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Re: constant volume gas bulb apparatus

From: Glenn Knapp <kahuna@vcn.com>
Date: Thu, 20 Aug 1998 21:25:51 -0700

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At 03:46 PM 8/20/98 -0700, DSCHROEDER@cc.weber.edu wrote:

Our department has a bunch of Winsco (Wabash Instrument Corp.)
gas bulb things, you know, the copper bulb attached to an
"absolute pressure" gauge for testing the relation between
pressure and temperature for an ideal gas (or for defining
a temperature scale based on an ideal gas). What bothers me
is this: We're at 4700' above sea level, where the pressure
should be about 15% less than a standard atmosphere, and yet
all 10 of these things are reading between 760 and 780 mmHg
when the valves are wide open. I tried the experiment, immersing
it in four different temperature baths, with three different
amounts of air in the bulb, and got three nice straight lines
that intersect at about -280 Celsius and +165 mmHg. Thus, I'm
pretty sure that the gauges are off by an additive constant
of roughly 130-160 mmHg. I'm wondering how they could all have
gotten off by this amount. As far as I know they've never been
used, and I can see no way of opening them up and changing the
calibration. I suppose it's possible that years ago, some well-
meaning technician popped all the covers off and twisted the
needles on the dials, but I doubt it. Is anyone out there
familiar enough with this apparatus to tell me how to recalibrate
it? Thanks.

Dan Schroeder
Weber State University
dschroeder@cc.weber.edu

Dan,

I've got one of those things here in Wyoming at around 4500 feet and it too
reads around 15 psi. The bulb has a constant volume - it's pretty rigid -
and,
as a result of that, does not respond to external pressure changes. Instead,
it responds to temperature changes, the pressure in the bulb varies with the
temperature independently of the external pressure of the environment. It's
not a barometer.

What bugs me is how the weather reports give the air pressure in inches of
mercury but they convert them to 30 inches of mercury rather than reporting
what the actual pressure is.

Glenn Knapp

-----------------------------------------------------------------
Physics Kahuna
Kahuna Physics Institute - on the flapping edge of physics research.
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<html><div>At 03:46 PM 8/20/98 -0700, DSCHROEDER@cc.weber.edu
wrote:</div>
<div>>Our department has a bunch of Winsco (Wabash Instrument
Corp.)</div>
<div>>gas bulb things, you know, the copper bulb attached to
an</div>
<div>>"absolute pressure" gauge for testing the relation
between</div>
<div>>pressure and temperature for an ideal gas (or for
defining</div>
<div>>a temperature scale based on an ideal gas).  What bothers
me</div>
<div>>is this:  We're at 4700' above sea level, where the
pressure</div>
<div>>should be about 15% less than a standard atmosphere, and
yet</div>
<div>>all 10 of these things are reading between 760 and 780
mmHg</div>
<div>>when the valves are wide open.  I tried the experiment,
immersing</div>
<div>>it in four different temperature baths, with three
different</div>
<div>>amounts of air in the bulb, and got three nice straight
lines</div>
<div>>that intersect at about -280 Celsius and +165 mmHg.  Thus,
I'm</div>
<div>>pretty sure that the gauges are off by an additive
constant</div>
<div>>of roughly 130-160 mmHg.  I'm wondering how they could all
have</div>
<div>>gotten off by this amount.  As far as I know they've never
been</div>
<div>>used, and I can see no way of opening them up and changing
the</div>
<div>>calibration.  I suppose it's possible that years ago, some
well-</div>
<div>>meaning technician popped all the covers off and twisted
the</div>
<div>>needles on the dials, but I doubt it.  Is anyone out
there</div>
<div>>familiar enough with this apparatus to tell me how to
recalibrate</div>
<div>>it?  Thanks.</div>
<div>></div>
<div>>Dan Schroeder</div>
<div>>Weber State University</div>
<div>>dschroeder@cc.weber.edu</div>
<br>
<div>Dan,</div>
<br>
<div>I've got one of those things here in Wyoming at around 4500 feet and
it too reads around 15 psi.  The bulb has a constant volume - it's
pretty rigid - and, as a result of that, does not respond to external
pressure changes.  Instead, it responds to temperature changes, the
pressure in the bulb varies with the temperature independently of the
external pressure of the environment.  It's not a barometer.</div>
<br>
<div>What bugs me is how the weather reports give the air pressure in
inches of mercury but they convert them to 30 inches of mercury rather
than reporting what the actual pressure is.</div>
<br>
<div>Glenn Knapp</div>
<div>> </div>
<br>
<br>
<br>
<br>

<font face="Times New Roman, Times" size=4>-----------------------------------------------------------------<br>
</font><font size=3>Physics Kahuna<br>
Kahuna Physics Institute - on the flapping edge of physics research.
</font></html>

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