Level Measurment : An Overview

Level measurement is to monitor and measure quantitatively the liquid content in vessels , reservoirs and tanks. The determination of the location of the interface between two fluids, separable by gravity, with respect to a fixed horizontal datum plane. The respective fluids may be any fluids, liquid or gaseous, which do not mix and have specific gravities significantly different from one another. Fluids include granular or particulate solids which are fluidized or handled like fluids. The most common level measurements are, however, between a liquid and a gas or vapour.

Different level measurement devices

Level measurement may be classified in the main categories: direct visual indication of interface location; remotely transmitted indication of interface location; interface location inferred from hydrostatic pressure; interface location inferred from fluid properties. Level measurements may be of an analog or on-off nature.

Direct visual measurement:

In many cases, fluid levels may be observed directly and consequently measured to obtain trends or magnitudes in volume.

Graduated scale:

Level is measured directly from a vertical graduated scale partially immersed in the liquid.

Glass window:

Level is observed through a transparent window in the side of a tank. The window may be graduated with a vertical scale.

Gauge glass:

Level is observed in a transparent vertical tube attached to a closed tank. The bottom of the tube is connected to the liquid space and the top of the tube to the gaseous space. The liquid level in the tube corresponds with the level in the tank and may be observed or measured against a graduated scale. Isolating valves usually are fitted in the upper and lower connecting pipes to allow for replacement of the transparent tube (which is usually of glass) without draining the tank (Fig. 1a).

Fig. 1  Direct visual and float-type measurements. (a) Gauge glass. (b) Float with electrical sensor. (c) Float with magnetic switches. (d) Float with buoyancy effect.

Closed tanks are often under some pressure, hence the use of external tubes able to withstand pressure rather than windows. In pressurized systems the upper and lower connecting tubes are fitted with ball check valves to avoid a dangerous discharge of fluid in the event of a tube rupture. For very high pressure systems, metallic ducts with thick glass windows are used. These windows are sometimes made refractive and artificially illuminated to show more clearly the difference between the two fluids (such as water and steam).

Remote measurement from float:

Where levels cannot be observed and hence measured directly, it is common to use a float and to indicate remotely the elevation of this float.

The float must be of an average density between that of the two fluids, the densities of which must be significantly different (such as water and air) to ensure that a sufficient buoyant force is generated on the float, with changing level, to activate the position-sensing mechanism.

The float may generate an analog signal which varies over the whole range of operating level, or may generate an on-off signal as the level rises above or falls below a predetermined elevation. A series of on-off sensors at different elevations can generate a digital type of level indication.

Float with mechanical indicator:

Several simple methods allow the float position and hence level to be observed indirectly from outside the vessel containing the fluids. A vertical rod attached to the float and protruding through a hole in the top of the tank can show the level by the amount of protrusion of the rod, which may be graduated. An external weight attached to the float by a rope or tape running over a pulley at the top of the tank can show the level on an inverse scale outside the tank. A rotating shaft passing through a sealed hole in the side of the tank can be connected to the float by a lever so that any rise or fall in the level rotates the shaft appropriately and so moves a pointer on an external graduated scale.

Float with electrical resistance sensor:

In a closed vessel a float having a lever connected to a variable-resistance sensor can cause a change in the electrical resistance as the level rises or falls. The change in electrical resistance can be used, via a suitable calibrated electrical instrument, to indicate the level or volume in the tank (Fig. 1b).

Float with magnetic switches:

In a closed tank a float pivoting about a fixed point can move a magnet close to the wall of the tank down or up as the level rises or falls about a selected level. A similar magnet just outside the tank is flipped by magnetic repulsion to operate electrical contacts to give a corresponding on-off signal. Magnetic repulsion rather than magnetic attraction is employed to create a definite toggle effect (Fig. 1c).

Float with buoyancy effect:

A float constrained in its vertical movement will exert a varying force on the restraining mechanism. This force in turn can be measured or converted into an analog electrical signal which can be calibrated to indicate the liquid level. In such an application, level can be measured only over the height of the float, so such floats usually have slender dimensions (Fig. 1d).

Measurement by hydrostatic pressure:

The pressure p at any depth h in a liquid of density ρ is given by the following equation, p = ρgh, where g is acceleration due to gravity. Hence, if the density of liquid is known, the depth of liquid above a selected point can be determined by measuring the pressure in the liquid at that point.

Pressure gauge:

In an application with a simple pressure gauge, it may be calibrated to give a direct reading of the depth of liquid. If the tank is closed and there is pressure in the space above the liquid surface, the difference in pressure between this space and the measuring point must be used.

Pressure diaphragm:

In applications where the liquid may be contaminated with aggressive impurities or contain solids or sludge, pressure gauges or their pressure-measuring tappings may become blocked and unresponsive. In such cases, pressure diaphragms installed flush with the inside surface of the vessel may be used. Movement of the diaphragm against a spring may be measured directly to give an indication of the pressure or depth of liquid. However, since such movement is limited, it is better to apply a corresponding pressure to the back of the diaphragm so as to restore the diaphragm to its neutral position. Measurement of this external pressure then can be converted into a measure of the depth of liquid (Fig. 2a).

Fig. 2  Hydrostatic pressure and acoustic-wave measurements. (a) Pressure diaphragm. (b) Bubble tube. (c) Manometer. (d) Acoustic.

Bubble tube:

In order to discharge gas from a pipe or vessel into a liquid at some depth, the pressure of the gas must be at least equal to that of the liquid at that depth. Hence the depth of liquid at the point of discharge can be determined from the gas pressure, provided the gas flow rate is low enough to eliminate dynamic or frictional effects. A pipe supplied with a steady but low flow of gas (such as air) may be inserted into a tank of liquid and the gas pressure in the pipe measured. This pressure measurement can then be converted into a measurement of the depth of liquid above the point of discharge (Fig. 2b).

Manometer:

A manometer may be used instead of a pressure gauge for measuring pressure (more correctly pressure difference). A manometer using mercury as the reference liquid reduces the level variation by a factor of about 13, making direct measurement more convenient, and is more sensitive than a pressure gauge. Its location relative to that of the vessel in which the liquid level is being measured usually necessitates twin pipes from each side of the manometer extending back to the measuring points and filled with the same liquid as in the vessel (Fig. 2c).

Measurement by fluid properties:

Certain fluid properties can be readily measured or used to determine the presence or extent of a known fluid. The presence or absence of a fluid can provide an on-off signal indicating whether a certain level has been reached or not, whereas the extent of a fluid can be used to determine the depth.

Conductivity:

If a liquid is a conductor of electricity, its presence can be detected by a pair of electrodes subject to a potential difference. When immersed on rising level, they can generate an off-on electrical signal.

Capacitance:

If a liquid is a dielectric, probes can be inserted into a tank and the capacitance between them measured. This will vary with the degree of immersion and can be converted to a measurement of level. 

Acoustic:

Most liquids conduct sound waves readily, and these are reflected from any interfaces, including the liquid surface. If an acoustic transmitter-receiver located at the bottom of a tank directs sound waves vertically upward and senses their reflection from the surface, the depth, and hence level, can be determined by the time taken for the sound wave to travel up and be reflected down (Fig. 2d). 

Nuclear:

Since gamma rays are absorbed by many liquids, the presence of liquid can be sensed by the attenuation of gamma rays emitted from a gamma-ray source and measured by a detector a short distance away. 

Thermal:

If an electrically heated thermistor is subject to immersion in a liquid, it will be cooled more effectively. The resulting drop in temperature will be reflected as a change in resistance which will indicate the presence of the liquid.  

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