Hi there, I have an RHB 0-80 analogue refractometer that does not want to calibrate today. I've looked for tips elsewhere online, but the calibration steps aren't solving my problem. When I put a few drops of distilled water on the plate and point towards the sun, there is no null line: instead the orange goes all the way to the bottom, past the 0 point on the scale. My calibration screw lock just keeps turning around - it doesn't lock. In the past, when the calibration has been off I've been able to tighten/loosen the calibration screw and that has worked. Help! Any advice would be most welcome. Thank you.