DHT22 & DHT11 innaccuracies

been playing with a number of DHT11s getting reasonable readings (within 7-8% of my test humidity instrument & 0.5ºC temperature)
Read that DHT22 is a better, more accurate, more durable sensor so bought a number of DHT22s & find temperature is slightly better but %RH is way off, typically 15 - 25% low.
In salt cell check NaCl should be round 75% depending on exact temp - I get readings about 50%. (Test instrument reading 72%)
Using Rob Tillaarts library version 0.1.20 in a Uno. (actuallt 3 different ones - with & without display shields. (with display shield & sensor connected, sketch uploads fail: another problem but minor )
Have I just got a batch of duds?

Good chance of that. I bought four last year and one was about 20% low in humidity. and unstable. Try a few more from a different seller.

If you bought them from Ebay, you are in luck that is it only 15 - 25% low. At least buy them from good sellers, like Adafruit, Sparkfun, Seeedstudio, and so on.

The capacitive humidity sensors are better.

Adafruit has the HTU21D-F and the SHT31-D and the BME280 humidity sensors, I think they all are better than the DHT22. The HoneyWell HIH-9000 Series should also be good.

http://www.ebay.ca/sch/m.html?_odkw=&_ssn=ca.th67&_armrs=1&_osacat=0&_from=R40&_trksid=m570.l1313&_nkw=dht22&_sacat=0

I bought my DHT22's from this seller, and their accuracy was no more than 1-3% on temperature, and a little worse (5%) for humidity.