Knowledge Base

Which High voltage detector to use DC or pulse DC - Video

 

Each and every high voltage detector is different, DC and pulse DC, so what to use and where.

 

 

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

PCWI porosity / Holiday Detectors. The instruments and support you need for your industry.

Two types of high voltage detectors are available. Which should you be using? DC or Pulse DC?

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PCWI manufactures high voltage porosity detectors and the wet sponge low voltage detector.

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We have a vast range of internal external brushware, flat brushes, pipeline coils. Specials can be made to order.

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The two we will be discussing today are the high voltage detectors with Pulsed DC and a DC unit, direct current, constant current.

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The display of the voltage differences, the pulse DC you can see the high peaks. The peaks are not there very long. There's somewhere between 26 and 30 pulses per second depending on what brand of detector.

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You can see it swings back around through the zero to the negative so it has a negative component to the positive charge so it swings backwards and forwards through the zero position. Very difficult to measure because the peaks are not there long and depends on how well the creast meter is suited to the detector.

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The Constant DC you can see is just a straight line across. The voltage that's set is all above zero. There is nothing below the zero. The diagram shows the Constant Current Flow. The moment the probe is put on the surface, the current flows and it's continuous. It's a little bit like having a piece of string tied both ends. If you pull too much on one end you're going to pull the voltage down.

As in conductive and contaminated surfaces, the units do have regulators to lift the voltage as it's pulled down but the end of the day everything has its limit. The pulse DC detector, it is pulses and it continues to pulse. That's the current flow. It contains some positive and negative charge, so charging some discharging the item under test.

dc_pulse_waveform_concept

Once the probe is lifted off the surface the unit continues to pulse, but the current flow stops. You can see here the difference. This is a pulse 20 and 40 and high voltage coils in the handle and the voltages produced in the hand. Industrial kit and there is a pipeline kit. The same goes for the DC 15 and the 30.

The voltage is produced in the main unit and transferred to the handle. We have an industrial kit and a pipeline kit. So there's two different uses here. One's pipeline, one's industrial for general coatings. It's for flat surfaces. It can be used on pipes as well.

So to use what detector where? This is not about DC versus Pulse DC. It's about what is the best Holiday / Porosity Test for the applied coating in its intended environment, to ensure that you have a porosity free coating system.

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Testing coatings use DC or Pulse DC. There are far too many coatings to have a hard and fast policy.

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I've made this up you can see on the thinner coatings, internal and external coatings we prefer to see DC used rather than Pulse DC. DC is smoother with more controls on the output. You can work off this chart. It is an indication.

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Pipe internals, tank internals, pipe externals and submerged structure. And then you have your different coatings. I believe below 500 microns you're better off using DC and above you can use Pulse DC. This chart here shows coatings that are prone to charging up and contaminated surfaces use Pulse DC detector. DC may work in some of these areas. If you have a DC detector try it before going pulse.

Some of the coatings that can be charged up. A static charge can build up in high voltage testing some pipe coatings and remain for a while. The discharge of the static is not harmful but may bite all the same. Not  all coatings do this - it generally happens in dryer atmospheric conditions.

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This is mainly aimed at nylon coatings. I would use Pulse DC for the simple reason you can charge the coating up to the voltage that's been applied, hence any discharge is not possible. A mat may give you a capacity of earth. That is generally enough to test these coatings. You can also, once you have a mat down you can also use a trailing earth in conjunction with it. 

When a project has to be high voltage porosity tested, to say use a Pulse DC detector because a direct earth is not available is incorrect. The substrate may be earthed at some point if it's not, it's own bulk would be an earth in itself. A trailing earth works on both DC and Pulsed DC high voltage detectors. A direct earth to the substrate is always a positive earth. It is always the best.

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Last minute decision to high voltage DC test these towers found one tower had a large amount of pinholes right down one side. The reason? Overspray from one spray painter down wind was deposited on the surface over the down wind side and sprayed over by the second spray painter. A lot of pinholes.

These are just some of the surfaces where you can use a DC detector. If the coating's thick enough you can also use a Pulse DC. Some of these atmospheric coatings ordinarily you would bypass and say 'well we're not going to porosity test these.'

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But you can see the result on some of those coatings there and they really needed to be high voltage tested because you can see the breakdown. Now that breakdown may not have occurred had it of been tested in the first place.

With all the environmental issues to contend with today, no one needs premature coating failure. Corroded or pitted steel that can be environmentally difficult and very costly to repair. Leaking or ruptured tanks and pipelines can leave everlasting environmental disasters. It's a different world we live in today. Maybe it's time to do a rethink on the level of testing that's needed for the future. PCWI Porosity / Holiday detector probes, flat brushes, rolling pipe brushes, internal and external pipe brushware. Special requirements made to order. Coils, high voltage leads, earth leads, probe extensions, adaptors to suit all brands. Contact sales@pcwi.com.au.

 

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

 

 

Categories: Videos