[BLDG-SIM] Wet Bulb Temperature

Samuel Hassid cvrhasd at techunix.technion.ac.il
Wed Jul 6 09:15:09 PDT 2005


I had more or less the same thoughts as you had - but after reading the FAO document I understand that our Meteorological service standing behind its definition is justified - the problem is of course that in most programs the algorithm used is teh one of ASHRAE.



This is from a FAO document I found in the Net. It justifies the Meteorological Srevice's attitude ...

The actual vapour pressure can be determined from the difference between the dry and wet bulb temperatures, the so-called wet bulb depression. The relationship is expressed by the following equation: 


  ea = e° (Twet) - g psy (Tdry - Twet) (15)


where 


  ea actual vapour pressure [kPa],

  e°(Twet) saturation vapour pressure at wet bulb temperature [kPa],

  g psy psychrometric constant of the instrument [kPa °C-1],

  Tdry-Twet wet bulb depression, with Tdry the dry bulb and Twet the wet bulb temperature [°C].


The psychrometric constant of the instrument is given by: 


  g psy = apsy P (16)


where apsy is a coefficient depending on the type of ventilation of the wet bulb [°C-1], and P is the atmospheric pressure [kPa]. The coefficient apsy depends mainly on the design of the psychrometer and rate of ventilation around the wet bulb. The following values are used: 

apsy = 0.000662

for ventilated (Asmann type) psychrometers, with an air movement of some 5 m/s,


0.000800 for natural ventilated psychrometers (about 1 m/s),


0.001200 for non-ventilated psychrometers installed indoors.

  ----- Original Message ----- 
  From: Chip Barnaby 
  To: cvrhasd at techunix.technion.ac.il ; BLDG-SIM at gard.com 
  Sent: Wednesday, July 06, 2005 4:59 PM
  Subject: Re: [BLDG-SIM] Wet Bulb Temperature


  I do not think this problem should be ignored.  It is often difficult to get everyone on the same page as to the exact definitions of data items, but it is very important or we'll soon be feeding unknown data into unknown algorithms ...

  ASHRAE algorithms are based on the thermodynamic wet bulb temperature, t* = the temperature of adiabatic saturation.  t* is a unique property of moist air (not dependent on velocity etc.) and is the input data required by most simulation programs.  It can be approximately measured with a psychrometer with a sufficient air flow rate or calculated from the dew point.  From your description, it sounds like the Met service is publishing a wet bulb defined differently than this (?).

  It may be possible to derive t* if you have the details of the Met service instrumentation and/or algorithm.  Alternatively, if the Met service data includes dew point, you should be able to work from that using ASHRAE algorithms.

  Regarding your comment that velocities inside buildings are usually low ... if a given simulation includes velocity-related algorithms (for comfort modeling, perhaps), those would need the true absolute humidity (or some variant) as input.  The models would provide velocity adjustments as required.  For HVAC calculations (coil models, evaporative cooling, etc.), t* or a suitable alternative is needed.  I am not aware of any situation where a "partial saturation temperature" is useful as primary data.

  In am curious how the Met service data relates to WMO standards.  I am not familiar in detail with these, but I am sure the wet bulb and its measurement are rigorously defined.  I am surprised that a national service would choose an alternative procedure (if in fact they have).  If you get more info, I would be interested in hearing about it.

  Chip Barnaby
  Chair, ASHRAE TC 4.2 (Climatic Information)


  At 10:09 AM 07-06-05, Samuel Hassid wrote:

       I was wondering if anybody has encountered a similar problem - and how it was solved
     
      In Israel the metorological years contain both the Wet Bulb Temperature and the the relatrive humidity. These, however, are not consistent with the ASHRAE algorithm - which is used in most simulation programs. The Meteorological service stands behind its algorithm for calculating the Relative Humidity from the Wet and Dry Bulb temperatures, on the grounds that the wet bulb temperature recorded is under natural ventilation, and therefore the incresed value of the hygrometric constant is relevant.
       What should one do
    a. Calculate from the Relative Humidity Readings a new wet bulb temperature, consistent with teh ASHRAE algorithm (or with the high velocity wet bulb temperature algorithm) ?
    b.  Not bother - on teh grounds that anyway the wet bulb temperature inside buildings is usually based on relative low velocity - as in the measured wet bulb temperature ?
     
      A quick check shows that the difference may be of the order of several percentage points for the relative humidity, or of the order of 1 oC for Wet Bulb Temperature.
     
      Thanks                 S.  Hassid




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