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Old October 13th 18, 03:16 AM posted to rec.bicycles.tech
Frank Krygowski[_4_]
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Default "torque wrench" pump/compressor

On 10/12/2018 6:54 PM, John B. Slocomb wrote:
On Fri, 12 Oct 2018 10:18:06 -0400, 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 :-?


When I was in school, years ago, we were quite strictly made to write
either lb_f (pound force) or lb_m (pound mass), and to include unit
conversions from one to the other using constants g (the nominal force
of graivty at the surface of the Earth) and g_c (a unit conversion factor).

The conversion is:

lb_f = lb_m * g / g_c

In English units g = 32.2 ft/s^2
g_c = 32.2 lb_m ft/s^2 lb_f

but if you didn't include the conversion, you failed.


Question. "Lb_m * g". how can you meaure 1 lb_m without gravety?


Interestingly, they do that! Or rather, they do it in orbit or in free
fall*, which is effectively the same thing.

On the International Space Station, they attach the mass to a spring
system. The frequency of oscillation allows them to determine the mass.

Here you go: https://www.youtube.com/watch?v=8rt3udip7l4

(* Free fall or stable orbit isn't exactly "without gravity" because
gravity is acting on the object. But since the object is accelerating
freely in response to gravity, it's effectively the same as if the
gravitational force were zero.)

--
- Frank Krygowski
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