Calculate the boiling point of a liquid under vacuum (reduced pressure) using the Clausius-Clapeyron relation, or find the pressure needed to reach a target boiling temperature.

Approximate pure-liquid vapor pressure using constant enthalpy. Use absolute pressure, not gauge vacuum. Check phase limits and substance-specific data before practical use.

Use a value appropriate near the reference boiling temperature.

Reference pressure

Blank uses 1 atm (101.325 kPa). Enter the pressure for your reference temperature.

Related Calculators

Vacuum Temperature Formula

The integrated Clausiusโ€“Clapeyron approximation relates absolute vapor pressures and boiling temperatures using a constant molar enthalpy of vaporization.

ln(Pโ‚‚ / Pโ‚) = - (Delta Hแตฅap / R)(1 / Tโ‚‚ - 1 / Tโ‚)
  • Pโ‚ and Pโ‚‚ are reference and target absolute vapor pressures in the same unit
  • ฮ”Hแตฅโ‚โ‚š is molar enthalpy of vaporization (J/mol); R = 8.31446261815324 J/(molยทK)
  • Tโ‚ and Tโ‚‚ are reference and target boiling temperatures in kelvin

Select boiling temperature or required absolute pressure. Enter the reference boiling temperature, enthalpy of vaporization, and target pressure or temperature. The optional reference pressure defaults to 1 atm (101.325 kPa).

What is a Vacuum Temperature Calculator?

Definition:

This calculator estimates a pure liquidโ€™s boiling temperature at a specified absolute pressure, or estimates the pressure corresponding to a target boiling temperature. It uses the integrated Clausiusโ€“Clapeyron approximation, assuming ideal vapor, negligible liquid volume, and constant enthalpy of vaporization. It does not calculate radiative equilibrium temperature.

How to Calculate Vacuum Temperature?

Example Problem:

The following example illustrates the constant-enthalpy model for water; it is an approximate calculation, not a steam-table value.

First, select required absolute pressure for a target boiling temperature of 50ยฐC (323.15 K).

Use a reference temperature of 100ยฐC (373.15 K) at 101.325 kPa and an assumed constant vaporization enthalpy of 40.7 kJ/mol.

Calculate the estimated absolute pressure using the integrated relation:

ln(P_2/P_1) = -(Delta H_vap/R)(1/T_2 - 1/T_1)

Pโ‚‚ = Pโ‚ exp[โˆ’ฮ”Hแตฅโ‚โ‚š/R ร— (1/Tโ‚‚ โˆ’ 1/Tโ‚)]

Pโ‚‚ = 101.325 ร— exp[โˆ’40700/8.31446261815324 ร— (1/323.15 โˆ’ 1/373.15)] kPa

Pโ‚‚ โ‰ˆ 13.311 kPa absolute. IAPWS gives about 12.35 kPa for real water at 50ยฐC; this constant-enthalpy estimate is about 7.8% higher.

FAQ

What does the vaporization enthalpy mean?

It is the energy required to vaporize one mole of liquid at a specified temperature. Use substance-specific data near the reference temperature. The calculator holds this value constant, although real enthalpy varies with temperature.

How does a Vacuum Temperature Calculator work?

Choose the desired result explicitly. The calculator relates a known liquidโ€“vapor reference point to a second point using the Clausiusโ€“Clapeyron approximation. Enter absolute pressure rather than a gauge-vacuum reading.

Why might the calculated temperature differ from expected values?

Constant enthalpy and ideal vapor are approximations. A stable liquidโ€“vapor region must exist: below freezing or the triple-point region, sublimation may replace boiling; near or above the critical point, this model is unreliable. No universal phase limits can be applied without knowing the liquid. Verify practical values against substance-specific saturation data.