Re: mass vs. volume measurement


"Andy Resnick" <andy.resnick@xxxxxxxxxxx> wrote in message
news:d6i2d8$5c5$1@xxxxxxxxxxxxxxxxxxxxxx
> Old Man wrote:
>
> <snip>
>>
>> Yes. The normal method of "calibration" consists of
>> parts replacement, but, first, they check for radioactive
>> contamination and refuse to touch such. Used properly,
>> contamiation isn't a problem. So why do they check ?
> <snip>
>
> No, I think the calibration is a dispense into a calibrated cuvette. At
> least that's how I would do it.
>
> Lots of lab equipment requires periodic calibration- I have a friend who
> works at Keithley, and they go way beyond anything I would ever do (and
> rightfully so, given their business). It's interesting talking to her
> about it.
>
>
>>
>> Old Man once used a vapor pressure osmometer on
>> plant tissue samples. Calibration every two hours, and
>> if room temperature was changing, it wasn't used at all.
>> Others in the lab used it (required, against their better
>> judgment) to check stock solution concentration. They
>> just couldn't understand the need for prior calibration
>> (five measurements) nor the need for repeats after
>> calibration. Fortunately for science, they weren't given
>> a choice.
>
> Yeah, I still don't really understand how osmometers work. Something
> about freezing point depression, but it seems kinda voodoo.

Not Voodoo. Lots of good physics !! It depends
upon the non-equilibrium water evaporation rate
from the thermocouple. Extremely small temperature
drop. You can roughly calculate the vapor pressure
over a solution from ion concentration. if NaCl is
1 / 2, then CaCl2 is 1 / 3. However, tables in the
Handbook of Chemistry & Physics are the standard.

Works great until some slob mucks-up the thermo-
couple with his solution. For fast response, the
thermocouple is about the size of a dust mote. The
electrical leads are virtually invisible. Dust motes get
brushed away by untutored slobs.

[Old Man]

> Andrew Resnick, Ph.D.


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