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Author: Subject: Testing a fume hood?
Draeger
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[*] posted on 16-8-2020 at 06:49
Testing a fume hood?


So, I've finally built myself a fume hood. But I don't quite trust working with more toxic chemicals in it yet. How can I test it? Does boiling ammonia and not smelling anything except a hint of it safe enough?



Collected elements:
Al, Cu, Ga, C (coal), S, Zn, Na

Collected compounds:

Inorganic:
NaOH; NaHCO3; MnCl2; MnCO3; CuSO4; FeSO4; aq. 30-33% HCl; aq. NaClO; aq. 9,5% ammonia; aq. 94-96% H2SO4; aq. 3% H2O2

Organic:
citric acid, sodium acetate, sodium citrate, petroleum, mineral oil
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JJay
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[*] posted on 16-8-2020 at 11:11


The test usually used for demonstrations is dry ice and water.

You actually shouldn't smell even a hint of ammonia. You'll want to calculate the velocity of the air going in when the sash is raised and check it against current safety standards, such as these: http://www.newtechtm.com/aspshtml/aspsnfpa45.html

There are also specific standards that apply to things like perchloric acid. The standards are constantly changing.
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CouchHatter
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[*] posted on 16-8-2020 at 18:05


You can use a vaneometer to measure the face velocity at several points on the sash. They aren't too expensive compared to their electronic counterparts.

vaneometer.jpg - 30kB
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macckone
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[*] posted on 17-8-2020 at 13:04


you can use a spray bottle on mist setting (not all have a mist setting).
If the droplets move toward the sash from outside you are good.

Another test is using amyl acetate (banana scent/flavor).
Place an open tube at the sash, if you can smell it then you need better air flow.

The third way is plastic streamers.
They should be substantially pulled into the fume hood.
Many labs have these permanently attached to the ends of the sash as a ready visible indicator that the hood is working.
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aromaticfanatic
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[*] posted on 17-8-2020 at 14:02


You could also look into getting a smoke bomb. Light it off and look for colored smoke leaking.
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Fyndium
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[*] posted on 17-8-2020 at 23:48


Sounds a bit complex. I think you should actually feel by hand the air flowing. Even just using a match or candle flame can show you the direction of the flow. I have 800m3 duct fan and at full power the suction can be felt by hand even when the hatch is wide open. It replaces the entire air of the fume hood every 3 seconds, basically.

Lighting up a smoke bomb indoors that produces thousands of m3 of impermeable smoke doesn't sound like a good idea.
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outer_limits
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[*] posted on 18-8-2020 at 00:18


Checking the airflow is one thing.
The other is to be sure that there are no leaks.
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Fyndium
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[*] posted on 18-8-2020 at 05:40


The negative pressure from suction makes leaks only flow into the cabinet.
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macckone
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[*] posted on 18-8-2020 at 07:42


Leaks aren't usually a concern with a negative pressure fume hood.
They can however be a concern if a reaction goes very wrong, fire, overwhelming vapor generation, fan failure, etc.
Leaks can be dealt with using silicone sealant and duck tape.

and trivia:
http://www.todayifoundout.com/index.php/2010/02/duct-tape-wa...
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outer_limits
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[*] posted on 18-8-2020 at 08:07


Quote: Originally posted by Fyndium  
The negative pressure from suction makes leaks only flow into the cabinet.


But besides the hood there is also a ventilation duct and blower.
If blower is on the end of the duct making negative pressure in whole system there shouldn't be any issues. Even if there is small hole everything will be sucked.

There is also a possibility that the blower is positioned just after the hood. In this case some nasty vapours could contaminate the room because they will be blown out of the ducting.
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Herr Haber
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[*] posted on 18-8-2020 at 11:54


Quote: Originally posted by Fyndium  
The negative pressure from suction makes leaks only flow into the cabinet.


What's negative pressure ? ;)

But yeah, and some of those leaks would be normal wouldnt they? A sink or any liquid outlet for example.




The spirit of adventure was upon me. Having nitric acid and copper, I had only to learn what the words 'act upon' meant. - Ira Remsen
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Frankenshtein
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[*] posted on 9-9-2020 at 16:37


For smoke tests, superior smoke is a good brand and here's another brand. You could make them yourself with potassium nitrate (some stump removers) and sugar. The King of Random has a video on this on youtube. But how good your fume hood is also depends on its construction and how it protects you and others. Measure out the area you're exhausting to (realizing it's not going to mix with air evenly) and the amount of gas youll generate, to calculate the ppm and see if you should adjust something to achieve a lower ppm and the level of safety you'd expect from others. Also, Ive thought of making something smelly in it after turning it on and seeing if I can smell it, as an extra test. A fume hood is just a fume hood and every chemical you work with has special safe handling needs. If you cant prove youll survive the kind of work you plan to do with this fume hood youve made, then you shouldnt continue until then.

Some other info:
The "FlinnScientific" channel on youtube has educational videos, some covering fume hoods and ventilation. I found them very insightful. I believe they say "suck, dont blow" (put the fan at the end of the ducting like other members are saying, not right on the fume hood). I read about smokestacks the other day, they're great and the gases are meant to be exiting the smokestack at at least 3,000 fpm according to this pdf, which is around 35 mph. To gauge, your average leaf blower is much stronger than this.



[Edited on 10-9-2020 by Frankenshtein]
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