How to Calculate mmWC Range of Level Transmitter with Examples?

How to calculate the inch or mmWC range of the level transmitter and explain with different examples.

We need to configure the Lower Range Value (LRV) and Upper Range Value (URV) for the level transmitters after installation in the field. The formula to calculate the LRV (4mA) and URV (20mA) will always change based on the transmitter mounting or installation position.

The below example shows the different types of level transmitters installation.

Remote Capillary Level Transmitter Mounted at Zero Level

Remote capillary Level Transmitter mounted at zero level

  • dH = Vertical distance from the transmitter to high-pressure seal
  • dL = Vertical distance from the transmitter to low-pressure seal
  • Lmax = the maximum level of the process above the high-pressure seal and typically the 20 mA lower range value
  • Lmin = the minimum level of the process above the high-pressure seal and typically the 4 mA lower range value
  • SGf = specific gravity of fill fluid
  • SGp = specific gravity of process fluid

Tank span = Lmax *SGp – Lmin *SGp

Tank span = (108 in.* 0.75) – (0 in.* 0.75) = 81 inH2O

4 mA = Lmin *SGp + dH *SGf

4 mA = (0 in. 0.75) + (0 in.* 0.934) = 0 inH2O

20 mA = Lmax* SGp + dH * SGf

20 mA = (108 in.* 0.75) + (0 in.* 0.934) = 81 inH2O

Span = 81 inH2O (81 to 0 inH2O)

Note: Both installations would have the same calculated range points.

Note: Silicone 200 has a specific gravity of 0.934.

Remote Capillary Level Transmitter Mounted below Zero Level

Remote Capillary Level Transmitter Mounted below the Zero Level

Tank span = Lmax* SGp – Lmin * SGp

Tan span = (108 in.* 0.75) – (0 in*. 0.75) = 81 inH2O

4 mA = Lmin *SGp + (dH* SGf)

4 mA = (0 in.* 0.75) + (60 in.* 0.934) = 56.04 inH2O

20 mA = Lmax *SGp + (dH *SGf)

20 mA = (108 in* 0.75) + (56.04* inH2O) = 137.04 inH2O

Span = 81 inH2O (137.04 to 56.04 inH2O)

Note: Silicone 200 has a specific gravity of 0.934.

Remote Capillary Level Transmitter Mounted Above Seal

Remote Capillary Level Transmitter Mounted Above the Top Seal

Tank span = Lmax SGp – Lmin SGp

Tank span = (108 in. 0.75) = 81 inH2O

4 mA = Lmin * SGp +(dH * SGf)

4 mA = (0 in * 0.75) + (–120 in *. 0.934) = –112.08 inH2O

20 mA = Lmax * SGp + (dH* SGf)

20 mA = (108 in * 0.75) + (–120 in.* 0.934) = –31.08 inH2O

Span = 81 inH2O (–112.08 to –31.08 inH2O)

Note The height of the transmitter (Hd Sg) should not be greater than approximately 394 inH2O (14.2 PSI) not to exceed the 0.5 PSIA sensor limits of a coplanar DP or GP.

Remote Capillary Level Transmitter Installed between Seals

Remote capillary transmitter installed between the seals.

Tank span = Lmax SG

Tank span = 108 in. 0.75 = 81 inH2O

4 mA = Lmin *SGp – (dL* SGF) +(dH * SGf)

4 mA = (0 in. 0.75) – (60 in.* 0.934) + (– 60 in.* 0.934) = –112.08 inH2O

20 mA = Lmax * SGp – (dL * SGf) +(dH SGf)

20 mA = (108 in. 0.75) – (60 in. 0.934) + (–60 in. 0.934) = –31.08 inH2O

Span = 81 inH2O (–112.08 to –31.08 inH2O) 

Note: Silicone 200 has a specific gravity of 0.934.

Remote Capillary Level Transmitter Mounted on Closed Tank

Remote capillary transmitter mounted on Closed Tank

Tank span = Lmax *SGp – Lmin *SGp

Tank span = (108 in.* 0.75) = 81 inH2O

4 mA = Lmin *SGp – (dL*SGf)

4 mA = (0 in.* 0.75) – (120 in.* 0.934) = –112.08 inH2O

20 mA = Lmax *SGp – (dL* SGf)

20 mA = (108 in.* 0.75) – (120 in.* 0.934) = –31.08 inH2O

Span = 81 inH2O (–112.08 to –31.08 inH2O)

Note: Silicone 200 has a specific gravity of 0.934.

Source: manual-rosemount-dp-level-transmitters

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Project Execution Engineer at Tree-Tech Engineers & Consultants | + posts

An astute professional with 25 years of service as an Instrumentation Engineer, started a career in Pulp & Paper. In the course of time, shifted to Power plant, Cellulose unit, Chemical plants (Carbon di Sulphide, Clo2, and Sulphuric Acid), Oil and Gas (Exploration & Production).

7 thoughts on “How to Calculate mmWC Range of Level Transmitter with Examples?”

  1. Why does Tank span = (180 in.* 0.75) – (0 in*. 0.75) = 81 inH2O? as far as I can tell, 180*0.75 = 135? What’s going on here that am I missing?

    Reply
  2. I’m in Concord with the initial commentor,My line of understanding that calculation,is blurry,won’t some light be shed?.I’m talking about what DN is commenting about

    Reply
  3. Not thinking
    Here in the examples all 180 are ,misplaced actually it was 108

    And in 3rd example dh & dl are also wrong position according to theory

    May this calculation will be correct by author

    Reply
  4. I thank everyone for their comments on the mistakes. Corrections have been done and updated in the article. I also thank Mr. Bharadwaj Reddy Garu for bringing various comments to my notice.

    Reply
  5. I need to know the minimum pipe size in high side used to measure fuel tank level in order not to cause Blockage due to rust dirty in fuel

    Reply

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