Steve. Thanks. This sounds like the Tank Tender system: www.thetanktender.com

"Tank Tender operates on air pressure. Select the tank, then pump a
small amount of air through tubing to the bottom of the tank. When the
air pressure in the tube equals the liquid pressure at the bottom of
the tank, the gauge displays this pressure in inches".

They list at US$375. for a single tank system. Have to find a non-
marine version. ?? www.grainger.com list several gages alone from
US$65. - $300.There are some lised on ebay as well -

It is an option but I only have access to a aft corner of one tank and
the fwd on the other tank - (belly tanks 168 cm L x 43 cm w X 28 cm
H). some people with the same boat have said they do not find the Tank
Tender very accurate - not that we really need precision - probably
because the tank is shallow and the tube is at the end..

foggy

On Jan 18, 10:56*pm, "Steve Lusardi" wrote:
As stated before, a flow meter gives indirect answers to your question and
therefore is prone to error. I have another suggestion that I think is not
only more practical, but less expensive. Use a "Differential Pressure Gage"
calibrated in inches of H2O. All this takes is a small diameter hose to the
tank bottom connected directly to the high side of the gauge and a method to
burp the line from time to time. These gauges can be found both new and used
at very freindly prices.
Steve