Can anybody recommend an accurate reference sensor or device what I can use to calibrate my other RH/Temp sensors against?
For temperature:
Ignore the fact that they mix up precision and accuracy in the description. It’s good for ±0.1°C (max) from –20°C to 50°C.
For relative humidity you are going to need some jars and some salts: Saturated Salt Solutions and control of Air Humidity
Method: Test and Calibrate DHT22 / AM2302 / RHT03 hygrometers
Sealing your sensor in a jar above a saturated solution of sodium chloride (table salt) at room temperature should give a RH of 75%, once equilibrium is reached.
Magnesium chloride is also (relatively) harmless and easy to obtain. It should give 33% RH at room temperature.
A two point calibration should be good enough but for three points you can use potassium carbonate or sodium hydroxide (draino), beware that this last one is quite caustic. Wear skin and eye protection. And don’t dunk your sensor in any of them 
thanks for this method 
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