[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
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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---------------------------------------------------------
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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