Deuterium Orifice Size

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Tom McCarthy
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Deuterium Orifice Size

Post by Tom McCarthy » Mon Jan 05, 2015 6:24 pm

Hi all,

First post in a while - Fusor is coming along very well, just haven't found the time to make a post on my blog or in Images Du Jour.

I'm about to purchase a lenoxlaser.com laser drilled conflat orifice for my Fusor. However, I'm not sure what width (microns) is appropriate. I've used their calculator (found on the website's sidebar), but I'm unsure of one of the variables - Inlet pressure. As the orifice will be very small regardless, I'm assuming my turbo pump and rotary vane pump won't have much of an effect on the pressure of the deuterium line. So, the pressure in the deuterium line shouldn't be too affected by that. Does anybody have a good idea of what pressure (roughly, I know this can vary a lot) I should be setting my regulator to when allowing Deuterium into the line? I have a 12" diameter spherical chamber and an Edwards RV3 and Oerlikon Leybold Turbovac 151 turbo pump - This can pull it down to 10^-10 torr, although I'll never be going down that much and pumping for Hydrogen is 115l/s.

If anybody can give some advice on the pressure in the deuterium line I'd really appreciate it.

Cheers,
Tom

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Rich Feldman
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Re: Deuterium Orifice Size

Post by Rich Feldman » Mon Jan 05, 2015 7:13 pm

Well, this is not from someone with fusor experience.

You are regulating and gauging deuterium pressure with respect to the atmosphere.
Moving from 0 to a few psi, which is typical for fuel gases fed to a torch,
produces little relative change in the differential pressure across the orifice.
If you can turn the regulator from 0 up to 15 or 30 or 60 psi,
the orifice differential pressure goes from 1 to 2, 3, or 5 atmospheres. Verstehen Sie?
Beware of un-clamped plastic hoses popping off of barbed fittings and rapidly wasting expensive D2.

Find a formula or online calculator for orifice flow. MW is the same as helium!
How does the mass flow vary with absolute inlet pressure, as outlet is held at vacuum?
All models are wrong; some models are useful. -- George Box

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Re: Deuterium Orifice Size

Post by Tom McCarthy » Wed Jan 07, 2015 9:59 pm

Rich, I'm not 100% following what you said, so I'm going to try and paraphrase it to see if I've got your gist.

I'm gauging the pressure of the deuterium with respect to atmosphere, yes. Regulating too, by this you mean that I'm changing the pressure of the deuterium by basing measurements of atmospheric pressure? I'm having a bit of trouble with your next point, but I think I have it - Before regulation, the pressure in the deuterium line is atmospheric. By introducing deuterium, the pressure rises, aye? On the orifice's differential pressure: Pressure difference between inlet and outlet, am I correct? If the outlet is high vacuum, it's pretty much 0 on that side and by having x bar pressure at the inlet, the differential is x bar.

My understanding isn't very sound, but what I can infer from your post is that the differential across the orifice is essentially the deuterium line's pressure. However, this doesn't really get me closer to what that pressure should be, under optimal circumstances (which we never seem to have!).

There's a calculator on the lenoxlaser.com site for calculating orifice width when you know the flow. I think Chris Bradley stated his flow was ~5 SCCM, but I could be wrong - This gives a width of 15 microns if I take outlet pressure to be .0001 PSI absolute and inlet pressure to be 45 PSIG (~60 PSI abs.) and using a calculator at tlv.com ...But this was for air which has a MW of 28.97, rather than 2.01. Can anybody with some experience lend some insight on this?

My regulator was in a box as I haven't got my deuterium yet, so I checked it out to see what the outlet range for it is. It can output at from 0-200 PSI. There's a lot of variance there though, so it doesn't really get me anywhere...I'll make sure to take care with gas fittings when the time comes.

Thanks for the help, Rich.

Cheers,
Tom

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Rich Feldman
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Re: Deuterium Orifice Size

Post by Rich Feldman » Thu Jan 08, 2015 12:04 am

Tom, I composed a long reply & then just lost it by clicking wrong. :-(
Let's try to be briefer and faster this time.

Adjustment knobs are good for people like us. Especially on experimental fusors.
Chamber pressure of deuterium (in microns) is set by equilibrium between supply flow (in sccm) and vacuum pumping speed.
The design of your rig can make one or both of those adjustable.

Orifice design formulas or calculators can teach you that volume flow
is proportional to orifice area and to the absolute pressure upstream.
The absolute pressure downstream, if less than around 1/2 atmosphere, has no effect. (See "choked flow".)

It's often easier to adjust the inlet pressure than the orifice diameter.
So if you are prepared to turn the regulator setting anywhere between 1 psig and 45 psig,
you have control within a 1:4 range of absolute pressure and, hence, volume flow into the vacuum.
I think that for air and 15 micron diameter, the sccm knob would go from about 2 to about 8.

Let's leave it to you to pick a flow range and figure the orifice diameter for deuterium.
Gas molecular weight matters much more than likely variations of absolute temperature or adiabatic gamma.
You got the MW of deuterium wrong. Hint: the orifice calculator at lenoxlaser site has a menu of common gases, including air but not deuterium. The best substitute in their list is not hydrogen.
All models are wrong; some models are useful. -- George Box

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Re: Deuterium Orifice Size

Post by Tom McCarthy » Sun Jan 11, 2015 6:02 pm

Ok, 2-8 SCCM sounds pretty good.

I know where I went wrong - deuterium's MW is 2ish but deuterium gas is 4 as it's D2.

Using the Lenox calculator:
Inlet pressure = 15 PSIA (1 atm)
Outlet pressure = Anything under at least 10 PSIA has no effect on calculations.
Gas type = Helium (4)
Temperature = 10 degrees Celsius
Flow rate = 2 SCCM

Orifice diameter = 9.0925 microns.

With this I can turn the regulator up/down to 60 PSI and can vary between 2-8 SCCM - Up to 60PSI that is.

Thanks Rich,
Tom

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Rich Feldman
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Re: Deuterium Orifice Size

Post by Rich Feldman » Wed Jan 14, 2015 10:23 pm

Sounds good, Tom.

In your shoes I would get a second opinion about orifice diameter, before shelling out $31 + shipping for a tiny bit of metal foil,
or calling in a favor from a sponsor. There are other online calculators, formulas, self-professed experts at forums, and customer service from orifice vendors. Carl Willis on fusor.net once named a value not wildly different from yours.
Did you see Chris Bradley's FAQ about flow control? viewtopic.php?f=6&t=3224

Here's a simple, quantitative flow test anyone can do after mounting orifice (or capillary) to impede flow between larger tubes.
Apply vacuum to outlet side. Put a hose on the inlet side, leading to a narrow clear tube you can dip into water or oil. Liquid should slowly rise in the tube, at a rate equal to the sccm flow of air at 15 psia. The standard volume flow through an orifice, and not coincidentally the speed of sound, will change in proportion to inverse square root of the gas molecular weight.
All models are wrong; some models are useful. -- George Box

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Re: Deuterium Orifice Size

Post by Tom McCarthy » Mon Jan 19, 2015 1:06 pm

Carl quotes a 4-6 micron orifice for a 1 SCCM flow in this post (http://fusor.net/board/viewtopic.php?f= ... ccm#p18129), so I'm in the ballpark area for a 2SCCM flow (I think!).

I have read Chris's FAQ, but didn't look for it again, very helpful to go back over.

Currently bolting up my vacuum chamber and getting my roughing pump set up - when I get my turbo-pump mounted and an orifice delivered I'll borrow a graduated cylinder or something similar from school and check out your experiment, Rich.

Would anybody with some experience in orifice matters like to chime in with in any advice on my calculations?

Cheers,
Tom

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