A common concern surrounding tank level measurement is selecting the right type of sensor technology. Selecting the proper sensor technology will ultimately be determined by the conditions within the tank. The first thing you need to determine is if your vented tank is above ground or buried in-ground. An above ground tank will typically have a process connection external near the bottom of the tank, providing easy access to the sensor. A tank that sits in-ground will typically have an access hatch, where you can install your tank level sensor. Hydrostatic (gauge), capacitance, resistive and ultrasonic sensors are most commonly used for more critical tank level applications. Although they all measure liquid level, they work in different ways.
A gauge sensor, used in the hydrostatic method of measuring tank level, employs simple physics to produce high accuracy readings. By using a liquid's specific gravity and column height, pressure is easily determined. Utilizing a gauge pressure transducer allows for real-time tank level measurement, even in rapidly changing tank applications.
Capacitance level sensors detect a change in the capacitance that occurs between two conductors when a liquid is present. An empty tank has a lower capacitance, while a filled tank has a higher capacitance. Unfortunately, as levels drop, some liquid remains on the sensor that can cause false readings. As a result, there is a lag in the response time, especially with liquids with a high viscosity. This type of sensor isn't ideal for applications that experience rapid changes in tank level.
Sensors that use resistance measurements are often made with a series of sensors submerged into the bezel. This method is similar to a dipstick in your car's oil reservoir, where sensor probes are located along the length of stick. These sensor probes are connected to circuitry that ties back to your alarm or control panel, altering an operator when to fill or drain a tank. The drawback to resistive sensors is that your tank level measurement is only as accurate as the number of sensor probes used in the application.
Ultrasonic sensors are typically mounted at the top of the tank. They emit high-frequency acoustic waves that reflect against the process media below and return to the transducer. The sensor then measures the signal’s transit time to determine liquid level height within the vessel. One advantage of this type of sensor is that it does not come in contact with the liquid and may make a good choice for applications that come in contact with corrosive media. If the media foams, these units will measure the top of the foam rather than the liquid level. One drawback of these sensors is that their accuracy can be affected by moisture, temperature and pressure.
CLICK HERE to learn about Setra's pressure transducers that are used in hydrostatic tank level methods.