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Discussion Starter #41
Andy if you had of said "bottle pressure 900psi" it would have made sence but you said "bottle temp 25 degrees"

now how does that tell anyone anything apart from the temp of the bottle?????


i think everyone reading this will make their own minds up as to what you ment by injecting gas but i think you ment exactly that?
 

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The relationship betwwen bottle preasure is directly proportional to bottle temprature....needs no more explaining.
I am happy for other peoples input and opinions, as long as they try to explain their position....
Regards, Andy.
 

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T.F.S.
When you say "so whats the pressure when a bottle of unknown content is at 25 degrees then?"
In what way do you mean unknown, truly unknown, or N2O, but amount unknown?
For instance take a bottle that can hold 10lb's of N2O, that bottle is not 'full' it could hold slightly more, but there is always room for a head of gassed N2O, and it is the preasure from that gas that keeps the rest in it's liquid state.
The actual preasure of that gas is directly proportional to it's temprature, and will remain so untill all the liquid has gone.
I'll look for a chart to paste on here, I am sure there is one...
As a PURE GUESS, at 10 degrees it will be say 600 PSI, and at 30 degrees it will be more like 950 PSI, and as above remains so untill all liquid has gone.
I hope that helps you understand, I am not the best teacher, but at least I take the time to try...
Regards, Andy.
 

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andy42uk said:
As a PURE GUESS, at 10 degrees it will be say 600 PSI, and at 30 degrees it will be more like 950 PSI, and as above remains so untill all liquid has gone.
Thats wrong!
 

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Discussion Starter #47
yes thats right andy the bottle pressure changes constantly (as pikey knows very well)






the point i was making was that bottle temp is nothing...what we need is pressure!, if you jet via temp your jetting is totally out

one day you can have 25 degrees and you are at 800psi but the next day the same temp could see 600psi due to bottle content (or even 0psi because its empty)

if what you say is true you would have perfect fuelling all the way to the last drip of the bottle but sadly this isnt the case im afraid

BTW pressure only tells us exactly that.........nitrous oxide is a liquid and as liquids cant be compressed you dont know how much is in there via pressure either, you have to weigh the bottle to find that out
 

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Ok that's enough from me, you are happy with how you see things, that is cool, no problem.
The varience in preasure you see at constant ambient temps. is due to useage, as N2O liquid leaves the bottle, the space is taken up by N2O that turns to gas, and as in refidgeration this cools the bottle, but very quickly, so you need a good fast thermocouple to see it happen, and the preasure as direct effect reduces.
That is why it is hard to get the fuel/N2O mixture constant, and back to the beggining, why using Nitrogen to fill that space in the bottle is a good idea...
if you do not understand fine, I am happy to leave it untill someone else has constructive input.
Regards, Andy.
 

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Pikey said:
andy42uk said:
As a PURE GUESS, at 10 degrees it will be say 600 PSI, and at 30 degrees it will be more like 950 PSI, and as above remains so untill all liquid has gone.
Thats wrong!
Which bit, the pressures stated or the "remains so until all liquid has gone" bit?

Regarding the actual pressures quoted, he did say "pure guess".

It's ages since I studied phyics so I'm pretty rusty on these things but I have made a few observations here.

Firstly, the boiling point of a liquid is where the vapour pressure equals the ambient pressure. Therefore, the lower the pressure the lower the boiling point (which is why you can get water to boil at room temperature if you put some in a vacuum flask and swich on the vacuum pump).

If you have a half full bottle of N2O at room temperature and (miraculously) atmospheric pressure, the temp is way above the boiling point at this pressure so some of the liquid boils, thus raising the pressure in the bottle. It also reduces the temperature due to latent heat of evaporation, so the vapour pressure drops. Eventually, the pressure will keep rising and the temperature will keep dropping until the vapour pressure and ambient pressure are the same.

However the bottle by now will be quite cold and will start to warm up by drawing heat from its surroundings. The pressure in the bottle will then start to rise further (Boyle's law: p1v1t1 = p2v2t2). I don't know whether some of the gas will condense back to liquid form as a result of this new higher pressure, thus lowering the pressure again (slightly) ... ?

Eventually the pressure and temperature will stabilise.

The vapour pressure is higher at raised temperatures so you would still expect a bottle at 25C with stabilised contents to have a higher pressure than one at 15C with stabilised contents, hence Andy's argument that bottle temperature has a direct effect on bottle pressure.

At the point some of the N2O is used, the pressure drops in the bottle. This causes some more of the liquid to evaporate, lowering the temperature again. More liquid boils, thus raising the pressure again. However, as it's now colder pressure is reduced for a while until the N2O temperature is raised back to its original level.

It therefore appears that, as I think Andy said, temperature does directly relate to bottle pressure but seeing as maintaining a constant bottle temperature seems very difficult in practice, using temperature to maintain a constant pressure seems fraught with problems.

Therefore, as T.F.S. says (and Andy agrees), using Nitrogen to provide constant pressure seems the wiser choice.

Maybe T.F.S. can confirm - if you're trying to use temperature to control N2O pressure, does delivery start out OK and then drop off a bit, resulting in a correct mixture initially but a rich one thereafter? If that's the case, then it also suggests that if it takes a few seconds of running to measure the mixture, you may unwittingly be setting up fuel flow to be correct for the 'dropped off' level, thus making the engine run lean for the first few moments of hitting the gas. Maybe this is where some of the problems stem from?

I'm sure this is simple stuff for Andy and T.F.S. but it's how I see it.
 

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Kingsley,
That's better, I didn't formaly study, and I know I do have problems getting things across, your explanation is perfect, and restores my faith a little.
THANK YOU.
Regards, Andy.
 

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Discussion Starter #51
cant argue with that!


when pressure returns the gas will indeed change phase into liquid

as for jetting via temp its just not anywhere near as good as jetting via pressure, we can fit a pressure guage and monitor pressure all the way down the strip but we cant do that with temp
(and as we have said its far better to have a cold bottle than a hot one)

you will also see massive temp difference across the bottle, the top can be hot to the touch and the bottom cold
(after refill)

the pressure will still be constant at this point though
 
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