Hello, I am taking readings in the greenhouse with a temperature and Humidity Meter that is measuring in Celsius and Relative Humidity. However, in an attempt to better understand the effect of humidity on the plant I would like to add a column to the chart that illustrates the VPD (vapor pressure deficit).
There are numerous equations for approximate calculation of water vapor pressure. This is a good starting reference list: Water Vapor Pressure Formulations Some are thermodynamic models valid over a wide temperature range but complex; others are s...
The Goff Gratch equation  for the vapor pressure over liquid water covers a region of -50°C to 102°C [Gibbins 1990]. This work is generally considered the reference equation but other equations are in use in the meteorological community [Elliott and Gaffen, 1993]. The measured atmospheric pressure is the sum of two terms, the partial pressure of dry air (p a) and the partial pressure of water vapor (p w). The water vapor pressure is a function of temperature, and the dew point temperature (T d), which is defined as the temperature at which the air is saturated with water vapor. The August-Roche-Magnus (or Magnus-Tetens or Magnus) equation, as described in Alduchov and Eskridge (1996). Equation 21 in  provides the coefficients used here. See also discussion of Clausius-Clapeyron approximations used in meteorology and climatology . (for which the unit is Celsius). Therefore, the August–Roche–Magnus equation implies that saturation water vapor pressure changes approximately exponentially with temperature under typical atmospheric conditions, and hence the water-holding capacity of the atmosphere increases by about 7% for every 1 °C rise in temperature. Therefore, the August–Roche–Magnus equation implies that saturation water vapor pressure changes approximately exponentially with temperature under typical atmospheric conditions, and hence the water-holding capacity of the atmosphere increases by about 7% for every 1 °C rise in temperature. where temperature T is in degrees Celsius (°C) and saturation vapor pressure P is in kilopascals (kPa). According to Monteith and Unsworth, "Values of saturation vapour pressure from Tetens' formula are within 1 Pa of exact values up to 35 °C." Murray (1967) provides Tetens' equation for temperatures below 0 °C:
Global climate change also puts high latitude region to be suffer with extreme low temperature . So it is necessary to evaluate the influence of Teten formula on determination of saturation vapor pressure, vapor pressure deficits and hence on the evapotranspiration. 2. Saturation vapor pressure formulas 2.1. The water vapour pressure is the partial pressure of water vapour in any gas mixture in equilibrium with solid or liquid water. As for other substances, water vapour pressure is a function of temperature and can be determined with the Clausius–Clapeyron relation . The temperature T is expressed in degrees Celsius and the vapor pressure P is in mmHg. Jump to the next section to read more about the constants in the Antoine formula. Magnus formula, also known as August-Roche-Magnus or Magnus-Tetens equation; Magnus_pressure = e^[(17.625 * temperature) / (temperature + 243.04)]