Thursday, November 13, 2014


At the Clear Spring School, students are studying weather and meteorology. In science lab they have been making various instruments and testing various soils, sand, water, and soil covered by fake snow to record the amount of solar radiation they absorb from heat lamps.

In wood shop, we've been making hygrometers based on the difference in expansion and contraction of wood, long-grain vs. cross-grained. The idea is that if you glue cross grained wood to a strip of thinner long grained wood, it will flex as it expands and contracts, acting like a needle on a dial. The woods used (elm and walnut) are very responsive to changes in relative humidity. One of my students noted, "mine's all bent." "It is supposed to be," I assured her.

As the relative humidity climbs, the assembled piece straightens or may even curve upwards. As the humidity falls due to changes of weather, or due to heating or AC, the  "needle" will curve downward. By using a professional level hygrometer to do our calibration, we will have made some useful instruments and will have demonstrated a property of wood: that it expands and contracts in response to changes in relative humidity across the grain, but not in length. That is a crucial thing to remember when designing furniture or even boxes from solid wood.

Make, fix and create...


  1. Anonymous3:54 AM

    Very nice project.
    If you don't have a professional hygrometer to compare with, you can use two thermometer and a psychrometric chart :
    or :

    the bulb of one thermometer is wrapped with moist cotton (saturation temperature); the other is kept "dry" (dry bulb temperature).
    The intersection of the two lines on the chart gives you various values, among those : the relative humidity.
    The saturation temperature is lower than the dry bulb temperature (unless you have 100% humidity)because the evaporation of the cotton moisture causes a temperature drop.
    The interest of the hygrometer is of course to have a direct reading.
    The bi-wood hygrometer could be enhanced if the end of the beam acts on a needle which would amplify the deflection. Take for example a stick 2"1/2 long, make a pivot at the mark 1/2". If the short side moves with the bi-wood beam, the extremity of the long side will have a 4X deflection.
    I hope this makes sense.

  2. Thanks for the suggestion on magnifying the deflection... And for the idea of using of two thermometers to determine relative humidity. We have a school weather station that will give indoor and outdoor relative humidity, so I plan to calibrate our wood hygrometers using that, as our science teacher already demonstrated relative humidity using the two thermometer approach.