Mixing Vessel Weight

[anthomson]

A large sealed chemical mixing vessel is weighed using load cells positioned under the vessel ( The vessel is weighed to accurately control the quantities being added). Suspended from the roof of the vessel is a large motorised agitator (fan). When the agitator is started will the weight remain constant, drop or rise ?

[j2buttonsw]

A very good question. I guess the crux of the question is that in a sealed container does the volatile components still exhibit the same "weight" whether they are in solution (or in solid: whatever the word is) or suspended in the air? The mind says stay the same but I can't envisage molecules suspended in the air having the same "weight" as molecules in solution or in solid (what is the proper word?). I can't see the weight rising, though.

[Squirrel]

I hope I'm right in picturing this "chemical mixing vessel" as a box. If so, and if it is filled with a liquid or gas (or mixture of liquids or gases, such as air) then the movement of some of the molecules in a certain direction within the box will instantly cause an equal number of molecules to move in the opposite direction to take their place. An example of this is if there was a helicopter hovering inside the box, or the fan in the question. Whether the helicopter was sitting on the floor or hovering, the downward force it exerts on the bottom of the box would be the same, so the load cells (whatever they may be) should show a relatively steady reading.

If there was some liquid (water, for example) in the bottom of this sealed box and you then heated it to evaporate some or all of the water, the overall mass of liquid water + gaseous water would be the same. The overall weight would also, therefore, be the same. It sounds strange because, obviously, if you tried to weigh (with an ordinary scale) a cubic metre of air that was simply mixed in with a whole room full of air, the scale would read zero. This is because all weighing scales used in our atmosphere have been calibrated to account for the constant downward force that gravity is causing the air to push with. I think that placing the scale in a vacuum (still on Earth, just in a room with no air) would make it possible to 'weigh' air, water vapour etc. You would just have to make sure that the scale was supporting the full weight of the gas (e.g. with a vertical tube or something). I hope I've come close to answering the question.

[doristwonk]

One of Newtons 'laws of motion' stated that to every action there is an equal and opposite reaction.Therefore the action of the fan blades pressing down would be nullified by the action of the fan body pressing upwards, and since the contained is sealed all within must remain within and of the same weight.

[Squirrel]

I wish I was capable of...just writing a concise answer like that.

[anthomson]

Thanks everybody - my worry was that the fan might be generating a force which would act with or against gravity to affect the weight measured. ( In the same way a helicopter rotor generates a lifting force ). Incidently this was for a software control system - but I was made redundant before I could ever test this out.

[bigblooze]

Pretty straightforward this one. A load cell arrangement on the tank is set-up with 4 load cells, one on each corner of the tank. Obviously the shape of the tank affects this (i.e circular or square) but it doesn't make too much difference. The reason why the movement will not affect it is down to the control & measurement system. All load systems operate a fairly complex algorithm to take into account minute disturbances on each load cell (bear in mind that a load cell is that sensitive, that on a 100 ton load cell, it can detect a fly landing on it!!!). Therefore, as the agitator moves around the tank it causes a tilting effect (not visible, but there anyway!) As one load cell gets heavier, its direct opposite becomes lighter. A bit like a see saw with a fat kid on one side. The computer control system recognises this & compensates its measurement. That means that it doesn't matter what the agitator does, the weight (as long as no one leans on top of the tank) is a true representation of what is inside it!!!

[bigblooze]

Errr, in case you were wondering, I do this sort of thing for a living. You could do a lot worse than use the Siemens Siwarex weighing system (shameless plug for my company!)