Correlating relative humidity outside and inside an enclosure

Question

I am building a device and I tried to Google an answer to this question but I am an electrical person and lack knowledge in this field.

I need to know what the relative humidity inside an enclosure (of my device) will be for a given ambient temperature. If I know:-

Relative humidity outside the enclosure (95% RH) The ambient temperature (40 deg C) The air temperature inside the enclosure (60 deg C)

Since the air inside the enclosure is hotter because of the electronics I know the RH inside will be lower but I do not know how to go about quantifying and justifying it on paper.

Answer 1

Using the +10 to +120°C psychrometric chart with sensible heating from 40°C, 95% RH to 60°C gives approximately 35% RH. If you need a more precise answer I would look up the formulas which can be found in ASHRAE Fundamentals or CIBSE Guide C.

See image below for how to look this up on the chart:

Answer 2

There is a psychrometric chart (ASHRAE) that will give you the relative humidity, but you will need the Dew Point or condensation temperature as well as the ambient temperature. This you can measure with a Wet and Dry bulb thermometer but you will need to access the thermometer to wet the wet bulb. Charts can be found here.

Perhaps your easiest solution will be one of the cheap meters available which will give you a reading directly in %rh.