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Feb 20, 2013 4:00:00 AM

Application Note: Liquid Level Measurement

Liquid Level

The pressure measured at the bottom of a liquid filled tank is proportional to both the height (head) and density of the liquid. For best results the pressure should be sensed by a pressure transmitter mounted at the lowest point possible on the tank, as shown in Figure 1.

Figure 1: Liquid Level Tank - Setra Application Note

Example #1: You want to monitor liquid level in a 30 foot high vertical water tank. A full tank would generate a maximum head pressure of 30 feet of water. Since one foot of water is equivalent to 0.4335 pounds per square inch (psi), the maximum pressure in psi is 13 psig. Per Table I below, a pressure transmitter with a range of 0-15 psig will accurately measure the liquid level within ±0.46 inches of water.

Table I:
Tank Level using a high accuracy (<±0.11% FS) Pressure Transmitter

Full Scale Pressure Range Tank Level
(Feet of Water)
Accuracy of Level Reading (inches of water)
0-15 psig 0-34.3 ±0.46
0-25 psig 0-57.7 ±0.76
0-50 psig 0-115.3 ±1.52
0-100 psig 0-230.7 ±3.04

For liquids with densities different than water (1g/cm3), pressure can be determined from liquid density (p), gravitational force (g) and liquid column height (h) with the formula: Pressure formula, for liquids with densities different than water

P = pressure in pounds per square inch (PSI)
p = liquid density in grams per cubic centimeters (g/cm3)
(See Table II)
g = gravitational force (constant) gravitational force (constant) formulah = liquid column height in centimeters (cm) (Note: 1 inch =2.54 cm)

For liquid column height (h) in feet the formula simplifies to:

Formula 1: P = p h (0.4335)

Example #2: You want to monitor liquid level in a 50 foot high vertical tank filled with Carbon Tetrachloride. Table II lists the density of Carbon Tetrachloride at 1.595 g/cm3 and we know that the maximum liquid height is 50 feet.

Using formula 1 the maximum pressure generated is:

P = p h (0.4335) = (1.595) (50) (0.4335) = 34.57 psig

Thus, a pressure transmitter with a range of 0-50 psig would be sufficient for monitoring levels in this tank.

Pressurized Vessels

A differential pressure transmitter is necessary for determining level in a pressurized vessel. Here the high side pressure port is connected to the lowest point possible on the tank and the low side pressure port is connected to the top of the tank. P in the formula would be the differential pressure measured.

Table II:
Density of various liquids

Liquid Density (g/cm3) Temp. °C
Acetone .792 20
Alcohol, Ethyl .791 20
Alcohol, Methyl .810 0
Benzene .899 0
Brine, 10% CaCi 1.091 0
Brine, 10% NaCi 1.078 0
Bunkers C Fuel Max. 1.014 15
Carbolic Acid 0.950-0.965 15
Carbon Disulfide 1.293 0
Carbon Tetrachloride 1.595 20
Chloroform 1.489 20
Distillate 0.850 15
Ether 0.736 0
Fuel 3 Max. 0.898 15
Fuel 5 Min. 0.966 15
Fuel 5 Max. 0.993 15
Fuel 6 Min. 0.993 15
Gasoline 0.66-0.69 -
Glycerin 1.260 0
Kerosene 0.82 -
Mercury 13.60 -
Milk 1.028-1.035 -
Naptha, Wood 0.848-0.810 -
Pentane 0.624 15
SAE 10 Lube 0.876 15
SAE 30 Lube 0.898 15
SAE 70 Lube 0.916 15
Salt Lake Crude 0.843 15
32.6° API Crude 0.862 15
35.6° API Crude 0.847 15
40° API Crude 0.825 15
48° API Crude 0.788 15
Sea Water 1.025 15
Turpentine 0.87 -
Water 1.00 4

Note: See Corrosion Resistance Table for media compatibility with 17-4 pH stainless steel.

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Topics: General Industrial OEM, Process/Mfg Tank Level, Water and Wastewater, General Industrial

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