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Old October 13th 18, 04:02 AM posted to rec.bicycles.tech
John B. Slocomb
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Posts: 417
Default "torque wrench" pump/compressor

On Fri, 12 Oct 2018 22:21:58 -0400, Frank Krygowski
wrote:

On 10/12/2018 7:01 PM, John B. Slocomb wrote:
On Fri, 12 Oct 2018 11:08:06 -0400, Frank Krygowski
wrote:

On 10/12/2018 10:18 AM, Radey Shouman wrote:
John B. Slocomb writes:

On Thu, 11 Oct 2018 15:36:04 -0400, Frank Krygowski
wrote:

On 10/11/2018 1:53 PM, Theodore Heise wrote:
On Thu, 11 Oct 2018 09:27:02 -0700 (PDT),
Frank Krygowski wrote:
On Thursday, October 11, 2018 at 5:00:33 AM UTC-4, John B. Slocomb wrote:
On Thu, 11 Oct 2018 08:54:38 +0200, Emanuel Berg
wrote:

Just out of curiosity, is there a "torque wrench" pump or
compressor? I.e., you would screw on the presta valve, set
the gizmo to e.g. 35psi, engage it, and instead of watching
the indicator, automagically at the right level it would
stop?

Most of the gas stations here use an air station that you can
set for your desired pressure and then just plug the hose onto
the tire valve
- there is a little clamp to hold it there. When the tire is
inflated to the specified pressure the inflation stops and a
bell rings.

Since they aren't manufactured here I had assumed that the
rest of the world had them too.

My experience from 50+ years ago says not to rely on those
things, although I suppose they may be different now.

Back then I blew a tire off the rim with one. I suspect the
problem was the volume of each pumping stroke. In a large sized
car tire, the volume surge with each big stroke would be
absorbed and barely raise the pressure. In a low volume bike
tire, it caused an explosion. That's my guess anyway.

I usually inflate using a manual floor pump with a gage. It's
easy enough to stop pumping when the dial reads the desired
temperature.

Don't you mean, when the dial reads the desired foot-pounds?

Oh geez, my mistake!

But: Neither! I stop when it reads the desired PRESSURE!

Around here we use psi = pounds per square inch. Weirdly enough, my
pump's pressure gauge is also graduated in kg/cm^2. I would have used
that as a bad example in my courses, since kg is properly used to
measure mass, not force. And pressure is force per unit area.

(This indicates that the SI system gets misused as much as the U.S. or
Imperial system.)

But isn't "pound" a measurement of mass also :-?

As I used to explain it to students: Properly speaking, a _force_ is a
push or a pull on an object. Properly speaking, _mass_ is a measure of
the amount of matter in an object. _Weight_ is a particular force, i.e.
the force of gravity on an object.

So in a U.S. grocery if you buy 2.2 pounds of cheese, you're buying the
amount of cheese upon which the earth's gravity exerts a force of two
pounds. It's a roundabout way of specifying the mass you want, but it
works as long as you're just talking cheese, etc.

In a European country, you'd specify you wanted a kilogram of cheese,
which is about 2.2 pounds worth. There, you're directly specifying the
amount of cheese you want.

I think I must have been out of school for too long. How is 1 kilogram
which equates to approximately2.20462262185 pounds a different
measurement than pounds? Aren't they both a measurement of the effect
of gravity on a certain amount of stuff?


First, don't trouble yourself. For ordinary everyday stuff it doesn't
matter.

But at its heart, mass is not the same as weight. For a simple example,
if you took a 1 kg mass to the moon its mass would still be 1 kg. But
its weight would be about 1/6 of what it was on earth. And in the ISS
the observed weight or effective weight of that object would be zero.

Again, if you're just (say) buying cheese on the surface of the earth
the difference doesn't matter. If you don't keep it straight in
calculations involving dynamics - as in "how much tension will be on
this connecting rod?" - you're apt to get answers that are off by a
factor of 32. Or in an SI system country, off by a factor of 9.8 or so.


O.K. mass would be a factor in the acceleration of an object when a
force is applied to it.

--

Cheers,

John B.
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