75:25 or 50:50 Test Solution for your CM Series?
In order to correctly select which test solution to use, it is necessary to know a little about the constituents of the solution.
The function of the water is to ensure a dissociation of ions taken into solution by the solvent action of the 2-propanol (IPA). It is the presence of these dissociated ions, which increases the conductivity of the test solution. As pure 2-propanol is a non-ionic hydrophilic solvent, its presence does not influence the reading except insofar as it “dilutes” the water bearing the dissociated ions.
The conductivity of the solution, for a given weight of ions, is therefore lower with the presence of the solvent, and the greater the quantity of 2-propanol, the lower will be the conductivity. The greater the proportion of 2-propanol, the lower will be the sensitivity of the instrument. On the other hand, the greater the proportion of 2-propanol, the greater will be the solvency of the solution. The choice of the test solution to be employed is therefore a compromise between the sensitivity of the measurement and the power to dissolve as many kinds of contamination as possible. If working to MIL standards a 75% solution is required. The MIL-P-28809 specified, a 75% solution of 2-propanol and this proportion has become more or less standardised in the U.S.A. This standard though was made obsolete in 1996.
The nearest equivalent is IPC TM650 method 2.3.25c. However, the conditions governing the dissolution of the contaminants in the ®Contaminometer are very different from those of the MIL specification. As discussed above, a higher alcohol content will reduce sensitivity but increase solvency. Thus the use of 75% solution is not necessarily the best choice for use with the ®Contaminometer. In light of this it has emerged that when employing immersion methods in particular, working with a 50% solution gives a much better compromise between the sensitivity and the solvency. This 50% solution is also specified by the UK DEF-STAN 00-10/3 standard whose referee method is virtually identical to the ®Contaminometer principle. Other related international standards are: IPC J-STD 001, IPC 6012 and others.