Converting Dew Point to other Measuring Units

Dew point is an important parameter for many industrial and scientific processes, especially where detecting trace measurements of moisture is essential for maintaining optimal levels of quality, safety or efficiency. 

As with most industrial process parameters, a variety of measuring technologies are available, each suited to different applications.  You can read more about these in this related blog, which gives you a definition of dew point, as well as associated parameters of humidity, relative humidity and moisture content.   

The most common method of measuring dew point is with a dew-point meter, transmitter or analyzer.  For example, instruments such as our Easidew PDP portable or Easidew EA2 transmitter are designed to make it quick and easy to obtain precise, repeatable measurements, often in real time.  

These instruments normally display results as temperature dew point or ^oCdp.  This is acceptable in many applications, but sometimes needs to be converted to other measuring units, such as parts per million by volume (PPMv), parts per million by weight (PPM_w), absolute humidity (mg/m³), water vapor pressure (Pa), or relative humidity (%rh). 

Why would I need to convert dew point to other measuring units? 

Converting dew point to different measuring units is often necessary for the following reasons: 

How to convert dew point to different measuring units 

The first step in all humidity conversions is to convert your source humidity measurement into water vapor pressure and, depending on the target unit of interest, convert temperature into saturation water vapor pressure. 

PST | Michell products use the Sonntag formula to convert dew/frost point into water vapor pressure, or temperature into saturation vapor pressure. The PST Humidity and Dew Point Calculator can be used to perform these calculations. 

Dew/frost point and temperature to water vapor pressure

For dew point or temperature > 0 °C:
ln ew(T) = -6096.9385 T-1 + 21.2409642 - 2.711193 x 102 T + 1.673952 x 10-5 T2 + 2.433502 ln T

For dew point or temperature ≤ 0 °C:
ln ei(T) = -6024.5282 T-1 + 29.32707 + 1.0613868 x 10-2 T - 1.3198825 x 10-5 T2 - 0.49382577 ln T

(Reference: BS1339:1, 3.2.2 Saturation vapor pressure [Sonntag])

T can either be a temperature or dew/frost point. The result is in vapor pressure in Pascal pressure units. 

2. Parts per million by volume (ppm_V) 

Parts per million by volume (ppm_V) 

ppm_V measures the concentration of water vapor in a gas mixture, making it an essential requirement in applications such as gas quality measurements and inert blanketing.  The formula used to convert dew point or frost point (when below 0 ^oC) to ppm_V is: 

Conversion formula:
ppmV = (e / P) * 106
where e is the water vapor pressure in Pascals, and P is the total system pressure in Pascals.

where e is water vapor pressure in Pascal, and P is total system pressure in Pascal. 

(ref. BS1339:1, 3.2.10 Relationships between mixing ratio, mol ratio and vapor pressure) 

For ppm_V in natural gas, the natural gas humidity calculator must be used. 

Parts per million by weight (ppm_W) 

An alternative method is to convert dew point to ppm_w.  This is preferred in applications where the precise weight of moisture in the air is critical for product quality and process efficiency, such as in the food processing, pharmaceutical or chemical industries.  The formula used is: 

Conversion formula:
ppmW = (18.0153 / 28.96) * (e / P) * 106
where 18.0153 g/mol is the molar weight of water, and 28.96 g/mol is the molar weight of air.

where the mol weight of water is 18.0153 g/mol and, for example, the mol weight of air is 28.96 g/mol. 

(ref. BS1339:1, 3.2.10 Relationships between mixing ratio, mol ratio and vapor pressure) 

For ppmW in natural gas, the natural gas humidity calculator must be used.

4. Absolute humidity (mg/m³)

Absolute humidity is measured in milligrams per cubic meter (mg/m³) and represents the total mass of water vapor present in a specific volume of air.  This unit is used in climate studies, drying processes and HVAC systems to measure and control the actual moisture content in the air.  The formula used is: 

To calculate absolute humidity:
mg/m3 = (2167 * e) / (R * Tk)
where e is the water vapor pressure calculated from dew/frost point in Pascal, T_k is the temperature in Kelvin, and R is the universal gas constant.

where e is the water vapor pressure calculated from dew/frost point in Pascal, and Tk is the temperature in Kelvin. 

This calculation is almost always performed under normal conditions, with a fixed temperature. 

(ref. BS1339:1, 3.2.12 Volumetric humidity) 

For absolute humidity in natural gas, the natural gas humidity calculator must be used. 

Relative humidity (%rh) 

Relative humidity (%rh) indicates the percentage of moisture in the air relative to the maximum amount the air can hold at a specific temperature.  It is a common measure in weather forecasting, indoor air quality assessments and various industrial applications.  

Like all other conversions, there is no direct formula for converting dew point to relative humidity.  Instead, it is necessary to convert dew/frost-point temperature to water vapor pressure, and temperature to saturation vapor pressure (see above) and then use the following formula: 

%rh = (e / es) * 100

Note that e/e^s defines the ratio between the amount of water vapor (e) that is in the air, compared with the amount of water vapor (e^s) that would be in the air if it was saturated at the same temperature and pressure. 

(ref. BS1339:1, 3.2.9 Relative Humidity) 

The simple approach to dew-point conversions

Clearly, converting dew-point temperature measurements to other parameters can be a complex and time-consuming task, requiring a sound understanding of scientific principles.  The good news is that we’ve done much of the work for you! 

Our interactive online calculator makes the process of conversion quick, simple and accurate.  Check it out for yourself! 

With 50 years’ experience in the development of moisture and dew-point technology, we are the application experts for all trace moisture measurement in industrial applications. If you would like to discuss your requirements, please contact our team today. 

Useful Information

Humidity and Dew Point Calculator 

Related Blogs

Dew Point Definition and How to Measure it for Industries

How a Michell Chilled Mirror Hygrometer Measures Relative Humidity

How to Calculate Pressure Dew Points (PDP) Accurately

How to Calculate Pressure Dew Point using Conversion Chart over Ice and Water