Sling Psychrometer Inaccuracy

Are there any special tricks to getting an accurate reading with a sling psychrometer? The instructions provided with the instrument seem quite basic: saturate the wet bulb wick, whirl the instrument until no thermometer movement is detected, and compare with a chart.

AccuWeather, my Vantage Pro, and a digital handheld psychrometer all suggest I should be getting more wet bulb depression, usually about 3 degrees F. What could I possibly be doing wrong? I avoid touching the wick, use reverse osmosis water and read the wet bulb first. Even refrigerating the instrument before using it makes no difference; the wet bulb temp will rise to the same (too high) reading. I usually whirl the instrument at about 2 revolutions per second, have tried faster/slower, but didn't see much difference. It seems the wet bulb has reached it's minimum temperature after about two minutes regardless how fast I spin it. I hold the psychrometer at arms length and upwind from myself. Wet days, dry days, hot, or cold, I always measure humidity higher than it really is.

Someone suggested spirit filled psychrometers are less accurate than the mercury type, so I purchased a mercury filled psychrometer as well. The two agree precisely.

Any suggestions would be greatly appreciated.