# Convert Dewpoint Temperature specification to RH (Relative Humidity)

#### Charles Wathen

##### Involved - Posts
Hi gang,
I'm performing a measurement uncertainty, and the chilled mirror hygrometer that I use has an accuracy of:
accuracy: ±0.2°C dewpoint
repeatability: ±0.05°C.

I need to convert the above dewpoint temperature spec to RH (relative humidity). I've searched the net for a simple conversion, but I could not find one. I need to maintain the RH value in order to convert that value back to DC volts so I maintain the same parameter.

#### Al Rosen

Staff member
Super Moderator
Charles Wathen said:
Hi gang,
I'm performing a measurement uncertainty, and the chilled mirror hygrometer that I use has an accuracy of:
accuracy: ±0.2°C dewpoint
repeatability: ±0.05°C.

I need to convert the above dewpoint temperature spec to RH (relative humidity). I've searched the net for a simple conversion, but I could not find one. I need to maintain the RH value in order to convert that value back to DC volts so I maintain the same parameter.
Hi Charles:

I don't think it's a simple conversion since it is temperature dependent. Try this link anyway.
http://www.paroscientific.com/dewpoint.htm

#### Charles Wathen

##### Involved - Posts
Hi Al,
Thanks for the reply. After spending additional time on the net researching this, you were correct: it's temperature dependent. I found an Excel Spreadsheet that has the complicated formula. I took this spread sheet and widened the column for the calcuation to give me the precision I needed.

Our Calibration Lab typically runs at 70°F. I took 70°F and using a dewpoint of 32°F, it calculated the humidity to be 24.3927%. I then took 70°F and using a dewpoint of 32.36°F, it calcualted the humidity to be 24.75%. The difference between these two values is 0.3573%RH or 0.0036 Vdc on the Fluke 8842A.

The formula used:
T = temperature deg F
D = dewpoint deg F
Code:
``6.108*EXP(17.27*(((D-32)*5/9+273.16)-273.16)/(((D-32)*5/9+273.16)-35.86))/(6.108*EXP((17.27*((T-32)*5/9+273.16-273.16))/((T-32)*5/9+273.16-35.86)))*100``
If anyone wants a copy of this Excel Spreadsheet, here is the link:
http://ggweather.com/wx_calc.xls