1% to 5% remover to water by volume. The recommended concentration for manual spray operations is
1% by volume, and in no case shall the concentration of remover exceed 5%.
b. Equipment. A practical and efficient way of handling the low remover concentrations is by
continuously metering the remover directly into the stream of water. This can be done with an aspirator
device that employs the water flow to create a vacuum (Bernoulli effect), drawing up the concentrate
directly from the container. The method is inexpensive and only requires a minimum of equipment and
provides intermittent, on/off operation. A disadvantage of this system is the variation in concentration
with water pressure. This requires the careful control of water pressure as well as the mixing ratio.
The most commonly used system is the installation of a three-way valve on the water rinse or wash line.
The aspirator is connected to one side; fresh or plain water to the second; while the third position is off.
This allows the existing wash tank to be used for both spray removal and fresh water rinsing.
c. Removal Mechanism. The modes of action are the same for both hydrophilic immersion and spray
remover techniques. However, the mechanism of spray removal is complicated by the relation between
the chemical and mechanical action. As the spray water pressure is increased, the rate of removal also
increases. A common misconception is that the increased rate of removal is due solely to the greater
mechanical action. The higher water pressure actually increases both mechanical and chemical action.
As the water pressure increases, more solution contacts the surface per unit of time, thereby increasing
the chemical action.
Following the penetrant removal step, the part SHALL be subjected to a plain water rinse or wash. The purpose is to
remove any remover residues that could contaminate the developer or interfere with the development process. The
rinse step is a water spray in the station or tank used for the pre-rinse. The process step is not critical and requires very
few controls. The cycle SHALL be a plain water spray of 30 to 60 seconds duration using a pressure of 10 to 35 psig
and a water temperature between 50°F (10°C) to 100°F (38°C). Rinsing of fluorescent penetrants SHALL be
accomplished under a black light. Further details of water spray removal are given in section 18.104.22.168. One of the
advantages of the hydrophilic technique is the ability to do touch-up removal on local areas after the initial application
of the water rinse. After touch-up, the part SHALL be fresh water rinsed.
Summary Comparison of Lipophilic and Hydrophilic.
A comparison of the physical, chemical and application differences between the lipophilic and hydrophilic techniques
is set out in Table 2-4. The hydrophilic method has the ability to remove surface penetrant with reduced effect on
penetrant entrapped in a crack. This ability results in several benefits compared to the lipophilic method. A major
advantage of hydrophilic removers is the increased process tolerance, i.e.; (hydrophilic) removal time is not as critical
as (lipophilic) emulsification dwell. Additional (hydrophilic) removal times of 1 or 2 minutes have little effect on
penetrant entrapped in a discontinuity, while additional (lipophilic) emulsification times as little as 10 or 15 seconds
can seriously degrade a flaw indication. Figure 2-21 is a cracked chrome plated panel that has been processed to show
the effects of optimum, insufficient and excessive hydrophilic removal. The cracks in the panel are progressively
smaller from left to right in the figure.