Interpreting the Psychrometer

Accurate readings are important when using the psychrometer to calculate moisture content. April 15, 2012

Question
Can someone can help me use a psychrometer that I just bought? The readings I got in a heated barn were:
RH 54.7
Wet Bulb 18.0C
Dry Bulb 24.3C

What would the EMC be of the wood? When I go in the Drying Hardwood Lumber book to the graph, I think I read that it would be around 9.8. Is this right? If it is, how do I get that to 6-8% without having to heat it to 115F?

The other reading I got was in a unheated open barn:
RH 65.3
Wet Bulb 66.0
Dry bulb 74.3

What would the EMC be of the wood in this barn? I just need to learn how to use this and what to do calculation wise. Thanks.

Forum Responses
(Sawing and Drying Forum)


From Professor Gene Wengert, forum technical advisor:
Pages 16 and 17 of Drying Hardwood Lumber have exactly what you need.

When using these charts, note that the depression (DB - WB) is a key. The temperature of the air is not so critical, so that is why it is given in 5 degree increments. In your example, the depression is 8.3 F, and the temperature is close to 75 F. That would give 11.7%. EMC and 65% RH. To lower the RH or EMC you can heat or dehumidify.

Because the depression is so important, if you have readings to the closest degree C, your calculation will be subject to variation. For example, if you have 30 C and 24 C, you could actually have 30.4 and 23.6 for a 6.8 depression. Or, you could have 29.6 and 24.4 for a 5.2 depression. That is quite an EMC and RH difference. We really need temperatures to the closest 0.1 degree, C or F.


From the original questioner:
Thank you so much. So what do I do to get that down to 6-8%? Heat it to 110F? That would be very hot to work in.

From the original questioner:
Also, in those kind of temperatures (74.3db), how long would it take to get to that EMC? This is important to me, as we dry a lot of wood and store a lot in a shed that is not heated and is just walls and doors. I tend to re-dry a lot of wood even before we run it. Maybe it is worth just drying to 11% MC, and then when ready to run, sticking it in the kiln for a day or two to get it to 6-8%.

From Professor Gene Wengert, forum technical advisor:
Four weeks or longer for 4/4.