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<DIV><FONT face=Arial size=2>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.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>This is from a FAO document I found in the Net. It
justifies the Meteorological Srevice's attitude ...</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV>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: <BR><BR>
<DL>
<DD>ea</SUB> = e° (Twet</SUB>) - <FONT face=symbol>g</FONT> psy</SUB>
(Tdry</SUB> - Twet</SUB>) (15)<BR><BR></DD></DL>where <BR><BR>
<DL>
<DD>ea</SUB> actual vapour pressure [kPa],<BR>
<DD>e°(Twet</SUB>) saturation vapour pressure at wet bulb temperature
[kPa],<BR>
<DD><FONT face=symbol>g</FONT> psy</SUB> psychrometric constant of the
instrument [kPa °C-1</SUP>],<BR>
<DD>Tdry</SUB>-Twet</SUB> wet bulb depression, with Tdry</SUB> the dry bulb
and Twet</SUB> the wet bulb temperature [°C].<BR><BR></DD></DL>The psychrometric
constant of the instrument is given by: <BR><BR>
<DL>
<DD><FONT face=symbol>g</FONT> psy</SUB> = apsy</SUB> P
(16)<BR><BR></DD></DL>where apsy</SUB> is a coefficient depending on the type of
ventilation of the wet bulb [°C-1</SUP>], and P is the atmospheric pressure
[kPa]. The coefficient apsy</SUB> depends mainly on the design of the
psychrometer and rate of ventilation around the wet bulb. The following values
are used: <BR><BR>apsy</SUB> = 0.000662<BR><BR>for ventilated (Asmann type)
psychrometers, with an air movement of some 5 m/s,<BR><BR><BR>0.000800 for
natural ventilated psychrometers (about 1 m/s),<BR><BR><BR>0.001200 for
non-ventilated psychrometers installed indoors.<BR></DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=cbarnaby@wrightsoft.com href="mailto:cbarnaby@wrightsoft.com">Chip
Barnaby</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=cvrhasd@techunix.technion.ac.il
href="mailto:cvrhasd@techunix.technion.ac.il">cvrhasd@techunix.technion.ac.il</A>
; <A title=BLDG-SIM@gard.com
href="mailto:BLDG-SIM@gard.com">BLDG-SIM@gard.com</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Wednesday, July 06, 2005 4:59
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [BLDG-SIM] Wet Bulb
Temperature</DIV>
<DIV><BR></DIV>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 ...<BR><BR>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 (?).<BR><BR>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.<BR><BR>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.<BR><BR>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.<BR><BR>Chip Barnaby<BR>Chair, ASHRAE
TC 4.2 (Climatic Information)<BR><BR><BR>At 10:09 AM 07-06-05, Samuel Hassid
wrote:<BR>
<BLOCKQUOTE class=cite cite="" type="cite"><FONT face=arial
size=2> I was wondering if anybody has encountered a similar
problem - and how it was solved<BR></FONT> <BR><FONT face=arial
size=2> 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.<BR> What should one do<BR>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)
?<BR>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 ?<BR></FONT> <BR><FONT face=arial
size=2> 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.<BR></FONT> <BR><FONT face=arial size=2>
Thanks
S. Hassid<BR></FONT><BR><PRE>
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</PRE><FONT face="Courier New, Courier"></BLOCKQUOTE><X-SIGSEP>
<P></X-SIGSEP>---------------------------------------------------------<BR>Chip
Barnaby
cbarnaby@wrightsoft.com<BR>Vice President of Research<BR>Wrightsoft
Corp.
781-862-8719 x118 voice<BR>394 Lowell St, Suite
12 781-861-2058 fax<BR>Lexington, MA
02420 <A
href="http://www.wrightsoft.com/"
eudora="autourl">www.wrightsoft.com<BR></A>---------------------------------------------------------</FONT>
</P></BLOCKQUOTE><PRE>
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