SB Probe psi

[bahed]

I was asked today by a client how much force the probe tip will be exerting on the surface of the object that I will be probing for them. I said not much, shrugged - and that I would find out.

Any quantifiable numbers ?

[thewoodcrafter]

Boy, that's an interesting one.

Yah, not much.
I would say you could probe clay and it might not mark it.

[dana_swift]

That question got my curiosity up.. so I went and measured it.

I used a postal scale to measure the force required for the LED to change color. That was 2.5oz on my postal scale. Then I measured the diameter of the probe I have installed, its diameter is 0.078" according to my micrometer.

D = 0.078"
R = 0.039"

The Area is:
A = Pi * r^2
A = 0.004778362 in^2

2.5oz = 0.15625lb

So..

psi = 0.15625 lb / 0.004778362 in^2
psi = 32.7

Much higher than I would have guessed. Its not much force, but its a very tiny area. I suspect the clay experiment would yield dents.

I hope that does not discourage your customer.. try using a larger diameter probe tip so the psi will drop radically. You can do the math from here-

Or you can tell them 2.5oz.

That was interesting.. a fun little experiment!

D

[cabindoors]

Dana,

You made the assumption that the full diameter of the probe tip (.078") made contact with your postal scale.

If it did not, then your calculations are in error.

[erik_f]

so the psi numbers would actually be higher if the probe only make partial contact

[dana_swift]

Tom and Erik- I was fully aware of that, I didn't have any way to correct for the actual contact area, it will enter the material until the material provides 2.5 oz of resistance. Then the bot will get the signal to pull back. The area will be determined by the material, with a MINIMUM of 32.7psi of pressure according to what I measured. I cant say all probes operate with the same amount of force, or that my postal scale is accurate, or even my micrometer.

Tom if you wish you can send a probe to NIST, they will be happy to get a definitive answer for any given material. Send a large pile of cash for the investigation, but the answer will be indisputable. Let us know what results they get.

My answer was free, take it for what it cost. Bret had a great question and I thought it interesting enough to go get an answer. What I post to the forum is "reasonably" accurate, and intended to be helpful but not definitive.

Fortunately most materials that get probed are able to provide the 2.5oz resistance required without any damage.

Bret- I hope the answer was useful for your purposes. It was for me.

D

[cutitout]

All probe sizes will have the same size surface contact point if the probe contact is a sphere, near microscopic. So the true PSI is very very vary high but the total pressure to trip is what your scale showed.

Cool idea -- and good math work.

[erik_f]

Yes, I was not bashing your answer Dana...I think you offer a lot on the forum. I was just trying to clarify what the previous poster was trying to say. I would guess it would be hard to probe a fresh sandwich, but a stale one might work out ok. I believe I have read you are an electrical engineer and enjoy your posts and admire your knowledge about things technical. I was kicked out of high school and quit college...a decision I regret at times. Now I know that anytime I can get a free lesson in something over my head, I ought not give the teacher a hard time.

[dana_swift]

Thanks, both Eriks

It's an interesting thing to get our arms around.

Real world lessons in math, physics and politics no less!

D

[cutitout]

Yes -- fun to work out stuff like this. it will haunt me from time to time until we have nailed it down.