I replaced my aging refractometer with a veegee stx 3. My question is about the calibration adjustment. I have the screw loose to the point it's about to fall out but the ring is super stuff and I can barely turn it by hand. It also feels gritty when turning. Just wondering if that's the way they all are. If so I guess I can live with it but it's not what I expected from a $150 refractometer. TIA