Spark Testing for Rubber Linings

Learn more about the holiday detection of rubber linings using high voltage.

What is spark testing?

Spark testing is a very effective form of high-voltage (< 1,000 volts DC) leak detection used for finding pinholes, cracks, or voids in a variety of materials. This nondestructive technique is primarily used for detecting vulnerabilities that are not visible to the naked eye in materials such as rubber, paint, or other coatings. As a result of its high-voltage, this form of leak detection is especially useful when testing rubber linings since they are usually quite thick or other coatings that exceed 500 microns or 20 mils in size.

It is important to regularly test the coating on metal used for pipelines and water tanks in order to prevent corrosion. Problems with these coatings usually come from three sources: the manufacturer, the installation process, and the age of the object. Spark testers can efficiently test large surface areas of these coatings which are sometimes as thick as 1/4 inch without damaging the underlying object or its coating.

When the electrode or probe encounters a hole, spark discharges occur from the probe to the tank wall through the air path defined by the hole in the dielectric tank lining. The spark discharge is usually easy to see running from the probe tip to the location of the hole on the top of the surface lining.

The spark test or discharge method is found reliable and nondestructive. The probe voltage should be large enough to achieve spark discharge through long, oblique holes. There is no danger from creating holes by dielectric breakdown in sound linings for material thicker than 1/16 inches, provided the test is done with Electro-Technic model BD-50E with its particular voltage and frequency characteristics. This statement is based on extensive published literature and some experiments with conventional thermoplastics and rubbers.

How do you use a spark tester?

02 Calibrate

Be certain that your voltage is set correctly and you are using the proper electrode. A calibration block is usually used to determine the necessary length of the spark. A good rule of thumb is that the spark needs to be at least twice the size of the lining you are testing. For large surfaces, typically a brush, T, or L shaped electrode is used in order to cover lots of ground more efficiently.

01 Inspect

Whether it's a pipeline coating or rubber tank lining, the first step is to thoroughly inspect the surface prior to testing. You want to ensure that the area is free of moisture, dirt, or any other type of foreign matter that might interfere with testing.

03 Test

Pass the probe over the surface of the material in a constant and interrupted manner at a speed of about 1 ft/sec. until the leak is found. Leaks are detected by the presence of white or blue sparks, while a leak free lining causes dark-blue or purple sparks. Once a leak is found, mark with chalk for later repair.