Knowledge Base

Understanding and setting test voltages of porosity / holiday detectors - Video


Getting the voltage right for the coatings and conditions in which you're working is critical to detecting all the flaws in your coatings without damaging the coating. This video shows you the types of flaws which may occur in your coatings and how to set your porosity / holiday tester to the right voltage so you find all the flaws without damaging the coating.



Visit our Porosity product page for more information about our porosity detectors and accessories


Here's the video transcript for those who prefer to read


The instruments and support that you need for your industry Porosity/Holiday Detector understanding test voltages. This is for the plant owner, specifying voltages. PCWI manufactures Porosity Holiday Detectors and a vast range of coils and brass brushware.


These are the 3 detectors. They come in various models. We have a wet sponge unit, a pulse DC detector, DC unit direct current, constant current. The wet sponge unit is not really part of this discussion. I will leave discussing the wet sponge unit in another session. I will also discuss maybe why it is still used on coatings today.

Note: Standards, methods of test, and voltage formulas are all surpassed here.  Should you be looking at using a standard read the forward and find if the voltage formula is calculated to find flaws that are exposing the substrate only. Many standards are written this way.

The coating needs to be put under some stress to find its applied weakness. The high voltage detectors, pulsed DC which are in 20kv and 40kv, the DC Direct Current models are 15kv and 30kv.


This is a range of brushes, internal pipe, external, flat surfaces, it is all fine brass bristle wire. The coils come in various pre-cut sizes and fits all pipes with a little pac-man to join them. A simple way of doing the test although not the best way.


This is a pipe coil that has been stretched too far. You can see the gaps here. Now those gaps are going to need more voltage because it needs to travel further. It is either that or you have multiple passes over the surface in order to make sure you cover all the surface.


This is a general layout of what the pulses on the pulse DC looks like compared to the DC. The DC is just a straight line of voltage. The pulse DC is these higher peaks, about 28per second. The peaks not there for long - only microseconds. You don’t want to be moving that coil too fast otherwise you could possibly skip over areas.


We have this concept drawing pin holed before and after testing. The pin hole after testing is charred and carbonised as the spark has obviously passed through it and burnt the coating. The flaw - now carbonised - is more easily found on subsequent passes. Also, the air in the area has been ionised. That also makes for easier finding of the next series of holes. It depends how fast you move and the circumstances but that is some of the reasons why it is easier to find it the second time rather than just the first time around.


Coating Dielectric strength test. Pass the brush over the surface multiple times while increasing the voltage until it burns through.

Depending on coating type and thickness, you may find that you do not have enough voltage available to burn through. It takes considerable amount of voltage to actually create a flaw.

You need to establish the true breakdown voltage of the specified coating is, to be comfortable with what test voltage that you are applying to the coating system.


These are some of the defects that you should be looking for.

1. A hole through to the substrate
2. Air voids within the coating. These are very difficult to find.
3. A crevasse which is partially through. Unless you have another test voltage, you may not find these two.
4. The dry sprayed coating is porosity can be a headache to find because you pass over it and pass over it and then all of a sudden the bristles of the brass wire will be in a certain position on the surface to reduce the overall distance the spark has to travel and you can find that it will be a real troublesome thing to find.
5. Is just the low area, you can reduce the coating to see if you can burn through that last little bit but you most certainly wouldn't do any of this with a coil.
6. Is just mechanical damage - you can see the coil laying over the surface, the gaps you need to bridge before you actually get to the coating. This is the coil laying on top of a coating to see all of the flaws below it.



A carbon rubber strip probe does fall into the gaps very little. It dries up over any bumps like as a coil does. It is not something that I would recommend to use.


A brass bristle wire brush is the best test method. The spark that leaves the brass is much longer than the spark that leaves the coil at the same voltage.


When you put some pressure on the brass bristle brush it does fall partially into some of these voids so that increases your chances of finding these areas. The brass bristle is much longer that has far greater reaching capabilities than what anything else has. You can see by the longer strand hanging 10mm down - should this have been a void or a low area it would have dropped into that.

Note: Using a brass wire strip probe with the voltage set to low you may not find all these low areas you may find the bare areas. Using a carbon rubber probe, you may not find any of the flaws as the probe would ride over the high spots. Using a percentage of the predetermined dielectric strength and using a brass strip brush you would find all these areas.

These are quite common to find flaws and corroded metal like this and these are always difficult areas to spray and the more coating you tend to put on the more it tends to run away from the surface. Always keep your brass bristle wire brushes in good condition. Anything that is tangled or matted should be discarded.

With the environmental issues today, no one needs coating failure or corroded pitted steel or leaking ruptured pipelines are a disaster. It is a different world today than what it was a few years ago - maybe it is time to do a little rethink.

PCWI Porosity/Holiday Detector probes, flat brushes, rolling pipe brushes, internal & external pipe brushware, and special requirements made to order. We have all the accessories and adapters to suit all brands. Contact


Visit our Porosity product page for more information about our porosity detectors and accessories



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