Closed tank level measurement calculation is a recurring question and a DP (differential pressure) transmitter is one of the best tools for it. In fact, the basic equation is easy, but it gets more complex once we start layering in factors like installation, type of differential pressure transmitter, and more.

## Impulse line vs capillary

### Impulse lines, wet leg and dry leg

In a closed tank, the product may create gas, vapour, or can have a pressured vessel. This requires compensation, using the low-pressure side of the differential pressure transmitter. That brings us to impulse lines.

We call the two different impulse lines wet leg and dry leg. Depending on the product, we have to use one of them. For example, if the product may create condensate, then you’ll use the wet leg. Otherwise, you can go with a dry leg.

First of all, a mechanical structure connects both sides of the differential pressure transmitter to the vessel. Wet legs have impulse lines filled with liquid, and we can have different liquids in them. Dry legs have columns filled with vapour, gas, or whatever may come from the process without condensation.

Deltabar M PMD55

Differential pressure transmitter with metal sensor for measuring pressure differences

We can certainly have issues with both installations, like condensate in dry legs and leaks or clogs in wet legs. We can avoid these issues with the solutions discussed in this article, but we just need to understand the concept for the math part here.

### Differential pressure transmitter with a capillary

Impulse lines have disadvantages, so somebody created the capillary system to solve this problem. Frankly, it’s difficult working with capillary transmitters, but they do solve those impulse line problems such as evaporation, condensation, leaks, clogs, and so on.

In a capillary setup, we mount a remote seal system and a sensing diaphragm in the process with oil-filled capillaries. When we have a force deflecting from the diaphragm of the remote seal, it sends the pressure through the oil. Then the transmitter captures that to show the process measurement.

When we talk about a capillary system, usually we mean a balanced system. What does that mean? Basically, we have the same remote seal and an equal length of the capillary on each side of the transmitter. In theory, we can avoid problems with temperature shifts and other issues.

### Fancy options

We can find still more solutions to address the above problems with impulse lines and capillary systems. For example, one solution has a new way to compensate the capillary, reducing issues with the balanced system.

We also have electronic remote sensors, which different companies call by different names. Rather than a mechanical structure or remote seal system, we have a sensor that communicates digitally. This option fixes all the problems we have with impulse lines and capillary systems.

## Calculating closed tank level from differential pressure

### The basics of differential pressure

First, we can find the level range using an impulse line with a dry leg. Below we have an example where we have only the standard gravity of the process liquid, with the transmitter installed at the same level as the zero level measurement.

Minimum = level at 0% =  HP (SGp * H) – LP (SGf * H)
Maximum = level at 100% =  HP (SGp * H) – LP (SGf * H)

• HP = high pressure
• LP = low pressure
• SGp = Specific gravity of the process
• SGf = Specific gravity of the fluid
• H = height

### Saved by zero

This example has a wet leg and the transmitter installed below the zero point, which we call suppressed zero.

Minimum = level at 0% =  HP (SGs * h1)
Maximum = level at 100% =  HP [(SGs * h1) + (SGp * H)]

• HP = high pressure
• LP = low pressure
• SGp =Specific gravity of process
• SGs =Specific gravity of seal
• H = height
• h1 = height of suppressed zero
• H2 = height of full tank

To know more about DP flow measurement, you can read our article on calculating flow rate

### Raising the bar

In this third example, we elevated the minimum level in a wet leg system.

Minimum = level at 0% = HP (SGp * h0) – LP(SGs * H2)
Maximum = level at 100% = HP (SGp * H+h0) -LP(SGs * H2)

• HP = high pressure
• LP = low pressure
• SGp =Specific gravity of the process
• SGs =Specific gravity of seal
• H = height
• h1 = height of suppressed zero
• h0 = height of the minimum level
• H2 = height of the full tank

### Creeping into capillaries

Now let’s check a capillary system. The math stays the same, but the data going into it changes.

Minimum = level at 0% =  HP (SGp * h0) – LP (SGs * h)
Maximum = level at 100% =  HP (SGp * H) – LP (SGs * h)

• HP = high pressure
• LP = low pressure
• SGp =Specific gravity of the process
• SGs =Specificgravity of seal
• H = max. level height
• h0 = min. level height
• h = height of full tank

### The one with the electronic sensor

Last but not least, when we have an electronic sensor, we go back to something like a dry leg equation. The diaphragm connects directly to the tank.

Minimum = level at 0% =  HP (SGp * H0)
Maximum = level at 100% =  HP (SGp * H)

• HP = high pressure
• LP = low pressure
• SGp =Specific gravity of the process
• H = height
• H0 = minimum level

## Conclusion

We can use differential pressure transmitters for closed tank level measurement calculation with various setups. We listed the most common ones, any of which we can use as a base to understand the concept and calculate even more complex processes.

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To know more about level measurement in a closed tank, you can get in touch with our engineers!

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