Refractometers and test strips account for the rest. The curious fact about this is that even a high precision, expensive laboratory hydrometer used by a trained laboratory technician, in a controller environment, using a calibrated and clean hydrometer, using fluids mathematically corrected for fluid temperature, can not obtain this level of accuracy. How can we reconcile this fact with the overwhelming feeling of satisfaction that service provides have regarding their hydrometers. Simply stated they do not realize how inaccurate the hydrometers really are. Facts Hydrometers measure specific gravity.
Specific gravity is extremely temperature dependent.
The same sample which is read at 150 F will read as having a 30 F better freeze point protection level than if it is read a temperature of 100 F. Temperature effects can be calculated. Hydrometers only work for the ethylene glycol based antifreeze coolant. Sampling techniques is critical to hydrometer use. Air bubbles in the sample will cause inaccurate readings. The float must be kept free from the wall of the hydrometer.