Professional Automatic Temperature Compensation Sea Water Salinity Refractometer for Aquarium, Hydrometer, 0-100ppt & 1.000-1.070 Salinity Specific Gravity https://www.amazon.com/dp/B00AZ1SMC2/ref=cm_sw_r_cp_apa_wFakAb27GPM7T
This ok?
more or less exactly what I use. From what I have seen from the cheapest to the most expensive there's not a lot of difference. until you get to the digital ones anyway. but just my opinion.
the key Is to calibrate as close to your reading point as possible. so you would want to get some 1.026 calibration fluid. I like to keep the fluid and the refractometer in the same place so there's not a huge temp difference when calibrating.
if you google Randy Holmes refractomer, you should see quite a lengthy nice article on calibrating, function and proper use.