No wonder Brian knows a lot: If I'm correct, Brian has built a complete
weather station somewhere near Limassol/ Cyprus.  Today I suspect Brian is
busy with digging a big whole in his garden, for hiding his euros from the
tax collector.  This time, he is not in the Sherwood, so there are eyes
everywhere.

On 16 March 2013 09:59, Gerry Gagnon <[log in to unmask]> wrote:

> As someone who now works PCB/PCBA in the "instruments" field I can tell
> you that Brian is absolutely correct.A very short note with a lot of
> information that probably should go to Inge's dropbox. Thank you Brian!
> Gerry
>  > Date: Sat, 9 Mar 2013 10:08:02 +0200
> > From: [log in to unmask]
> > Subject: Re: [TN] Humidity Measurements: Relative or Absolute
> > To: [log in to unmask]
> >
> > Just to put a cat amongst the pigeons, meteorologists measure the water
> > vapour content of the air as dew point (I point out, not Dewey!) as
> > being the only accurate method. This done with a thin metal mirror
> > thermally attached to a Peltier junction. A beam of light impinges on
> > the mirror at 45° and is reflected into a photodiode. A current passes
> > through the Peltier junction to heat it until the detected light reaches
> > a threshold value, then the current is reversed and the mirror is cooled
> > at, say, 2°C/min until the light beam diminishes due to dew forming on
> > the mirror and its temperature is accurately measured with a Pt100
> > resistance thermometer attached to it. This method is very accurate.
> > Conversion to RH is done through tables or an algorithm.
> >
> > The next most accurate method is through a wet and dry bulb thermometer,
> > using Pt100 thermometers, with the disadvantage that daily maintenance
> > is required to ensure the reservoir is full of pure water and that the
> > sock is clean and moist.
> >
> > Then comes capacitive sensors, consisting of two electrodes separated by
> > a hygroscopic dielectric. As water has an enormous dielectric constant
> > (permittivity), the more moisture absorbed, so the value of the capacity
> > increases.
> >
> > The second least accurate method, and the one most commonly used for
> > electrical measurement, you have the resistive sensor, where the
> > resistance of a piece of plastic doped with a fairish quantity of an
> > organic hygroscopic substance is measured. This drifts after a couple of
> > years or so.
> >
> > The least accurate and cheapest is the hair hygrometer where the length
> > of a hygroscopic fibre (originally human hair but now synthetic) acts
> > directly on the needle of a dial. This is not reliable or accurate.
> >
> > I have a semi-pro weather system that uses capacitive sensors for both
> > inside and outside. The dew point is claimed to be accurate to ±1.5°C
> > between -76°C and +54°C, translating to ±3% from 0% to 100%.
> >
> > No matter which system is used, it must be placed in an enclosure which
> > allows free access of the air being measured with no risk of a current
> > of air exceeding, say, 0.05 m/s. This is why double- or triple-louvred
> > housings are commonly used for professional use. For the electronic
> > types, these are usually shared with the temperature sensors, to avoid
> > errors from direct solar or other heat source radiation and they are
> > moulded in bright white plastic. I can say that, as I write at 10:00,
> > the outside temp is 15.6°C ±0.5°C and the dew point is 7.9°C ±1.5°C,
> > giving a RH of 58% ±3%. Fully professional weather systems would halve
> > the accuracy tolerances and would cost 20 times more! Similarly, cheapo
> > amateur systems can be obtained for 1/5th cost of mine and their
> > accuracy tolerances are never mentioned! The ratio of 100:1 from a <$100
> > amateur weather station to a >$10,000 professional one is very eloquent.
> > This ratio can also apply to humidity sensors.
> >
> > As has already been pointed out, the unit for absolute humidity is g/m³
> > and the conversion AH>RH is temperature dependent. For applications we,
> > as production guys, are likely to need and encounter, the unit is
> > percent relative humidity at a given temperature.
> >
> > Brian
> >
> > On 08/03/2013 18:23, Robert Kondner wrote:
> > > Hi,
> > >
> > >
> > >
> > > As I start looking at humidity measurement devices I started to wonder
> if
> > > IPC humidity numbers are Relative or Absolute? Does anyone know off
> hand. I
> > > need to go buy some specs.
> > >
> > >
> > >
> > > I would have thought Absolute but the cardboard indicators are
> temperature
> > > sensitive as are the desiccant drying systems in dry cabinets.
> > >
> > >
> > >
> > > If the temperature is high, say > 100C then a part will "Bake Out"
> > > regardless of the absolut humidity, is that right?
> > >
> > >
> > >
> > > Bob K.
> > >
> > >
> > >
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