homeGeek CultureWebstoreeCards!Forums!Joy of Tech!AY2K!webcam

The Geek Culture Forums


Post New Topic  New Poll  Post A Reply
my profile | directory login | | search | faq | forum home
  next oldest topic   next newest topic
» The Geek Culture Forums   » Techno-Talking   » Science!   » a box question (Page 1)

 - UBBFriend: Email this page to someone!  
This topic comprises 2 pages: 1  2 
 
Author Topic: a box question
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 04, 2007 07:17      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
Imagine a box.

It has a balance in it.

It has a hole in the top

It is filling from the bottom with Argon.

Argon is heavier than air so the argon is pushing the air out the top.

The question is as the box fills with argon, does the zeroed balance show an increase.

Some argue that it does not as the box has a whole in the top, there is no way for air pressure to build up in the box.

I say it does because the argon is heavier that the air there is more weight to push down on the balance.

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged
TheMoMan
BlabberMouth, a Blabber Odyssey
Member # 1659

Member Rated:
4
Icon 1 posted October 04, 2007 08:52      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Ashitaka ___________________ I need a little more info. is the balance double sided much like lady Justices balance. Or is it single sided balance like a step on scale?

Now for the kicker it doesn't make any difference, as the pressure below the scale platform will be equal to the pressure on the platform because the gas can flow around the scale.

If the scale would be disrupted then why doesn,t barometic pressure change our weights?

--------------------
Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.


Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 04, 2007 09:42      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
 -

It has this balance in it. It is a load cell (resistance of the cell changes as pressure is applied.) If the pressure in the box increases the resistence in the load cell would change because it is being pushed on from all sides. An old fashioned hanging balance would not have this problem.

The basic argument is wether the hole in the top prevents a higher pressure in the box. I say it doesn't. A coworker says it does.

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 04, 2007 09:55      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by TheMoMan:
orm because the gas can flow around the scale.

If the scale would be disrupted then why doesn,t barometic pressure change our weights?

I say barometric pressure does not change our wieght but on a modern electric load cell would add or subtract the change of barometric pressure from the weight displayed.

If I weighed a kilogram on a load cell, if the barometric pressure changed, unless i rezeroed the load cell, the weight of the kilo would be off.

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged
Xanthine

Solid Nitrozanium SuperFan!
Member # 736

Member Rated:
5
Icon 1 posted October 04, 2007 09:58      Profile for Xanthine     Send New Private Message       Edit/Delete Post   Reply With Quote 
I think MoMan's right. Even if you're thinking of a nice, new, modern digital lab balance, the apparatus itself is not airtight (I looked). As you evacuate the box, the air within the balance is also getting replaced with argon and it zeroes out on either end.

ETA: I agree with your co-worker.

--------------------
And it's one, two, three / On the wrong side of the lee / What were you meant for? / What were you meant for?
- The Decemberists

Posts: 7670 | From: the lab | Registered: Mar 2001  |  IP: Logged
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 04, 2007 10:14      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Xanthine:
I think MoMan's right. Even if you're thinking of a nice, new, modern digital lab balance, the apparatus itself is not airtight (I looked). As you evacuate the box, the air within the balance is also getting replaced with argon and it zeroes out on either end.

ETA: I agree with your co-worker.

Imagine then a empty fish tank and a fish tank full of water.


If you have the same load cell in each fish tank, will the resistence of that load cell for a given voltage be the same in each tank?

I say no, I say the pressure is greater under the heavier water than the less heavy air. I also contend I have a similar situationin my argon box. AS the heavy argon replaces the air (albiet on all sides of the load cell) the pressure in the box increases.

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged
GrumpySteen

Solid Nitrozanium SuperFan
Member # 170

Icon 1 posted October 04, 2007 10:35      Profile for GrumpySteen     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Ashitaka:
Imagine a box.

It has a balance in it.

It has a hole in the top

It is filling from the bottom with Argon.

Argon is heavier than air so the argon is pushing the air out the top.

The question is as the box fills with argon, does the zeroed balance show an increase.

Some argue that it does not as the box has a whole in the top, there is no way for air pressure to build up in the box.

I say it does because the argon is heavier that the air there is more weight to push down on the balance.

You're right, but you owe royalty fees now

--------------------
Worst. Celibate. Ever.

Posts: 6364 | From: Tennessee | Registered: Jan 2000  |  IP: Logged
Xanthine

Solid Nitrozanium SuperFan!
Member # 736

Member Rated:
5
Icon 1 posted October 04, 2007 10:43      Profile for Xanthine     Send New Private Message       Edit/Delete Post   Reply With Quote 
You sure?

We all agree that what a blance is really measuring is the amount of force on a pan. Gas of any kind will exert foce on the pan in the form of pressure, but if there's pressure on the opposing side, it neutralizes. Furthermore, pressure is directly related toi the number, not the mass, of the molecules banging against a surface (PV=nRT ring any bells?). Lastly, if your box is the slightest bit open, pressure WILL equalize (go pop a ballon if you don't believe me; you can even fill it with argon first). Therefore, if you pump a bunch of argon into your box and leave an opening in the box, the pressure in the box will equilibrate to the outside. This is how the regulator on an argon tank is working, BTW...

--------------------
And it's one, two, three / On the wrong side of the lee / What were you meant for? / What were you meant for?
- The Decemberists

Posts: 7670 | From: the lab | Registered: Mar 2001  |  IP: Logged
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 04, 2007 11:16      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
I think my balance is not really measuring the amount of force in a pan but the resistance a substance that changes with the pressure exerted on it. If I hold my hanf over the whole in the top to prevent the escape of gas, the mg reading on the balance increases. The question is, why does the balance not zero itself when the whole in not blocked. The mg reading on the balance keeps increasing in this case and I think it is becuse the argon is heavier and creates no more pressure in the box. I unfortunaly do not have two barometers for inside and outside this box.

p=rho*g*h

p=pressure
rho=density
g= acc. of gravity
h=height of the fluid.


So instead of saying that the argon is heavier, i should say the argon is denser. If the argon is denser and g and h are constant, then the pressure is increasing as the box fills.


post scriptum

thank you all for the argument, I will goto work tomorrow much better prepared for the coming discusion.

Now to go find some documentaiton that not only is argon heavier than air, but also denser.

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged
Stereo

Solid Nitrozanium SuperFan!
Member # 748

Member Rated:
5
Icon 1 posted October 04, 2007 12:07      Profile for Stereo     Send New Private Message       Edit/Delete Post   Reply With Quote 
Ok, those things I keep mixing up and getting confused about, so forgive me if I say something stupid.

Couldn't you just put some water in your bath, set the zero on a standard scale, then put it in the bath and see what happens? (NOT an electronic scale - I'm sure you don't need to learn that water and electronic devices don't match well.) If you're not sure the scale is sensitive enough to show any difference, you should be able to find a mechanical kitchen scale; that should do neatly.

Oh, and you may want to shake/turn it upside-down to make sure there is no air stuck under the plate to create a buoyancy effect.

--------------------
Eppur, si muove!

Galileo Galilei

Posts: 2289 | From: Gatineau, Quebec, Canada | Registered: Apr 2001  |  IP: Logged
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 04, 2007 12:15      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Stereo:
Ok, those things I keep mixing up and getting confused about, so forgive me if I say something stupid.

Couldn't you just put some water in your bath, set the zero on a standard scale, then put it in the bath and see what happens? (NOT an electronic scale - I'm sure you don't need to learn that water and electronic devices don't match well.) If you're not sure the scale is sensitive enough to show any difference, you should be able to find a mechanical kitchen scale; that should do neatly.

Oh, and you may want to shake/turn it upside-down to make sure there is no air stuck under the plate to create a buoyancy effect.

The whole point of my argument is that we are using an electric scale and they function differently from a mechanical scale. i.e. They are sensitive to atmospheric pressure.

My argument is based on two things.
Pascals law.
electric load cells are sensitive to atmospheric pressure.

Does anybody have a good attack on the two points of my argument?

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged
Xanthine

Solid Nitrozanium SuperFan!
Member # 736

Member Rated:
5
Icon 1 posted October 04, 2007 12:40      Profile for Xanthine     Send New Private Message       Edit/Delete Post   Reply With Quote 
Like I said before, if the box is not sealed, the pressure in the box *will* equalize with the outside air and you won't be able to measure anything. That's just what gases do.

--------------------
And it's one, two, three / On the wrong side of the lee / What were you meant for? / What were you meant for?
- The Decemberists

Posts: 7670 | From: the lab | Registered: Mar 2001  |  IP: Logged
GrumpySteen

Solid Nitrozanium SuperFan
Member # 170

Icon 1 posted October 04, 2007 13:09      Profile for GrumpySteen     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Xanthine:
Like I said before, if the box is not sealed, the pressure in the box *will* equalize with the outside air and you won't be able to measure anything. That's just what gases do.

If you let it set long enough and the container isn't too deep, yes, it will do that. That doesn't mean you can't take advantage of the tendancy of a heavier gas to stay put even in an open container for long enough to do the measurements.

--------------------
Worst. Celibate. Ever.

Posts: 6364 | From: Tennessee | Registered: Jan 2000  |  IP: Logged
TheMoMan
BlabberMouth, a Blabber Odyssey
Member # 1659

Member Rated:
4
Icon 1 posted October 04, 2007 13:24      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Ashitaka _____________________ Lets go with Air density (AD) now if you are on an airliner that is at the upper load limit, and the runway is the shortest that that airplane can take off from. do you want hot, humid and low Barometer readings or would you prefer cold, dry high barometer readings.

Remember the air pressure is the same on top and below the parked aircraft.

If your load cell is hermetically sealed then and only then would it show baro type readings, As to the load cell being able to detect such small differences as a a foot of argon/a foot or breatin air, I really doubt you have access to one.

--------------------
Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.


Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 04, 2007 13:46      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by TheMoMan:
Ashitaka _____________________ Lets go with Air density (AD) now if you are on an airliner that is at the upper load limit, and the runway is the shortest that that airplane can take off from. do you want hot, humid and low Barometer readings or would you prefer cold, dry high barometer readings.

Remember the air pressure is the same on top and below the parked aircraft.

If your load cell is hermetically sealed then and only then would it show baro type readings, As to the load cell being able to detect such small differences as a a foot of argon/a foot or breatin air, I really doubt you have access to one.

@0.00200g d=0.001 , balance works fine without the argon on. Turn it on and it doesn't. I say this is because the pressure is changing as the box fills. More specifically as the box fills the air/argon mixure's density changes, which changes the pressure (slightly) due to pascals law. I also agree that given time (few weeks?) the argon would mix completly with the rooms air and the outside pressure would reach the inside pressure.

So it is a good point to wether my balance would pick up the difference between 1.23g/L and 1.7g/L in just less one meter of gas. I think the problem is the computer is waiting for the fifth digit after the decimal point to stop changing for three seconds before it considers tah balance tared, and the fifth decimal point just keep increasing slowly. If the flow of agron is turned down the balance tares.

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged
TheMoMan
BlabberMouth, a Blabber Odyssey
Member # 1659

Member Rated:
4
Icon 1 posted October 04, 2007 14:11      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Ashitaka __________________ I worked in a plant that built Mass Air flow sensors, Airmeters for the auto industry, We had labs full of very fine equipment, I worked on (repaired) the test stands to make sure that we did not ship bad airmeters. I think that your cell is still filled with air and has not swapped out its N2O2CO2 mix with the argon. Yes the fish tank anology is correct but this next time flood the sensor first so that it is filled with argon.

Could you find a piece of pipe big enough to set over the scale and get several feet of depth for your experiment.

--------------------
Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.


Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
Xanthine

Solid Nitrozanium SuperFan!
Member # 736

Member Rated:
5
Icon 1 posted October 04, 2007 15:31      Profile for Xanthine     Send New Private Message       Edit/Delete Post   Reply With Quote 
Are you sure it's the argon that it's reacting to and not the fact that you've got gas blowing through? 'Cuz our analytical balance doesn't read worth shit unless the doors are closed and keeping the air currents from coming in. Heat convection can also screw with your system as well. So can looking at it funny.

I avoid the analytical balance when I can.

--------------------
And it's one, two, three / On the wrong side of the lee / What were you meant for? / What were you meant for?
- The Decemberists

Posts: 7670 | From: the lab | Registered: Mar 2001  |  IP: Logged
littlefish
BlabberMouth, a Blabber Odyssey
Member # 966

Member Rated:
4
Icon 1 posted October 05, 2007 00:34      Profile for littlefish   Author's Homepage     Send New Private Message       Edit/Delete Post   Reply With Quote 
^^^^This^^^^
Posts: 2421 | From: That London | Registered: Nov 2001  |  IP: Logged
TheMoMan
BlabberMouth, a Blabber Odyssey
Member # 1659

Member Rated:
4
Icon 1 posted October 05, 2007 03:40      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Ashitaka _________________________ For your scale to act like a baro is a strech.

Take a large beaker and place on scale. knowing that argon is heaver than air slowly dispense argon into beaker. There should be a curtin or shield around the scale and beaker to prvent the gas from being drafted out of the beaker. While the beaker fills you should, if it is a good enough balance see a rise in the mass reading.

I am not sure that the unit you are using uses a carbon pile resister or a piezo element, the difference in behavior of the two types could explain your reading difference

--------------------
Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.


Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 05, 2007 04:46      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
The balance has two glass boxes between the outside and the weigh-in grate. (see pic) The argon "breeze" is unlikely affecting it.

moman____ a 5 ml volumetric flask ( the largest volume you can use withthis balance) is ~2.3mg heavier filled with air ratehr than with argon.

I checked the math and this is is abot right. The difference in density of argon and air is about
0.5 g/L. 5mL is 1/200th of 1 L so the difference in wieght divded by 200 is 0.0025g or 2.5 mg. Close enough to my mesurements. (I obviously didn't get pure Ar into the vial).

In the end I think what solution is going to be is to fill the box wth argon and then turn off the argon flow before doing the measurements. If it is either the breeze of Ar or the changing density, the problem shoudl go away.

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged
TheMoMan
BlabberMouth, a Blabber Odyssey
Member # 1659

Member Rated:
4
Icon 1 posted October 05, 2007 09:18      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Ashitaka _________________________ I have some piezo cells and will hook them up in a pressure chamber to se if they will read pressure.

Carbon pile load cells are plates of carbon retained by a spring mechanism. Argon molocules being slightly different size than N2O2 air would change the resistance of the pile.

I also have a baro. element from a car gastank (US emissions laws) that I am trying to find a use for.

--------------------
Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.


Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
Moe Monkey
Geek
Member # 1900

Icon 1 posted October 05, 2007 19:21      Profile for Moe Monkey     Send New Private Message       Edit/Delete Post   Reply With Quote 
Ultimately, your balance is relying on the downward force the pan (and object being weighed) exerts on the sensor.

The net downward force is a combination of gravity and the buoyant force exerted by the fluid (air) in the box.

Since argon is more dense than air, the pan and any object on it will appear less massive because of the greater buoyant force.

Fun questions to consider:

Pretend the pan is made of wood, as is the sample you are weighing. What happens to the reading when you fill the container with water?

Does an electronic balance measure the mass of a helium-filled balloon correctly?

More serious question:
Do buoyancy differences when weighing in argon as opposed to air make a significant difference to the object being weighed? (Your experiment with the vial suggest they do) If you need an inert atmosphere, why not fill the chamber with nitrogen (close to the density of air)? Or a nitrogen-argon mix adjusted to the density of air?

Posts: 145 | From: The couch in the living room | Registered: Dec 2002  |  IP: Logged
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 06, 2007 01:31      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by Moe Monkey:


More serious question:
Do buoyancy differences when weighing in argon as opposed to air make a significant difference to the object being weighed? (Your experiment with the vial suggest they do) If you need an inert atmosphere, why not fill the chamber with nitrogen (close to the density of air)? Or a nitrogen-argon mix adjusted to the density of air?

Ultimatly, nitrogen is cheaper, but just not good enough. We use Argon so that as the box fills, it pushes the lighter air out the top. Hopefully taking as much of the CO2, 02 and H20 with it as possible. (as these three things react with what we are trying to measure)

(Before someocomments that this will not work well because of mixing effects, I know it does as we have a humidity sensor, o2 sensor and a co2 sensor inthe box)

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged
TheMoMan
BlabberMouth, a Blabber Odyssey
Member # 1659

Member Rated:
4
Icon 1 posted October 06, 2007 03:48      Profile for TheMoMan         Edit/Delete Post   Reply With Quote 
Ashitaka _____________________________ Now that more of the project is comming to light,

Think of it this way, take a clear bottle half filled with salad or cooking oil, now slowly add water down the side until the bottle is near full. The water and oil will stay seporate until disterbed, the problem you have is keeping the gasses from mixing, tempature will even be your enemy.

Do you do calculations for the increased bouyance of the argon?

--------------------
Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety.


Benjamin Franklin,

Posts: 5848 | From: Just South of the Huron National Forest, in the water shed of the Rifle River | Registered: Sep 2002  |  IP: Logged
Ashitaka

SuperFan!
Member # 4924

Member Rated:
4
Icon 1 posted October 06, 2007 06:33      Profile for Ashitaka     Send New Private Message       Edit/Delete Post   Reply With Quote 
quote:
Originally posted by TheMoMan:
Ashitaka _____________________________ Now that more of the project is comming to light,

Think of it this way, take a clear bottle half filled with salad or cooking oil, now slowly add water down the side until the bottle is near full. The water and oil will stay seporate until disterbed, the problem you have is keeping the gasses from mixing, tempature will even be your enemy.

Do you do calculations for the increased bouyance of the argon?

The gasses do mix but not much as after 20 minutes the O2, CO2, and H2O are all mostly gone.

I also don't see thermal currents as the culprit as the colder argon is coming in at the bottom through six seperate points.

I will do the expirement moday of sealing thebox and seeing if four bars of pressure shows up on the balance.

--------------------
"If they're not gonna make a distinction between Muslims and violent extremists, then why should I take the time to distinguish between decent, fearful white people and racists?"

-Assif Mandvi

Posts: 3089 | From: Switzerland | Registered: Feb 2006  |  IP: Logged


All times are Eastern Time
This topic comprises 2 pages: 1  2 
 
Post New Topic  New Poll  Post A Reply Close Topic    Move Topic    Delete Topic next oldest topic   next newest topic
 - Printer-friendly view of this topic
Hop To:

Contact Us | Geek Culture Home Page

2015 Geek Culture

Powered by Infopop Corporation
UBB.classicTM 6.4.0


homeGeek CultureWebstoreeCards!Forums!Joy of Tech!AY2K!webcam