These Economy Refractometers are an excellent choice when occasional dissolved solids measurements are needed. The instruments feature adjustable focus and a zero-adjustment screw that make reading and calibration a snap. To use, just aim the front end of the instrument at a bright light, turn the adjusting ring until the reticule can be clearly seen, open the cover plate and add a few drops of distilled water to the prism, and then close the cover plate and press lightly. The correcting screw can be used to align the light/dark boundary with the null line. Choose from standard units calibrated and adjusted to read at 20°C or models with Automatic Temperature Compensation (ATC) for applications where the temperatures of your sample vary. Each refractometer includes hard case, calibration screw driver and instructions.