28 FEBRUARY 2003
DATA SET PAGE 3: 30-YEAR PSYCHROMETRIC SUMMARY. Figure 2-3 is an
example of Data Set Page 3, a graph summarizing the site's psychrometric data.
Explanation of Graph
The graph displays the joint cumulative percent frequency of temperature and
humidity ratio. Hourly observations are grouped into bins of 5 Fahrenheit degrees and 10
grains per pound (gr/lb) (or 3 Celsius degrees and 1.5 grams per kilogram [g/kg]), centered on
each value of temperature or humidity ratio. For example, the 70 F temperature bin collects
all observations between 67.5 F and 72.5 F. The bin is depicted as a gridline on the chart;
the vertical lines represent the temperature bins and the horizontal lines represent the
humidity ratio bins. The intersection of temperature and humidity ratio lines represent a further
subdivision of the observations into groups meeting both temperature and humidity ratio
criteria. For example, the intersection of the 70 F bin line and the 40 gr/lb bin line represent
the observations when temperature was between 67.5 F and 72.4 F and the humidity ratio
was between 35 gr/lb and 44 gr/lb. Thus, a joint-frequency table is created for all temperature
NOTE: The psychrometric graph is intended as a visual tool only. Its purpose is to allow a
quick visual comparison between climates at different locations. Extrapolation of data directly
from the graph is not advised due to the approximate plotting routine used to generate the
graph from the binned data. This is evident where values of humidity appear past their
saturation point. This discrepancy between the actual data and the graph is the result of the
plotting routine used to generate the graph and not from errors in the original hourly data used
to create the binned summary.
The contours on this chart represent the areas containing 99%, 97.5%, 95%, 80%,
and 50% of all observations (cumulative percent frequency or percentiles). The contours are
centered on the most frequently occurring bins (50% contour), spreading outward until almost
all observations (99%) are grouped. Contours are defined by calculating a percent frequency
for each bin (relative to the others), and then accumulating these percent frequencies (from
most frequent to least frequent) until the 50% value is passed, and thus the first set of bins is
grouped. The accumulating continues until the 80% value is passed, and the second group of
bins is grouped. This process continues until the 95%, 97.5%, and 99% values are passed.
Consequently, the least frequent (most extreme) bins, which when accumulated
amount to less than 1 percent of the total observations, are outside of the 99% contour. Any
bins outside the 99% contour thus have either not occurred, or have occurred so infrequently
that they should not be taken into consideration for sizing equipment.
Suggestions For Use
The Data Set Page 3 graph displays the long-term history of temperature and
moisture at each station (a total of 262,800 hourly observations if the POR is 30 years and if
the data is complete over that period). The engineer can use this graph to ascertain the most
common temperature and moisture conditions that will be encountered over the operating life
of the mechanical equipment.