Protecting instruments against fouling

Antifouling paints and other agents are often used to avoid transducers that get covered with barnacles or other evil crustacés during long deployments.  Depending on the water temperature, fouling can be a problem already after 2 weeks in tropical areas or it can take 6 months in cold areas.  Sunlight is also a big factor - fouling is generally much less of a problem below 100 m than it is at 5m.

Here are some things people do (cut from various newsgroup postings and from emails we have received):

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In general when antifouling any [acoustic current meter] for a long deployment we use plastic bags. Thin plastic bags of the type used as liners for small household or office bins work really well. Stretch them over the [acoustic current meter] and then bind them in place with ducting tape. You need to put some tiny pinprick holes in the top of the bags so any trapped air can escape but the holes are too small to allow any organisms to enter.

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We use zinc oxide as can be found in children's ointments.  A small amount on the high-frequency transducers and we see no sign of fouling

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We have been pleased with Pettit's Trinidad anti-fouling paint (75.8% cuprous oxide).

Best regards,

Atle Lohrmann

Quote from email we received:

I am intrigued by the idea of using plastic bin bags as anti-fouling and will try it out.
Zinc oxide cream - is this not somewhat messy?
The old hands at IOS used to smear vaseline on their aluminium housing tubes and this works well.

I have a problem with pressure sensors both with NDPs and with Aquadopps.
We have solved the NDP problem and can give people details of our solution on request.
With Aquadopp (single point and Profilers):
- cover caps pop off...
- small barnacles enter, lodge on the pressure sensor membrane and GROW.
I think we need to address this problem.
FYI see three Aquadopps immediately following recovery after 8 weeks deployment in a tidal river at stations about 1.5km apart. The one deployed in freshes water is least barnacled,

Aquadopp recovery

We have a run a test with three different zinc oxides, a zinkpaste with 40% zinc oxide, Silulen (Nycomed) with 20% zinc oxide and Desitin (Pfifer) 40% zinc oxide.  They all work well but the Silulen a little less so.  Our conclusion was that a higher zinc oxide percentage is probably better. 

We also tested the effect on the transmission of the 2 MHz Aquadopp and we could find no appreciable change when we applied a thin (1 mm or so) layer on the transducers.  We also have reports from SiO that they successfully have used the same type of material on 6 MHz Vectors in their nearshore experiments.

Best regards, Atle Lohrmann

PS For those without kids, zinc oxide is used as ointment against diaper rash and can be purchased at the pharmacy or in the pharmacy section of the supermarket.