Both relative humidity and dew point temperature are measure of the amount of moisture in air or other gases. Some applications require the moisture content expressed as relative humidity while others require the moisture content as dew point temperature. Often these applications are industry specific. For instance, the compressed air market typically prefers moisture content in terms of dew point temperature while the HVAC industry more often defines moisture content in terms of relative humidity. Although instruments are readily available by IOThrifty as well as other manufacturers to provide measurement of both relative humidity as well as dew point temperature, the instruments that measure only relative humidity are generally less expensive and more commonly available. Converting back and forth from relative humidity to dew point temperature can be done via mathematical calculations but the formulas can be quite complex. To simplify the calculation process, we have created an Excel Dew Point/Humidity calculator. There are other online calculators, but we have released ours in spreadsheet form which we believe provides flexibility which allows you to modify it for other than a single point calculation. The spreadsheet can even be adapted to applications where continuous measurements are being made via real time data acquisition.

There are several versions of Dew Point equations in literature. Some are simpler, others more complex. I chose this one as the results agreed with several commercial products. Just note that it does not take barometric pressure into account so at high altitudes it may be off by a few degrees.

Tdp =(243.04 × [ln(RH/100) + ( (17.625×T) / (243.04+T) )]) / (17.625 - [ln(RH/100) +

( (17.625×T) / (243.04+T) )])

Where Tdp is Dew Point Temperature(°C)

RH is the relative humidity(%)

T is the temperature (°C)

The two calculators in the spreadsheet accompanying this blog will either calculate the Dew Point by inputting Dry Bulb Temperature and Relative Humidity or calculate the Relative Humidity by inputting Dry Bulb Temperature and Dew Point.

DJ