Why is it necessary to start this experiment with the water level at 400mL?

[Aurelius120]

Homework Statement

What is the reason for placing the spirometer at 400mL mark at the start of the experiment?

Relevant Equations

NA

This is a water spirometer. You have to exhale as hard as possible the pulley rotates and the pointer shows the volume exhaled.

The procedure wants us to keep the pointer at 400 and later subtract 400 to get the correct volume.

Why can't we start at zero?

Teacher says it is easier to exhale when there's more air. As far as I know, the role of water is to only act as an air tight seal.
And I don't see why more air makes any difference in resistance to expansion. We have to always blow against air pressure anyways?

 

[DaveC426913]

Why can't we start at zero?

the role of water is to only act as an air tight seal.

You answered your own question.

 

[Aurelius120]

You answered your own question.

But how does it change anything? Less air or more air in inner cylinder shouldn't change resistance, right? I still have to exhale against atmospheric pressure.

 

[kuruman]

Why can't we start at zero?

Why can't you? This is an experiment after all. Try starting at zero and see what happens. Do you get a consistent reading with what you got when you started at 400 mL? Is it just as easy to exhale at zero as it is at 400 ml? If the answer to both questions is "yes", then you might confront your teacher with the information that you gathered and ask for additional explanations.

 

[DaveC426913]

But how does it change anything? Less air or more air in inner cylinder shouldn't change resistance, right? I still have to exhale against atmospheric pressure.

If it's at zero, then it can't form a proper seal. Air could escape, ruining the calibration.

 

[kuruman]

If it's at zero, then it can't form a proper seal. Air could escape, ruining the calibration.

I think (without being 100% sure) that "zero" means that the bell is all the way down with its bottom just above water level. OP is asking why one cannot start at this initial position in which case the amount of air that one blows in is what the indicator points.

 

[DaveC426913]

...with its bottom just above water level.

Yes.

OP is asking why one cannot start at this initial position...

Yes.

So:

If it's at zero, then it can't form a proper seal. Air could escape, ruining the calibration.

 

[votingmachine]

It is an arbitrary scale. The counterweight balances the weight of the bell, so adding air to the trapped volume is perfectly against the atmospheric pressure. As long as the water is keeping a seal, it doesn't matter what your starting volume in the bell is. It matters that the bell has available expansion for your lung expiration volume.

You could indeed "tare" the system, by starting at: volume = zero, and then arriving at: volume = lung capacity.

It is important in any experiment to describe the setup, materials and methods, and calibration procedures. I think you are correct in observing there is an alternate way to make the measurement of the bell volume change from lung air.

If you were publishing your data, the important thing would be that you can clearly show the drawing and describe how you took your measurements. Someone else should be able to exactly trace your path. Then the data matters.

Do the experiment by rotating the volume scale to start with the pointer at Zero. You will get the same result. Experimentally, people are right to say the water seals the bell, and without a seal, you won't get a valid measurement.

Take a 100 ml syringe and inject 100 ml of air into the system with different starting settings. You should always measure 100 ml if the set up is correct.

My guess is that someone has determined that 400 ml is a good seal, and they wanted to make sure you KNEW you had a good seal. If it was a really small depth of water, maybe blowing would see some escape ...

 

[Aurelius120]

Why can't you? This is an experiment after all. Try starting at zero and see what happens. Do you get a consistent reading with what you got when you started at 400 mL? Is it just as easy to exhale at zero as it is at 400 ml? If the answer to both questions is "yes", then you might confront your teacher with the information that you gathered and ask for additional explanations.

Now the problem is. This experiment is already over. And they aren't going to allow us to do this anymore.

But I had tried it at zero, the value certainly came out lesser than the regular way. Maybe a 100mL less. But most of the apparatus were likely unused or something because most of us got errors.

 

[Aurelius120]

Do the experiment by rotating the volume scale to start with the pointer at Zero. You will get the same result. Experimentally, people are right to say the water seals the bell, and without a seal, you won't get a valid measurement.

In the theory, it was asked to set it at 400-500 mL and do the experiment. In the practical lab, teacher asked us to lift it to 400-500mL and rotate the pointer to zero

 

[Aurelius120]

If it's at zero, then it can't form a proper seal. Air could escape, ruining the calibration.

So hypothetically if the bell rests on the mouth of the inner metallic pipe, the water won't form a proper seal? Why so? Is it because we will be blowing air into the water?

 

[256bits]

Homework Statement: What is the reason for placing the spirometer at 400mL mark at the start of the experiment?
Relevant Equations: NA

The procedure wants us to keep the pointer at 400 and later subtract 400 to get the correct volume.

Ask yourself for what reason is there a counterweight.
And what does bouyancy affect the apparent weight of the inverted graduated cylinder as it moves upwards when someone exhales.
And calibration procedures ( for neutral bouyancy )

That's all I got..
The reasons behind the directions should have been explained in the outline of the experiment.

Homework Statement: What is the reason for placing the spirometer at 400mL mark at the start of the experiment?
Relevant Equations: NA

Teacher says it is easier to exhale when there's more air.

Not easier.
To make less one source of error is more in line with starting with a greater volume of air.

 

[DaveC426913]

So I would say there are several reasons:

• You want to ensure that there is a failsafe margin of error. If the apparatus is jostled, or the patient inhales briefly, or any number of other incidents, you want your setup to be robust, enough to handle them gracefully without going out of calibration. Staring with zero leaves you no margin of safety.
• You want to ensure your setup is consistent with standard practice. i.e. you wet it to 400mL because that's the standard. That way, if the method turns out to have a flaw in it (say, 400mL turns out to be too easily compressible or something), at least the flaw is consistent across all tests in the data sets. Your data will be compatible with all other data from tests that used the identical setup. It's arguably more valuable that the data's accuracy is comparable than that its accuracy is absolute.

But I'm not in my wheelhouse.

 

[votingmachine]

When you are putting the bell into the starting position, You can raise and lower it freely, with the air going out-or-in the rubber tube. If you wanted to start with no air in the bell, I can think of 3 things that might go wrong.

1. You might squish some water into the rubber tube, and that could introduce an error.

2. You might push the bell into the water slightly, such that the water displacement introduces an error.

3. A person might accidentally inhale water and choke.

If I had to guess, I would bet a combination of 1 and 3 are considered a reason to keep some air in the bell.

Mostly #3. I can imagine that a person is instructed to inhale, then exhale into the tube. But humans are prone to mistakes.

Sometimes the reason for a setup is not because the data collection is easier, it is for safety or other reasons. The diving board on a pool could be recessed from the pool rather than extending, and a physics-of-diving experiment still work. But safety has to be considered. Someone might fall off.

You are probably not wrong about the experiment working equally well with a slightly different setup.

EDIT: The comment on consistency was coincident with my typing. But I would echo that. The goal is to make the method completely repeatable. Specifying things helps achieve consistent results.

 

[jbriggs444]

bell is all the way down with its bottom just above water level.

Which means that the bell will not float and, therefore, the indicator needle will be unreliable.

With positive buoyancy, the apparatus has a stable equilibrium point.

 

[Aurelius120]

Which means that the bell will not float and, therefore, the indicator needle will be unreliable.

With positive buoyancy, the apparatus has a stable equilibrium point.

Why would it be unreliable?
Because we have to blow air against mass of bell which would make it difficult to exhale all air??

Not easier.
To make less one source of error is more in line with starting with a greater volume of air.

Ask yourself for what reason is there a counterweight

I think to make the net force zero on the bell?

 

[jbriggs444]

Why would it be unreliable?

If the inverted jar sinks to the bottom, the pointer will spin as it does so. The pointer is where you measure the results, right? You want to measure the volume of exhaled air, not the depth of the tank.

 

[Aurelius120]

If the inverted jar sinks to the bottom, the pointer will spin as it does so. The pointer is where you measure the results, right? You want to measure the volume of exhaled air, not the depth of ri3the tank.

But if it is at zero when the bell is resting at the water level,bell can't sink any further? Blowing air into it will still make it float, and give the required reading right?

 

[DaveC426913]

But if it is at zero when the bell is resting at the water level,bell can't sink any further? Blowing air into it will still make it float, and give the required reading right?

How do you know blowing air will make it float? In order for that to be true, the Bell jar must already be neutrally buoyant, which it is not, because it is resting on the bottom.

 

[Aurelius120]

How do you know blowing air will make it float? In order for that to be true, the Bell jar must already be neutrally buoyant, which it is not, because it is resting on the bottom.

So it will be like trying to blow away a chunk of solid metal?

 

[jbriggs444]

So it will be like trying to blow away a chunk of solid metal?

I am not sure what mental picture you have of the setup.

Exhaling into an inverted flask sitting at the bottom of a tank of water will require a pressure corresponding to the depth of the water. Like blowing bubbles in the bottom of a glass of water with a straw.

At first, you will be displacing water from the flask, creating a bubble within. Once the bubble is large enough, the flask may float back to the surface.

 

[Aurelius120]

I meant this. (Water level at level of bell and bell resting on air pipe)
Since normal reaction is balancing the bell. We have to blow against (weight-buoyancy), Right?

Similar to if we had to lift/move a metal block with only force of exhaled air?

 

[erobz]

View attachment 357249
I meant this. (Water level at level of bell and bell resting on air pipe)
Since normal reaction is balancing the bell. We have to blow against (weight-buoyancy), Right?

Similar to if we had to lift/move the bell with only force of exhaled air?

That sounds like you understand the mechanics. You would have to generate some net pressure over the area of the tube to get $N$ (the normal support from the pipe) to go to zero. That pressure would depend on the tube area and the material density of the bell. It could be that the bell is just barely touching the tube end (bell made of plastic or something of low density), or maybe it's a steel bell with high density and large weight.

How detailed are you trying to get tracking the progression of change from that point on is up to you.

 

[erobz]

My opinion:

The fine details of what transpires from some initial state is complex and not without nuance. By starting from some value that has been tested in which less things dramatically change in your model you are attempting to minimize systematic error.

If you don't believe it. Try looking at the fine details of this measurement model for a bit from some initial state. I think you will see its actually complicated when we zoom in. I know we've done some similar stuff with overturned glasses in very large surroundings, but this adds more complexity because the surroundings of the bell are finite.

 

[jbriggs444]

View attachment 357249
We have to blow against (weight-buoyancy), Right?

Weight of what and buoyancy of what, exactly?

In the initial state, I see an inverted beaker supported by nothing other than the rim of the vertical pipe.

The upward pressure of water on the flat upper surface is exactly equal to the downward pressure from the atmosphere. The upward pressure of the air in the pipe is exactly equal to the upward pressure from the air in the pipe. Zero net upward force from buoyancy on the bulk volume of the beaker.

We do have some buoyancy from the upward pressure of water on the lower rim of the beaker which is not entirely matched by the downward pressure on the upper rim of the beaker.

So yes, we have the weight of the glass (or metal) beaker minus the buoyancy from the volume of the beaker's glass (or metal) in water. A beaker made of ordinary glass or from borosilicate will have a specific gravity of about 2.4 or 2.5. Stainless steel is three times that.

So for a glass beaker, it would be about 60% as hard as blowing against the same beaker resting on top of the vertical pipe with an airtight seal but no water. ($\frac{2.4 - 1.0}{2.4} \approx 0.6)$. For a stainless steel beaker it would be about 87% as hard as blowing against the same beaker resting on top of the vertical pipe with an airtight seal but no water ($\frac{7.5-1.0}{7.5} \approx 0.87)$

The diameter of the pipe will be a critical factor in how much air pressure we will need to do this.

If the seal is leaky, we may be able to introduce air more easily than that. The diameter of the beaker will then become important.

 

[Aurelius120]

So yes, we have the weight of the glass (or metal) beaker minus the buoyancy from the volume of the beaker's glass (or metal) in water.

So if some air already exists for example at 400mL, it will be easier to blow into it (than doing so at 0mL) because there is more buoyancy?

 

[jbriggs444]

So if some air already exists for example at 400mL, it will be easier to blow into it (than doing so at 0mL) because there is more buoyancy?

Again, buoyancy of what, exactly?

But yes, with the seal broken, the uplifting force it is from the pressure of the air in the pipe multiplied by the cross-sectional area of the beaker.

When one has sealed volumes and multiple pressures, it is probably a better idea to sum up pressure forces rather than summarizing them as "buoyancy".

 

[Aurelius120]

Again, buoyancy of what, exactly?

Upthrust exerted by water on the bell (including the air trapped within,right) because that's the volume displaced.

 

[jbriggs444]

Upthrust exerted by water on the bell (including the air trapped within,right) because that's the volume displaced.

No. Buoyancy does not apply to that. You'll need to sum pressure forces.

One could do a force accounting that includes buoyancy. But one would have to be careful in identifying the volume for which one is performing the calculation. Ordinarily "buoyancy" is used in evaluating the force from static pressure on an object floating or immersed in a fluid which is allowed to reach all the way under and which has attained equilibrium.

Notoriously, the buoyant force on a suction cup at the bottom of the bathtub is undefined.

 

[Aurelius120]

The

No. Buoyancy does not apply to that. You'll need to sum pressure forces.

Then as long as water level is at the level of upper rim of the bell or lower, we have to blow against atmospheric pressure since downwards pressure of water is zero. So the amount of exhaled air is same in 400mL or 0mL.

Problem will be if water level is above bell at 0mL. Because of additional water pressure.

Right??

How do you know blowing air will make it float? In order for that to be true, the Bell jar must already be neutrally buoyant, which it is not, because it is resting on the bottom.

If it doesn't float, then the pointer will turn differently from floating condition, correct?

The other error is if air tight sealing of water breaks. Any specific mechanics for this?