instant ocean hydrometer
INSTANT OCEAN HYDROMETER:
Easy to use and inexpensive. Range of 0-40% ppt (1.000-1.030 sg). Accurate to 1.5 ppt. Direct reading in ppt and specific gravity.

#HYD10: Instant Ocean Hydrometer $12.95

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Someone needs to make phone app for this. After a series of beeps it could announce, "You have three inches of saltwater in your bilge" or "You have water in your bilge, but don't worry, it's fresh water" or "You have a foot and a half of saltwater in your bilge. Get your ass down here now or I'll call the Coast Guard"