Subject: identify this device

From: [email-masked] (Michael Edelman)
Date: 20 May 1996 15:36:33 GMT

I have had come in to my posession an unusual device- it appears to be for
calibrating pressure gauges. It consists of a brass balance arm, a large
horizontal brass lfywheel driven by a rubber wheel, a couple of stopcocks,
a 3" diameter brass cylinder with a a floating piston, and various adapters.
The maker's name escapes me- wrote it down and left it at home- but it
began with C, I think- and it was made in Boston.

Does this sound familiar to anyone? Can you venture a guess as to its
age and value? Is it still a useful device?

--mike  [email-masked]

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From: Dave Knight <[email-masked]>
Date: Mon, 20 May 1996 16:03:38 -0400

Michael Edelman wrote:
> a 3" diameter brass cylinder with a a floating piston, . . .

If it looks like you could attach a paper chart to the cylinder and
it has a pen or scriber that could mark the paper, it might be a steam
engine "indicator".

An indicator is used to graphically show the power curve of an engine
for calculating power/efficiency of an operating engine. . .

Dave.

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From: [email-masked] (John Dammeyer)
Date: Wed, 22 May 1996 03:57:16 GMT

>began with C, I think- and it was made in Boston.

Sounds a bit like a primary pressure standard.  If you have a cylinder
of known dimension filled with light oil and a piston that fits if
precisely then placing a mass on the piston will produce a pressure
out a tube connected to the cylinder. Trouble is the ugly co-efficient
of static friction rears its ugly little head.  Remember?  Takes more
force to get something moving then to keep it moving so adding mass to
the piston will appear to not have any effect until you overcome the
static friction forces between the wall of the cylinder and the
piston.  This means that the reading will always be suspect.

The solution is to spin the piston by driving it with the rubber
wheel.  Now,  given a known mass placed on the turntable you can
generate a pressure independant of static friction because the mass is
already moving.  These are normally very expensive devices.

Cool eh?

John.