ATC Refractometer temperature compensation

Question:

I just read a post on your site about temp compensation when using a refractometer. I am using a refractometer with ATC, does this still mean I have to adjust for the temperature differnce in the aquarium water. I calibrate at 20oC but my aquarium water is usually 26oC. How would the refractometer work in this case?

regards
Phill

Answer:

Phill

The water temperature is 26 but this is irrelevant to the refractometer which will always compensate and measure as if the waterwas at 20C.

You need to look for the starting point or reference that you want to run your tank at. Most people want to run their tank at a specific gravity of 1.025 when the tank is at a temperature of 25c.

As specific gravity is temperature dependant then the SG will increase as you cool down the water because the water contracts and reduces in volume but the weight always remains constant. At 20C the same water will have an SG of 1.0264 and as the refractometer always reads at 20C then this is the reading you should really be looking for.

Basically if you calibrate your refractometer at an instrument ( not water temp) of 20C with RO or DI water so that it reads zero and then look for a reading of 1.026 you will not go far wrong.

The most important thing is how consistent the SG is over time.

I hope you can get your head around this as it is not immediately obvious and most people thing the ATC refers to the water temperature.

Regards

Stuart