Level measurement

What is Level

The level is a measure of the difference in altitude of two points.

The height of the water column, liquid, or powder in the desired range between the minimum and maximum values is called their level.

There are different measurement methods implied for the level measurement.

Units of level

Meter

Foot

Percent

Types of level measurement

There are two main methods to measure level.

1. Direct or visual method

2. Indirect method

Direct Methods (Visual Methods)

The direct method is used to measure the height of a substance above a point marked as zero points.

Direct methods for level measurement are preferably used when level changes are slow and small, for example, to measure the level of Sump tanks and Bulk storage tanks.

Direct level measurement methods are simple and are considered very reliable.

Types of direct methods

Direct level measurement devices are divided into four types:

a. Dip-sticks & Dip-Rods

b. Weighted gauge tape

c. Sight Gauges

d. Magnetic type Sight gauges

a. Dip-sticks & Dip-Rods

These are sticks and rods marked with a scale so by inserting the dip sticks and dip rods a measurement of level is done, a very simple method used in daily life.

b. Weighted gauge tape

Another variation of direct level measurement is the weighted gauge tape. dipstick and weighted gauge tape are used similarly, but weighted auger tape is used for deep vessels and tanks.

Floats are sued for a direct readout of liquid level. These floats are connected to a rope or mechanical linkage and a scale is used to measure the level.

In this type, a zero level indication is at the top and the maximum level indication is at the bottom of the gauge.

c. Sight Gauges

Sight glasses are available in various shapes, common types being used are:

1. Sight glass; in a flat, tubular, and reflex shape

2.Magnetic sight gauges.

Flat glass sight gauge

It is used for non-pressurized vessels. An application of this type is in hot oil tanks, where excessive foam contaminated oil may be detected.

It is made up of a vessel and a glass or plastic window.

Tubular Sight Glasses

Tubular sight glasses are used for non-corrosive and non-toxic liquids. These types of glasses can be used for moderate temperatures and pressures.

In this type of glasses, light is refracted from the vapor portion of the column and shown in a color usually white color.

Light is absorbed by the liquid portion in the column and is shown as a dark color portion.

d. Magnetic type Sight gauges

The magnetic type sight gauge is consists of a magnetic float that travels up and down on the inside of a long stainless steel pipe.

The pipe is connected to flanged nozzles on the side of the tank so that the fluid can enter the pipe. A visual indicator, consisting of triangular wafer elements is used to indicate level.

These elements flip over from one color to another color so by sighting the color change we can check the level of fluid. alarms and tripping options can be used in these magnetic-type level instrumentation.

2. Indirect (inferential Methods)

These are called the indirect or inferential method of measurement because we measure the level by easurmingmeasuring or inferring other values.

This type of level instrument can be used for low as well as high levels measurement.

There are different types of indirect level measurement, some types are listed below.

1. Hydrostatic Pressure Methods

2. Displacement devices

3. Capacitance Level

4. Radiation-Based Level Gages

1. Hydrostatic Pressure Methods

It is a known fact that a vertical column of fluid always exerts pressure at the bottom of the column.

Column height and fluid pressure are directly proportional regardless of vessel shape and width.

So we can infer the level of a vessel by measuring the pressure at the bottom of the vessel.

D.P. transmitter applied to a close tank?

In the close tank, the bottom of the tank is connected to the high-pressure side of the transmitter, and the top of the tank is connected to the L.P. side of the transmitter. In this way the vessel pressure is balanced.

D.P. transmitter applied to an open tank?

On an open tank level measurement, the L.P. side is vented to the atmosphere. Whatever pressure acts is on the H.P. side which is a measure of level.

D.P transmitter applied to a close tank & open tank with Dry leg?

Span = (X) (GL)

HW at minimum level = ( Z ) ( GS

) + ( Y ) ( GL )

HW at maximum level = ( Z ) ( GS

) + ( X + Y ) ( GL )

Where:

GL = Specific gravity of tank liquid.

GS = Specific gravity of seal liquid.

HW = Equivalent head of water.

X, Y & Z are shown in fig (1.1)

Example:

Open tank with X = 300 inches

Y = 50 inches

Z = 10 inches

GL = 0.8

GS = 0.9

Span = (300) (0.8) = 240 inches

HW at minimum level = ( 10 ) ( 0.9 ) + ( 50 ) ( 0.8 ) = 49 inches

HW at maximum level = (10 ) ( 0.9 ) + ( 50 + 300 ) ( 0.8 ) = 289 inches

Calibrated range = 49 to 289 inches head of water

2. Displacement devices

These are the same as discussed above weighted sight tape.

This type of measurement should only be used for liquids of specific gravity, in a process where errors due to process variations are acceptable, and where there is no crystal or solid particles in the process liquid.

Coating or dirt that clings to the displacer may affect the accuracy of the measurement.

By inserting electrical equipment like a potentiometer we can take an electrical signal and use it to indicate to ccr and also configure alarm and tripping option

3. Capacitance Level Instrumentation

This type of level instrumentation work on the principle of change in capacitance due to the fluid is used as a dielectric, so when the area between the two electrodes of a capacitor changes due to fluid quantity changes the dielectric constant so the capacitance varies and by measuring this change in capacitance we can measure the level. The capacitance will vary only as a function of the dielectric constant of the substance filling the gap between the plates.

Capacitance Probe is used as a Leve Switch, an insulator with a low dielectric constant is used to isolate the conductive probe from the housing, which is connected to the vessel wall.

The probe is connected to the level sensor via the conductive threads of the housing.

Measurement is made by applying a supply signal between the conductive probe and the vessel wall when the material level changes it causes a change in the capacitance and gives switching to show alarm or warning.