For my project i need to measure time required for a ball(small ball made of metal) to pass two points in a water cylinder. Please see the image below.
Is it possible to use IR sensor in those points (A & B point) to determine when the ball passes those points and calculate the time (in seconds). As the cylinder is made of plastic and there is water present will it be a problem for the sensor ? or there is any other way to do so.
Thanks in advance.
The sensor will detect the passage of the ball, but getting the ball to pass between the IR LED and the sensor might prove to be tricky, depending on the size of the cylinder.
Perhaps multiple sensors will be needed to ensure the ball passes between at least one pair.
How large is the ball in relation to the IR source, the IR sensor and the diameter of the cylinder.
If the ball is magnetic then you may be able to use a Hall effect sensor to detect it passing but that too will depend on sizes.
So if i use a ball big enough (almost same dia of the cylinder or some less ) then it will be okay ?
rdbhaxor:
So if i use a ball big enough (almost same dia of the cylinder or some less ) then it will be okay ?
It would certainly be easier I suspect but the size of the ball in proportion to the size of the cylinder will alter the speed at which it falls.
Timing the ball between 2 points presupposes to me that you will then change some factor in the experiment and do a comparison. Is that what you will be doing ?
I see it's from a Wikihow. Perhaps you could give us a link to that, so we have more flexibility in suggesting solutions.
UKHeliBob:
It would certainly be easier I suspect but the size of the ball in proportion to the size of the cylinder will alter the speed at which it falls.Timing the ball between 2 points presupposes to me that you will then change some factor in the experiment and do a comparison. Is that what you will be doing ?
I am doing on a project to measure viscosity . So i need the velocity of a falling body in liquid . thus need the time to calculate.
aarg:
I see it's from a Wikihow. Perhaps you could give us a link to that, so we have more flexibility in suggesting solutions.
Here is the wikihow link - How to Measure Viscosity: 10 Steps (with Pictures) - wikiHow
Though i will use a different law for more accuracy but the method is same.
I would probably experiment with two coils wrapped around the cylinder at the two measuring points.
And detect the inductance change when the ball passes through them.
But that might be too involved for a school project.
Leo..
If you provide details of the size of things as asked in reply #2 it may be easier to give advice.
UKHeliBob:
If you provide details of the size of things as asked in reply #2 it may be easier to give advice.
I dont have exact size. after determining how the sensors work i will make/buy required size tools
It seems to me that there are several factors that will influence your ability to detect the ball passing a sensor. In your OP you say that the ball is small which, if you mean small in relation to the tube, will make it more difficult to detect. If you make the ball larger in relation to the tube then it will make it easier to detect but this will begin to affect the speed at which it falls through the liquid. The tube as illustrated is clear. Detection by light would probably be made easier if the majority of the tube were opaque with light only shining through at the detection points.
I am afraid that you seem to be at the point in your project where practical experimentation is required so you need to acquire some hardware to test what is possible.
UKHeliBob:
Detection by light would probably be made easier if the majority of the tube were opaque with light only shining through at the detection points.
That's a pretty good idea.
Anyway rdbhaxor, I'd just grab a couple inexpensive phototransistors and a couple IR LEDs and get to work. You can google how photointerrupters work for the circuit and code that you need.
Personally I don't think you'll have much trouble getting this to succeed, but the bigger the ball is, and the more beam it can block, the better.
UKHeliBob:
It seems to me that there are several factors that will influence your ability to detect the ball passing a sensor. In your OP you say that the ball is small which, if you mean small in relation to the tube, will make it more difficult to detect. If you make the ball larger in relation to the tube then it will make it easier to detect but this will begin to affect the speed at which it falls through the liquid. The tube as illustrated is clear. Detection by light would probably be made easier if the majority of the tube were opaque with light only shining through at the detection points.I am afraid that you seem to be at the point in your project where practical experimentation is required so you need to acquire some hardware to test what is possible.
Thank you i will start experimenting 
Chagrin:
That's a pretty good idea.Anyway rdbhaxor, I'd just grab a couple inexpensive phototransistors and a couple IR LEDs and get to work. You can google how photointerrupters work for the circuit and code that you need.
Personally I don't think you'll have much trouble getting this to succeed, but the bigger the ball is, and the more beam it can block, the better.
Okay i will post the results 
Hi,
Chagrin:
Personally I don't think you'll have much trouble getting this to succeed, but the bigger the ball is, and the more beam it can block, the better.
I agree, not only will you get good detection, the smaller space between ball and container sides will make the time differences for different viscosity broader, hopefully minimising detection errors.
Tom....
Another problem... Infrared light it's not visible light... Infrared light is for the most part absorbed by water, and by glass, too.
so... i don't know, try, but I'm not really sure that IR works.
paofanello:
Another problem... Infrared light it's not visible light... Infrared light is for the most part absorbed by water, and by glass, too.
so... i don't know, try, but I'm not really sure that IR works.
You are thinking of far infrared (longwave IR).
Even Near IR can be absorbed between 10 to 1000x more than the visible.
Chose a visible light sensor. Cover the tube in black paper and make two holes...use 2 visible spectrum (white) LEDs.
Ideally around 500nm (blue) is best:
http://th.physik.uni-frankfurt.de/~scherer/Blogging/ColourWater/absorption.png
Okay, let's actually read your chart. Choose an 850nm IR emitter. That's 0.85um. From the chart, the absorption, alpha, would be about 10exp0 * 3/m = 3/m. That means the transmission should be about 1-(1/(3/m)) = 2/3 or 66.7% through 1 meter of water.
Now tell me it will make a big difference through about 2 centimeters.
What this chart proves, is the "blueness" that you see underwater. But when you see underwater photographs taken at close range, there is very little colour shift.
