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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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