Level Sensors | Vented Tank Monitoring with Hydrostatic Pressure

Example: AST Submersible pressure transducers 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.

Liquid Level Measurement Overview

Liquid level can be determined using a submersible pressure transducer by taking a continuous pressure measurement from the bottom of the tank or measurement point where the transducer's diaphragm is placed. The pressure existing at a certain depth within a liquid is directly proportional to the column of liquid above above it (measured as Water Column or WC). This is different from differential pressure, which is determined by an up and down stream measurement. By calibrating a submersible pressure transducer to the respective liquid density, it's output signal is tuned to the column of liquid to be measured. Submersible pressure transducers are cost effective, offer excellent compatibility with liquids, do not have issues regarding reflection or foaming, and can offer low current consumption for battery powered applications.

How to Calculate Liquid Level with Submersible Pressure Transducers

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 water column 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

Submersible Pressure Transducers

AST4500 / AST4510 - Submersible Pressure Transducer - The AST4500 and AST4510 submersible liquid level sensors are offered in pressure ranges starting at 1 PSI. Available with voltage and 4-20mA output signals, these submersible pressure transducers are an accurate and cost effective solution for monitoring liquids such as: water (canal, fresh, drinking, salt water holding tanks, etc.), diesel fuel, ballast water, oil, heating oil, and more. Each sensor is ABS certified for use on ships and intrinsically safe for Groups C and D. Applying the information outlined above is simple. The AST4500/AST4510 will rest at the bottom of the tank with the density of the liquid and level providing the change in output signal.

AST4520 - Non-Clogging Level Sensor - Featuring a steel cage, all 316L SS and non-clogging diaphragm, the AST4520 level transmitter is designed for viscous media such as waste water level. Starting at 2.5 PSIG pressure ranges, the AST4520 can monitor the level of liquids for various media compatible with 316L and PVDF material. Each sensors is ABS certified for use on ships and intrinsically safe for Groups C and D. The AST4520 has a standard 1/2" NPT conduit fitting at the electrical connection for added weight or rigid piping to protect against turbulance. PVDF cable is the preferred cable due to applications where chemicals are added in close proximity to the transmitter.