Tank Level Sensors - Liquid Level Measurement

level sensor example with 4-20mA output signal showing 4mA empty up to 20mA ful

Example: AST Submersible pressure transducers offer a linear output signal for tank level 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

Conversion Table (Based on Specific Gravity of Water @ 0.433 psi = 1 foot*)
Sensor Measuring - Range in PSI Depth In Feet - 100% of Full Measurement Range Depth In Feet - 60% to 80% Full Measurement Range
0 to 1 2.31 1.38 to 1.85
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

diesel tanks

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 it (measured as Water Column or WC). This is different from differential pressure, which is determined by an up and down stream measurement or the difference in pressure between the top and bottom of a tank.

By the liquid density, the height of the liquid can be measured accurately. For vented tank applications, the transducer is vented to the atmosphere through a tube that goes from inside of the housing to the end of the cable. This allows the sensor to compensate for changes in barometric pressure outside of the tank, which is more critical in shorter tank installations.

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. Fresh water, diesel fuel tanks and chemical totes all use submersible pressure transducers for level monitoring.

How to Calculate Liquid Level with Submersible Pressure Transducers

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

27.679 inches of WC is the approximate equivalent of 1 PSI at 100°F. The volume of water or shape of the tank or vessel 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. Modern PLC's and HMI's can calulate the liquid level of a tank by entering the geometry of the tank and the specific gravity of the liquid.

Water density at 100°F (37.8°C) = 60.00 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 volume.

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

Booklet Featuring Capabilities in the Water Industry

Submersible Pressure Transducers

submerisble pressure transducer

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, diesel fuel, oil, heating oil, and more. Each sensor is ABS certified for use on ships and intrinsically safe for Groups C and D and ATEX Exia IIB. 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.

flush diaphragm submersible transmitter

AST4520 - Non-Clogging Level Sensor - Featuring a steel cage or a flush diaprhagm and 316L SS, the AST4520 level transmitter is designed for viscous liquids such as waste water level. Starting at 2.5 PSIG pressure ranges, the AST4520 can monitor the level of liquids for various liquids compatible with 316L and PVDF material. Each sensor 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.

flush diaphragm submersible transmitter

AST4530 - PVDF Submersible Pressure Transducer - The AST4530 submersible pressure transducer is constructed using PVDF material and a PTFE diaphragm. Designed to measure liquid level of harsh liquids or corrosive liquids such as salt water or chemicals, the AST4530 has submersible PVDF cable, cord grip and housing. The AST4530 features a conduit connection for turbulent installations such as on-board ships, turbulent tanks, and rail cars. Each sensor is rated laser engraved with intrinsically safe and ATEX markings.


 

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