What do you fill a Raymarine P79 Depth Transducer with?

Yep… a quick search confirms it's anti-freeze ( https://en.wikipedia.org/wiki/Antifreeze ). Can't imagine concentration is vitally important, surely it's more to do with having a fluid there to transmit the pulse from the transducer and using something that won't freeze? No doubt others more expert than me will surely correct me if I'm wrong ( jfm where are you when we need you<g> )
 
I was told by a marine electronics engineer to use cheap frying oil. It works fine giving a good picture even at high (20+ knots) speeds

I just read this on a marine engineers site lol. (no mention of frying oil though):D
Tank should be filled with non-toxic antifreeze (propylene glycol). Do not use mineral oil.
 
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