>Hello,
>I have question about manometer calibration.  In my lab I have conventional
>(old fashioned) carbonate, water equilibration, silicate, and sulfide
>vacuum extraction lines.  On the carbonate line I have a mercury manometer
>which I calibrated long ago ( so long ago I have forgotten exactly how I
>did it) and I use measured amounts of gas from it to calibrate Baratron
>Gauges for manometers on the other lines.  Today the mercury
>manometer  broke (what a mess) and I wonder if I should use this
>opportunity to replace it with a Baratron.   My worry is about
>calibrating.  Is it possible to calibrate the manometer from first
>principles using a Baratron Gauge with out knowing the actual displacement
>in terms of mm of Hg?  It would be wonderful to get rid of the Hg if there
>are alternative ways to calibrate.
>Thank you for your suggestions.
>
>Andy
>
>Andy,


The calibration is relatively straight-forward.  Get a sample tube
and measure its volume as precisely as possible - for example by
filling it with a liquid and weighing it before and after or by
titration.  Sample tubes with a regular greased stopcock work best
because then you don't have to worry about the stem location for
example when you are using a Teflon stopcock.

Next, freeze or expand CO2 into the sample tube.  Start with as much
gas as you can (but less than an atmosphere of course).  Attach your
sample tube to the manometer system - it should have a coldfinger
next to the manometer.  Once the gas has reached room temperature
(measure the temperature) open the sample tube to the manometer.
Read the pressure off the manometer.  Since you know the volume of
the sample tube you can now use the ideal gas law to calculate the
number of micromoles in your sample tube.  Close the sample tube and
evacuate the manometer and coldfinger.  Next freeze the gas from the
sample tube into the coldfinger and let it reach room temperature
(record the temperature).  Now you can read the pressure of the
manometer and you have a pressure-micromole relationship (your first
data point).  Now allow the gas to expand into the sample tube from
the manometer.  Read the pressure (and temperature) and close the
sample tube.  Now you know again from the pressure how much gas is in
the sample tube.  Repeat the process until you gas pressure become to
low to record.  You should be able to get 4-5 readings from a single
gas sample this way.  You can fill the tube again and repeat the
process with another gas sample.

I use pressure tranducers  and for all practical purposes we get
linear correlations between pressure and micromoles.  Don't forget
that you calibration curve will be for a certain temperature (e.g
25C) so you must correct your values if the temperature for the
unknown is different from that.

Good luck,

Hal Karlsson

>
>**********************************************
>Dr. Andrew R. Campbell                       *
>Professor of Geology                         *
>Dept. of Earth and Environmental Science     *
>New Mexico Tech                              *
>Socorro NM 87801                             *
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Dr. Haraldur R. Karlsson
Associate Professor
Department of Geosciences
and Department of Chemistry and Biochemistry (joint)
Texas Tech University
Lubbock, TX 79409


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