Help, I don't know anything! Flow rates and pouring time...

[jayjay]

Hello,

 

Obviously I'm not a science person, but I have a question that one of you geniuses can probably help me with-

 

say you had a container with a thin gooseneck spout (about .5cm diameter) that restricted the pouring flow rate of water out of the container, would the initial amount of water in the container affect the flow rate?

 

if so,

 

what if you were dealing with only 250g or 700g of water in the container, for example- would the flow rate change significantly between these two initial amounts? how much would the flow rate decrease as the volume inside the container decreased?

 

I'd like to know if the time it takes per gram of water to be poured will be affected very much by the volume inside the container. obviously the pressure would be higher with more water, but would it be a significant difference with such small initial volumes? ie 200-700g

 

Thanks so much in advance!

 

[CaptainPanic]

Yes, it would probably affect the flow rate. It does matter whether your container is closed or open (e.g. a bottle with just 1 opening, or a watering can, which has two openings). What matters is not so much the volume, as the difference in height between the top of the water level, and the height of the opening of your container where it pours out.

 

If you want to do some calculations, you should read up on Bernoulli's principle, although it may be a little complicated, since you indicate that you're not a "science person". If you explain a little more what you want to achieve, we can give feedback at the right level of complexity.

[jayjay]

Thanks for the answer, Captainpanic!

 

Think of a watering can for plants- long and thin spout coming out from the bottom of the container, with an opening at the top for filling (ie no suction).

 

I want to install a flow restrictor onto the end of the spout to adjust flow rate incrementally in order to pour a certain amount of water per minute. Say for example, 125g/minute.

 

I would like it to be quite accurate, and therefore I'd like to know how much the initial volume inside the can would affect the flow rate with said flow restrictor installed.

 

I've tried to test how much I can pour in 10 seconds with a full can vs a half full can, but it's altogether too inaccurate, thus my post here with the science folks.

 

I really appreciate your help!

 

J

[CaptainPanic]

If you make a relatively long tube, and a relatively large and flat reservoir at the top, the water level should not drop much as the container empties, so you take that out of the equation altogether. Then at least you don't have to adjust your 'flow restrictor' while it empties. Btw, a "valve" is probably the most common "flow restrictor".

 

Can I ask what the project is aiming to achieve? Is it a hobby project? Or something for school?

[studiot]

If this is a school project, go to your local hospital (possibly the medical physics dept) and ask for someone to show you how I V and syringe pump drivers work and flow contollers work.

 

I'm sure you will find some useful practical help there.

[jayjay]

This is for a hobby project. Normally in this application, one uses a scale to measure the liquid poured in a certain time frame. But I'd like to eliminate the scale and instead use only time and a precise flow restrictor which can be adjusted to pour a certain amount of liquid in a given time. I'm thinking something like this:

 

http://apps.boschrexroth.com/products/compact-hydraulics/Sleeve-Catalog/pdf/RDF.pdf

 

But this project will only work if the flow rate remains constant regardless of volume inside the container. So if I begin with 500g of liquid, or 250g of liquid, the flow rate will remain (relatively) constant. Obviously there will be some decrease in flow depending on the weight pressure, but will it be appreciable?

 

Thanks again! You're great!

 

J

[studiot]

I don't think you have quite understood what Cpt Panic was trying to tell you.

 

The flow rate depends on the pressure pushing it along and the size of the minimum cross sectional area of pipe.

It does not depend directly the volume available from the reservoir tank.

 

Your 'flow restrictor', valve, tap, or whatever you want to call it adjusts the minimum pipe area to provide the desired flow rate for a given pressure.

 

The flow restrictor does not control the pressure. That is up to you and how you arrange your pipework and tank.

 

My hospital example they put the tank(Bag) up a large height in relation to the size of the bag so this mounting height controls the pressure.

 

Captn Panic offered another way - to have a large flat reservoir.

 

There are yet other ways some mechanically quite complex.

 

But to have constant flow you need to arrange constant pressure.

[Endy0816]

This should give an idea of the issues.

 

http://www.efunda.com/formulae/fluids/draining_tank.cfm#calc

 

Simplest thing would be to float your outlet. More expensive would be to use a pump or valve set up.

[jayjay]

Thanks for the answers!

 

I understand that a valve will not adjust the pressure, yet if the opening where the water exits is located at the bottom of the container (it is not possible in this project to use height to add constant pressure as in an intravenous, not is it possible to change the location of the spout), and the container is full of water, say with 500g, then the pressure into the exit when pouring will be greater than when there is only 50g of water left in the container... right? or am I still misunderstanding? Since in a container like this one, http://www.pernellgerver.com/hawshandywateringcan.htm ,the more liquid is in the container, the higher the depth of water. Right? (assuming only atmospheric pressure is present).

 

In my container, the spout is at the bottom, and the base has a diameter of about 20cm, and then tapers increasingly to about 10cm diameter at the top.

[studiot]

Well Captn Panic did ask what you are trying to do.

 

If you can't control the pressure in the tank, the only other place you can control it is in the piping.

 

This means that you don't take the fluid from the outlet directly, but lead it in some sort of pipe that has a flow rate controller attached.

This may include a pump.