# Measuring Velocity based on change in Pressure

Tags:
1. Nov 23, 2014

### UYasher

1. The problem statement, all variables and given/known data
There is an air-tight 1L Canister filled with 90% water and 10% N2O. This canister is attached to some clear tubing which is not entirely air-tight. At the end of this tubing are some tungsten particles averaging ~1µm in diameter with total mass of 60mg. The contraption is shaken until the pressure inside the canister is 40-60 bars and then the pressure is released into the tubing, but not all at once, dislodging the tungsten and propelling it towards a target. Your goal is to find a way to determine the speed of the tungsten particles when they hit the target. You may determine the distance of the device from your target and the orientation relative to your target to best suit your method (so long as that distance is less than 1/3 meters).

P.S. Your experiment will be set up and tested in engineering class.

2. Relevant equations
Bernoulli's Law?

3. The attempt at a solution

Attempt 1: Shine a light through the tubing at the tungsten and hope it reflects light back. Then use a slow motion camera to see how quickly the particles are traveling. This idea would not work because a) the tungsten might not reflect light when dispersed; b) after discussing it with some other students i determined that the particles might be moving faster than a slow motion camera can capture (300m/s +); c) we don't have a slow motion camera

Attempt 2: After more discussion, we determined that the small mass and size of the particles would make them act similarly to particulate matter in the air. From this I propose a setup where the device is discharged towards a vat of liquid with a pressure gauge in it. We measure the change in pressure when the blast of air hits the liquid and use Bernoulli's Law to find the velocity of the air, which should be equivalent to the velocity of the tungsten.

Would this method work, and if so how would I use Bernoulli's Law to solve for velocity in this scenario?

2. Nov 24, 2014

### CWatters

Perhaps you allow the target to move and apply conservation of momentum? The problem is knowing if any of the gas also hits the target - but perhaps you could run it twice with and without the tungsten powder?

Edit: Might have to make the target sticky.

3. Nov 24, 2014

### Staff: Mentor

A soft surface to catch them (let them move into the target) and a light target to measure its velocity change? Looks possible.
The target mass can be measured before and after to estimate the amount of tungsten that hit it.

4. Nov 30, 2014

### UYasher

I discussed this with some of my peers and we came to these conclusions:
a) What surface could we use to catch it? According to some of the upper class-men, the particles are going at some hundred m/s.
b) How would we suspend the target such that it would be able to move, and friction would be so minimal as to be negligible?
c) The gas hitting the target should not have a significant effect on measuring the velocity of the particles, unless the target catches a lot of air. Which it might in the case of a soft, light surface.
d) The tungsten would be traveling at the same speed as the air. Taking this into account, finding the mass of the air, or using the method I proposed with pressure, would eliminate the need for a second test, which the instructor may not allow us to do.
e) Going back to my second attempt, if I were to use the equation ½v2+gz+p/ρ = constant, how do I determine the constant? (the two unknowns are the constant and velocity.)

5. Nov 30, 2014

### Staff: Mentor

a) Some hundred m/s? That's the speed of bullets. Wood, rubber, cardboard, ...
b) A long pendulum? Or even a torsion pendulum?
c) Some distance between origin of the bullets and target could help. Maybe with some shielding?

I don't understand your second concept. Do you have a sketch? The stream of air should be very brief.

6. Dec 1, 2014

### CWatters

I don't understand concept 2 either.

I wonder if you could blow the tungsten powder between two sets of high voltage electrodes eg triggering two discharges one after the other, then use time over distance? Would need a high voltage source (2?) and a storage scope.

7. Dec 1, 2014

### dragoneyes001

a smoke curtain with high speed photography and a scale should make measuring possible.

no camera eh? hmm then breaking two circuits at a set distance becomes the next way.as was pointed out above

Last edited: Dec 1, 2014
8. Dec 3, 2014

### UYasher

After looking into it, the electrical discharge idea seems pretty effective, thanks CWatters. Just one question: What is a storage scope?

And about my original idea, although the stream of air would be brief, we have barometers which supply digital feed to a computer. Is there any specific idea which you don't understand, or would you prefer a general diagram?