Fred Bloggs said:
Aubrey McIntosh wrote:
[..snip...]
Just stop wasting time and use two opamps- one for your voltage
reference and a second to servo the output- and what purpose does that
ultra-large base resistor serve except to test the output range limits
of the opamp. What is the circuit supposed to do anyway?
Fair enough.
The larger problem that I am working on is this. Design, build, and
characterize a "microdistillation" probe for mass spectrometry.
Interpret the data with factor analysis, according to "On the
discrimination and quantitation of isomeric mixtures of compounds by
mass spectrometry," Dissertation, McIntosh, 1999. Using known and
unknown mixtures, report assay values. Do subtle chemistry to enable
chiral determinations. Build at least one mass spectrometer which
allows such determinations to be performed quickly.
In use, the probe will be in a high vacuum chamber, 3E-7 Torr
pressure. Sample will be on the probe, and the temperature programmed
specifically to facilitate interpretation by the Arrhenius (or
Clausius-Clayperon) equation. Multiple compounds will be mixed, and
the exact amounts are not only unknown, but the object of
determination. In replicate runs, known amounts of the compounds can
be added to the sample (Method of Standard Additions) or else run in
pure form.
Total amounts of material will be small. Typically a 1 uL deposition
of solution is made, and the solvent evaporated. The solution
deposited is typically 25 mg / 25 mL w/w concentration. This is
general purpose, so the analytes are whatever comes in the door.
However, you may wish to Google for
to get an idea of the likely
candidates.
This probe is structurally similar to a "hot wire" probe that is in
common use, with more finesse. That is, it will be a helix with a
diameter of about 0.5 -- 1.0 mm and a length of 1-2 mm. The wire
diameter is small. It can be fabricated from Wollaston wire and
etched. The dimensions should be such that a droplet of solution
should be held by the wire prior to solvent evaporation. Various
solvents, such as water, ethanol, and methylene chloride should be
accomodated.
Preliminary constraints are:
1. Go from -50C to 300C as quickly as 30 seconds.
2. Report the actual temperature of the probe.
3. Manage the temperature profile so that the value (1 / Kelvin)
changes at a linear rate.
4. Deliver sufficient power so that the analyte evaporates without
deviation from the temperature profile.
5. Log the power delivered to the probe. Log enough information so
that the power given to the sample, as distinct from heating the wire,
is recorded. For reference, read up on Differential Scanning
Calorimetry. Adapt so that the reference scan is performed on the
same hot wire, at a different time. Be able to calculate Enthalpy of
Sublimation.
6. Oxygen sensitivity contra-indicates tungsten construction
material. Rhodium and platinum are commonly used and easily
available. Nickel is also a platinum group material, and may be
preferred for the early prototypes, depending on availability and cost
of well characterized material. Platinum thermometers are likely to
be the cheapest solution.
7. In the (predominant) operation mode where cooling is not
energized, record temperature with a minumum of self heating and
regain control when servo requests heating.
8. Allow a cleaning cycle where the wire reaches incandesence. While
not necessary chemically, this is an important human interface design
aspect.
9. Respect two emergency input signals: cut power, and maintain
current power.
The current area of effort is to move from linear-temperature-in-time
control without power logging, to linear in "reciprical Kelvins"
control with power logging and energy integration.
My choices are to learn enough electronics, or to hire a EE postdoc
and teach him vacuum systems and chemistry. So I am working through
various stable voltage controlled power supply concepts. You may
choose to view the LEDs as a red herring, a double-dipping project, or
a way to make the project seem interesting to the readership of s.e.d.