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TIP 0808-06 OLD NUMBER 018-10
ISSUED - 1984 REVISED - 1996
CORRECTED - 1999 REVISED - 2001 REVISED - 2005
©2005 TAPPI The information and data contained in this document
were prepared by a technical committee of the Association. The
committee and the Association assume no liability or responsibility
in connection with the use of such information or data, including
but not limited to any liability under patent, copyright, or trade
secret laws. The user is responsible for determining that this
document is the most recent edition published.
TIP Category: Data and Calculations
TAPPI
Relative humidity tables Scope This technical information paper
provides an easy method to determine relative humidity. Tables are
provided to determine relative humidity from dry bulb and wet bulb
temperature readings and barometric pressure. An equation is also
provided to be able to calculate relative humidity from these
readings. Introduction The tables presented in this technical
information paper provide an easy way to obtain relative humidity.
For may years, the only available procedure for determining
relative humidity from dry and wet temperature readings was to use
tables, charts, and graphs. This involved calculations and locating
the numbers in a table or picking out points in a graph. They also
did not provide factors for barometer correction. TAPPI T 402
“Standard Conditioning and Testing Atmospheres for Paper, Board,
Pulp Handsheets, and Related Products” prescribes the proper
temperature and humidity for the testing of paper products:
Fahrenheit 73.4 1.8 (range 71.6 to 75.2 ) Celsius 23 1 (range 22 to
24 ) Relative humidity 50% 2% (range 48% to 52%) The tables
presented are for the range of temperatures and relative humidities
prescribed. The relative humidities in the tables are calculated
for the barometric pressures indicated at the top of each table.
Add 0.055% to the relative for each 0.1 psi increase. This is not
an exact correction, but any resultant errors will be less than
0.1% RH. Safety precautions Broken thermometers must be discarded
according to safety rules. Procedure The procedure for determining
relative humidity is more detailed in ASTM E337 “Relative Humidity
by Wet and Dry Bulb Psychrometer” (1). The dry and wet temperature
figures are obtained with an Assmann psychrometer; then the
Fahrenheit or Celsius degrees are located in the appropriate table
columns. The relative humidity is found directly.
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TIP 0808-06 Relative humidity tables / 2
NOTE 1: Formula for conversion of F to C or vice versa:
( )3295 −= FC oo
3259 +
= CF oo
For those who wish to calculate the relative humidity at
different barometric pressures or to write a computer program for
doing the same, the following equation is provided: Psw – {0.000367
* [1 + 0.00064 (Tw - 32)]} * Pb * (Td - Tw)
%RH = 100 Psd where Td = dry bulb temperature in F Tw = Wet bulb
temperature in F Pb = barometric pressure in inches Hg Psw =
saturated vapor pressure for wet bulb temperature from Table 11 Psd
= saturated vapor pressure for dry bulb temperature from Table 11
This equation is valid within the following restrictions: Dry bulb
and wet bulb temperatures must be between 47 and 78 F (Td - TW) 5 F
Barometric pressure range of 28.9 - 31.0 inches Hg To do a more
exact calculation, see Chapter 6 in ASHRAE Handbook (3).
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Table 11. Saturation Vapor Pressure for Water from 47 to 78°F F
47-51 52-56 57-61 62-66 67-71 72-76 77-78 .0 0.32386 0.39028
0.46840 0.55994 0.66681 0.79112 0.93523 .1 0.32508 0.39172 0.47009
0.56191 0.66911 0.79379 0.93832 .2 0.32631 0.39316 0.47178 0.56389
0.67142 0.79648 0.94143 .3 0.32754 0.39461 0.47349 0.56588 0.67374
0.79917 0.94454 .4 0.32878 0.39607 0.47519 0.56788 0.67607 0.80187
0.94767 .5 0.33002 0.39753 0.47691 0.56989 0.67840 0.80458 0.95081
.6 0.33126 0.39900 0.47863 0.57190 0.68074 0.80730 0.95395 .7
0.33251 0.40047 0.48036 0.57392 0.68309 0.81003 0.95711 .8 0.33377
0.40195 0.48209 0.57394 0.68545 0,81277 0.96028 .9 0.33500 0.40343
0.48383 0.57598 0.68782 0.81552 0.96346 .0 0.33629 0.40492 0.48558
0.58002 0.69019 0.81828 0.96665 .1 0.33755 0.40640 0.48732 0.58205
0.69256 0.82102 .2 0.33882 0.40790 0.48907 0.58410 0.69495 0.82378
.3 0.34009 0.40940 0.49082 0.58615 0.69734 0.82656 .4 0.34137
0.41090 0.49259 0.58821 0.69973 0.82934 .5 0.34265 0.41241 0.49436
0.59028 0.70214 0.83213 .6 0.34394 0.41392 0.49613 0.59235 0.70455
0.83493 .7 0.34523 0.41544 0.49791 0.59444 0.70698 0.83774 .8
0.34652 0.41697 0.49970 0.59652 0.70941 0.84056 .9 0.34782 0.41850
0.50149 0.59862 0.71185 0.84339 .0 0.34913 0.42004 0.50330 0.60073
0.71430 0.84624 .1 0.35044 0.42157 0.50509 0.60283 0.71674 0.84907
.2 0.35175 0.42311 0.50690 0.60494 0.71920 0.85192 .3 0.35306
0.42466 0.50871 0.60705 0.72166 0.85477 .4 0.35440 0.42621 0.51053
0.60918 0.72414 0.85764 .5 0.35570 0.42777 0.51236 0.61131 0.72662
0.86501 .6 0.35703 0.42934 0.51419 0.61345 0.72911 0.86340 .7
0.35837 0.43090 0.51603 0.61560 0.73160 0.86629 .8 0.35971 0.43248
0.51788 0.61775 0.73411 0.86920 .9 0.36105 0.43406 0.51973 0.61992
0.73662 0.87211 .0 0.36240 0.43565 0.52159 0.62209 0.73915 0.87504
.1 0.36375 0.43723 0.52344 0.62426 0.74167 0.87795 .2 0.36510
0.43882 0.52531 0.62643 0.74420 0.88039 .3 0.36646 0.44042 0.52718
0.62861 0.74674 0.88383 .4 0.36782 0.44202 0.52906 0.63080 0.74928
0.88678 .5 0.36919 0.44363 0.53094 0.63300 0.75184 0.88974 .6
0.37056 0.44524 0.53283 0.63521 0.75440 0.89271 .7 0.37194 0.44686
0.53473 0.63472 0.75698 0.89569 .8 0.37332 0.44849 0.53663 0.63964
0.75956 0.89868 .9 0.37471 0.45012 0.53855 0.64187 0.76215 0.90169
.0 0.37611 0.45176 0.54047 0.64411 0.76475 0.90470 .1 0.37750
0.45339 0.54238 0.64634 0.76735 0.90770 .2 0.37890 0.45504 0.54430
0.64859 0.76995 0.91072 .3 0.38030 0.45669 0.54623 0.65084 0.77257
0.91375 .4 0.38171 0.45834 0.54817 0.65310 0.77519 0.91679 .5
0.38313 0.46000 0.55011 0.65536 0.77782 0.91984 .6 0.38454 0.46167
0.55206 0.65764 0.78046 0.92289 .7 0.38597 0.46334 0.55402 0.65992
0.78311 0.92596 .8 0.38740 0.46502 0.55598 0.66221 0.78577 0.92904
.9 0.38883 0.46671 0.55795 0.66451 0.78844 0.93213
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Keywords Humidity, Humidity measurement, Dry bulb temperature,
Wet bulb temperature Additional information Effective date of
issue: January 24, 2005. Working Group Members: John W. Walkinshaw
- Chairman, University of Massachusetts James C. Abbott, Consultant
Sue F. Jakubsen, Herty Foundation References 1. ASTM E337 “Relative
Humidity by Wet and Dry Bulb Psychrometer,” American Society for
Testing and
Materials, Philadelphia, PA. 2. TAPPI PRESS Report 121 “Relative
Humidity: Thermodynamic Charts.” 3. ASHRAE Handbook 1993 –
Fundamentals, Chapter 6. g