After concentration, the most important measurable associated with Metalworking Fluid management is pH.  This symbol is defined as the activity of hydrogen ions in solution expressed as a negative logarithm.  Small “p” is the mathematical symbol for the negative logarithm and the “H”, or more properly the “[H+]”, is the chemical. In laboratory pH is often measured with a pH meter which is the most accurate method if proper techniques are used.  In the field pH paper is often used and while it is not as accurate on a theoretical level but can give a fair idea about the health of fluid.  Reasonably priced digital meters are easier to use and reliable it properly maintained (keep the probdbe wet).  STLE helps us understand importance of pH in depth.