Correction of flux density

Introduction
Fluxes for soldering supplied by flux manufacturers must sometimes be diluted to bring the flux to the specified density for the soldering process. On the other hand, flux solvents do evaporate from the flux, increasing the flux density.
In case of the use of so called low solid content fluxes other means of flux control than the hydrometer with sensor may be necessary to keep the flux within its specifications. In this case the tolerance of the conventional controls may be too large for accurate flux control. The flux supplier can give adequate information about this matter.
The main issue of this sheet is to give some calculation examples in case a flux has to be changed or the flux level and density control unit has to be filled.
Example 1
The flux in the can has a density of 0.84 g/ml.
The density for the soldering process must be set at 0.82 g/ml. The diluent is isopropyl alcohol with a density of 0.79 g/ml. We will calculate how much diluent must be added to bring the flux to the desired density, expressed in litre diluent per litre flux (l/l).
Calculation:
If we add x litres diluent to one litre flux, the total amount will be (1 + x) litre. We start with a flux density of 0.84 g/ml to which we add x l/l diluent with a density of 0.79 g/ml. This must give in total a flux amount of (1 + x) with a density of 0.82 g/ml.
This can be expressed in the following equation:
1 . 0.84 + x . 0.79 = (1 + x) . 0.82 giving
0.84 + 0.79x = 0.82 + 0.82x
0.02 = 0.03 x --> x = 2/3 l/let 014
Example 2
A fluxing system is completely filled with a flux with a density of 0.835 g/ml. The wanted density is 0.825 g/ml. We want to calculate how much flux of 0.835 g/ml has to be exchanged for diluent with a density of 0.79 g/ml, expressed in litre per litre.
Calculation:
The original flux 0.835 g/ml has to be partly exchanged for diluent 0.79 g/ml. The total amount may not change but must be brought to a density of 0.825 g/ml. If we exchange y litre/litre of flux to y litre/litre of diluent, we can set up the following equation:
(1 - y) . 0.835 + y . 0.79 = 1 . 0.825 giving
0.835 - 0.835y + 0.79y = 0.825
0.01 = 0,045y --> y ± 0.22 l/l