U. S. DE

I NATIO

These charts of humidity fi ranging from

tended to anj

described here

tations of the

the Fahrenhei

which has bee

United States e=e

in which t and

bulb thermom

metric pressun

e' is the satui^ mercury at trj water vapor in in inches of m^

a formula of t

in which P, e' cury, and t anc has been take]

mula in the lig tom

c

[o 8jnss9jd jo

'qmq-c^M 8U^ i -nq QAi^Bpi %

uoT^muxio^ep

8Jn^BJ9dUI9^ A

9q; JO UOI}T3UT

Example 1 (Centigrade Chart —Low Range)

Suppose a psychrometer reads 20° C on the dry bulb c

_ on the wet bulb when the total (baromet-ic) pressure is 6( j

To find the pressure of water vapor, place a straightedge s_j_ intersects the " Observed Dry Bulb Mintis Wet Bulb, C' ; 20 —8 = 12. Adjust it so that it intersects the "Barometer, /\ scale at 60, and note the intersection with the "Sea Levels

lent Dry Bulb Minus Wet Bulb, C" scale (9.5). Hoi

straightedge at this point, swing it so that it intersects the

nearly vertical) "Wet Bulb, C" scale at 8, and read the

of water vapor (3.30) on the "Humidity, mm Hg" seal right. A computation from the psychrorietric formula gi- mm Hg for this value. In obtaining the pressure of water vapor when t bulb temperature is below freezing, on either the ] heit or centigrade low-range chart, the scale marke

Bulb" is employed if the psychrometer was us

either subcooled water or a frozen wick. Howe

more accurate results can be obtained with an ic( H thermometer, using the "Ice Bulb'' scale (to the

the "Wet Bulb" scale).

Determining Relative Humidity

To determine the relative humidity, place a strai;)1 so that it intersects the "Humidity mm (or in.) H

at the value of the pressure of water vapor, and in

' the (diagonal) "Dry Bulb" scale at the value of W\

bulb temperature. The relative humidity is give

point of intersection with the inner left scale. Example 2 (Centigrade Chart—Low Range)

Continuing the example started above, hold the straight^

at the value 3.30 on the ".Humidity, mm Hg", and swing v

it intersects the (diagonal) "Dry Bulb, C" scale at 20. "By

ing this line to the vertical "Relative Humidity, Perce)

(inner left), the relative humidity is founi (18.8%). Con

gives 18.87% for the psychrometric data given originally.

Determining Dewpoint

To obtain the dewpoint, determine the pressure j vapor as in the first example. Connect this poin

"Humidity, mm (or in.) Hg" scale by straighten

100% relative humidity and read the dewpoint

"Wet Bulb" scale. Example 3 (Centigrade Chart—Low Range)

Continuing with the data of example 1, connect 3.3C "Humidity, mm Hg" scale by straightedge with 100 on t

1 tive Humidity, Percent" scale. The intersection of the str with the "Wet Bulb, C" scale (-4.35 C) gives the de

U. S. DEPARTMENT OF COMMI

Daniel C. Roper, Secretary NATIONAL BUREAU OF STANDAR

Lymon J. Briggs, Director

These charts were developed for use in tin

of humidity from psychrometric observatk

in which ( and t' are the temperatures of th bulb thermometer.-;, in degrees Fahrenheit

metric pressure at the psychrometcr, in ine

e' is the saturation vapor pressure of wat mercury at the temperature and e is water vapor in the atmosphere, or atmosp!

in inches of mercury. The centigrade char

a formula of the same type

e =e'-0.000G52 P(t-t') (1+O.C

in which P, e', and e are expressed in niilli cury, and t and (' in degrees centigrade. T

has been taken to modify the constants u

to be more accurate. Hence tho values in mula above are used rather than the valu

correspond to those used in the first formu

PSYCHROMETRIC
lent Dry Bulb Minus Wet Bui

neurlv o-rlicull "Wet Bulb, C"

of water vapor (3.30) on the "I

* the pressii it of water vapor when 1 J *«.- wi-t- bulb temperature is below freezing, on either the Fahrenheit or centigrade low-mine chart . ihe soale marked "Wet

Bulb" is employed if the psychronicler was used with

either subcooled water or a frozen wick, Howei-er (4), more accurate results can be obtained with an ice-coated thermometer, using the "Ice Bulb" scale (to the right of the "Wet Bulb" scale).

Deter ling Relative Humidity

Chart—Low Range)

Continuing the example

Ikknnlning De.wpoint To obtain the dew-point, determine the pressure of water

vapor as in the first example. Connect this point on the "Humidity, mm (or in.) Hg" scale by straightedge with 100% relative humiditv and read the dewpoint on the "Wet Bulb "scale.

•;Humidity"mmHg

<^-,,

l |^b> ^rairilu!i.:o wit°h 10^n t{e "Re'lawith the "Wet Bulb, C" seal.- <-!.:;,-> C) uives Hie .levuuiul- la

Approximate Determinations: Error Involved Numerous other uses for these charts will suegest themselves to those familiar with psvehrumetric work. For

example, when working at pressures near one atmosphere,

I iCeiitiunidu Chart—Low Range)

iromcter reads 74 :jr. nu 1 1 u- . Tl

eon the "Sea Level Equiv

Tims this approximation introduced a departure from

Hie formula i>f m.I:; inm Il'_' intn (ho iv-u!t. Xote (hat such departure refill. - from (he iw of all p- ychrornrf rie .-harts which do not include

A useful guide a- to wli.-fi tli.' aho\ approximation can

safely be used is „ (t-f) (P-760)

Departure, mm Hg=

1533

or thi' rule ran he used, that lie 1 depart un- in tin- humidity

will not exceed 0.1 mm Hg when the product of the wetIndb depiction by the diU'etcnee between the observed barometric pressure in cm Hg and 76 does not exceed 15.

Similarly, for the Fahrenheit charts,

Doputure>. H6=(tm)

nltil llir ilrjiLirtlllP ill tin' 1 1 llll lii I i t V will lli-l L'Xrceil IM) L 111

II~ if tin/ wet-bulb ili'|»]o-ion t i [lies tlic ditierenre between

the observed bnrometric pressure, in. Hg, and 30 does not

exceed 27.

Vapor from Helat

The use of these charts for the determination of

(saturation) vapor pressure of water at any tempera

is too obvious to require an example. This determination

is made by setting a straightedge on 100% relative midity and on the required temperature on the wet-t

(or dry-bulb) scale, and reading the vapor pressun water on the humidity scale nt the right.
It is also obvious that by reversing the procedure for

finiling tlie relative humidity when the dry-bulb temperature and the pressure of water vapor are given, the latter

can bo found for any given relative humidity.

Example 5 (Centigrade Charts-Low Range)

.mining Ttm.pt ,,.hl,i

cqucntly is that of maintaining

e humidity lit a given dry-bulb

temperature, the humidity tn he indicated by a psychrometcr. To find from the formula what wet-bulb reading

corresponds !o tin' required humidity involves tedious approximations, or double interpolation in tables. This value

can be found from these charts quite simply. If a certain

relative liuniidity is required, determine as in the above

example the corresponding pie. so re of water vapor. Keeping the straightedge fixed on this value, swing it until the sum of the values on the "Wet Bulb" and "Sea Lovel

Equivalent Dry Bulb Minus Wet Bulb" scales equals the

given dry-bulb temperature. The value indicated by the

intersection of the straightedge and the "Wet Bulb" scales

is the required wet-bulb rendiiie if operation is to bo at

sea-level pressure. For other pressures, the operation is

slightly more complicated, but can bo done by reversing

the procedure used in example 1.

A similar problem is that of bringing a given atmosphen

to a desired relative humidity by altering its dry-bulb tem

perature. To determine the desired dry-bulb temperature

connect by straightedge the pouit on the "Humidity, mn

(or in.) Hg" scale represent ing t he pressure of water vapo

present with the point representing the desired relativi

humidity on tho "Relative Humidity, Percent" scale The required dry-bulb temperature is given by the inter

section of the straightedge with the "Dry Bulb" scale.

i

y. To detern

. [ t f i '. i j Ihmii.l

; value 5.20° C is obtain.

JitUruoinui) I'ncivinii Required

Another use of these rharls is in del'Tinininu; tin1 prerir-ini] rvqiii-itr in th' 1 psvhr ''trie data (o a:\vf a desired

prei iision in the result. This can be done most readily by

-.ii' cr-.ivi'ly n-sumim: a L'ivrn error, say one unit in the

residing, in each of the data, and finding the resulting change in the final value From the ehanges so found,

the requisite precision is apparent.

vapor is found to be 2.05 mm Hg. By o

jes 25° C on the dry bulb, 9° C on the \\

; following sequence of values is found.

TME SiSue

0.58C

0. 22 C Wet bulb

0.39 0. 26 C

II 111 C

^

Use of the Claris at Other Pressures These charts can be used at pressures other than thoso

marked on tho pressure scale by either of two expedients.

Since the pressure scale operates to multiply ""' "'.served

wet-bulb depression by the ratio of observed pressure to

standard pressure, to obtain the "sca-leve .undent "

wet-bulb depression, tin- operation con be performed by

compulation instead of graphically.

tipk lie Dry Bull

Oprr.lt! 'II plT^ioil

"values of the "Olwrvcl Wet Bulb MiniThe

opriilti .11 of lin.liliL' tin' -t-l-l'.'Vfl i'<|ilivali'lif \M'l-billi. '['-

\ s then carried out graphically as usual. Note

pressure scale is in terms of gage pressure, not

absolute pressure and this method consequently cannot be
2

md 8° C

i cm Hg.

o that it scale at cmHg"

Equivading the

(central, pressure

e at the

res 3.312

he wetFahrend "Wet

3d with

ver (4), )-coated

right of

g'htedge

5" scale

tersects he dry-

.1 at the

;dge fixed

it so that

v extendlt" scale lputation

)f water

: on the

ge with on the

) on the

Le "Relaaightedge

vpoint in

the chart can be used to determine humidity approximately by neglecting the " Observed Dry Bulb Minus Wet

Bulb " and the pressure scales. This amounts to assuming

that the observed dry bulb minus wet bulb is equal to the

sea-level equivalent dry bulb minus wet bulb, and proceeding accordingly.

Example 4 (Centigrade Chart—Low Range)

A psychrometer reads 20° C dry on the bulb and 8° C on the wet bulb when the barometer reads 74.35 cm Hg. The humidity

can be obtained approximately by neglecting the pressure correction,

and using the observed wet-bulb depression as the sea-level equivalent depression. Place a straightedge on the "Sea Level Equivalent

Dry Bulb Minus Wet Bulb, C" scale at 20 —8 = 12, and adjust so

it intersects the "Wet Bulb, C" scale at 8. The approximate humidity is read on the "Humidity, mm Hg" scale as 2.05 mm Hg.

Computation gives the value 2.050 for a 76 cm Hg pressure (as is assumed in this approximate method) and 2.180 for the actual pressure of 74.35 cm Hg.

Thus this approximation introduced a departure from

the formula of 0.13 mm Hg into the result. Note that such

a departure results from the use of all psychrometric charts

which do not include a correction for barometric pressure. A useful guide as to when the above approximation can

safely be used is (t-f) (P-760)

Departure, mm Hg=

1533

Departure, in. Hg

2725

and the departure in the humidity will not exceed 0.01 in. Hg if the wet-bulb depression times the difference between

the observed barometric pressure, in. Hg, and 30 does not

exceed 27. The above formulas are exact only when the wet-bulb

temperature is 0° C or 32° F; however, they are suffi- ciently close for practical purposes over the working range

of temperature.

Determining Pressure of Water Vapor from Relative

or the rule can be used, that the departure in the humidity

will not exceed 0.1 mm Hg when the product of the wetbulb depression by the difference between the observed

barometric pressure in cm Hg and 76 does not exceed 15. Similarly, for the Fahrenheit charts,

(f-f) (P-30)
It is also ol finding the rel ture and the p can be found

Exai

When the dr midity is 40%,

straightedge so Percent" scale w

read the pressure mm Hg" scale

Determining 1 A problem v a desired relati temperature, t eter. To find

corresponds to proximations, can be found f relative humic

example the co ing the straigh

sum of the v£ Equivalent Di

given dry-bull

intersection of

is the requirec

sea-level press

slightly more

th

OHf

in o

ro1 1 1 1 

ro

in 1 1 1 1 1 1 O




6

oh *n i ALiamnH

j _ O °> CO

- - - o o o snNm gnng Aba a^AbGsgo
Example 8 (Centigrade Chart—Low Range)

A psychrometer operating at a pressure of 6 atmospl

15° C on the dry bulb and 13° C on the wet bulb. T

humidity, multiply the numbers on the atmosphere pre by 2, and divide those on the " Observed Dry Bulb IN Bulb" scale by 2. Proceeding as usual, the "Sea Level i Dry Bulb Minus Wet Bulb, C " value is found to be 12. humidity 5.22 mm Hg. Computed values are 12.00 and

This procedure can also be applied to the gage

scale, provided 14.7 (n— 1), (where ?i=factor by 1 merics on gage pressure scale are multiplied) is si from the observed pressure in entering the revised

scale.


8

REFERENCES

^i.eres reads u

o find the

ssure scale linus Wet

Equivalent

00 and the

5.210. pressure

vhich nuibtracted

pressure

(1) Report of the Chief Signal Officer, War Department, 1886,

appendix 24, pages 233-259.

(2) Psychrometric Tables, WB no. 235, 10 cents.

(3) Feuchtigkeitsmessung, by Dr. Hermann Bongards.

(4) Journal of the Washington Academy of Sciences, March 15, 1933, pages 121-134.

(5) Centimeter-inch paper scales for convenient use with these

charts are available from many manufacturers of drafting

and scientific supplies.

This procedure ran also he applied to the pip- pie-smo

scale, provided 14.7 (n — 11, (where ;i = factor by which nnmerio^ on pi ire pn>s-uiv scalo aiv 1 1 1 < i ! (

i

) >t i - t ) i- ^ahtmetrd

from the observed pres>nre in entering the revised pressure