Have you ever measured, say, air temperature and relative humidity and wondered how accurate your estimate on dew-point really is? You can find out by simply running the program dbrh2rms.exe located on the Pychrometric Web Page, under Applications of the PTM. The URL is: http://www.bae.uky.edu/~gates/psych/PTM_apps.htm under section 4 at the bottom.

To demonstrate how to use by means of an example, let's say your temperature/humidity instrument has a rated accuracy of plus/minus 0.5C and 4%rh. (This is pretty good!). What is the dew-point, and error in dew-point, if we measure temperature and humidity to be 20C and 50% rh with this instrument?

Download the program "dbrhrms.exe" on the web page. Save it to disk, say in your temp directory. Now Start/Programs/MSDos Prompt. In the DOS window, type cd..\temp (assuming you did save the file there). Your prompt should say "c:\temp".

To run the program type: dbrhrms
You will be asked to enter dry bulb, relative humidity, error in dry bulb and error in relative humidity, each on a separate line. Enter 20, 50, 0.5 and 4.
The program outputs the following information:

What does this mean? The air is at a dew-point of 9.3C. The probable error, called Root Mean Square (RMS) is 1.28 C. The possible error is 1.65C (this is unlikely, but possible, for some combinations of dry bulb and relative humdity). Thus, your computed dew-point is 9.3 plus/minus 1.3C, even though your dry bulb measurement error is only 0.5C. Surprised?

Some combinations of temperature and relative humidity cause problems. Basically, if you are "high and dry" you will have larger dew-point errors.

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