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. Both are determined in field testing to be more accurate and reliable that the typical hydrometer. The difference between using a refractometer versus using a test strip is the desired result. Emissions control is impossible without the proper concentration.