Measuring Osmosis?

[Wardy]

How is osmosis actually measured?

My thoughts are that two probes are pressed into the inside of the hull, a set distance apart, and a high voltage is applied and the resistance measured in Megohms.
It would certainly measure the conductivity of the insulative material grp (lower resitances meaning wetter hulls) and an altered scale might be able to give a percentage readout.

Presumably I am well off the mark with my simplistic approach, so what's the reality?

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[BrendanS]

OK, this is a bit pedantic, but what surveryors measure when they stick their little electronic devices onto the hull, is the amount of water in the hull material.

To measure 'osmosis' is a completely different thing. Osmosis is effectively a pressure difference across a semi permeable membrane (in this case, gel coat or similar) and would be a test of the pressure differential between the GRP and the outside water - no one measures this, and it would be of little practical use

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[Wardy]

Excused for being a pedant; i did mean how is the water content of the GRP measured, I assumed it was the same thing.

So what principle does the device use that measures the water content?
Is it possible to use a Megger and achieve results that joe public might use to save a few quid?

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[BrendanS]

There seem to be two types. One is very simple, and works pretty much as you describe, with two pins that stick into the hull.

The other type is a bit more sophisticated, and uses RF transmission to detect changes in capacitance, caused by the presence of water, two probes, one transmitting aerial, one receiving


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[fastjedi]

I believe some can provide reasonable readings within an hour of a lift (Time for the hull to dry dry) and others should only be used after circa 2 days ashore?

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[macd]

As I understand it osmosis (a term many reputable surveyors will not use, preferring something like 'blistering') and water content are very much not the same thing, although they can certainly be related. It's self-evident that blisters can easily be quantified (proportion of hull area affected, size of blisters), whilst 'osmosis' can't so readily, if at all. Equally self-evidently, it's the blisters that we ought to be worried about, not some chemical process that seems only partially understood. I suspect that insurers like to know moisture readings since they've little else to go on. But I bet if you went round your local boat park taking moisture readings, you'd find a pretty inconclusive correlation between high readings and observable 'osmosis'.

I've found what seems to me a deal of good sense on http://www.yachtsurvey.com/blisters.htm and related links. Would recommend a look-see to anyone worried about their GRP.

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[macd]

Out of interest, why would osmosis concern anyone who sails a cast iron bath? Or have you installed a GRP shower tray, you old softy?

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[BrendanS]

I'm looking at buying a brand new, super speedy planing bath tub of GRP or plastic manufacture, which raises obvious concerns! I might replace the rubber band while I'm at it, it's getting a bit stretched and manky, and isn't giving the performance it used to.

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[Birdseye]

I think that the sovereign meter measures the dielectric constant of the material under the probe. The dielectric constant of grp is affected by its water content, as indeed are other permiable materials - wood or plaster for example.

The meter reading does not correspond directly to a percentage of water in a laminate, which is one reason why you are advised to check the figure on a known dry part of the hull for comparison purpose. And you need to be careful of what is on the inside - for example a timber bulkhead or even some condensation on the inside of the hull will often affect the localised reading.l And you also need to ensure that the probe is flat against the surface or you will get a false good reading because you are partially reading air.

And as someone has pointed out, none of this is a measure of osmosis. It merely indicates a damp hull which may or may not cause an osmosis problem.

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[charles_reed]

Re: Measuring water content of grp

Is not at all simple and requires expert knowledge and careful calibration of the equipment used.

It is possible to get a reading from just about any hull and I guess that 50% of the treatments that are carried out are unnecessary.

Far better to look for those blisters and then only use an induction meter (the more accurate) to monitor the drying-out process.

In answer to your question - no unless you're prepared to learn a new trade with plenty of examples on which to practice.

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