Maximum Comfortminder - Manual (Page 4)

Because of the high cost of the other two types of instruments, we will consider only the sling psychrometer. If
you buy an official US Weather Bureau sling psychrometer, it will contain the proper operating instructions and
tables for high accuracy usage. If the unit you obtain does not contain tables for various pressures, then you
should obtain the official pamphlet from the Superintendent of Documents at the US Government Printing Office.
The pamphlet’s name is “Psychrometric Tables for Obtaining the Vapor Pressure. Relative Humidity and
Temperature of Dewpoint from Readings of the Wet and Dry Bulb Thermometer, No. 235, Reprint 1941”: by
C.E. Marvin.
The calibration check is made by swinging the psychrometer as close as possible to the instrument under test.
You should swing the psychrometer at a rate of two revolutions per second for at least one minute. The readings
of the wet and dry bulbs should then be recorded and the procedure repeated until at least two consecutive
identical readings are obtained. You should then record the hygrometer’s reading, making sure the unit is in the
vertical position and lightly tapping the instrument. Also record the time of observation. Now following the
instructions supplied with the tables, determine the % RH and record it. Because of the relatively slow response
of most hygrometers compared to a psychrometer, you need to repeat the checking procedure every 15 to 30
minutes until the psychrometer and hygrometer readings are stable (usually within an hour).
Now that you have your comparative data, you must take into account the specified accuracy of both instruments.
As an example, we will use our COMFORTMINDER compared to an official weather bureau psychrometer. The
COMFORTMINDER has a rated accuracy of ±5% RH from 20 to 80% RH. The psychrometer is accurate to ±2%
RH from 0 to 100% RH. So, if the psychrometer reads between 20 and 80% RH, then the two instruments should
read within 7% RH of each other. If, however, the humidity is below 20% or above 80%RH, then no comparison
can be made because it is outside of the COMFORTMINDER’S accurate range. The seven percent span is
derived by adding the ± tolerances together, ±5% and ±2% = 7% (i.e. the COMFORTMINDER reads +5% and
the psychrometer reads –2%, then the difference is 7% RH). This shows us that if you were to compare two
COMFORTMINDERS they could read as much as 10% RH different and still be within calibration tolerances.
In conclusion, humidity measurement is one of the most difficult and least understood environmental
measurements made. We hope that this essay has helped you to understand humidity and its measurement.
However, this is far from a complete explanation, in fact, there is a branch of the sciences called Hygrometry that
deals with humidity measurement. If you want more detailed information, please refer to the publications listed
below:
Introduction to Meteorology Instruments for Physical Environmental
Author – Sverre Petterssen, PHD Measurements, Vol. I 2
nd
Edition
Publisher – McGraw Hill, Inc. Authors – J.Y. Wang and C.M.M. Felton
New York, NY Publisher – Kendall/Hunt Publishing
Library of Congress #68-15476 Dubuque, IA
Library of Congress #83-81613
A Field Guide to the Atmosphere ISBN #0-8403-3098-7
Authors – Vincent J. Schaefer and
John A. Day Humidity and Moisture, Vol. I to IV
Publisher – Houghton Mifflin Co. Publisher – Reinhold Publishing Corp.
Boston, MA New York, NY
Library of Congress #80-25473 Library of Congress #65-13613
ISBN #0-395-33033-5
Climate and Weather
Author – Hermann Flohn
Publisher – McGraw Hill, Inc.
New York, NY
Library of Congress #67-22978