Open Tank Level Measurement
In the industry liquid, gas is stored in a tank before and after processing. The level measurement plays an important role in the industry. As many application needs a constant supply of process liquid and hence the tank level has to be maintained at a normal level to achieve the complete cycle of the process. The open tank level measurement is mostly used to collect liquids like water as it is not affected by environmental conditions.
The differential pressure transmitter is commonly used in the industry in open tank level measurement. As the level of the tank gets increased the pressure transmitter tapping taken at the bottom of the tank also gets increased. This pressure difference is used to scale the level of the tank. The diagram below will explain how the tank is scaled according to the pressure.
Each and every liquid has a different specific gravity. As you saw in the above diagram water has a specific gravity of 1.And the height of the tank is considered as 1000 mm. As we know the pressure is equal to the multiplication of specific gravity, height, and acceleration due to gravity. Applying these value to the equation we can easily able to find the LRV and URV value.
Water pressure = Tank height * specific gravity of liquid
When the level is 0% then the liquid height will also be 0.So the pressure acting at the transmitter end will be
Water pressure = 0 * 1
LRV = 0
Similarly when the level of the tank is 100%. The pressure acting at the high pressure side of the transmitter will be.
Water pressure = 1000 * 1
URV = 1000
Now we have found both the value required to configure the transmitter. After that, the pressure signal will be transmitted using a scaled level. By this method the pressure measurement is indirectly used to measure the tank level.
For example, when the level of the tank is reduced to 50%. The pressure of the water will be around 500 mmh2o it is sensed by the differential pressure transmitter. And the respective 12 mA is sent from the transmitter to the control room. As 500 mmh2o is scaled as 50% tank level it will get displayed in the transmitter as well as in the configured SCADA display.
