🚀 go-pugleaf

Many years back(for my thesis) I made an ozone-photometer with similar
characteristics, Because of the UV-light(254nm) I used a simple
quartz-glass(Schott) 45� in the beam as a splitter. Around 8% are reflected
to the reference diode. (I remember it was a Hamamatzu-part) I kept the 2
receivers embedded into an aluminum block, to get the same temperature. then
the ratio of the 2 receivers was taken.
I remember the trick with that beam-splitter worked over a large range of
frequencies. The accuracy was better than anything available that time. BTW
the photometer is still sold :-)

--
ciao Ban
electronic hardware designer
http://thinkamove.com/


"Bill Sloman" <bill.sloman@ieee.org> schrieb im Newsbeitrag
news:7c584d27.0204260038.5e480482@posting.google.com...
> aseychell@vet.com.au (Adam Seychell) wrote in message
news:<21a524b0.0204251637.5d74991f@posting.google.com>...
> > 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:
>
> <snipped the good oil from Tony>
>
> > >  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.
>
> Search on dual beam spectrometers ... I got lectures on the subject as
> a chemistry undergraudate in 1962.
>
> > 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 ?
>
> Yes. The biggest single source of temerature variation in the system
> is likely to be self-heating in the LED junction - a normal-ish
> non-red LED running at 20mA is dissipating 40-50mW in the junction,
> and IIRR, that corresponds to some ten or twenty degrees Kelvin of
> self-heating, with a thermal time constant of a few minutes, which in
> our system cuts the LED output by about 20% as it warms up.
>
> At one point in the development of our system, I allowed myself to be
> seduced by the relative cheapness of open-loop compensation, and we
> stuck a thermistor into a hole drilled in the back of a 10mm diameter
> LED, not too close to the junction, and arranged for the LED drive to
> rise as it got hotter. This reduced the excursion on the LED output
> over warm-up to about 1%, but stretched the settling time to about an
> hour - a very bad idea.
>
> Now we glue a bit of glass fibre onto the side of the LED package with
> UV-setting optical glue, passively monitor the light output with a
> second photodiode at the otehr end of the (plastic) fibre, digitise to
> 16-bits (an over-kill, but the A/D converter was cheap) and get the
> microprocessor to computer the ratio. Seems to work.