T.O. 33B-1-1
1-36
panel without allowing them to contact each other. However, this is not always possible so care should
be taken to keep the area of mixing to a minimum. An overlapping of the two emulsifiers of less than
1/4 inch is acceptable; however, any overlap greater then 1/4 inch will reduce the accuracy of the test,
requiring the panels to be reprocessed.
c. Allow an emulsifier dwell time of 2 minutes and a wash time of 60 seconds. The water spray should be
applied equally to both panel halves with the nozzle at a constant distance from the surface. The bare
metal should be examined for signs of fluorescent background after the process has been applied.
Developer must be applied before evaluation.
d. A distinct difference in residual background indicates excess penetrant contamination of the working
bath lipophilic emulsifier. It is acceptable to extract a quantity of used emulsifier, for example 55
gallons, and replace it with fresh unused emulsifier. At least 25 to 50% of the tank volume should be
extracted and replaced. This procedure SHALL be done only once before changing the entire tank.
Following this procedure the emulsifier mixture shall be retested as described in paragraphs 1.5.5.3 to
ensure proper functioning of the emulsifier bath.
1.5.5.4
Testing Hydrophilic Remover.
1.5.5.4.1
Immersion Baths.
NOTE
This test is valid only on new hydrophilic remover baths.
Freshly mixed (new) hydrophilic remover is characterized by a pinkish-red color that varies in intensity with the water
content. There are three methods to verify initial remover concentration. The first method utilizes an instrument
known as a refractometer. The refractometer measures the refraction or refractive index (Snell’s Law) of the material
utilizing the refractive index scale which ranges from 0 to 320, with water having refractive index of 0. The second
method that may be used is known as colorimetry. It is a procedure of chemical analysis that deals with the
measurement of the light absorption by colored solutions. The fundamental principles of colorimetry state that the
amount of light absorbed by a given substance in a solution is proportional to the intensity of incident light and to the
concentration of absorbing material. Visual colorimetry is a simple method and is fairly precise. It matches the color
of a standard solution with an unknown; when they become identical they must contain the same amount of colored
substance. An instrument used to perform this task is known as a colorimeter. The final method involves the use of a
hydrometer to determine the concentration by specific gravity. This method is very similar to the method used to check
developer. Procedures for the refractometer and hydrometer are detailed below.
a. A refractometer is supplied in the penetrant process control kit and is the recommended method to use
in determining the initial water content concentration. The refractive index and light transmission
properties of removers vary from batch to batch, even with the same type and manufacturer. This
makes it necessary for each NDI lab to develop a graph of concentration versus refractive index number
or colorimeter reading for each batch or lot of remover.
b. When using a refractometer perform a water content test in accordance with the following procedure:
(1)
Dip the plastic rod supplied with the refractometer in the solution of new hydrophilic remover
and water being tested. Do not use a metal or glass rod as it may scratch the refractometer prism
face. The test solution should be well mixed for accurate results.
(2)
Raise the cover plate on the refractometer and place two or three drops of the test solution on the
prism face. Close the cover plate, making certain the test solution film completely covers the
prism face.
