blockTWetBul_TDryBulXi

Information

This block computes the wet bulb temperature for a given dry bulb temperature, mass fraction and atmospheric pressure.

If the constant approximateWetBulb is true, then the block uses the approximation of Stull (2011) to compute the wet bulb temperature without requiring a nonlinear equation. Otherwise, the model will introduce one nonlinear equation. The approximation by Stull is valid for a relative humidity of 5% to 99%, a temperature range from -20°C to 50°C and standard sea level pressure. For this range of data, the approximation error is -1 Kelvin to +0.65 Kelvin, with a mean error of less than 0.3 Kelvin.

Otherwise a calculation based on an energy balance is used. See #474 for a discussion.

For a model that takes the relative humidity instead of the mass fraction as an input, see Buildings.Utilities.Psychrometrics.TWetBul_TDryBulPhi.

References

Stull, Roland. Wet-Bulb Temperature from Relative Humidity and Air Temperature Roland Stull. Journal of Applied Meteorology and Climatology. Volume 50, Issue 11, pp. 2267-2269. November 2011 DOI: 10.1175/JAMC-D-11-0143.1

Parameters

Connectors

Contents

Name	Description
Medium	Medium model

Revisions

• July 29, 2026, by Michael Wetter:
  Added nominal attribute for temperature.
  This is for IBPSA, #2155.
• March 6, 2023, by Michael Wetter:
  Added a constant in order for unit check to pass.
  See #1711 for a discussion.
• May 1, 2017, by Filip Jorissen:
  Revised computation of iWat such that it does not require an initial algorithm. See #759.
• November 3, 2016, by Michael Wetter:
  Changed icon.
• May 24, 2016, by Filip Jorissen:
  Corrected exact implementation. See #474 for a discussion.
• April 11, 2016 by Michael Wetter:
  Corrected wrong hyperlink in documentation for issue 450.
• November 17, 2014, by Michael Wetter:
  Removed test on saturation pressure that avoids it to be larger than p. This test is not needed as it is only active near or above the boiling temperature, and the result is only used in the computation of rh_per. I do not see any negative impact from removing this test.
• July 24, 2014 by Michael Wetter:
  Revised computation of rh_per to use Buildings.Utilities.Math.Functions.smoothMin rather than min.
• November 20, 2013 by Michael Wetter:
  Updated model to use Buildings.Utilities.Psychrometrics.Functions.saturationPressure() and Buildings.Utilities.Psychrometrics.Functions.saturationPressureLiquid() as these functions have been moved from the medium to the psychrometrics package.
• September 10, 2013 by Michael Wetter:
  Added start value and nominal value for XiSat as this is an iteration variable in OpenModelica.
• October 1, 2012 by Michael Wetter:
  Revised implementation to change the dimension of the nonlinear system of equations from two to one. Added option to compute wet bulb temperature explicitly.
• February 22, 2011 by Michael Wetter:
  Changed the code sections that obtain the water concentration. The old version accessed the water concentration using the index of the vector X. However, Dymola 7.4 cannot differentiate the function if vector elements are accessed using their index. In the new implementation, an inner product is used to access the vector element. In addition, the medium substance name is searched using a case insensitive search.
• February 17, 2010 by Michael Wetter:
  Renamed block from WetBulbTemperature to TWetBul_TDryBulXi and changed obsolete real connectors to input and output connectors.
• May 19, 2008 by Michael Wetter:
  Added relative humidity as a port.
• May 7, 2008 by Michael Wetter:
  First implementation.