FROM: "Mark Folsom" <folsomman@redshift.com>
SUBJECT: Re: Dew Point , formula, effect on coatings ...
DATE: Mon, 16 Oct 2000 09:37:34 -0700
ORGANIZATION: Consultant
NEWSGROUPS: sci.optics

"Bill Davy" <Bill@SynectixLtd.com> wrote in message
news:_izG5.3136$_B5.33836@NewsReader...
> We are proposing to cool a laser diode to the bottom of our product's
> specified operating temperature range (15 to 25 deg C).  Someone has
pointed
> out that we may be below the dew point, so we need to tie this in with a
> relative humidity value.

The dew point is the temperature where the vapor pressure of water would be
equal to the current partial pressure of water vapor in the air.  Relative
humidity is the ratio of the partial pressure of water vapor in the air to
the vapor pressure of water at the current temperature---

RH = Pw/Pv

The Clapyron equation for approximating vapor pressure for liquids is of the
form

Pv = a*e^(-b/T).

For water at around room temperature, I recently found the coefficients of
this equation:

Pv = (1.670275717·10^11)·ê^(- 5306.774193/T).

You can find the partial pressure of water in the air at the ambient
temperature from the relative humidity and the vapor pressure (from the
above equation) and then you can solve the above equation for the
temperature at which Pv = Pw.

> I have very complicated formulae involving arcane properties, but I would
> like a simple way of plotting (ideally) or tabulating dew point, ambient
> temperature (from 15 to 30 deg C say) and relative humidity for typical
> conditions (normal pressure, for example).

For $125.00 per hour (1 hour minimum) I can write a little spreadsheet for
you.

>
> Incidentally, am I right in assuming that condensation could permanently
> damage typical coatings (for use at about 400 nm), and that they should
> therefore be stored in non-condensing conditions?

If you operate a diode with liquid water on the facet, it will generate
severe thermal stress and probably cause damage.

Mark Folsom






FROM: "Bill Davy" <Bill@SynectixLtd.com>
SUBJECT: Re: Dew Point , formula, effect on coatings ...
DATE: Wed, 18 Oct 2000 08:28:49 +0100
NEWSGROUPS: sci.optics

Herewith Dew Point program.  Did what I wanted ...


//
// Jensen et al. 1990. ASCE Manual #70 (pages 176 & 177).
// Assume sea level and P = 100 kPa.
//

double es(double TdegC) // TdegC is dry bulb temperature
{
 return 0.611*exp(17.27*TdegC/(TdegC+237.3));
}

double Tdewpoint(double e) // e is ambient vapour pressure in kPa
{
 //
 // Return dewpoint temperature in degrees Centigrade.
 //
 return (116.9+237.3*log(e))/(16.78-log(e));
}

inline double round(double v)
{
 return (v>0.0) ? floor(v+0.5) : ceil(v-0.5);
}

int main(int argc, char* argv[])
{
 printf("Hello World!\n");

 {
 double e = es(25);
 printf("e=%f (should be 3.17 kPa)\n",e);
 double td = Tdewpoint(e*0.5); // should be
 printf("td=%f (should be 13.85 deg C)\n",td);
 }

 {
 printf("Dew Point Temperatures tabulated for RH and Ambient\n");
 for(int RH=10; RH<100; RH += 5)
  printf("\t%i",RH);
 printf("\n");
 }

 for(int TambDegC=15;TambDegC<=35; TambDegC += 5)
  {
  double esTamb = es(TambDegC);
  printf("TAmb=%i",TambDegC);
  for(int RH=10; RH<100; RH += 5)
   {
   double e = RH*esTamb/100;
   double TdewDegC = Tdewpoint(e);
   printf("\t%.1f",round(TdewDegC*10)/10);
   } // next RH
  printf("\n");
  } //next TambDegC
 return 0;
}