Is this dew point calculation correct using the DHT-11?

Other Hardware Sensors
1

I converted a spreadsheet dewpoint calculation I found on http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html to a function, is this correct? I've seen several much simpler calculation on the forum and would like to know the accuracy of this find.

Requires the inclusion of math.h for the log(x) function.

/*
 * Calculates the dewpoint base off of the temperature and relative humidity
 * calculation of Dewpoint as provided on http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html
 */
float calcDewpoint(float temp, float rh)
{
  //TD: =243.04*(LN(RH/100)+((17.625*T)/(243.04+T)))/(17.625-LN(RH/100)-((17.625*T)/(243.04+T)))
  return 243.04*(log(rh/100)+((17.625*temp)/(243.04+temp)))/(17.625-log(rh/100)-((17.625*temp)/(243.04+temp)));
}

Also why are the other calculations so simple where this calculation seems more complex?

Thanks in advance for your responses.

2

It looks correct but the easiest way to check is if you write a little test code and feed it some numbers that you can compare with the online calculator.

Pete

3

I just did that and noticied I should clarify that that temp as required by this function must be provided in degrees F and not C. Can be modified as follows if temp is provided in Celsius.

/*
 * Calculates the dewpoint base off of the temperature and relative humidity
 * calculation of Dewpoint as provided on http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html
 */
float calcDewpoint(float temp, float rh)
{
  float tempf = (temp * 1.8) + 32;
  //TD: =243.04*(LN(RH/100)+((17.625*T)/(243.04+T)))/(17.625-LN(RH/100)-((17.625*T)/(243.04+T)))
  return 243.04*(log(rh/100)+((17.625*tempf)/(243.04+tempf)))/(17.625-log(rh/100)-((17.625*tempf)/(243.04+tempf)));
}
4

I tried your code and it gave the same result as the online calculator when I gave it the temperature in Celsius

(• T and TD inputs/outputs to the equations are in Celsius)

Also why are the other calculations so simple

Which one(s)?

Pete

5

el_supremo:
Which one(s)?

https://forum.arduino.cc/index.php?topic=504940.msg3445563#msg3445563

and external sites too:

are just a few, I just used a google search of "arduino dht 11 dewpoint" before posting, some do seem more complex than the one I found on the here on the forums.

6

japreja:
I just did that and noticied I should clarify that that temp as required by this function must be provided in degrees F and not C. Can be modified as follows if temp is provided in Celsius.

Tried this and although it allows degrees C for the conversion, the output is still in degrees F.

You need to take the output and -32 then * 0.5556 to get dew point in C.