Refractometer calibration not stable

Question:

Hi
I have owned my d-d refractometer for 2 years now and have had no problems but lately it does'nt seem to hold its calibration. Set to zero with R.O then test tank water, when i check the zero afterwards it has either gone above the zero or below, its got to the point that i can't trust the reading i'm getting.The clamp is tight and the flap looks square. What could be wrong?

Answer:

Alan

It is not that the zero is not holding it is that the auto temperature calibration is adjusting the scale.

The refractometer MUST be calibrated with the INSTRUMENT at 20C and not the water as the small amount of water will take the temperature of the unit straight away. 20C is the temperature that the refractometer is designed to be calibrated at.

What you are now finding with the weather is that the ambient temperature and therefore the refractometer is now changing in temperature and adjusting a bimetallic strip inside which moves the scale. You will therefore see that the zero point is not always in the same position.

Have a read of an article that I wrote on refractometers
http://www.theaquariumsolution.com/measuring-salinity-and-specific-gravity-your-aquarium

Cheers
Stuart