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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