As in much of life, you get what you pay for.

 There are two main type of antifreeze testers available on the market today.  Hydrometers and Refractometers.  Hydrometers measure the specific gravity of antifreeze and refractometers measure the speed with which light travels through a sample of antifreeze.

 Hydrometers account for nearly 75% of all antifreeze testers sold.  A recent survey showed that 95% of service personnel believed that their antifreeze testers were accurate to + or - 5 deg. F, and that was an acceptable degree of error.  Plus or minus 10 deg. F was considered unacceptable by 90% of service personnel.  Unfortunately, most people do not realize just how inaccurate hydrometers can be.   Under controlled laboratory conditions, with a very high quality hydrometer, the ASTM (American Society for Testing and Materials) found that the best accuracy that is achievable with a hydrometer is plus or minus 8 degrees F.  ((ASTM) Method D1124)  That means that, even under ideal circumstances, 50/50 antifreeze, with a freeze point -34 deg F, could read anywhere from -26 to -42 deg. F.  What causes this inaccuracy?  Well, there are a number of items that could contribute.  Remember that Hydrometers measure specific gravity. Specific gravity is extremely temperature dependent. The same sample that is at -34 deg F. at 100 degrees F will read as having a freeze point of -4 deg. F at a temperature of 150 degrees F.  Obviously, the temperature of the solution being tested will greatly affect the reading that shows in the hydrometer, and can result in considerable error.

 There are other factors that can affect the reading a hydrometer will show as well.  Air in the solution can cause bubbles to attach themselves to the floating balls or dial and affect the reading.  Also, over time, oil emulsified in the antifreeze can coat the floats and affect the reading.  And if all this isn't bad enough, experience has shown that hydrometers always seem to err on the side of showing the mixture to be weaker than it really is, and requiring more pure antifreeze to be added.  Whether this is due to liability concerns, or because the testers are made by the same companies that make and sell antifreeze is unclear. 

 What is clear, however, is that you aren't doing yourself or your customers any favors by running a 70/30 antifreeze mix in their cooling system.  Increasing concentrations to roughly 60 percent improves freeze point protection level. Above 70 percent, freeze point protection level becomes progressively worse. In this case more is not better.
Cavitation corrosion, water pump failure, scale formation, gelation, inefficient heat transfer, boil over, freezing and cracking engine blocks, solder bloom - all are problems defined by the Society of Automotive Engineers (SAE) for over and under concentration of engine coolant/ antifreeze.  More than 26 percent of all repair costs can be directly attributed to cooling system maintenance issues. There are other problems associated with higher concentrations as well. Water pumps are designed to work with a specific viscosity of fluid. Increasing concentration increases the viscosity of the fluid, thus water pumps have to work harder, potentially reducing life of the water pump. There is also a phenomenon known as cavitation corrosion associated with concentrations exceeding 70 percent. This is the formation of microscopic bubbles within the coolant/antifreeze. These bubbles and the silicate particles from the additives contained in many coolant/ antifreezes act almost like sandpaper. They can rapidly wear away at the cylinder heads, liners, head gaskets, water pump impeller and even the radiator.