Actually already said everything, anyway:

bought three units and compared: Two show identical values, the third + 3% RH and + 1 ° C. So absolutely in the tolerance. After many bad buys, or appropriate freebies this device is the drug of choice and my wholehearted recommendation. Who has rooms with wet (cellar) to fight and bring the necessary concern for the physical relationships, the dew point indicator will be appreciated! Simply only the wall temperatures, or the monitor of "critical" cold spots and one is on the safe side and know exactly whether you should take action (air, dehumidifier) ​​or not.