Smart Temperature and Humidity Gauge

This prototype displays temperature, relative humidity, dew point temperature and absolute humidity

As shown in previous posts on the CarnotCycle blog, it is possible to compute dew point temperature and absolute humidity (defined as water vapor density in g/m^3) from ambient temperature and relative humidity. This adds value to the output of RH&T sensors like the DHT22 pictured above, and extends the range of useful parameters that can be displayed or toggled on temperature-humidity gauges employing these sensors.

Meteorological opinion* suggests that dew point temperature is a more dependable parameter than relative humidity for assessing climate comfort especially during summer, while absolute humidity quantifies water vapor in terms of mass per unit volume. In effect this added parameter turns an ordinary temperature-humidity gauge into a gas analyzer.

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Compute dew point temperature from RH&T

Relative humidity (RH) and temperature (T) data from an RH&T sensor like the DHT22 can be used to compute not only absolute humidity AH but also dew point temperature T_D

There has been a fair amount of interest in my formula which computes absolute humidity (AH) from measured relative humidity (RH) and temperature (T) since it adds value to the output of RH&T sensors. To further extend this value, I have developed another formula which computes dew point temperature T_D from measured RH and T.

Formula for computing dew point temperature T_D
In this formula (P Mander 2017) the measured temperature T and the computed dew point temperature T_D are expressed in degrees Celsius, and the measured relative humidity RH is expressed in %

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Strategy for computing T_D from RH and T

1. The dew point temperature T_D is defined in the following relation where RH is expressed in %

2. To obtain values for P_sat, we can use the Bolton formula^{[REF, eq.10]} which generates saturation vapor pressure P_sat (hectopascals) as a function of temperature T (Celsius)

These formulas are stated to be accurate to within 0.1% over the temperature range –30°C to +35°C

3. Substituting in the first equation yields

Taking natural logarithms

Rearranging

Separating T_D terms on one side yields

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P Mander August 2017