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Q & A: pressure-based fuel gauge

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Most recent answer: 10/22/2007
Q:
On an upright diesel fuel storage tank (15 ft high), would it be possable to have a pressure gauge at the bottom and by reading the pressure, be able to accurately tell the amount of fuel in the tank. Would air pressure (barometric pressure) be a factor? What is the density of diesel?
- Lewis
OK
A:
The answer is yes, but as you guessed you would have to make corrections for the varying air pressure. One way would be to have two pressure gauges, one at the tank bottom and one above the liquid level, and compare the readings.Obviously, this is a situation where you want to make sure that the wiring for the gauge NEVER gives sparks. Another way would be to have a differential gauge, with one side open to the atmosphere. I wouldn't recommend that because it seems like an invitation for leaks.
I think that the density of diesel fuel runs around 0.85 kg/liter, but you should Google around to get more accurate numbers.

Mike W.

If the tank is sealed, the pressure inside can be very different from one atmosphere (especially if fuel is drained or pumped from it). My car's gas tank has a pressure that's noticeably less than one atmosphere after a long trip. When I fill up the tank after driving all day, I can hear air rushing into the tank when I remove the gas cap. The two-pressure-sensor technique should solve this problem. A lot of pressure gauges read the pressure of something minus atmospheric pressure, because they measure the displacement of a spring-loaded diaphragm, one side of which is open to the atmosphere. You could arrange your differential pressure gauge on the bottom of the fuel tank with its reference pressure pipe going to the top of the fuel tank, closing the system.

There is a great amount of information from companies which sell tank level gauges, and a great variety of products. Because your application is diesel fuel, be sure to get something that is rated for this purpose. Be careful with leaks and also the fact that fuels will dissolve some kinds of rubber.

Tom

(published on 10/22/2007)

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