🚀 go-pugleaf

Tony Williams <tonyw@ledelec.demon.co.uk> wrote in message news:<4b2cc6c7e6tonyw@ledelec.demon.co.uk>...
> In article <21a524b0.0204250322.19119138@posting.google.com>,
>    Adam Seychell <aseychell@vet.com.au> wrote:
> 
> > The problem lies in a mismatch in temperature sensitivities between 
> > the two optical paths. When I touch one leg of the LED with a 
> > soldering iron tip (1 second) the effect is easily produced. I can
> > also see the output takes several seconds to settle after power
> > is applied, as  if the LED is heating up and stabilizing. I 
> > suspect the LED is changing its radiation pattern with temperature,
> > thus altering relative amounts of light to the feedback and 
> > measuring photodiode. 
> 
>  I've been here Adam.... Six weeks of piddling about,
>  trying to get an absorption meter stabilised.
> 
>  I ended up with the LED, stabilising photodiode,
>  and measuring photodiode mounted in an aluminium
>  block, stabilised at 35C (+/- a fraction). The
>  output/input light was via 5mm fibre-optics.
>  I even had a special bundle made, where the servo
>  takeoff was a 1% Y-piece off the main transmitting
>  fibre-optic bundle.
> 
>  And still the bleeding thing was not stable enough.
> 
>  What caught me out was that the transmission through
>  the measured medium was itself not stable, being
>  sensitive to temperature and *tiny* shifts in the
>  wavelength of the generated light.
> 
>  The real answer to the problem is to have two
>  identical light channels, one as a reference (with a
>  known absorption), and the other being the measuring
>  channel. The output is the ratio of the two channels.


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. 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. Was there a need for a stabilizing photodiode even 
though temperature remained at a constant 35�C ?