Keeping your world up and running.®

Evaluating relative humidity: Key factors and measurements

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.

To evaluate relative humidity, wet bulb temperature, and dew point, HVAC technicians traditionally used a sling psychrometer and psychrometric chart. Nowadays they use "humidity" meters that are accurate, more convenient, and usable in confined locations unsuitable for sling psychrometers.

Temperature-humidity meters
From dry bulb temperature and relative humidity measurements, temperature humidity meters such as the Fluke 971 can calculate wet bulb temperature and dew point temperature, psychrometric points that are essential for HVAC evaluations and diagnostics.

  • Wet bulb is very closely related to enthalpy, or the total heat in the air (dry bulb and wet bulb). In a psychrometric chart, the wet bulb lines are nearly parallel the enthalpy scale values. Return wet bulb temperature is mandatory for accurately charging a cooling system that incorporates a fixed restrictor metering device.
  • Supply and return wet bulb temperatures across an evaporator can be used with a psychrometric chart or enthalpy table to calculate total cooling capacity, sensible and latent capacity, and S/T ratio.
  • Total heat may be found by multiplying cfm x 4.5 x enthalpy difference across evaporator (Qt = cfm x 4.5 x Δh).
  • 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. If condensation appears on window or wall surfaces, condensation hidden within envelope walls will be likely.

Standards adoption
Many states have adopted ANSI/ ASHRAE Standards 552004 on humidity and 622004 on IAQ into their building codes. Since both standards have been newly updated, inspectors and contractors may need to update practices to meet new requirements.

For standards details and measurement techniques applying to relative comfort, humidity levels, fungus, and condensation, as well as for a description of related terms and resources, please download the complete article »