Measuring level in an open tank with a differential pressure transmitter is relatively straightforward. On the downside, pressure solutions come with a long list of limits and problems we could avoid with newer solutions.

DP transmitter level measurement calculation

We base DP level measurement in open tanks on the Pascal equation for hydrostatic pressure. Therefore pressure (P) equals the liquid’s density (ρ) times acceleration due to gravity (g) times the liquid column’s height (h), or P = ρ * g * h.

Although different processes may call for more complicated calculations, don’t worry. You can build from here and learn as you go. With that in mind, let’s go through a simple situation now, an open tank process, and later we can go to a more complicated one.

To know more about differential pressure transmitters, you can read the Visaya Article on DP Transmitters Applications

Open tank level measurement

First, we need to talk about how differential pressure transmitters work. Here you have two cells, one for high pressure and one for low. With level measurement based on hydrostatic pressure, the high-pressure cell goes at the bottom of the tank. And in an open tank, the low-pressure cell should be open to the air.

Some companies sell gauge transmitters for open tanks. A gauge transmitter will work fine, but instead of having the low cell open to the air, you have just a vent in the transmitter.

Endress+Hauser
Deltabar M PMD55 Differential pressure transmitter with metal sensor for measuring pressure differences
in the shop from 1150 €
PMD55

Calculating level from pressure

In this setup, we have a transmitter installed at the tank’s zero level, dead easy. Also, you have the level connection filled with the process product. Still with me?

So we just need to figure the minimum and maximum measurements.

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

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

Here, the minimum will always equal zero because you have zero on both sides. On the 100 percent, we multiply the height of the liquid in the full tank with the gravity for the high pressure. For the low pressure, we’ll still have zero, so we can leave it.

Open tank with suppressed zero installation

Now, let’s try a different installation, with the transmitter below the zero level. Because the transmitter shows a value higher than the real value, we call this suppressed zero. Our scenario has a transmitter without a seal pot and the process connection filled with the process product.

level measurement
Courtesy of Visaya.solutions

We’ll use the same equation, but we need to pay attention to the new height with the transmitter below the bottom of the tank.

Minimum = level at 0% =  HP (SGp * h1) – LP (SG * H)
Maximum = level at 100% =  HP (SGp * h1 + H2) – LP (SG * H)

  • HP = high pressure
  • LP = low pressure
  • SGp =Specific gravity of process
  • H= height
  • h1 = height of the suppressed zero
  • H2 = height of the full tank
Endress+Hauser
Deltabar S PMD75 Differential pressure transmitter with metal sensor for continuous measurement
in the shop from 1683 €
PMD75

Next, imagine a situation where you have a seal pot on the HP side of the transmitter. Because the seal pot’s density differs from the process product, this difference will add to your math.

level measurement
Courtesy of Visaya.solutions

Minimum = level at 0% =  HP (SGs * h1) – LP (SG * H)
Maximum = level at 100% =  HP [(SGs * h1) +( SGp * H2)] – LP (SG * H)

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

    To know more about absolute, gauge and differential pressure, you can read the Visaya Article here

Conclusion

Thus, we can start with an easy equation and build on it. However, we always need to pay attention to each detail in the setup. Otherwise, we may set up with the wrong range or wrong zero references and get wrong results.

To know more about pressure transmitters and level measurement, you can get in touch with our engineers!

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