Tank level sensors that use differential pressure...how do they work?

Question

What physical principles make them work? I know the equation that the differential pressure equals the (tank level) height times the specific gravity and know how to calculate it...but I don't have the background in the engineering sciences to really understand WHY this works. Could someone explain in simple layman's terms the physical principles about pressure, etc. that make it possible to measure the tank level by measuring the difference between atmospheric pressure (or the pressure at the top of a closed tank) and pressure at the bottom of the tank? Thanks so much!

Answer 1

There's no particular magic about it.  Imagine putting a cylindrical tank on the platform of a weighing machine.  It doesn't take much imagination to realize that the weight is related to the height of liquid in the tank and the density of that liquid.  You could in fact re-zero the weighing machine to allow for the weight of the empty tank and then calibrate its scale directly in depth measurements.

The complication is the calibration is only good for that particular size of tank.  If you use a fatter tank, the readings are off because an inch (or centimeter) depth of liquid in the wider tank WEIGHS more.  You can work out a correction factor by calculating the weight force per unit area of the base of the tank. For each square inch (or square centimeter) of base, the weight of the column of liquid above it is the same. But force per unit area is given a name.  It's called the pressure.  If you measure the pressure, rather than the weight, it doesn't matter how wide the tank is.

The last issue is that your tank is actually at the bottom of a column of air that is miles (kilometers!) high and contributes a pressure of its own which varies with the weather.  What you want is the pressure at the bottom of your tank, minus the pressure at the top.  Hence the differential bit.

Answer 2

hey rather than using such complex idea..u can try this out..

let there be n sensors separated by distance 1 meter and let them be arranged in top to bottom order...chose sensors such that as soon as u immerse these sensors they start conducting...and connect each sensor with a LED..so that if the the water level is 5 meters from bottom...first five led's glow...and if water level goes to 6th meter ..then 6th sensor start conducting and 6th LED lits on...

hope u got my technique

Answer 3

The column of liquid has weight and exerts pressure in the pressure sensing device. The device is located in the bottom of the tank. The more liquid in the tank the more pressure that will be exerted on the pressure sensing device. A correlation can be made between the pressure sensed by the device and the tank level. This relationship will be linear If the tank is symmetrically shaped about the vertical axis.

I got a  laugh from one of the other submitters who said that the column will change if the tank is widened. If you had a glass of water that was 10 feet tall and filled to the top with water. The pressure of a column of water  at the bottom of the glass will be exactly the same as in tank 100 miles wide and ten feet deep.

The only difference is, of course the correlation between pressure and level.