Relationship between dew heater power and dew point?

[daz]

Having acquired a Mount Hub Pro and having an MBox Weather station - both of which I can interface to - I got to thinking about incorporating a simple (?) app to regulate the power to the dew strap.vRather than just being on at some arbitrary setting - which of course, may not be enough - I thought about finding out how the relationship should work.

So, I understand that the optics needs to be above dew point for dew not to form, but how should that compare to the temperature and then the heater power % ?  I am thinking that I need to understand the characteristics of the dew tape, so if I set up a test to measure the temperature increase for every 10% of heater power, that would give me a known scale for the band, at least.

Then I can measure the air temp (aT) and the dew point (dT) and do some calculation that takes these, and the change measurement above, into consideration to come up with a sensible power setting for the dew strap...

The question is - what should that calculation look like...?

I can find plenty of calculations for working out the construction of a band, but not for the amount of power to apply...

 

Your thoughts appreciated

 

[michael8554]

Hi Daz

There's an old circuit using precision temperature sensors - one measures the optics temperature, the other ambient.

The circuit then ensures that optics are at a set temperature above ambient.

In that situation dew can't form.

"Automatic Heater Control for the Prevention of Dew designed by Don Clement"

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&uact=8&ved=0ahUKEwjN1PnBnMDSAhVhFMAKHeyZDqwQFgiXATAG&url=http%3A%2F%2Fwww.clementfocuser.com%2Fimages%2FTemperature_Control_Print.pdf&usg=AFQjCNFQPR12qhCEisqf8VsDmD3rgz4N8g&bvm=bv.148747831,d.ZGg

Michael

[astronymonkey]

Hi,

I would think you would need to take into account various things such as the sepcific heat capacity of the glass and metal tubing, and the rate of conduction of the heat relative to the radiated loss etc. The tape itself may get warm but if there isnt enough energy going into the optics to raise the temparature then dew will still form and larger optics have more glass so will need more power.

Cheers

[Alpollo]

I like Micheal8554's suggestion of using a little circuit that measures the glass and compares it to ambient with a couple of temp sensors.

Another option would be a PWM driver to create a pseudo variable voltage supply. This would have the advantage of gentler heat, you are probably better off to have "just enough" heat with the heater running fairly steady as opposed to quick, hot on/off cycles. 

That being said the Clement circuit above probably works pretty good with the right size heater strip.

[redtail]

http://www.iceinspace.com.au/forum/showthread.php?t=144679 would this help

 

[daz]

Thanks guys!

I don't need the circuit, as I already have a controller on the Mount Hub Pro - it was about programmatically being able to set the power % based on ambient and dew temps.

The one that redtail links to uses a 5 deg C threshold (0% at 5 deg above dew through to 100% at 0 deg above). I found another arduino based one that used a 10oF delta in much the same way.

Ultimately, you would indeed want precision temp sensing at the optics, but I think this will suffice for now...

[Alpollo]

OK gotcha, I didn't fully understand what you are asking.

While I'm here I may as well ask why you are referencing the dew point instead of just ambient? Is this so your mirror could cool below ambient and not have to have the heat come on so long as the dew point is low enough?

I've been looking at it too simplistically I guess, that if the glass is just above ambient, dew cannot form.

I can see the advantage of not having to turn the heater on unless dew is actually going to form. Thereby letting the optics cool below ambient at times. Interesting.

[MarkyD]

I use this void/subroutine on an Arduino.  If you can make sense of it.  The whole code measures ambient humidity and temperature and the temperature of the specific sensor (secondary mirror in this case - DB18B20 stuck to the rear of the mirror).  The dew heater is usually powered at 10 - 20% power for most of the time so the mirror never gets to the dewpoint

int getpwr( float channeltemp, int trackmode )
{
  int pwrlevel = 0;
  if ( trackmode == DEWPOINT )
  {
    if ( channeltemp >= (dew_point + 6.0 + dewconfig.offsetval) )
    {
      pwrlevel = 0;
    }
    else if ( channeltemp >= (dew_point + 5.0 + dewconfig.offsetval) )
    {
      pwrlevel = 10;
    }
    else if ( channeltemp >= (dew_point + 4.0 + dewconfig.offsetval) )
    {
      pwrlevel = 20;
    }
    else if ( channeltemp >= (dew_point + 3.0 + dewconfig.offsetval) )
    {
      pwrlevel = 50;
    }
    else if ( channeltemp >= (dew_point + 2.0 + dewconfig.offsetval) )
    {
      pwrlevel = 75;
    }
    else pwrlevel = 100;
  }
  else if ( trackmode == AMBIENT) {   // assume trackmode is AMBIENT
    if ( channeltemp <= (tval - 9.0 + dewconfig.offsetval) )
    {
      // is the OTA temperature way below ambient
      pwrlevel = 100;                         // then set the pwr level to 100%
    }
    else if ( channeltemp <= (tval - 7.0 + dewconfig.offsetval) )
    {
      // is the OTA temperature 7 degrees or less below ambient
      pwrlevel = 75;                         // then set the pwr level to 75%
    }
    else if ( channeltemp <= (tval - 5.0 + dewconfig.offsetval) )
    {
      // is the OTA temperature 5 degrees or less below ambient
      pwrlevel = 50;                         // then set the pwr level to 50%
    }
    else if ( channeltemp <= (tval - 3.0 + dewconfig.offsetval) )
    {
      // is the OTA temperature 3 degrees or less below ambient
      pwrlevel = 20;                         // then set the pwr level to 20%
    }
    else if ( channeltemp <= (tval - 1.0 + dewconfig.offsetval) )
    {
      // is the OTA temperature 1 degrees or less below ambient
      pwrlevel = 10;                         // then set the pwr level to 10%
    }
    else pwrlevel = 0;
    // if ota close to ambient then pwr level to 0%
  } // end of if trackmode
  else if ( trackmode == HALFWAY)
  { // assume trackmode is MIDPOINT
    if ( channeltemp >= (tval - (((tval - (int)dew_point) / 2) - 0) + dewconfig.offsetval))
    {
      pwrlevel = 0;
    }
    else if ( channeltemp >= (tval - (((tval - (int)dew_point) / 2) - 2) + dewconfig.offsetval))
    {
      pwrlevel = 20;
    }
    else if ( channeltemp >= (tval - (((tval - (int)dew_point) / 2) - 4) + dewconfig.offsetval))
    {
      pwrlevel = 50;
    }
    else if ( channeltemp >= (tval - (((tval - (int)dew_point) / 2) - 6) + dewconfig.offsetval))
    {
      pwrlevel = 100;
    }
    else pwrlevel = 100; // its lower than midpoint - 7
  } // end of if trackmode
  else
    pwrlevel = 0;       // set pwrlevel to 0 if there is an unknown tracking mode
  return pwrlevel;
}

 

Credit to Robert Brown for this snippet of code

Here is the idea I heavily modified for my own needs to include intervalometer, LCD touch screen, voltage monitors etc contained in one box all running on a Mege2560

https://sourceforge.net/projects/arduinonanodewcontrollerpro/files/v300/Documentation/

[hughgilhespie]

Well, you did ask.............................................

Regards, Hugh

 

A well-known approximation used to calculate the dew point, T_dp, given just the actual ("dry bulb") air temperature, T (in degrees Celsius) and relative humidity (in percent), RH, is the Magnus formula:

{\displaystyle {\begin{aligned}\gamma (T,R\!H)&=\ln \left({\frac {R\!H}{100}}\right)+{\frac {bT}{c+T}};\\T_{dp}&={\frac {c\gamma (T,R\!H)}{b-\gamma (T,R\!H)}};\end{aligned}}}

The more complete formulation and origin of this approximation involves the interrelated saturated water vapor pressure (in units of millibar, which is also hPa) at T, P_s(T), and the actual vapor pressure (also in units of millibar), P_a(T), which can be either found with RH or approximated with the barometric pressure (in millibar units), BP_mb, and "wet-bulb" temperature, T_w is:

Note: unless declared otherwise, all temperatures are expressed in degrees Celsius.

{\displaystyle {\begin{aligned}P_{s}(T)&={\frac {100}{R\!H}}P_{\text{a}}(T)=a\exp \left({\frac {bT}{c+T}}\right);\\[8pt]P_{\text{a}}(T)&={\frac {R\!H}{100}}P_{s}(T)=a\exp(\gamma (T,R\!H)),\\&\approx P_{s}(T_{\text{w}})-B\!P_{\text{mb}}0.00066\left[1+(0.00115T_{\text{w}}\right)]\left(T-T_{\text{w}}\right);\\[5pt]T_{\text{dp}}&={\frac {c\ln(P_{\text{a}}(T)/a)}{b-\ln(P_{\text{a}}(T)/a)}};\end{aligned}}}

For greater accuracy, P_s(T) (and, therefore, γ(T,RH)) can be enhanced, using part of the Bögel modification, also known as the Arden Buck equation, which adds a fourth constant d:

{\displaystyle {\begin{aligned}P_{s:m}(T)&=a\exp {\bigg (}\left(b-{\frac {T}{d}}\right)\left({\frac {T}{c+T}}\right){\bigg )};\\[8pt]\gamma _{m}(T,R\!H)&=\ln {\Bigg (}{\frac {R\!H}{100}}\exp {\bigg (}\left(b-{\frac {T}{d}}\right)\left({\frac {T}{c+T}}\right){\bigg )}{\Bigg )};\\T_{dp}&={\frac {c\gamma _{m}(T,R\!H)}{b-\gamma _{m}(T,R\!H)}};\end{aligned}}}

(where {\displaystyle \scriptstyle {a=6.1121\ \mathrm {millibar} ;\quad \;b=18.678;\quad \;c=257.14^{\circ }\mathrm {C} ;\quad \;d=234.5^{\circ }\mathrm {C} .}})

There are several different constant sets in use. The ones used in NOAA's presentation^[9] are taken from a 1980 paper by David Bolton in the Monthly Weather Review:^[10]

{\displaystyle {\begin{aligned}a&=6.112\ \mathrm {millibar} ;\quad \;b&=17.67;\quad \;c&=243.5^{\circ }\mathrm {C} ;\end{aligned}}}

These valuations provide a maximum error of 0.1%, for

-30°C ≤ T ≤ +35°C;

1% < RH < 100%;

Also noteworthy is the Sonntag1990,^[11]

{\displaystyle \scriptstyle {a=6.112\ \mathrm {millibar} ;\quad \;b=17.62;\quad \;c=243.12^{\circ }\mathrm {C} :\quad -45^{\circ }\mathrm {C} \leq T\leq +60^{\circ }\mathrm {C} \quad (\pm 0.35^{\circ }\mathrm {C} )}}

Another common set of values originates from the 1974 Psychrometry and Psychrometric Charts, as presented by Paroscientific,^[12]

{\displaystyle \scriptstyle {a=6.105\ \mathrm {millibar} ;\quad \;b=17.27;\quad \;c=237.7^{\circ }\mathrm {C} :\quad 0^{\circ }\mathrm {C} \leq T\leq +60^{\circ }\mathrm {C} \quad (\pm 0.4^{\circ }\mathrm {C} )}}

Also, in the Journal of Applied Meteorology and Climatology,^[13] Arden Buck presents several different valuation sets, with different minimum accuracies for different temperature ranges. Two particular sets provide a range of -40 °C → +50 °C between the two, with even greater minimum accuracy than all of the other, above sets (maximum error at given |C°| extreme):

{\displaystyle \scriptstyle {a=6.1121\ \mathrm {millibar} ;\quad \;b=17.368;\quad \;c=238.88^{\circ }\mathrm {C} :\quad \quad \!0^{\circ }\mathrm {C} \leq T\leq +50^{\circ }\mathrm {C} \;\;(\leq 0.05\%)}}

{\displaystyle \scriptstyle {a=6.1121\ \mathrm {millibar} ;\quad \;b=17.966;\quad \;c=247.15^{\circ }\mathrm {C} :\quad -40^{\circ }\mathrm {C} \leq T\leq 0^{\circ }\mathrm {C} \quad \!\;\;(\leq 0.06\%)}}

[daz]

23 hours ago, Alpollo said:

OK gotcha, I didn't fully understand what you are asking.

While I'm here I may as well ask why you are referencing the dew point instead of just ambient? Is this so your mirror could cool below ambient and not have to have the heat come on so long as the dew point is low enough?

I've been looking at it too simplistically I guess, that if the glass is just above ambient, dew cannot form.

I can see the advantage of not having to turn the heater on unless dew is actually going to form. Thereby letting the optics cool below ambient at times. Interesting.

I am taking the same simplistic approach! If the temperature at the lens (or close to it, at least) is getting close to the dew point, then I need to apply power to the heater - this is my basic thinking.

The trick is determining how much power!

@MarkyD - interesting - and yes, perfectly understandable . So again, this uses an arbitrary value of ambient vs dew point to determine how much power to apply.

@hughgilhespie - unfortunately, your post does not appear to have pasted well from the source, so it's difficult to tell whats equation and what's formatting! Still, I think it's to do with calculating dew point. Which I don't need. My weather station does that for me, I just need to work out how much power to apply to the dew strap to compensate.

 

Thanks everyone that has responded! I think I'll go with 5 deg off set from an earlier post and see how that works! I can always adjust it - I'll keep you posted

[hughgilhespie]

Hi Daz,

Yes - it is nicely formatted in Wikipedia. Shame it didn't come over properly as it was a most impressive lot of equations!!

Good luck with the 5 degree offset - should work OK. I have also built Robert Brown's dew controller but never used it in anger.

Regards, Hugh