Q.  It looks like the humidity sensors in our plant are not accurate anymore. How is the interior plant humidity related to outside humidity values?

Answer: First, it is common to find relative humidity (RH) sensors lose calibration in a few years, especially in a dusty environment. So, I do suggest evaluating their every six months. A local heating/air conditioning person should be able to do this quickly.

In a plant, especially with dust collectors, open doors or windows, and so on, will draw outside air into the plant. Oftentimes, this air will be cooler than the plant’s desired temperature, so heat is added. When heat is added to air, its humidity drops. 

For example, start with foggy air at 100 percent RH and 50F. This condition will result in about 28 percent MC within wood, with the surface and ends changing within a few hours, while the core may take a month.

To give us woodworkers a good feel for the air’s condition, we use the term EMC (Equilibrium Moisture Content); at 50F and 100 percent RH, the air is at 28 percent EMC.

If this air is heated to 60F, the humidity will drop to 70 percent RH, which is 13 percent EMC. When heated to 70F, 49 percent RH and 9 percent EMC. If we have some finishing ovens that heat the air to 110F, the humidity is 14 percent RH and 3 percent EMC.

Another example: Assume the outside is sleeting with 99 percent RH and 30F.  Heating this air to 70F results in 24 percent RH and 5 percent EMC. Note that if we have some oak parts that came from lumber that was at 8 percent MC and are now exposed to this 5 percent EMC condition, the wood will lose rather quickly (days) because parts have a lot of surface area, With a 3 percent MC loss in oak in this example comes roughly 1 percent shrinkage in width and thickness and sometimes warping.

Obviously, it is important to match the lumber’s MC and the plant’s EMC, as well as the EMC in the office or home, so that very little or no size change occurs in our wood product.

In case you were wondering, wood essentially does not change size with temperature changes, if the EMC is constant; it is all about humidity and moisture.

Of course, when the plant is too dry, to avoid shrinkage defects, we add moisture to the air. Ideally, we want to match exactly the EMC in the customer’s home or office…or at least be within 2 percent EMC, as most of the time a 2 percent EMC difference is not too serious.

So, how can you determine when you will likely have to adjust your plant humidity?  You can use a humidity sensor, but they are not always accurate.  Plus, the sensor tells you that you are out of range after the fact.  It might be more helpful to know when a moisture situation is developing. So, here is a DIY technique.

Step 1. Measure the outside temperature and RH, or call the local weather station or look on-line. Many counties have local agricultural meteorological stations; call you county Coop Extension office.

Step 2.  Plot this outside condition on a hydrometric chart. I like the one on page 19 of Drying Hardwood Lumber. Report FPL-GTR-118 as it contains the EMC as well as temperature and RH and absolute humidity. Copy from


Determine the absolute humidity for this plotted point. (Using the first example, 100 percent RH and 50 F is at the top of the chart and is intersected by the 4 grains per cubic foot line.)

Step 3. Followed the curved absolute humidity line until it insects with the desired temperature (vertical line). Going horizontally to the let from this line will give the RH; going slightly upward to the right will give the EMC.

Special note 1: The absolute humidity seldom changes during a day or two unless there in a cold front or warm front that moves through. So, knowing one value each day will be adequate.

Special note 2: Do you turn the heat down over a weekend? You can predict the plant EMC over the weekend using the cooler plant temperature and then see the big drop in EMC when the heat is turned back on Monday morning.

Special note 3: If you do have Rh sensors, it would seem to make more sense to report the EMC values and not the RH.


Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.


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