HVAC Procedure and Performance Guide

Measure relative humidity

Training for this procedure can be found in these courses:

                         131 – Oil Heat l  and  161 – Boilers l


Relative humidity in space or duct must be determined using psychrometer.   Psychrometer must be used according to manufacturer’s instructions or following accepted practice.


    1. Review psychrometer instructions, inspect system and determine correct locations for measurements.
    2. Put on safety and protective equipment and apparel.
    3. Select instrument to use:
    4. Wet-bulb and dry-bulb thermometer in moving stream (duct).
    5. Sling psychrometer in static space (open room).
    6. Take temperature measurement:
    7. Wet wick.
    8. Insert wet-bulb and dry-bulb elements into the duct or whirl sling psychrometer.
    9. Record wet-bulb and dry-bulb readings.
    10. Determine relative humidity using readings from Step 4 and psychrometric data, and document findings.
    11. Leave equipment in required operating status.
    12. Remove safety and protective equipment and apparel.
    13. Gather tools, equipment and leftover materials, and leave work site clean.