Did you buy it new ? The new style ones look like this
I set up my heaters in a bucket with a powerhead and thermometer before using ...
The red ring has an arrow pointer
If you want 78F;
Hold the tube ( preferably by the black section ) and twist the blue ring to 78 and, after hours, check it against a thermometer
Now you use your fingernail to move the red pointer to match the thermo and it'll be calibrated
Now you can twist the blue ring to your desired temp and recheck it