When using my refractometer if I take a reading of the sample immediately and then 30 seconds later I get a different result, which one is correct?
When you place the sample on the unit you need to wait around 30 seconds to allow the water to equalise with the temperature of the unit. As the density of water changes with temperature it needs to sit for a while on the unit to match its calibrated working temperature.
When calibrating the Refractometer the unit itself needs to be at 200C, the sample of RO water should then be put on to the refractometer and then left to equalise for 30 seconds before setting the unit to zero. After this point it does not matter if the refractometer is at a different temperature as long as the sample is left for 30 seconds to equalise. This is due to the unit being auto temperature compensating (ATC).