I have the same hygrometer and have found it to be fairly accurate. However, I sure wish some clever person would come up with a "guitar friendly test kit" that works at 45% (instead of 75%) humidity because the accuracy of these relatively inexpensive digital hygrometers typically varies over their measurement range. Thus it could read 5% high at 75% humidity, and 5% low at 45% humidity.
Al
Indeed. I've seen response curves for similar capacitance sensors (dew point in this case) and they are generally non-linear s shaped curves ... so you could easily have a sensor in a hygrometer that reads 3% low at 75% and 3% high at 40% and still be within a +/- 3% error limit --- But, if you did a salt test at 75% on such a unit and noted that it was 3 points low and decided to always 3 points to the reading and were trying to maintain a 40% level, you'd let the unit go to 37% (cause you're adding 3), but its at the top of its error curve and the humidity is only 34%.
An inexpensive way to double check relative humidity is with a wet bulb/dry bulb psychrometer -- which can be easily improvised as shown here:
http://www.miamisci.org/hurricane/psychrometer.htmlcheers,
andrew