The "90psi" graph...

[Kweasi]

...I was giving that graph a good stare and
noticed that it shows the barrel as being in
a vacuum for a good portion of the sample time.
WTF!?
.
.
.



^Right along the bottom here; notice how the
scale shows the pressure as being less than
zero? How can this be?

-Kweasi

[nerobro]

It would appear that the gauge wasn't "zeroed"

[314159]

maby it was the "vacume assisted feeding" that was listed on the boxes of the first angels

[aut911]

i can just see it now, the vacum asisted angel.

comes complete with super tight vacumefeed and charger.


ittl even vacume up that nasty coke spill!

[nicad]

that is pretty funny..
but did you know that the PGI Mayhems have a vaccume assist feed? its in the bottom of the breach directly below the ball feed hole. actually pretty cool- too bad they drink air like a mule. :)

[314159]

to bad that stock mayhems max out at about 9 balls/sec =[

[Kweasi]

So I take it you guys agree that the graph is inaccurate?

That leads me to be suspicous of that 90psi peak pressure figure.

Can anyone clarify?

I greatly appricate the REAL DATA that this forum provides, but I'm a brutal skeptic and can't accept what it suggests without some explanation.

-Kweasi

[nerobro]

The data is fine. the scale is off. the machines used to do this kind of data collectoin either require re-zeroing frequently, or you need to use some logic and place the graph in the right place.

[AGD]

The sensor is dynamic in order to be able to respond quickly. With our setup the baseline of the dynamic sensor moves around so you do need to shift the graph up or down to zero to get the right numbers.

AGD

[Kweasi]

Thank you for your helpful replies. I figured it was something long those lines.

Does that 90psi figure include the scale compensation?

Also, I played around with that 90psi figure and by applying P=F/A I found that at 90psi the ball is exposed to over 30lbs of force (see below). That seems funny to me, I'd think that it wouldn't take nearly that much force to get a ball to start moving, seems to be that subjecting a poor little paintball to 30lbs would bust it in short order. Anyone have any thoughts on this?

-Kweasi

The Math:
P=F/A
Since the back half of the paintball is the area under influence of the pressure, and since it's dome shaped, if we draw vectors perpendicular to the surface of the ball then all the vertical components will cancell, and we're just left with the horizontal vectors. For that reason we can consider the area of the ball under influence to be circular. Thus:
A= (pi*D^2)/4 where D= 0.68
Thus A= 0.363in^2

so:
F= PA
F= (90)(0.363)
F= 32.7lbs

[Redkey]

Sure you can get the ball moving with much less pressure/force. However, you'll have to apply that lower force for a much longer period of time. Because the ball has to accelerate quickly you have to hit it pretty hard to get it up to speed before it leaves the barrel.

The theory behind "low pressure" guns is that they hit the ball with less pressure for a longer period of time.

[Redkey]

AGD

The pressure transducer is just like a load cell that uses a strain gauge bridge to "sense" the load. There are two ways to "zero" the transducer.

You should be able adjust the zero point and gain of your signal conditioning unit to have the output voltage at zero pressure always be the same. This is only possible if you transducer does not have the signal amp build into it... if so, you are stuck with the second option...

The other way to do it is to have labview perform a software zero before data is collected. Basicially this just shifts the entire dataset so that your initial voltage levels correspond to zero pressure.

What is the max capacity or your transducer? If you are trying to measure 100 psi using a 1,000 psi transducer that could also explain some of the low pressure drift you are seeing.

[bjjb99]

30 psi busting paint?

Kweasi,

The even distribution of pressure against the rear surface of the paintball is what keeps it from breaking. A similar effect can be seen if you take a plain old grocery store egg and squeeze it in your hand (take any rings off first); it is surprising how hard you have to squeeze before you make a mess. Smooth, convex structures are quite resistant to compressive forces of this type.

BJJB

[SLICEnDICE]

Transducers

I was under the impression from my reasurch that most pressure transducers come from the factory calibrated already.

[bjjb99]

Factory Calibration

Yes, most sensors are calibrated at the factory. Some pressure sensors are pre-calibrated for linearity (good dV/dP), leaving the zero offset to be corrected at the time of collection. All sensors are subject to drift with external pressure and temperature. It is good laboratory practice to calibrate your sensitive instrumentation immediately prior to, and immediately following, any data collection. You then know how the instruments were drifting at the time you ran your test, and can correct for that drift during data processing.

BJJB