Getting an exact amount of Methane out of a cylinder

[mkbent]

TL;DR

I want to fill a tedlar bag with an exact amount of methane from a cylinder.

Hello,

I am currently working on a research project that involves filling a 10L tedlar bag with about* 8 liters of methane gas from a compressed gas cylinder. I'm trying to work out the best way to fill the bag with a reasonable degree of accuracy while also minimizing (or hopefully avoiding altogether) the leakage of methane into the air. I do have access to mass flow controllers that maybe we could calibrate and use, but was wondering if there was a more precise way of filling them since the mass flow controller would fill the bag for a bit with lower flow while it gets up to the set point. I also could use a 1L gas syringe to fill the bag manually... but I am going to be filling a lot of these bags and that would give me a bit more of an arm workout than I was hoping to get for the next few months. Does anyone know of a better solution than either of these two methods?

Thanks,
Mike

 

[berkeman]

Welcome to PF.

Summary: I want to fill a tedlar bag with an exact amount of methane from a cylinder.

involves getting filling a 10L tedlar bag with exactly 8 liters of methane gas from a compressed gas cylinder.

8L at what temperature and pressure? How will you control the temperature?

https://www.supplymylab.com/Supplie...CMP7jkK-M8Wto6fvWYTO17NbF_hxA9UwaAm-TEALw_wcB

 

[Lnewqban]

Welcome!
Could you place the empty bag inside a graduated recipient and measure the increase of liquid level equivalent to a volume of 8 liters?

 

[mkbent]

Welcome to PF.8L at what temperature and pressure? How will you control the temperature?

View attachment 312949
https://www.supplymylab.com/Supplie...CMP7jkK-M8Wto6fvWYTO17NbF_hxA9UwaAm-TEALw_wcB

Room temperature and around an atmosphere of pressure. I should probably not have used the word exactly, as we don't need it to be incredibly precise. +/- 5% would be just fine I think.

 

[mkbent]

Welcome!
Could you place the empty bag inside a graduated recipient and measure the increase of liquid level equivalent to a volume of 8 liters?

That's a pretty good idea if I can find a graduated recipient big enough, thanks for the help!

 

[Lnewqban]

That's a pretty good idea if I can find a graduated recipient big enough, thanks for the help!

It could be any recipient with regular shape, if you can calculate the proper level differential for 8 liters or 8000 cubic centimeters, or 488 cubic inches.

 

[mkbent]

It could be any recipient with regular shape, if you can calculate the proper level differential for 8 liters or 8000 cubic centimeters, or 488 cubic inches.

True I could just put marks in a trash can for 22 L and 30 L and technically would be fine. Gotta see if my boss / the sponsors would be OK with it but it seems cheap fast and easy.

 

[hutchphd]

It could be any recipient with regular shape, if you can calculate the proper level differential for 8 liters or 8000 cubic centimeters, or 488 cubic inches.

Or make two marks: one before you add the measured extra 8 liters of water and one after !

 

[mkbent]

Or make two marks: one before you add the measured extra 8 liters of water and one after !

That's what I was saying, thanks again for the idea!

 

[hutchphd]

If you really only need 5% precision then you could measure the buoyancy in air. At STP 8 liters is $\frac 8 {22.4} mole$ and methane has a molecular mass of 16 while dry air is 28.97. So you would have about 4 grams buoyancy to work with.

 

[Baluncore]

This is how I would do it.
1. Set up an 8 litre clear bottle in a water tank, open neck at the bottom, two valves at the top. One top valve is for methane supply, the other for a tedlar bag.
2. Sink bottle into water tank with a top valve open, it fills with water through the neck.
3. When the bottle is full of water, close the top valve.
4. Open the methane supply valve at the top, as the bottle fills with methane, water is pushed out through the neck at the bottom. Raise the bottle as it floats up.
5. When water is at the 8 litre mark inside the neck, and water outside is at the same level, then methane is at atmospheric pressure in the bottle.
6. Connect tedlar bag to top port, open valve to bag, sink the bottle pushing all 8 litre of methane into the bag.
7. Close top valve as bottle is now full of water, ready for next methane charge.
8. Seal and remove the previous tedlar bag.
9. Go to step 4, filling the bottle with methane for the next bag.