Ah ... I was thinking more towards the OP question of mixing new saltwater.yes and no, it’s calibrated at 25c so the water needs to be 25c when you take the reading, my tank is 26c so time I pour 500mls into measuring cylinder it’s 25c, you can use a chart to get a reading if say the water was only 20c, you can also just measure it straight from the tank but the flow needs to be off to read it, you can also easily break the hydrometer in the tank on a rock etc but for all that the reading it gives is extremely accurate and it never needs calibrating, good to know your salinity is correct.