply #9 on: December 05, 2018, 08:27:09 PM »
Dave, for my purposes I just need to know if I'm working with fuel that's 20%, 25%, or 29% oil.  If I'm in that ballpark, I'm good.

Hi Tim   use the  boil down method  and  stir  the  fuel  to help get all the  water and  alcohol out

PS  I knew you were  teasing, that was  why my PS  :=)

Offline Brett Buck

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Re: Fuel mix how is weight vs volume different
« Reply #10 on: December 05, 2018, 08:43:15 PM »
The part about the open flame was a joke.  Really.  Honest.

    You can do it that way, outside, presumably. But as Randy notes, it doesn't work nearly as well as boiling it - you never seem to be able to burn it all off without also boiling it to get the volatile compound vaporized. But once you do that, it just evaporates without burning it. Everytime I did a burn-off, I got a higher oil content than with boiling, probably because it never got rid of all the nitro and methanol.

    Brett

   

Offline Dave Hull

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Re: Fuel mix how is weight vs volume different
« Reply #11 on: December 05, 2018, 10:31:41 PM »
Tim,

With careful measurement, you should be able to tell the 20% all-castor from the 25% using just a 4 oz. sample. The delta is about 5.7 grams. If the accuracy of your scale is <0.5% of full scale (based on a 500 gm capacity) then the error would be less than 2.5 gm. It is near zero endpoint, so nulling the scale should improve that. The rest of the error can be allocated to the volume measurement. You need something a lot better than a 5 fl oz "horse syringe" because you are trying to differentiate between 0.8 fl oz of oil and 1.0 fl oz, or 0.2 fl oz. So you have to be able to measure the initial sample, say, 10 times more accurately than that. In other words, down to 0.02 fl oz or 0.6 ml. Of course, we don't really know how complete the evaporation process is, which is another error source.


10N-20C-70AL --->  22.74 gm castor (23.66 ml) in a 4 fl oz sample

10N-25C-65AL --->  28.42 gm castor  (29.57 ml) in a 4 fl oz sample

Will be curious to see how your test turns out.

Dave

Offline Motorman

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Re: Fuel mix how is weight vs volume different
« Reply #12 on: December 06, 2018, 07:36:25 AM »
Properly labeling everything in the shop and knowing it's right has it's advantages.


Motorman 8)
There will be a sunny day and we will fly our airplanes.

Offline Dave Hull

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Re: Fuel mix how is weight vs volume different
« Reply #13 on: December 06, 2018, 11:48:17 AM »
Mr. Motor,

What are you using to label your plastic jugs?  I have been cleaning with alcohol, then using a wide Sharpie pen, followed by taping over it with clear packing tape. It seemed to work until recently, when I lost track of the Fox fuel and some other because the ink seemed to have washed away. My next thought is to wire tag a card to the jug handle. But if you have a better system....

Divot

PS--I found out my mistakenly switched fuel jugs when my Fancy Pants won the unofficial Fox Sport Speed non-contest recently. It sure cleaned up the varnish, though....

Online Tim Wescott

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Re: Fuel mix how is weight vs volume different
« Reply #14 on: December 06, 2018, 02:13:47 PM »
Tim,

With careful measurement, you should be able to tell the 20% all-castor from the 25% using just a 4 oz. sample. The delta is about 5.7 grams. If the accuracy of your scale is <0.5% of full scale (based on a 500 gm capacity)...

It's a 5-ounce scale (so, 150gm) that I got when I was building indoor freeflight stuff.  Or dealing meth.  Dang, I can't remember.  It involved white powder and funny smells and money flowing away from me.  Oh wait -- money flowing away -- musta been modeling, then.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline RandySmith

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Re: Fuel mix how is weight vs volume different
« Reply #15 on: December 06, 2018, 05:45:32 PM »
Mr. Motor,

What are you using to label your plastic jugs?  I have been cleaning with alcohol, then using a wide Sharpie pen, followed by taping over it with clear packing tape. It seemed to work until recently, when I lost track of the Fox fuel and some other because the ink seemed to have washed away. My next thought is to wire tag a card to the jug handle. But if you have a better system....

Divot

PS--I found out my mistakenly switched fuel jugs when my Fancy Pants won the unofficial Fox Sport Speed non-contest recently. It sure cleaned up the varnish, though....

I have better  luck writing on the  cap with a thin sharpie

Randy


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