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Pumukli
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[*] posted on 15-5-2019 at 09:11
Gas pressure/flow regulator


I've been thinking about building a simple gas chromatograph for org.chem analysis.

I'd use N2 as carrier gas from a high pressure cylinder.

Would a regular N2 pressure reducer on the top of the gas tank be fine as a flow controller or would I need something more elaborate? (Of course, I know the professional equipment is a "bit" more complicated than that, but think of the first chromatographs from the 50-ies!)

My question basically is how could I get a reliable, repeatable slow gas flow in the range of 0.1 - 1 l/min?
With amateur equipment on a strict budget.

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RedDwarf
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[*] posted on 15-5-2019 at 10:29


I suspect it would be difficult at that low flow rate to get a repeatable flow just using a standard regulator (I'm thinking that at 0.1- 1l/min would mean the valve would be close to being closed and as such any minor variation in valve position would result in a significant variation in flow).
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markx
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[*] posted on 15-5-2019 at 10:30


A decent rotameter with an adjustable needle valve after the pressure reducer be perhaps the simplest solution for your problem.

https://en.wikipedia.org/wiki/Rotameter

Just a "garden variety" pressure regulator/reducer alone shall not work reliably to supply a stable flow rate. Especially at low secondary pressures to achieve slow flow rates. There are specialty regulators designed to operate at low secondary pressures, but these are quite rich pricewise. And you need a way to measure and visualize the flow, otherwise it is a shot in the dark.

Trying to use simple pressure regulators (as the kind used for welding gases) to supply a slow flow rate by turning down the secondary pressure is usually going to end badly. There is a bear trap in the construction of cheap pressure reducers: if the bottle pressure is over about 50bar the regulator operates flawlessly at even minute secondary pressures. But as the bottle empties over time and the pressure drops below a critical point, the internal pressure reduction system fails to do the job and dumps the full primary pressure through the system and into the secondary output side. Blown tubes, damaged equipment, empty gas bottle, tears and sobbing shall follow as a result....
I've learnt it the hard way and seen others do the same over the years.

E.g. a glove box in my university chemistry building suffered major damage for exactly the abovementioned reason. A cheap simple pressure regulator was used at the Ar supply cylinder and the secondary pressure was turned down to nothing to supply a slow gasflow into the glovebox. All nice and well until the cylinder emptied to a critical pressure, the regulator "shorted out" and everything was blown apart with an impressive bang.

My own mishap of the same nature happened with a planted tank that used pressurised CO2 enrichment system as a carbon source for the plantlife. I made the classical mistake of trying to regulate the gas flow with the secondary pressure and of cource the same scenario of events followed....CO2 cylinder emptied below a critical pressure, regulator catoed and blew the hoses off the secondary side. Fortunately the break occured outside the fish tank. The cylinder emptied itself into my apartment and not into the fish tank. Otherwise it would have left none of the inhabitants alive. Several of my fellow aquatic enthusiasts were not so lucky and dumped their half empty CO2 cylinders into the fish tanks when the regulators failed. Not to mention it was an extinction level event for the poor inhabitants of the tank...

So long story short: do not use pressure reducers to regulate gas flow directly and do not turn the secondary pressure down to barely noticeable level. Use at least a proper needle valve for the purpose and design the system in such a way that the weakest rupture point would be in a location that does not harm the rest of the system if a pressure peak should happen.




Exact science is a figment of imagination.......
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Pumukli
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[*] posted on 15-5-2019 at 10:44


Thanks for the info on the regulator weekness!

It seems that I should use a needle valve with either a rotameter or a "moving bubble" flow meter after the main regulator to achieve stable flow.

Any other thoughts, suggestions?
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TheMrbunGee
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[*] posted on 15-5-2019 at 13:39


How about hole to limit air flow, like from aerated gas burners, like windproof lighters. This would be super cheap and kind of easy if you have some tools.



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wg48temp9
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[*] posted on 15-5-2019 at 16:59


Two stage regulators tend to have better pressure regulation as the bottle pressure changes as the final regulator is operating in the regulated pressure of the first one.

Most regulators used for tig and mig welding convert the pressure to a flow rate with an orifice . The gauge is even calibrated in flow rate see the example below
https://www.r-techwelding.co.uk/argon-regulator/?utm_source=...

There are also electronic flow controllers (mass flow controller) available but they are hundreds of pounds. Ebay occasionally has used ones for a round £50, Below is a used one for £25.

https://www.ebay.co.uk/itm/Celerity-Unit-UFC-8165-Mass-Flow-...:De4AAOSw1ZNbyqhV

Sorry the board is misinterpreting part of my link. Just search ebay for mass flow controller.







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Sulaiman
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[*] posted on 15-5-2019 at 18:00


I guess that a 'normal' regulator followed by a peristaltic pump would control and meter flow ?



CAUTION : Hobby Chemist, not Professional or even Amateur
(suffering from separation of me and my chemistry stuff)
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Pumukli
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[*] posted on 15-5-2019 at 21:04


What I found so far (besides used mass flow controllers on Ebay) is CO2 reductors combined with a needle valve. They are used in aquagardening to supply a VERY slow flow of CO2 for the plants.
I found one with 250 bar max input pressure, 6 bar max out pressure before the needle valve. The valve can reduce this 6 bar to about 1 bubble/2-3 sec into an aquarium as minimum. (For a GC a higher, steady flow would be required of course.)

I think it would be good enough IF it could be used with N2 too. Maybe the threads are different size/diameter on CO2 and N2 cylinders though. I don't see any chemical incompatibility between the two. What do you think?

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wg48temp9
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[*] posted on 16-5-2019 at 00:43


Quote: Originally posted by Pumukli  
What I found so far (besides used mass flow controllers on Ebay) is CO2 reductors combined with a needle valve. They are used in aquagardening to supply a VERY slow flow of CO2 for the plants.
I found one with 250 bar max input pressure, 6 bar max out pressure before the needle valve. The valve can reduce this 6 bar to about 1 bubble/2-3 sec into an aquarium as minimum. (For a GC a higher, steady flow would be required of course.)

I think it would be good enough IF it could be used with N2 too. Maybe the threads are different size/diameter on CO2 and N2 cylinders though. I don't see any chemical incompatibility between the two. What do you think?



Yes the threads can be different. In general connections to pub type CO2 bottle are female while those on argon nitrogen and oxygen are male. See BS341 for the details: https://www.ftipv.com/bs341-british-standard-cylinder-valve-...

However there are other standards and special connectors. Ebay sells adapters to connect between different threads on bottles and regulators.

Double check your regulator is suitable for your intended bottle. Chinese regulators can be non standard (not to BS341) and have unusual threads on gas outlet connection.

The following link is for for an adapter to connect a CO2 regulator with a female connection to a typical argon or nitrogen bottle with a female thread.

BS compliance tends to be in the UK your country may be completely. different

https://www.ebay.co.uk/itm/Argon-Adaptor-to-fit-a-CO2-Regula...

PS: for safety you have to make certain the regulator is rated for the maximum pressure of the bottle and if you connect the output of the regulator to a closed system the pressure in that system will raise to the maximum output pressure of the regulator. Some used regulators are leaky (do not shut off completely) so when they are connected to a closed system the pressure at the output can raise to the bottle pressure and not all regulators have safety valves or burst discs.





[Edited on 16-5-2019 by wg48temp9]




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Pumukli
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[*] posted on 16-5-2019 at 02:15


Hm, this "thread converter" seems promising!
I'll check the datasheet of the CO2 regulator when I find it. Or ask the aquarium guys about the actual threads. :-)

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[*] posted on 16-5-2019 at 03:12


There is no technical or chemical incompatibility between CO2 and N2 pressure regulators, but the threads can be a pain. There are various standards and often they are incompatible. Also watch for the maximum allowed pressure on the primary side. CO2 has lower pressures compared to N2 which can be 200bar in a full bottle.

I've seen CO2 reductors that upon measuring the threads (diameter/pitch) seem identical, but one fits the bottle valve freely and the other one locks up in about one turn. So go figure...




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Pumukli
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[*] posted on 16-5-2019 at 03:39


Yeah, threads, threads...

I checked with Messergas and they say that (in my country at least)

CO2 cylinders have: Withworth 21.8 x 1/14" right hand threads
N2 cylinders have: Withworth 24.32 x 1/14" right hand threads.

So they are incompatible as is. They need a thread converter.

Incidentally, Ar cylinders have the same thread as CO2 cylinders, they fit. But Ar is a "bit" more expensive than N2... :-)

Btw the above data is true for cylinders under 300 Bar initial pressure. Above that the threads are different.
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[*] posted on 30-5-2019 at 06:25


Couldn't you just get a simple thread adapter made in a machine shop, or make one yourself if you have a lathe and know how to use it?
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[*] posted on 30-5-2019 at 08:15


Yes, thread adapters can be made or bought from e.g. Ebay.
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[*] posted on 30-5-2019 at 09:32


Quote: Originally posted by Sulaiman  
I guess that a 'normal' regulator followed by a peristaltic pump would control and meter flow ?


Badly, but it would work.
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[*] posted on 30-5-2019 at 09:43


Did it occur to anyone to wonder why they use different threads in cheap CO2 regulators that only have to handle the vapour pressure of CO" and the expensive high pressure regulators used for nitrogen?

It's usually a bad idea to bypass safety measures.
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[*] posted on 30-5-2019 at 11:13


Actually lots of CO2 regulators are sold/advertised as CO2/Argon regulator! These tanks (Ar and CO2) have the same type of threads. The pressure gauges on these regulators are usually show scales up to 250 bar!

The thread differencies are based more on the intended usage than vapour pressure. Nitrogen seem to be "a different kind in its own" with other, nitrogen containing mixtures for packaging food. They are called "food gases" and have the same thread.
Toxic gases (H2S, CO, Cl2, etc) have a different thread. Oxygen has its own thread.
At least this is my understanding.
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[*] posted on 30-5-2019 at 12:39


Flammable gases often have a left hand thread.
It's to stop people coupling the wrong things together.
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[*] posted on 30-5-2019 at 15:00


Quote: Originally posted by unionised  
Flammable gases often have a left hand thread.
It's to stop people coupling the wrong things together.


I wounder what would happen if you connected a full oxygen bottle to a full acetylene bottle? I guess it would be a good chance of a very big explosion even if the acetylene bottle was empty but still full of acetone.




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[*] posted on 12-7-2019 at 19:14


Typically proportional valves are used for this type of control. You can often get these on ebay for a couple hundred dollars used. Note the type of gas that it is calibrated for.

For measuring flow there are some relatively inexpensive flow modules from IDT. See FS1012 (https://www.idt.com/products/sensor-products/flow-sensors/fs...) Datasheet is on that page for download.

You might be able to find a linear solenoid valve and work with a microcontroller. Also, you could potentially automate a needle valve, too using that flow sensor for feedback.

I'm often up for collaboration on scientific endeavors, so I can contribute with CAD, software development, and embedded hardware design should you want to pursue this project seriously.
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[*] posted on 12-7-2019 at 20:59


Quote: Originally posted by wg48temp9  

I wounder what would happen if you connected a full oxygen bottle to a full acetylene bottle? I guess it would be a good chance of a very big explosion even if the acetylene bottle was empty but still full of acetone.


J.F.C. :o

I'm imagining when you say bottle you mean tank, but this is a cool idea. I hope no youtubers try it.
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wg48temp9
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[*] posted on 13-7-2019 at 01:54


Quote: Originally posted by happyfooddance  
Quote: Originally posted by wg48temp9  

I wounder what would happen if you connected a full oxygen bottle to a full acetylene bottle? I guess it would be a good chance of a very big explosion even if the acetylene bottle was empty but still full of acetone.


J.F.C. :o

I'm imagining when you say bottle you mean tank, but this is a cool idea. I hope no youtubers try it.


I don't know if your thinking really big or if my sloppy use of terminology has confused you. I should have used "cylinder" as that was the term used in the thread.

On the assumption you don't mean a Challenger tank or a M1 Abrams and you do mean something like the things below, then I hope no youtubers try it too. But if they do I want to see the video.



Sorry for the huge picture blame wiki.





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[*] posted on 13-7-2019 at 05:20


No, now we're at the total opposite extreme!

You are correct that I was confused. I actually meant a high-pressure cylinder, but I am used to welders calling them tanks I guess, or maybe it's a local thing.
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