Why this method called as indirect method because level measurement is depends on other parameter like pressure, capacitance, speed of sound, etc. through this parameter we get a level measurement.



Below are list of method which detect pressure in order to measure level Variation of level in a process tank or vessel causes the head pressure of tank to vary. By measuring this head pressure we can find the level in the tank. There are several types of indirect measuring devices that are operated by pressure thus measuring level which are listed below,

1. Gauge pressure measurement

2. Differential pressure type

3. Air bubble purge system

In this post we will discuss about 3 of 2 method, Air bubble method i will cover in next post.


In this type a pressure gauge is located at the zero level of the liquid in the process vessel. Any rise in the level causes an increase of pressure, which can be measured by the gauge. As the tank fills, the pressure of the liquid naturally increases. This increase of pressure can be read on the gauge in feet or inches of level. with known density of liquid, Gravitational force we can easily detect pressure.


The differential pressure (DP) detector method of liquid level measurement uses a DP detector connected to the bottom of the tank being monitored. The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure (usually atmospheric) when tank is open. This comparison takes place in the DP detector. Figure illustrates a typical differential pressure transmitter attached to an open tank.

The tank is open to the atmosphere therefore, it is necessary to use only the high pressure (HP) connection on the DP transmitter. The low pressure (LP) side is vented to the atmosphere; therefore, the pressure differential is the hydro static head, or weight, of the liquid in the tank. The maximum level that can be measured by the DP transmitter is determined by the maximum height of liquid above the transmitter. The minimum level that can be measured is determined by the point where the transmitter is connected to the tank.

Not all tanks or vessels are open to the atmosphere. Many are totally enclosed to prevent vapors or steam from escaping, or to allow pressurizing the contents of the tank. When measuring the level in a tank that is pressurized or the level that can become pressurized by vapor pressure from the liquid, both the high pressure and low pressure sides of the DP transmitter must be connected.

The high pressure connection is connected to the tank at or below the lower range value to be measured. The low pressure side is connected to a "reference leg" that is connected at or above the upper range value to be measured. The reference leg is pressurized by the gas or vapor pressure. The filled reference leg applies a hydro static pressure to the high pressure side of the transmitter, which is equal to the maximum level to be measured. The DP transmitter is exposed to equal pressure on the high and low pressure sides when the liquid level is at its maximum; therefore, the differential pressure is zero. As the tank level goes down, the pressure applied to the low pressure side goes down also, and the differential pressure increases. As a result, the differential pressure and the transmitter output are inversely proportional to the tank level.

we will discuss more about dry leg and wet leg and its calculation in upcoming post.

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