🚀 go-pugleaf

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)? 

 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.

-- 
Tony Williams.