Here's the video transcript for those who prefer to read
The instruments and support you need for your industry. Low voltage wet sponge testing its use and history. PCWI Manufactures Porosity Holiday Detectors and a
vast range of brushware and coils.
The 3 units available are a wet sponge unit upholster dc and a dc direct current constant current unit.
The wet sponge unit is a very simple instrument. It has 3 voltage ranges: You simply wet the sponge clip on the earth pass the damp wet sponge, over the
surface and any flaws that have the substrate the exposed and available will show as a beep and the alarm will allow you to work exactly where that flaw is.
Now the types of flaws that you generally find in coatings are holes through to the substrate like as in number one; you will find the air voids in the
coating or in the crevasse like as in number three. Four is a very difficult area, dry sprayed – porosity this is very difficult to find. Five is a low spot
and six can be chemical landing.
Now low voltage wet sponge tests would only find flaw 1. That would be dependent on the floor diameter, the coating thickness and the flaw's capability of
being wetted out.
Now, if you are satisfied with that result, then low voltage wet sponge test maybe suitable for your project. That is, providing you can get back to revisit
a few years later to evaluate the coating system, and/or to carry out recoating and repairs. I would not like to see this used on the internal/external of
pipelines and varied structured. This test does not guarantee a flaw-free coating system. It all depends on what the flaw is.
Now, the wet sponge tester, and how it came to be.
Well in the early days all it took was a battery, a speaker/horn (like out of an old phone), some wire and a wet sponge mounted on an insulated handle. Clip
the battery negative wire to the substrate, the battery positive to the horn, and the other terminal of the horn is wired onto the sponge. When the wet
sponge/moisture touches the bare substrate the circuit is completed and the horn sound.
Simple, yes, but primitive. By today’s standard this system had a lot of electronic flaws. Some of the old style batteries that we use, like the 9 volt, has
always been around.
Now onto the voltages used.
Why do we have these 3 voltages settings, 9, 67.5 and 90 volt? 67.5 volts is an odd number isn’t it? More than odd. It goes a little like this:
They were all batteries, 9.0 volt battery was/has always been in existence, but why 67.5 and then 90 volt? Simple, these battery voltages were made up of
standard radio batteries. The 22.5 volt was a standard battery back in those good old valve radio days.
So 3 x 22.5 volt made up 67.5 volts. Or you could get a 67.5 volt battery. And the same is true with the 90 volt. This just shows again how simple this
system was to manufacture, although these days you tend to be looking at the input voltage being stable and the alarm being set at a certain resistance.
Handheld with three preset voltages - it's simple, but is it suitable? The wet sponge has an extendable handle that can reach out beyond 1.2 metres and a
simple wet sponge - easy to wet and run over the surface. But, surface wetness and dampness can vary, so your results can vary too. You can have round
sponges, you can make sponges up in any shape you like and any size you like. Whether the unit has the capabilities of actually handling that size sponge is
something you'll need to determine.
Low voltage wet sponges.
So why was it needed, when High Voltage was about? The High Voltage Pulsed DC units of the day were basic - a battery hooked up to an ignition coil. It was
unstable - high floating voltage that dropped considerably when the brush was placed on the surface, with the set voltage changing with battery condition.
The alarms would not work at all at the lower voltage settings, where they were needed most.
The High Voltage DC units suffered similar issues, with alarms not working very well as test current diminished at the lower voltage settings. They were DC
(Direct Current), not Constant Voltage Constant Current as they are of late.
You can virtually go to one of the latest DC units to replace a wet sponge, but you must first work out exactly the coating dielectric strength.
Once having done that, you can then use a DC unit dry. I wouldn't recommend a pulse unit, but you can use a DC unit to replace the wet sponge unit. You need
to establish what the true breakdown voltage of the specified coating is to be comfortable with what test voltage you are applying to the coating.
So where to for the future?
Selectable voltage DC (direct current), stable voltage, constant current from the selectable voltage range that is available - wow, it's 2015. We need to
move on. Make the coating systems last longer. We need to be smarter.
With the environmental issues today, we need to be smarter. No one needs coating failure or Corroded Pitted Steel or Leaking and Ruptured Pipelines - it's a
It’s a different world today than what it was a few years ago. Maybe it’s time to do a little rethink on this porosity detector.