[BLDG-SIM] Wet Bulb Temperature

Chip Barnaby cbarnaby at wrightsoft.com
Wed Jul 6 07:59:06 PDT 2005

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 

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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Chip Barnaby                   cbarnaby at wrightsoft.com
Vice President of Research
Wrightsoft Corp.               781-862-8719 x118 voice
394 Lowell St, Suite 12        781-861-2058 fax
Lexington, MA 02420            www.wrightsoft.com

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