Level Sensors | Vented Tank Monitoring with Hydrostatic Pressure

Example: AST Submersible sensors offer a linear output signal for tank monitoring. With the use of a 4-20mA output, the tank level will show a 4mA signal when the tank is empty and rise to 20mA when the tank is full

PSI to Foot Conversion Table (Based on Specific Gravity of Water @ 0.433 psi = 1 foot*)

Sensor Measuring Depth In Feet Depth In Feet Range in PSI 100% FS 60% - 80% FS 0 to 2.5 5.77 3.46 to 4.62 0 to 5 11.54 6.92 to 9.23 0 to 7.5 17.32 10.39 to 13.86 0 to 10 23.08 13.84 to 18.46 0 to 15 34.63 20.78 to 27.70 0 to 20 46.18 27.70 to 36.94 0 to 30 69.26 41.56 to 55.41 0 to 50 115.45 69.27 to 92.36 0 to 100 230.89 138.53 to 184.71

* Water density at approximately 60° F/15.6°C will have results vary slightly with fluctuations in temperature.

Define

Water level can be determined using a hydrostatic pressure sensor by taking a continuous pressure determination at the bottom of the Water Column (WC) at which point the sensors diaphragm is placed. The pressure existing at a certain depth within a liquid is directly proportional to the column of water above. This is different from differential pressure which is determined by an up and down stream measurement. By calibrating a hydrostatic level sensor to the respective liquid density, it's output signal is tuned to the column of liquid to be measured. Hydrostatic pressure sensors using the relationships stated below.

One method of determining a liquid level in a tank, pond or well is to utilize a pressure sensors. There is a direct relationship between liquid level and "head" pressure. Depending on the installation requirements, pressure sensors are available in submersible, screw on, flange mounted and other configurations.

How It Works

The pressure at the bottom of the tank or body of liquid (WC) is related to the height of the liquid. This level pressure is called hydrostatic head pressure. Typical units of measure for hydrostatic pressure are inches of water column (WC) or feet of WC.

27.679 inches of WC is the approximate equivalent of 1 PSI at 40°C. The volume of water will not affect the hydrostatic head pressure; it is the height that affects the pressure. Whether 27.679 inches deep in the middle of a large body of water or a small bucket of water the head pressure is the same.

Water density at 60°F (15.6°C) = 62.37 pounds per cubic foot. As you can see in this example the pressure exerted by a column of water is dependent only upon the height of the WC and is independent of the affected area.

The pressure exerted by a column of water one foot in height is:
pounds	x	square foot	=	.4331 pounds
cubic foot		144 sq inches		sq inch - foot WC
Or, what height of WC is required to generate one pound of pressure per square inch?
1 pound	x	sq. inch - ft	=	2.3089 ft. WC
Sq. inch		.4331 pound

 

 

 

 

 

 

 

 

AST4500 and AST4510 Submersible Level Sensors - Click image for more details

AST4520 Flush Diaphragm Wastewater Level Sensor - Click image for more details

The AST4500 and AST4510 submersible pressure sensors are approved to UL/cUL913 (CSA 157) Class 1 Div 1, Groups C and D for use in intrinsically safe areas with an approved barrier. For pressure ranges from 0-1 to 0-100 PSI that require a wide range of media compatibility, the submersible series is an excellent solution to level monitoring for indoor and outdoor applications.

The AST4520 Flush Submersible Series is the cost effective solution for level monitoring of turbulent tanks with viscous media. Approved to UL/cUL913 Class 1 Division 1 IS, Groups C and D with an approved barrier, the product ensures a safe, reliable source for level measurement. The AST4520 is offered with pressure ranges from 0-2.5 to 0-15 PSIG. The AST4520 steel cage front end design allows for proper flow of media while keeping the sensor at the bottom of the tank or well. With an engraved 316L housing and Kynar PVDF cable, this sensor is built to handle the toughest environments.

AST's Water Capabilities Booklet - Click image to download PDF version