Application Note

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Application Note

Understanding the relationship between dry bulb

temperature, wet bulb temperature, relative humidity,

and dew point temperature is essential in all facets of

air conditioning. These psychrometric processes play

an especially important role in building and materials

integrity, occupant health and comfort, and overall

indoor air quality.

From the Fluke Digital Library @ www.fluke.com/library

Evaluating relative

humidity: Key factors

and measurements

Relative comfort

ANSI/ASHRAE standard 55-2004,

Thermal Environmental

Conditions for Human Occupancy,

sets an upper limit to absolute

humidity levels (0.012 humidity

ratio, or 0.012 x 7000 = 84 grains

moisture/lb dry air, also equiva-

lent to a dew point (DP) of 62 °F),

above which most occupants

become uncomfortable.

Since all occupants won’t be

satisfied by the same thermal

conditions, especially all at the

same time, the standard attempts

to identify a norm based on a

V (Predictive Mean V

ote) of

80 % satisfaction. From that, a

PPD (Predicted Percentage

Dissatisfied) of 1

0 % is calculated

for general thermal comfort

dissatisfaction and 10 % PPD

from local (“my ank

les are cold”)

comfort dissatisfaction.

The standard lists six primary

factors that affect thermal c

om-

fort: metabolic rate, clothing

insulation, air temperature, radi-

ant temperature, air speed,

humidity.

Understanding the combined

affects of these factors can help

technicians configure building

systems appropriately.

The good news, strangely

enough, is that poor humidity and

temperature levels are likely to

cause occupant discomfort.

Occupant complaints open a win-

dow of opportunity for the HVAC

contractor to proactively discover

related undesirable psychrometric

effects on materials integrity and

indoor air quality, including

microbial propagation.

To evaluate relative humidity,

wet bulb temperature, and dew

point, HVAC technicians tradition-

ally used a sling psychrometer

and psychrometric chart.

Now-days they use “humidity”

meters that are accurate, more

onvenient, and usable in

onfined locations unsuitable for

sling psychrometers.

Standards adoption

Many states have adopted ANSI/

E Standards 55-2004 on

humidity and 62-2004 on I

into their building codes. Since

oth standards ha

ve b

een newly

updated, the following descrip-

tions may help inspectors and

ontractors update practic

es to

meet new requirements.

The Fluke 971 Temperature Humidity Meter measures tempera-

ture from -20 °C to 60 °C (-4 °F to 140 °F), dewpoint, wetbulb,

and relative humidity from 5 % to 95 %.

Humidity levels

NSI/ASHRAE Standard 62-2001,

Ventilation for Acceptable Indoor

Air Quality, spec

ifies that

“Relative humidity in habitable

spaces preferably should be

maintained b

etween 30 % and

60 % relative humidity to mini-

mize growth of allergenic or

pathogenic organisms

.”

The updated ANSI/ASHRAE

Standard 62.1-2004, Ventilation

for Ac

eptable Indoor Air Quality,

is more specific. Now, relative

humidity upper limits are based

on peak values

. “O

cupied space

relative humidity shall be

Summary of content (4 pages)

PAGE 1
Evaluating relative humidity: Key factors and measurements Application Note Understanding the relationship between dry bulb temperature, wet bulb temperature, relative humidity, and dew point temperature is essential in all facets of air conditioning. These psychrometric processes play an especially important role in building and materials integrity, occupant health and comfort, and overall indoor air quality.
PAGE 2
Psychrometrics has a language all its own. To better understand how the various parameters interact to support thermal comfort, here are some of the more common terms described in this document: Wet bulb temperature: Represents the cooling effect of evaporating water, the temperature air will cool to when water evaporates into unsaturated air. Dewpoint temperature: The temperature under which water will condense out of the air. Dry bulb temperature: Air temperature determined by an ordinary thermometer.
PAGE 3
Condensation equipment). Water pipes can be insulated. Air handling equipConditions that allow condensament and ductwork must be tion to form on surfaces are more sealed air tight and insulated obvious, so action can be taken with no breaks in the vapor barimmediately. When a surface rier especially when located temperature is at or below the outside of the conditioned envedew point temperature, condenlope. Ductwork in all walls must sation will form.
PAGE 4
• • • Sensible vs. latent cooling and S/T ratio can be found by plotting conditions on a psychrometric chart or from a psychrometric calculator. Dew point is critical in both summer and winter evaluations. Duct surface temperature must be maintained above dew point to prevent condensation whether inside or outside of the conditioned space. Winter indoor relative humidity must be kept low enough to ensure inside wall and window surface temperatures do not approach dew point.