Starpath Humidity Resources

Humidity, the moisture content of the air, is one of the most important atmospheric properties for weather analysis and forecasting. Accurate relative humidity, however, is not easy to measure.

There are many inexpensive hygrometers on the market, electronic and mechanical, that purport to measure it, but they generally do not work. Visit a store than sells many models and read what they say. Or search the YouTube for numerous videos of folks making some attempt to calibrate them with salt mixtures and solutions. Notice how they always find that the instruments are not right—though often the measurements are not right as well, but that does not affect the point at hand.

Instruments that do work, cost more than these decorative models, and they must be traced to a reputable manufacturer. We sell two devices that are dependable, one is a Taylor Sling Psychrometer and the other is the Fischer Precision Synthetic Hair Hygrometer.

A sling psychrometer is a simple, dependable instrument, but it takes some time and effort to get a good reading for Relative Humidity. A hygrometer, on the other hand, shows the Relative Humidity on its dial, just as you would read a thermometer or barometer. The hygrometer should be periodically rejuvenated and its calibration checked. A sling psychrometer is an excellent way to check a hygrometer at the existing ambient relative humidity.

A good sling psychrometer measures the wet bulb and dry bulb temperatures of the air and from these you can compute both the dew point and the relative humidity. Here is an app from Vaisala that will do that for you: Humidity Calculator. It can also be used to compute relative humidity from the given dewpoint and air temp. You might need this when using marine weather reports as they give temp and dew point, but not relative humidity as a rule.

The accuracy of the sling psychrometer depends on the accuracy of its thermometers, assuming the wick is kept tight and clean and only distilled water is used. Relative humidity is sensitive to the tenth of a degree in the two temperatures, so read the values as carefully as possible. The Taylor instruments we sell can be estimated to about ±0.2°. The measurements still require care. First the wick must be kept free of salt spray and dirt or any chemical contaminant. Use distilled water. And you must swing them longer than the instructions say. At least a full minute, preferrably a bit more. And be careful reading the therometers. You may see the dry bulb rise as you bring it close to your body to read... a few Farhenheit tenths is not uncommon. Play with the calculator to show that this can have a big effect on the humidity results. For use in a fixed location, you might rig a compuer cooling fan (Radio Shack) to blow on the wet bulb. This configuration is then called an Assmann Psycchrometer.

Hair hygrometers, synthetic or real, need to be periodically rejuvenated at high humidity. An easy way to do this is to place the instrument inside a sealed plastic bag with a small open container of water as shown below. Let it sit over night. The instrument should then read about 96% (94 to 98). This rejuvenates the fibers and provides a rough high-end calibration point on the instrument. If you note that it reads below 96, then consider the difference as the offset of the instrument and apply this correction to readings at lower RH values. This method, however, is not a very accurate calibration point because the result you get in a bag like this will depend on the room temperature to some degree.

Some instruments like the Fischer have an adjustment screw available to apply an offset. Remove the instrument from the bag and wait several hours till the instrument comes back to stable ambient RH reading before making the adjustment. If it read 88% at saturation, then it is nominally 8% low, so if the ambient RH read 45% it should be increased to as close to 45+8 = 53 as possible.

Below is a Fischer instrument in a plastic bag with a demitasse half filled with distilled water after setting all night. It reads 98% RH.

We are currently working on various salt and water combinations for testing at other RH values. We are also testing several commercial humidor calibration kits and will post our results here when conclusive. Our results so far are not very good, but we are learning.

News: as of May 30, 2011, Starpath now owns a Vaisala PTU300 instrument with high accuracy relative humidity and temperature. We will certainly learn more in the near future and will post it here.