Measuring Liquid Level Using Externally Mounted Gauge and Differential Pressure Transmitters

Gauge and differential pressure transmitters are reliable, cost effective instruments to measure the hydrostatic level of a liquid in a vented or sealed tank.

For a vented tank, the pressure transmitter should be mounted to the bottom. Typically, the transmitter will be threaded into the outlet line of the tank, a side bung, or flange. Since a pressure transmitter provides a linear output signal, e.g. 4 to 20mA signal from zero pressure to the full scale pressure, the level can be monitored based on the liquid density and the height of the tank.

For Example:
Density of water at 4°C = 1
Height of tank = 35 feet = 420 inches
27.68 inches of water column = 1 PSI
(420 inches / 27.68 inches of water column) x density of 1 = 15.17 PSI

Here, the height of the tank is converted into a measurement range. Since the pressure cannot exceed the height of the tank while considering the density of the liquid, the user can use the maximum amount of signal for the specified range.

The only difference between a gauge and differential pressure transmitter in this example is that the differential pressure transmitter will have P2 vent to the atmosphere, where the gauge pressure transmitter will typically have a tube within the cable to allow for changes to barometric pressure. The vent tube of the gauge pressure transmitter should be protected from humidity, especially if there are wide temperature changes between the liquid and atmosphere. This is typically accomplished by venting the sensor inside a control box that is protected with desiccant or by applying a desiccant filter to the end of the vent tube.

The selection of the output signal is always application specific. 4 to 20mA output signals are most common in monitoring applications that require long distance transmissions. The current can be read at the controller without signal loss for hundreds of feet. The downside is that the sensor will require more power and more importantly, current. For either battery powered or solar powered applications, voltage output signal are ideal. Typically the solar panel will be located close the pressure sensor, so there is no signal loss in the cable.

Sealed tanks, such as gas storage tanks, are required to be sealed from the atmosphere. Under this condition, a differential pressure transmitter should be used to calculate the difference in pressure between the top and the bottom of the tank. P1 of the differential would be installed at the bottom of the tank. A pipe connected to P2 would be connected to the top of the tank. When full, the differential pressure will be the maximum output signal. As the tank level is reduced, P2 will increase and the differential pressure will decrease along with the output signal.

AST Product Solutions:

The AST44LP is a intrinsically safe gauge pressure transmitter that can be installed externally to measure the level of a tank. The AST44LP series is Class I Division 1 intrinsically safe for Groups C and D with an approved barrier. With measurement ranges from 0 to 1 and up to 15 PSIG, this voltage or 4-20mA output signal pressure transmitter is a cost effective level measurement solution with a variety of configurations.

The AST5100 Wet / Wet Differential Pressure Transmitter is an excellent solution for low pressure differential measurements for external tank monitoring. With pressure ranges starting at 0 to 5 inches of water column, the AST5100 can be used to monitor the level of sealed and vented tanks; it also offers the accuracy and resolution to be installed on bubbler systems for level measurement.

The AST5400 Wet / Wet Differential Pressure transmitter can be used to monitor level in both sealed and vented tanks. Featuring AST’s new digital electronics, the AST5400 DP offers excellent performance at a competitive price for pressure ranges of 0-10 to 10,000 PSID with a 15 to 1 turndown ratio.