🚀 go-pugleaf

Tony Williams <tonyw@ledelec.demon.co.uk> wrote in message news:<4b2d2046aetonyw@ledelec.demon.co.uk>...
> In article <21a524b0.0204251637.5d74991f@posting.google.com>,
>    Adam Seychell <aseychell@vet.com.au> wrote:
> 
> > I'm glad you mentioned this Tony. I thought I was doing something 
> > horrible wrong, but I have now learned that stabilizing absorption 
> > photometers is not a trivial task.
> 
>  Instability in the optical path(s) is potentially
>  more of a problem than that of the electronics.
> 
>  MarkII of the instrument went for plane optics.
>  A large-area light source (tungsten bulb at dull
>  red), big fibre-optic bundles on the transmitter
>  side, and all optical paths meeting at a right
>  angle (to avoid problems due to reflections or
>  a change in the refractive index).
> 
> [text shuffled around slightly]
> > Was there a need for a stabilizing photodiode even 
> > though temperature remained at a constant 35�C ?
> 
>  To try and stabilise the frequency of the
>  transmitted light.
> 
>  The medium was water and the light source was
>  at about 900nm (in the near infra-red). Water
>  contains a nasty ambush around here, it has a
>  series of large dips in the light transmission,
>  as though there were a series of high-Q notch
>  filters in there.   Bill Sloman mentioned the
>  technical reason for this a few years ago.. was
>  it absorption due to hydroxyl ions (or something)? 

Must have been water vapour - the absorption around 900nm would have
been hydrogen-oxygen stretches (IIRR - it's been thirty-five years
since I used this stuff) and the comb-like fine structure would be the
P Q and R bands that you get due to the interaction of the quantised
rotational energy of the water vapour molecule with the much more
coarsely quantised vibrational energy.
 
>  If I was 'lucky' enough with the LED frequency it
>  would sit on the slope of one of these notches, so
>  that the tiniest shift in LED frequency would cause
>  a large change in the apparent absorption.  That's
>  the main reason why MarkII went for a tungsten bulb.
> 
> > Well you seemed to eliminate your  optics problem with a
> > temperature stabilized housing, but there still remains 
> > the stability of the medium you are trying to measure. I'm 
> > not yet sure if this will be a problem in my case. I like your idea of a 
> > two channel device. I'd rather build two simple devices than a single 
> > complicated one.
> 
>  Paul Mathews' remark is right... the reference
>  channel should share as much of the optical path
>  as possible.  The priority there is to get the
>  reference channel to 'see' exactly what the
>  measuring channel is seeing.  Short lengths of
>  plastic fibre-optic cable may be handy as light
>  guides with red light.
> 
>  I had an idea that looked a possible, but did not
>  get time to try it (losing too much money on the
>  job).  The idea was to have one light source into
>  the medium and to look at the apparent absorption
>  over two different path-lengths in the medium. 
>  The (log of the) difference between the two results
>  should be the absorption per unit path length.
> 
>  Note: Log-ratio circuitry on the receiving side
>        is useful.

See J.F.Gibbons and H.S.Horn, IEEE Transactions on Circuit Theory
CT-11 frpm page 378 (1964) - reference 71 in my Ph.D. thesis.

National Semiconductor put the circuit into their application notes
from time to time. The LM3046/CA3046 contains a useful matched pair of
transistors - and some clever people (was it you Tony?) have been
known to use the other transistors as heaters and temperature sensors
to stabilise the temperature of the matched pair.

----
bill Sloman, Nijmegen