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

• 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

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

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

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

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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