🚀 go-pugleaf

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.

-- 
Tony Williams.