EmonTH humidity value


I tried the salt test.I put a cup with seasalt and water inside a sealed bag with emonth and 2 other meters.

After 3 days my analog RH meter shows 80%, digital humidity/temp meter shows 69% and emonth shows 64,8% when 75% is the desired value. 

Analog meter is easy to calibrate. But how to calibrate the emonTH ?