1. Background

The accurate measurement of liquid levels in storage tanks using pressure measurement is a widespread application. Many of these tanks are not under pressure or not covered by an inert gas atmosphere: In the case of these so-called "open" tanks it does not necessarily mean they are tanks without covers; however, unlike the closed tanks they are above ventilation with the external atmosphere and are thus in contact with atmospheric pressure.

2. Measurement requirements

The level of a stationary, open, symmetrical storage tank for raw water should be continuously controlled. Due to the construction of the tank, frothing and available connections, the use of radar or mechanical level meters is not possible at a reasonable cost; level measurement should thus be carried out using a pressure gauge.

3. KROHNE solution

When it comes to level measurement using a pressure gauge, the hydrostatic pressure created by the liquid is measured using a measuring connection at the level of the tank outlet or at the height of the bottom of the tank. To eliminate any impact of changes in atmospheric pressure (e.g. changes in air pressure due to weather conditions), an OPTIBAR P 3050 C is used as relative pressure measurement with 4…20 mA analog output. As the density of the liquid and thus the specific weight is known, the required pressure range, reflecting 0…100% of the level, can be calculated using the maximum fill level of the tank.

The measuring device is set by approaching the minimal (tank completely empty) and then maximum level. The OPTIBAR saves the corresponding pressure values at the touch of a button and emits the actual value as an analogue signal between 4mA (minimum value) and 20mA (maximum value). As an option, the measuring range can be set directly via a local display without having to approach the measuring points. Since it is a stationary, symmetrical storage tank and the tank volume is known, the current tank content can be calculated in the control system.

4. Customer benefits

With this solution, the current level of the storage tank can be continuously monitored in a cost-effective way using tried and true technology. The measuring task was completed to its full extent.

5. Product used