Enter calculation values
Formula used
1) Pure-water vapor pressure:
log10(P_mmHg) = A - B / (C + T)
With Antoine constants for water: A = 8.07131, B = 1730.63, C = 233.426.
2) Ideal NaCl-equivalent activity model:
n_water = (1000 - S) / 18.01528
n_salt = S / 58.44
a_w = n_water / (n_water + 2 × n_salt)
3) Empirical activity model:
a_w = 1 - 0.000537 × S
4) Seawater vapor pressure:
P_seawater = a_w × P_pure_water
5) Vapor pressure depression:
ΔP = P_pure_water - P_seawater
6) Percent reduction:
% reduction = (ΔP / P_pure_water) × 100
7) Air-side partial pressure at entered humidity:
P_air = P_seawater × RH / 100
These are practical engineering approximations. Real seawater composition, non-ideal behavior, and very high salinity brines can shift actual values.
How to use this calculator
- Enter the liquid temperature.
- Select Celsius, Fahrenheit, or Kelvin.
- Enter salinity as g/kg, PSU, or ppt.
- Choose the activity model you want.
- Optionally enter relative humidity for air-side partial pressure.
- Press Calculate.
- Review seawater pressure, pure-water reference, depression, and reduction percentage.
- Use the chart for trend comparison.
- Export the calculation as CSV or PDF.
Example data table
| Temperature (°C) | Salinity (g/kg) | Water activity | Pure water VP (kPa) | Seawater VP (kPa) | Depression (kPa) |
|---|---|---|---|---|---|
| 15 | 35 | 0.97813 | 1.6976 | 1.6605 | 0.0371 |
| 25 | 35 | 0.97813 | 3.1579 | 3.0889 | 0.0691 |
| 35 | 35 | 0.97813 | 5.6090 | 5.4863 | 0.1227 |
| 45 | 35 | 0.97813 | 9.5598 | 9.3507 | 0.2091 |
Example values above use 35 g/kg salinity with the ideal NaCl-equivalent activity model.
Frequently asked questions
1) What does seawater vapor pressure mean?
It is the equilibrium vapor pressure of water above saline water at a given temperature. Dissolved salts reduce water activity, so seawater usually has lower vapor pressure than pure water.
2) Why does salinity lower vapor pressure?
Salt ties up part of the water’s escaping tendency. That lowers water activity, reduces the number of water molecules entering the vapor phase, and depresses equilibrium vapor pressure.
3) Which model should I choose?
Use the ideal NaCl-equivalent model for a more chemistry-style estimate. Use the empirical model for quick engineering work near normal seawater salinity. Compare both when sensitivity matters.
4) Are PSU, ppt, and g/kg identical here?
In this calculator, they are treated numerically the same for practical engineering use. For precise oceanographic work, laboratory definitions and reference scales can differ slightly.
5) Why is atmospheric pressure not an input?
Saturation vapor pressure at a fixed temperature is primarily a liquid-vapor equilibrium property. Atmospheric pressure becomes more important when you study boiling, evaporation rate, or mass-transfer conditions.
6) Can I use this for brines or hypersaline water?
Only as a rough first estimate. Very high salinity systems can deviate strongly from these simplified models, especially when mixed salts, non-ideal behavior, or temperature-dependent activity effects become important.
7) What is the air-side partial pressure output?
It multiplies the seawater equilibrium vapor pressure by the entered relative humidity. This helps estimate the water-vapor partial pressure in nearby air under the stated humidity condition.
8) Why does the chart show two curves?
One curve is the pure-water reference. The second is the salinity-adjusted seawater pressure. Their gap visualizes vapor-pressure depression and shows how salinity changes the full temperature trend.