Easy way to calculate Range for Differential
Pressure Level Transmitter?
Basic of Instrumentations
Another installation method for level measurement using a Differential Pressure
level transmitter is by utilizing the diaphragm seal plus capillary tube. To determine
the calibrated range for the Differential Pressure level transmitter, the filling fluid
density within the capillary must be taken into account in the calculation since the fill
fluid also creates hydrostatic force exerting both the DP level transmitter ports.
The density of fill fluid must be known, and the information could be obtained from
manufacturer specifications.
The example below describes the calculation in determining the calibrated range for
Differential Pressure level transmitter with a capillary tube.
Simple Differential Pressure Level Calculation
Diaphragm
Seal DP Transmitter Level Measurement
Sample Process Data
Process fluid density = 0.96 g/cm3
Fill fluid density = 1.05 g/cm3
100% from Process Connection (h) = 4 m = 4,000mm
Height Difference between Process Connection (h2) = 5 m = 5,000mm
When level at 0% = 4mA (Left Image)
∆P at 0% level
∆P = P1 – P2
P1 = 0 mmH2O
P2 = 1.05 x 5,000 = 5,250 mmH2O
Therefore Range at 0% is
P1 – P2 = 0 – 5,250 = -5,250 mmH2O
When level at 100% = 20mA (Right Image)
∆P at 100% level
∆P = P1 – P2
P1 = 0.96 x 4000 = 3,840 mmH2O
P2 = 1.05 x 5,000 = 5,250 mmH2O
Therefore ∆P at 100% is
P1 – P2 = 3,840 – 5,250 = -1,410 mmH2O
Scaling to set to Differential Pressure Transmitter
0% to 100% = -5,250 ~ -1,410 mmH2O
Level Calculation with Remote Seals
by
Diaphragm seals for tanks under Vacuum
The transmitter must be mounted level with or below the lowest tap to
ensure positive pressure at the transmitter.
Dist. Between Taps = H
DP = Hside – Lside
DP = (L*SGp + h*SGf ) – (H+h)*SGf
DP = L*SGp – H*SGf
Double Remote Seal Application
Calibration Range
LRV or 4mA point = Phigh – Plow
LRV = Head2 – Head1
LRV = (0.9*30”) – (0.9*100”)
LRV = -63”H2O
URV or 20mA Point = Phigh – Plow
URV = (Head2 + Head3) – Head1
URV = [(0.9*30”) + (1.1*50”)] – (0.9*100”)
URV = -8”H2O
Cal. Range = -63 to -8 in H2O
Density Calculation with Remote Seals Transmitter
Max. Allowable ΔS.G = 0.2
Dist. Betw. Taps = 10 ft = 10 x 12” = 120”
Calibration Range
LRV or 4mA point = 120” * (SGp – SGf)
LRV = 120” * (1.1 – 0.95)
LRV = 18”H2O
URV or 20mA Point = 120” * (SGp – SGf)
URV = 120” * (1.3 – 0.95)
URV = 42”H2O
Cal. Range = 18 to 42 inH2O
Interface Calculation with Remote Seals Transmitter
Application Example: To determine % of interface of Liquid A with respect to
Liquid B.
Dist. Betw. Taps = 10 ft = 10 x 12” = 120”
Calibration Range
LRV or 4mA point = 120” * (SG1 – SGf)
LRV = 120” * (1.1 – 0.95)
LRV = 18”H2O
URV or 20mA Point = 120” * (SG2 – SGf)
URV = 120” * (1.3 – 0.95)
URV = 42”H2O
Cal. Range = 18 to 42 inH2O
How to Calculate mmWC Range of
Level Transmitter with Examples?
by R Jagan Mohan Rao
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