Moisture Meters; good & bad moisture!!

[pcatterall]

I use my cheapo moisture meter all the times in buildings but wonder what use these meters ( and more expensive ones) have on yacht hulls.
Inside a building I am measuring damp which has penetrated or risen so as to make the surface damp where it is being measured. I assume that current flows across the electrodes better where the suface is damp. On a boat hull I guess that we are trying to measure the damp inside the hull rather than on the surface, if the surface is at all damp then won't the reading be completly distorted?
My instinct would be that to get a reading through a hull you would need an eletrode outside and one exactly opposite inside ( impossible to acheive)
So how does this wizardry work on our yacht hulls.
I understand that the better meters ( soveriegn?) can be set to give readings near the surface or deeper inside the material and wonder how this is acheived?
Good and bad moisture?? I assume that bad moisture is the salty stuff penetrating the gell coat and all ready to react with the chemicals in the resin. 'Good' ( well harmless) moisture could be associated with internal fresh water tanks. So some high readings may be ok if we are sure they originate from inside?
I know we get lots of repeat threads on the dreaded osmosis but I would be really interested to understand just how the meters work in a boat situation.

 

[savageseadog]

I have a Tramex buildings moisture meter which works fine on boats for what I want to do. It doesn't use point contact electrodes but a couple of rubber pads. I'm not sure of the exact method of measurement but it does work through an insulating barrier. The marine version is here:

http://www.tramex.ie/Our_Products_Details.aspx?ID=29

 

[boatmike]

I have a Tramex buildings moisture meter which works fine on boats for what I want to do. It doesn't use point contact electrodes but a couple of rubber pads. I'm not sure of the exact method of measurement but it does work through an insulating barrier. The marine version is here:

http://www.tramex.ie/Our_Products_Details.aspx?ID=29

The "rubber" pads are indeed electrodes so I guess made to conduct electricity although I am not sure how. They would better be termed "soft" electrodes because I don't think they are just rubber (someone else might know). I think the idea is that they make contact without scratching.
To answer the main point however there is no "good" or "bad" moisture and the actual reading will depend on many factors. The best way to use them is to take comparative readings. In other words take a reading on the hull well above the waterline where the laminate should be dry and compare with a reading below the waterline (after antifoul has been removed locally). Big differences indicate high moisture content. Small differences ignore.

 

[pcatterall]

Yes, I use the 'mapping method' in buildings, the exact (numerical) reading is less important than the differences and I try to build up a picture of damp sources by looking at the range of readings. Still not sure,though, how my system where the damp is on the surface relates to finding damp which is inside the surface being examined.

 

[Poignard]

Those things are useless. I bought one and wherever I tried it on my boat the needle was always in the red, so I sold it on ebay.

 

[Cymrogwyllt]

It's far more important to select the gauge user properly than the gauge. I bought a stone built house with a 20 year warranty damp course installed by a nationally reputable firm five years earlier. Due to lack of ventillation, poor windows and poor heating the place had a condensation problem. Four of the five firms I called in poked the wallpaper with the probes once and said 'you need a damp course and replaster to x height. One of the four said complete replaster in and out' . I did not feel it necessary to tell them about the installed damp course. The fifth spent more time investigating and pin pointed the condensation rather than damp. He informed me that no major wrok was needed and offered advice on short term measures followed by longer term measures including heating policy etc. No charge. I considered a tip of enough for several evenings' beers very well deserved.

edit - I'd bet that at least 80% of damp problems are actually condensation

 

[savageseadog]

Those things are useless. I bought one and wherever I tried it on my boat the needle was always in the red, so I sold it on ebay.

I've used my Tramex to identify moisture saturated areas and leaking fittings quite successfully. As already pointed out it's a case of making relative measurements so as one works round a boat it becomes apparent what is good and bad.

 

[danielefua]

I don't know about Tramex but the moisture meters for fiberglass do not measure electrical conductivity via contact electrodes. They measure the average dielectric constant of the material by capacitance sensitive circuitry. In this way they are able to measure even in depth, probably down to at least half inch from the surface, the presence of very small amounts of trapped water molecules whose dielectric constant is very different from that of fiberglass.

Daniel

 

[mitiempo]

The Electrophysics CT-33 meter is the most popular in North America for measuring moisture in boats. It measures to a depth of 1". It can not be used effectively on a deck that has dew on it nor can it be used in temps below freezing. I have found it to be accurate when used properly. Most surveyors use this meter.
Here's a great link explaining its use in detail with pics.
http://www.pbase.com/mainecruising/moisture_meter

 

[yacht surveyor]

I don't know about Tramex but the moisture meters for fiberglass do not measure electrical conductivity via contact electrodes. They measure the average dielectric constant of the material by capacitance sensitive circuitry. In this way they are able to measure even in depth, probably down to at least half inch from the surface, the presence of very small amounts of trapped water molecules whose dielectric constant is very different from that of fiberglass.

Daniel

Correct. It is the same method used by the iphone to detect your finger.

Care needs to be taken with readings as damp in the boat, water in tanks and the presence of conductive materials such as steel and carbon affect the readings. The GRP or core can be dry, but the readings can be off the scale in way of a small puddle of water internally!

The depth of measurement depends on the type of meter and the settings.

 

[pcatterall]

Thanks guys, I am a little more in the picture. I may have to invest in a meter of my own so that at least I can form my own opinion.
The cautions about using on damp surfaces are interesting, the 3 surveyors I have contacted about a hull survey seemed quite gung ho about measuring in mid winter.
I have been a surveyor for over 40 years but doing mapping, settlement and building work, boats are not my area of expertise but I do have a little understanding of the rigour involved in the survey and in drawing any conclusions from it.
I'm still interested in the good/bad moisture idea as I thought that only salty water set up the osmosis blistering and that 'clean' water would not have the same effect.
That idea was shot down twice so I'll have to conceed.
Thanks again

 

[yacht surveyor]

The cautions about using on damp surfaces are interesting, the 3 surveyors I have contacted about a hull survey seemed quite gung ho about measuring in mid winter.

The damp surfaces I was referring to are on the inside of the hull, and get picked up with deep reading meters such as the Tramex or the Sovereign Quantum meters. The older Sovereign meters don't read as deep.

Moisture meters need to be used with understanding and care. Experienced surveyors understand the conditions where readings need to be interpreted and do so. What I was trying to say in my previous post is that there are many occasions where a high reading does not necessarily mean that there is a problem. I have previously abandoned moisture meter readings and gone to other methods of diagnosis when the conditions were not suitable (4 deg 98% humidity for example is not a good time to take readings).

The moisture meter is only a tool, the use of which needs to be understood. You can draw all sorts of wrong conclusions if you don't undertand what it is trying to tell you.

The best time to take readings is of course in summer when the boat has been out of the water for a few weeks. Surveyors don't generally get that luxury - overnight is about as long as we get and often it is just a hour or so on a lunchtime lift out that the prospective owner and broker will allow us. (Some yards are more flexible than others).

I'm still interested in the good/bad moisture idea as I thought that only salty water set up the osmosis blistering and that 'clean' water would not have the same effect.
That idea was shot down twice so I'll have to conceed.
Thanks again

Osmosis is worse in fresh water as the concentration difference is higher between the blister and the outside fluid. (see page 175 of Fibreglass Boats, Hugo De Plessis ISBN 978-0-7136-1395). Higher temperature water also accelerates the process.

To help understand osmosis and basics of the use of moisture meters, have a read of read of this by my good friend Nigel Clegg.

Martin

 

[pcatterall]

Thanks Martin for that link to that really excellent article by Nigel Clegg. Its really good to get an in depth technical overview that a layman can understand.
Most of my stupid questions are now answered and I know where to come back to!!
The comment made about not reading on damp surfaces related to the link provided by mitiempo rather than to your previous thread.
Thanks again

 

[Bajansailor]

Excellent article by Nigel Clegg, thank you Martin for posting the link.

For more bedtime reading (you might even stay awake until the end) here is an osmosis opinion by Jan de Groot - http://uk.yachtworld.com/sealake/sealake_10.html

(His book 'No shoes allowed' is also a good read)

 

[mitiempo]

I know of Jan de Groot and have read that article before - interesting reading.

To me the best reason to have a moisture meter and know how to use it is:

1. If you are looking for a boat it can be used to weed out problem boats. Probably 98% of the fiberglass boats of any size have cored decks and cabin tops. Water ingress around fastenings on deck, such as stanchions, cleats, or anything else attached with through bolts or screws or passing through the deck as with chainplates can leak either when not bedded properly or when strained over time as with stanchions. The manufacturer in most cases didn't remove the core around these penetrations leaving it exposed on the many boats I have owned, worked on, or seen while being worked on. It takes a while but there are many boats 20, 30 or more years old so it has had the needed time. Also on some boats the manufacturer used sealant or bedding compound on the backing plates or washers making the problem worse. Any leak past the deck should not be stopped on the inside as there is nowhere else for the moisture to go except into the core. And no evidence of a leak below to warn of the problem. Rather than hiring a survey at large expense a meter used properly can eliminate the problem boats. As far as cored hulls, far fewer in number, I would leave that to an expert unless there was an obvious problem and then I would pass on that boat.

2. Once you own a boat with a cored deck or cabin top you can use the meter to check periodically around deck attachments and chainplates to check for problems. A table showing readings each year can help identify problem areas before they become worse.